US 5668884

Patent Overview — US 5,668,884 A

Title: Enhanced concert audio system

Source: https://patents.google.com/patent/US5668884/en

Assignees/Inventors: Original and current assignee listed as Clair Bros Audio Enterprises Inc.; inventors Roy Barry Clair, Jr. and Ronald D. Borthwick.

Key dates: Priority date 1992-07-30; filing date 1995-01-30 (application US08/380,511); publication/grant 1997-09-16; anticipated expiration 2014-09-16. Legal status: Ceased. A related continuation was filed 1999-09-16 leading to US RE38,405 E1.

Purpose: To enhance sound received by audiences at varying distances from main loudspeakers. The excerpt explains that as loudspeaker sound travels, frequency/time distortion and non-linear distortion occur, and loudness decreases with distance; raising source SPL to compensate adds further distortion. The invention addresses this by producing "augmentation sound" near listeners that reinforces, rather than degrades, the main arriving sound.

Plain-language scope: A sound system has at least one main loudspeaker (or cluster) at a first position producing primary sound from an audio source. A transmitting subsystem wirelessly sends a signal representing the audio-source signal to portable receiver/transducer subsystems (e.g., headphones or small speaker systems) carried or worn by listeners at remote locations. Time delay means delay the electrical signal to the transducer by a period corresponding generally to the time primary sound takes to travel from the main loudspeaker to the remote location, so augmentation sound and primary sound reach the listener's ears in substantial synchronism. In a multi-zone embodiment, N transmitters and N receiver/transducer subsystems use N wireless channels, each zone's signal delayed a predetermined amount based on that zone's distance from the main source; listeners tune to their zone's channel. The method claim covers providing a portable augmentation system, producing primary sound, wirelessly transmitting a corresponding signal, and delaying output so both sounds arrive in synchronism.

What the excerpt does not establish: The excerpt omits full claim language, the detailed description of transmitter/receiver circuits, FIGS. 1–3 content, and the complete prior-art discussion. It does not establish litigation, ownership transfers, validity, or the full prior-art record. The listed assignee/status are explicitly noted in the source as assumptions, not legal conclusions.

I'll research litigation involving US patent 5668884. Let me run several targeted searches.

The initial searches returned mostly false positives (the "888" in OxyContin cases refers to a different patent). Let me search patent-litigation-specific sources.

Bottom line

I found no litigation involving U.S. Patent 5,668,884 in any of the sources I was able to search. No district court docket, Court of Appeals for the Federal Circuit appeal, PTAB proceeding, ITC investigation, or foreign counterpart action naming US 5,668,884 (or its reissue RE38,405) surfaced. I cannot rule out that such a case exists in a paywalled database, but I will not invent one.

Litigation table

Important analytical point: why '884 itself would rarely appear as an asserted patent

US 5,668,884 is part of a three-patent U.S. family, and the family was reissued:

• US 5,432,858 — appl. 08/213,136, filed 1994‑03‑15, granted 1995‑07‑11 (continuation of appl. 07/922,448, filed 1992‑07‑30, abandoned)
• US 5,668,884 — appl. 08/380,511, filed 1995‑01‑30, granted 1997‑09‑16 (continuation of 08/213,136) — the patent you asked about
• US RE38,405 E — appl. 09/398,021, filed 1999‑09‑16, granted 2004‑01‑27 (reissue of the '884 lineage)

Under 35 U.S.C. § 252, once the reissue granted, the original '884 patent was surrendered. Consequently, any enforcement activity during the patent's commercially relevant window (roughly 2000–2014) would almost certainly have been pleaded on RE38,405, not on 5,668,884. That is likely the single biggest reason a litigation search keyed strictly to "5,668,884" returns nothing. Anyone running a real clearance/FTO or damages search should search the reissue number and the '858 parent as well — and, as noted, those searches also returned no hits.

Note also the family's non‑US counterparts, which would be the vehicles for any ex‑US enforcement: EP0653144B1, JP3134254B2, AU673372B2, CA2139866A1, DE69322393T2, WO1994004010A1. All are shown as expired/ceased/abandoned, and no opposition or nullity litigation was found.

What I actually searched (for transparency)

• Google Patents family page for US5668884 (https://patents.google.com/patent/US5668884/en) — the authoritative record you supplied. It shows the full "Cited By" and "Citations" lists, family members, and country status, but contains no litigation field/entry for the patent. Its "Cited By" list is prior-art citation data, not litigation.
• Unified Patents litigation portal (https://portal.unifiedpatents.com/litigation/caselist) — no case list mentioning 5,668,884.
• Targeted litigation queries combining the patent number and the assignee: "Clair Bros Audio Enterprises" / "Clair Brothers Audio" + infringement/lawsuit; "RE38405" + litigation. The only hits were false positives — most notably the OxyContin antitrust/MDL rulings, where "the '888 patent" is Purdue's U.S. Patent No. 6,488,888-family drug patent, a completely unrelated patent. I did not auto-correct those; I simply excluded them as non-matching.
• Third-party patents that cite 5,668,884 as prior art (e.g., US 7,110,552 "Personal listening device for arena events," US 9,504,335, US 2012/0087507 by Clair Brothers Audio Enterprises) appeared in results — but a citation relationship is not litigation of '884.

Caveats and recommended next steps

Please treat this as "not found," not as "none exists." My searches did not reach: Docket Navigator, Lex Machina, Darts‑IP, MaxVal/Litigation Datasource, or PACER directly, all of which require subscriptions or per‑docket access, and none of which I could query here. The instruction to search PACER/CAFC could not be fully satisfied with the tools available.

To close this out properly I'd recommend:

1. Docket Navigator or Lex Machina — search patent number 5668884 and RE38405 and 5432858, in all district courts and the Federal Circuit.
2. PTAB / USPTO PatentCenter — check the '884 and RE38,405 file histories for reissue, disclaimer, or prior-art protest activity.
3. USPTO Assignment records — verify whether the Clair Bros. Audio Enterprises portfolio was ever transferred to an assertion entity; a change of ownership is the usual trigger for enforcement suits.
4. Foreign: Darts‑IP / national registers for EP0653144, JP3134254, AU673372.

If you'd like, I can draft the specific Docket Navigator / Lex Machina search strings (patent-number plus reissue-number variants) for a paralegal to run.

I'll verify the structured ODP result with targeted searches before reporting.

Proceedings overview

Count: zero AIA trial proceedings on file. The structured ODP block reports no AIA trial proceedings for US 5668884, and my independent searches returned no IPR, PGR, or CBM petition (instituted, denied, or terminated) naming US 5668884 or its reissue US RE38405 as the challenged patent — so the breakdown is 0 active / 0 claims invalidated / 0 claims sustained / 0 settled / 0 institution denials. The bottom-line defensive posture is unusual: this is not a "hardened" patent, it is an expired one. The record shows an anticipated expiration of 2014-09-16 and a current legal status of "Ceased" (source: https://patents.google.com/patent/US5668884/en), while the AIA IPR regime opened in September 2012 — meaning for roughly the final two years of the patent's life it was IPR-eligible and attracted no petitioner. A defendant today cannot win an IPR race, because there is nothing left to cancel; the defense is temporal, not invalidity-based.

Because the structured data is authoritative and discloses no proceedings, I am not going to generate per-proceeding sections. Under your constraints I will not invent proceeding numbers, panels, or dispositions to fill the template. What follows is the honest negative finding, the reason it matters, and the residual risk surface.

No proceedings to report — verification trail

• Structured source (canonical): USPTO ODP via the supplied "PTAB proceedings on file" block — returns no AIA trial proceedings as of most recent ingest.
• Independent web checks run: searches for US5668884 IPR PTAB, "5,668,884" IPR petition, "RE38405" IPR, and Clair Bros Audio ... IPR petition returned no PTAB petition, institution decision, FWD, or termination for this family. Results instead surfaced the patent only as prior art cited by others (e.g., Unified Patents' portal lists US-5668884-A among citations against later Sonos-family patents: https://portal.unifiedpatents.com/patents/patent/US-[10359987](/patent/10359987)-B2).
• Two "hits" I deliberately did not treat as evidence: search results mentioning IPR2025-00819 and IPR2025-00372 are unrelated patents (Align/US 11,154,384; Skyworks/US 9,917,563). Neither is US 5668884. Likewise the "RE38405" hits include a Drosophila cDNA clone ID — same alphanumeric string, unrelated art.
• Caveat on completeness: USPTO's Office of Patent Legal Administration has no public IPR index I can query directly from this environment, so my confirmation rests on the ODP block plus search. If you need a belt-and-suspenders check, run the patent number through PTAB E2E (https://www.uspto.gov/ptab) and confirm the "no proceedings" result yourself.

Strategic summary

Claim status. All eight claims of US 5668884 — independent claims 1, 4, and 6, and dependent claims 2, 3, 5, 7, and 8 — are UNTESTED in any AIA trial. None is canceled; none was adjudicated valid. "Untested" is materially worse than "sustained" for a patent owner here, because the reason is not that the Board vindicated the claims but that the patent's term ran out before anyone had to bring a petition. The claims were never substantively examined by the Board, and I found no FWD, so I cannot tell you the claims are strong. Verbatim claim language is available at https://patents.google.com/patent/US5668884/en (Claims (8)).

Why no IPR, and why that is not a validation signal. US 5668884 issued 1997-09-16 with a priority date of 1992-07-30. It is a pre-AIA patent, so PGR was never available (PGR applies only to patents with effective filing dates on or after 2013-03-16), and CBM was never available — the claims are directed to concert audio augmentation (H04R 27/00, H04R 5/00 per the classification block), not to a "financial product or service," so the AIA § 18 gate never opened. That leaves IPR as the only real post-issuance AIA lever, available from 2012-09-16 until the patent's term ended on 2014-09-16. A two-year window with no filed petition is a weak signal: likely drivers include the patent owner's low assertion activity, the difficulty of monetizing an expired asset, and the fact that the claims carry a narrow "low maximum output sound level … heard by people only within a range of several feet" limitation that invites non-infringement rather than invalidity work. Do not read "no IPRs" as "unassailable."

Estoppel landscape — § 315(e)(2) is a non-issue. With no IPRs, no petitioner ever incurred § 315(e)(2) estoppel, and every prior-art ground remains available to a defendant — subject to the far more important point below. The complete 14-reference citation set from prosecution is public on the patent's face (https://patents.google.com/patent/US5668884/en): US 2,567,431 (Halstead); US 3,235,804 (McIntosh); FR 2,006,116 (Matsushita); US 4,165,487 (Corderman); JP S55-77295 (Matsushita); JP S57-202138 (Nippon Hoso — "Headphone concert system"); US 4,610,024 (Sony); US 4,618,987 (Deutsche Post); US 4,829,500 (Saunders); US 4,899,388 (Koss); US 4,993,074 (Carroll); WO 1992/005673 (Nouveaux Studios Merjithur); US 5,058,169 (Temmer); US 5,131,051 (Yamaha), plus the non-patent literature Popescu, "Sound Reinforcement Systems in Early Danish Churches," J. Audio Eng. Soc., Vol. 28, No. 10, Oct. 1980, pp. 713-717. Several of these (Halstead, McIntosh, Corderman, the Japanese "headphone concert system," and Popescu) map onto the core wireless-broadcast-plus-delay concept and would be the natural starting kit for an invalidity contention — but see the next paragraph for why you probably never need them.

Pattern signals. No repeat-petitioner pattern exists because there is no petitioner at all. No defensive aggregator (Unified Patents, RPX, et al.) appears in the chain. On the patent-owner side, the family shows active prosecution rather than aggressive post-grant litigating: a broadcast reissue was sought (app 09/398,021 filed 1999-09-16; US RE38405 granted 2004-01-27, https://patents.google.com/patent/USRE38405/en) and the same assignee later filed a continuing disclosure, US 2012/0087507 A1 / WO 2012/048299 A1, "Method and System for Enhancing Sound" (2012-04-12, Clair Brothers Audio Enterprises, Inc.). One timing detail worth flagging, because it cuts in a defendant's favor if you are ever dragged into a past infringement dispute: the reissue application was filed exactly two years after the original grant date, which sits at the 35 U.S.C. § 251(d) deadline for a broadening reissue. I have not verified from the reissue file history whether RE38405 in fact broadened the claims (the published RE38405 claim 1 tracks original claim 1, and the truncated listing suggests a possible added claim 9 that I could not read in full). Verify this before relying on it. If RE38405 did broaden, § 252 intervening rights are live for anyone who made substantial preparations between 1997-09-16 and the reissue grant.

Recommended next steps

1. Lead with expiration, not invalidity. US 5668884's anticipated expiration is 2014-09-16 and its status is "Ceased." If a demand letter today cites claims 1-8, the patent owner cannot recover for any post-2014-09-16 conduct. Damages would be confined to acts during the term and within the 35 U.S.C. § 286 six-year lookback — and since expiration was over eleven years ago, the entire actionable window is at least a decade stale. Ask the sender to identify the specific accused acts and their dates; most such letters cannot.
2. Force them to name the right patent. Before responding, confirm whether the assertion is on US 5668884, on US RE38405 (same family, same expired term), or on a descendant such as US 2012/0087507 A1 / WO 2012/048299 A1. A demand citing the '884 patent while relying on a live descendant is a drafting error worth exposing in your response.
3. Do not spend money on an IPR-based defense. There is no trial to file, no PTAB activity to check for trial-stage deadlines, and no institution deadline to track — the structured ODP record is empty and the patent expired. The absence of any proceeding here is not "hardened patent" evidence; it is an expired-asset signal, and your cheapest disposition is a term-expiration and § 286 response rather than an invalidity campaign.
4. Preserve the invalidity record anyway, cheaply. If past-term damages are genuinely in play, the prosecution citation set above (especially Halstead, McIntosh, Corderman, JP S57-202138, Popescu 1980, and WO 1992/005673) is the roadmap for a § 102/§ 103 theory against claims 1, 4, and 6. Note that with no prior IPR, no estoppel attaches — but also no Board construction exists, so any construction fight will be de novo.
5. Independent double-check, if the stakes justify it. Run US 5668884 and US RE38405 through PTAB E2E (https://www.uspto.gov/ptab) and through CourtListener (https://www.courtlistener.com/) for any district-court or Federal Circuit proceedings this patent's absence from the PTAB record would not capture. I found no CAFC appeal to report — there is no FWD to appeal from — but district-court dockets are outside what I could confirm here, and that is the one gap in this analysis I would close before advising a client to ignore a demand.

Confidence and limits: High confidence in the negative PTAB finding (two independent sources agree). High confidence in the expiration date and "Ceased" status as reported. Moderate-to-low confidence on the broadening-reissue and added-claim-9 inferences — those are flagged above and should be verified against the RE38405 file history before use. I found no judge panels, petition grounds, institution decisions, FWDs, settlements, or appeals to report, because no proceeding exists; I have not fabricated any to fill the template.

I'll research the assignment chain for US 5668884, starting with the USPTO Assignment Center and cross-referencing sources.

Let me try the USPTO Assignment Center directly and look into Clair Bros' corporate history.

Let me probe the assignment database directly and check the reissue sibling and any holding entities.

Ownership Chain — US 5,668,884 ("Enhanced concert audio system")

⚠️ Access limitation up front (read before the tables). I was not able to retrieve the recorded assignment records for US 5,668,884 from the USPTO Assignment Center / Assignment Search. The searches I could run returned only generic Assignment Center landing/help pages (e.g., the new assignmentcenter.uspto.gov system and the note that legacy Patent Assignment Search and Assignments-on-the-Web were retired on 2025‑09‑27 and 2025‑10‑20 respectively), not a chain-of-title record for this patent number. Consequently I have no reel/frame numbers, no correspondent names, and no assignor/assignee execution dates to report. I will not invent them. Everything below that is not sourced to a primary record is flagged as such. Where Google Patents or Trustoria surfaces ownership data, I cite it.

One important note on the litigation section I previously generated: it flagged that enforcement of this family would run through the reissue RE38,405, not the '884 patent, because the '884 patent was surrendered on reissue. That analysis is consistent with what follows and materially affects how any assignment search should be scoped (search 5668884, RE38405, and 5432858).

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Inventors

Unusual-pattern check (all inventors leaving within 12 months of filing): No such pattern. The inventor set is a company principal (Barry Clair, son of founder Roy Clair) plus a long‑tenured engineer. This is the opposite of the "inventors bail out right after filing" fire‑sale precursor — it reflects an operating company protecting an in‑house development. Note the two inventors are not the company's founders (Roy Clair / Gene Clair are not named), so this is a mid‑level technical‑leadership filing, not a founder assignment.

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Original assignee

Clair Bros. Audio Enterprises, Inc. (a/k/a "Clair Brothers Audio Enterprises"), Lititz, Pennsylvania (Google Patents "Current Assignee / Original Assignee"; Trustoria confirms the assignee of record on RE38,405 as Clair Bros. Audio Enterprises, Inc., Lititz, PA).

• Primary line of business: professional sound‑reinforcement / live‑touring production support, plus a fixed‑installation business. Incorporated 1970 (per Wikipedia/Clair Global history); the clairbrothers.com site notes "For 57 years, Clair Brothers was a trusted name in professional audio."
• Did they ship product embodying the claims? The patent claims a personal, wirelessly‑fed, delay‑synchronized augmentation headset system for concert audiences (claims 1 and 6, plus dependent claims 2, 3, 4, 5, 7, 8). I found no product literature evidencing a commercialized embodiment of these specific claims, and the specification is largely aspirational/architectural (it cites off‑the‑shelf third‑party gear such as the T.C. Electronic 1280DDL delay, Lectrosonics PRS‑72 receiver, and an Etymotic T‑72 transmitter). This is a genuine uncertainty, not a proven absence of product — I could not verify a market‑available "personal augmentation headset" from Clair.
• Current status: Operating. Rebranded to Clair Global in 2008; the separately‑branded loudspeaker manufacturing arm Clair Brothers ceased loudspeaker manufacturing in 2023 (per clairbrothers.com) while Clair Global continues as a privately held, debt‑financed operating company (PitchBook: latest deal $100M debt, 2025‑07‑09; multiple acquisitions through 2025). Co‑founder Gene Clair died December 2013; Troy Clair is president/CEO. No bankruptcy or Chapter 7/11 filing found for the original assignee.

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Assignment timeline

I cannot produce a reel/frame assignment timeline. As stated above, the USPTO Assignment Center records for US 5,668,884 were not retrievable with the tools available to me, so I have no executed dates, recording dates, reel/frame numbers, conveyances, or correspondents to list. Per the task's instruction I am stating that plainly rather than fabricating entries.

What the primary and secondary records do establish about the chain of title, without reel/frame granularity:

• The application lineage (Google Patents, front matter) shows the family stayed inside the same assignee throughout prosecution:
  • 07/922,448 — filed 1992‑07‑30, abandoned
  • 08/213,136 — filed 1994‑03‑15 → US 5,432,858 (granted 1995‑07‑11)
  • 08/380,511 — filed 1995‑01‑30 → US 5,668,884 (granted 1997‑09‑16) ← the patent at issue
  • 09/398,021 — filed 1999‑09‑16 → US RE38,405 E (granted 2004‑01‑27)
• A reissue is a strong ownership datum. A reissue application may only be filed by the current owner of the patent (35 U.S.C. § 251). Because RE38,405 was filed 1999‑09‑16 and names Clair Bros. Audio Enterprises, Inc. as assignee, the original assignee still owned the family as of 1999 — i.e., no third‑party assignment had intervened between issuance (1997) and the reissue filing (1999).
• No post‑issuance assignment surfaced in any indexed source I could reach (Google Patents legal events, Justia, Trustoria, general web). That is consistent with the original assignee retaining ownership — but it is not the same as a confirmed "no records" finding, because I could not query the Assignment Center itself.

If the Assignment Center genuinely returns no post‑issuance records for this patent, that is itself the finding (original assignee retained title). The honest state of my evidence is "not retrieved / not found," not "proven absent."

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Timeline diagram

Encoded from the prosecution/ownership data above (not from reel/frame records, which I could not obtain):

timeline
    title Ownership timeline for US 5668884
    1992 : Priority application filed
         : Inventors Clair Jr and Borthwick
    1994 : Continuation application filed
    1995 : Second continuation filed
    1997 : US 5668884 granted to Clair Bros
    1999 : Reissue application filed by owner
    2004 : Reissue RE38405 granted
    2008 : Clair Bros rebrands as Clair Global
    2012 : Related Clair Brothers application filed
    2014 : Anticipated patent term expiration

(The 2012 entry cross‑references US 2012/0087507 A1, "Method and System for Enhancing Sound," assignee Clair Brothers Audio Enterprises, Inc., filed 2010‑10‑07 — evidence the operating company continued to pursue this technology area long after the '884 issued.)

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NPE / troll-pattern signals

Given the retrieval failure, several signals cannot be affirmatively cleared with reel/frame-level evidence. I mark them accordingly rather than defaulting to "not present."

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Verdict

Insufficient data — justified because the definitive evidence (the Assignment Center chain of title with reel/frame entries and correspondents) could not be retrieved, so I cannot confirm the terminal owner with record‑level proof. What is affirmatively established points away from an NPE posture and toward continued operating‑company ownership: the only assignee of record surfaced is Clair Bros. Audio Enterprises, Inc. of Lititz, PA; the company was still the owner when it filed the reissue application 09/398,021 on 1999‑09‑16 (a § 251 owner‑only filing); the company is a going concern rebranded as Clair Global; and it was still filing related applications in this space as late as 2010‑10‑07 (US 2012/0087507 A1). No shell entity, no known asserter, no cascading transfers, and no bankruptcy appear in the record — but I did not find a single reel/frame entry, so I will not upgrade this to a positive "operating‑company assertion" call.

To close the gap (recommended next steps)

1. USPTO Assignment Center → Assignment Search (https://assignmentcenter.uspto.gov/), search by patent 5668884 and RE38405 and 5432858. Capture reel/frame, execution date, recording date, conveyance type, assignor, assignee, and — critically — correspondent/firm for each hit. If the result set is empty, record that as the finding.
2. USPTO PatentCenter file histories for 08/380,511 and 09/398,021 — the reissue file will contain the § 251 ownership declaration, which names the owner of record as of 1999 and any intervening transferees.
3. USPTO Assignment records / SEC — confirm whether the Clair Bros. → Clair Global reorganization (2008 rebrand) generated a Change of Name recording, and whether any Clair patent assets were later assigned to a financing/secured‑party (the PitchBook debt financings, e.g., $300M in 2022 and $100M in 2025, could have produced Security Agreement recordings against the portfolio).
4. Because of the reissue surrender point from my earlier litigation section, any FTO/damages search must be keyed to RE38,405, not the '884 number alone — the '884 claims were surrendered on 2004‑01‑27.

I'll research the prior-art references cited on the face of US 5,668,884. Let me run parallel searches on the most substantive citations.

Let me gather details on the remaining citations, focusing on the headphone/patron broadcast references most relevant to the claims.

Prior-Art Analysis — US 5,668,884 ("Enhanced concert audio system")

Scope and method note (read first)

Source of the citation list. The authoritative full text you supplied (fetched from Google Patents, which mirrors the USPTO record) lists exactly 14 patent citations and 1 non-patent citation on the face of US 5,668,884. I treated that list as the definitive universe of "each patent citation for 5668884" and did not substitute similar numbers. No other "5668884"-like identifier surfaced that could be mistaken for it.

What I could and could not retrieve. I retrieved substantive full text or detailed abstracts for four of the references (US 4,165,487 Corderman; US 5,058,169 Temmer; US 5,131,051 Yamaha; US 4,829,500 Saunders). Tool-call budget ran out before I could pull full text on the remaining ten; for those I rely on the title, inventor/assignee, and dates as printed on the '884 face plus, where limited search snippets appeared, the partial detail noted. I flag each such case explicitly rather than filling gaps from memory.

Critical framing for the §102 question. All 14 references pre-date the '884 priority date of 1992-07-30 (the latest, US 5,131,051, issued 1992-07-14 — 16 days earlier). So all are date-qualified as prior art under pre-AIA §102. But date-qualification is not anticipation. The examiner cited these references as the closest art; the '884 claims then issued over them — meaning the Office did not treat any of them as anticipating. My evaluation below confirms that reading: no single cited reference discloses all elements of any independent claim (1, 4, or 6). The appended references are best characterized as teaching individual sub-elements of the claimed combination, which raises a §103 obviousness question, not §102 anticipation.

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The 14 cited patent references

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Per-reference analysis

Reference 4 — US 4,165,487 (Corderman) — strongest "wireless-to-patron" art

Full citation: US 4,165,487 A, Corderman, Roy C., "Low power system and method for communicating audio information to patrons having portable radio receivers," filed 1978-04-10, issued 1979-08-21.
What it discloses (confirmed from full text): An audio signal is tapped from a public-address system, modulated onto an RF carrier, and transmitted at very low power via a loop/strip antenna to portable AM receivers carried by patrons in an auditorium; a "tuning aid" ferrite antenna near the entrance lets patrons tune for maximum signal. It is expressly aimed at hard-of-hearing patrons and the receivers are conventional AM radios.
Claim overlap:

• Claim 1 / Claim 6 — teaches "transmission means for converting said first electrical signal into a transmission signal and for wirelessly transmitting" and a "receiver for receiving and converting said transmission signal." This reference maps cleanly onto the transmit/receive sub-elements.
• Does not teach the delay synchronized to the main-loudspeaker sound-propagation time, the low-maximum-output transducer limited to a several-foot listening range, the single predetermined channel in the claimed arrangement, or a body-worn personal unit with in-unit delay.
• §102 verdict: no anticipation of claims 1, 4, or 6 — it lacks the delay/synchronization element entirely. It is, however, the most pertinent §103 teaching for the broadcast-to-personal-receiver leg.

Reference 13 — US 5,058,169 (Temmer) — strongest "delay-by-propagation-time" art

Full citation: US 5,058,169 A, Temmer, Stephen F., "Public address system," filed 1989-11-01, issued 1991-10-15.
What it discloses (confirmed from full text): A tubular-member distributed speaker system for large reverberant spaces (subway stations etc.) in which "delay element 36" delays the audio signal to a corresponding speaker "by a differential time period selected to compensate for time that it takes for sound to travel from one speaker to the next," optionally increased ~14–20 ms for the Haas/precedence effect. Claim 15 claims "delay means for delaying the electrical signals to at least one speaker by a preselected duration."
Claim overlap:

• Directly addresses the claimed "delay means for delaying … by a period of time substantially equal to an amount of time required for said primary sound to reach [the listener]" — the conceptual heart of '884.
• But Temmer's delayed transducers are hard-wired fixed speakers in a tubular array, not a wireless personal unit; there is no transmission means, no receiver, no low-output personal transducer.
• §102 verdict: no anticipation. It is the closest art on the delay-to-compensate-propagation-time element and a strong §103 reference against that limitation, but it cannot alone render claims 1/4/6 anticipated.

Reference 7 — JP S57-202138 (Nippon Hoso) — "Headphone concert system"

Full citation: JP S57-202138 A, Nippon Hoso K.K., "Headphone concert system," 1981-06-06 / 1982-12-10.
Significance: By title alone this is the closest "headphone-at-a-concert" reference on the face of the patent — it was cited by the examiner and appears in the "Cited By" lineage of later Clair-family patents. Its title indicates a system delivering concert audio to headphones.
Limitation: I could not retrieve its specification/disclosure text within the tool budget. I therefore cannot state what it does or does not disclose with confidence. Based on the title and the fact that '884 issued over it, it presumably discloses wireless headphone delivery of a concert program without the in-unit, sound-propagation-matched delay that the '884 claims require, but this is an inference, not a verified reading. If you want a defensible §102 statement against claim 1/6, this is the reference to pull in full (JPO/J-PlatPat) first.

Reference 8 — US 4,829,500 (Saunders) — wireless/portable transducer

Full citation: US 4,829,500 A, Saunders, Stuart D., "Portable wireless sound reproduction system," filed 1982-10-04, issued 1989-05-09.
What it discloses (confirmed): A portable housing with two transmitters broadcasting to detachably-connected remote speaker assemblies with receivers; the speakers may be detached and located remotely for stereo. Centralized volume/tone/balance control; low-power local transmission.
Claim overlap: Teaches "transmission means … wirelessly transmitting" and "personal electro-acoustic transducer unit … a receiver." Does not address a main concert loudspeaker, propagation-time delay, or a low-maximum-output several-foot listening range. §102 verdict: no anticipation; §103 interest only for the wireless-transducer leg.

Reference 14 — US 5,131,051 (Yamaha)

Full citation: US 5,131,051 A, Yamaha Corp. (Kishinaga et al.), "Method and apparatus for controlling the sound field in auditoriums," 1989-11-28 / 1992-07-14.
What it discloses (confirmed): Assisted-acoustics / active field control — microphones and loudspeakers coupled through FIR/IIR filters to equalize power-decay coefficients between stage and audience areas (reverberation and early-reflection control). Uses delay/matrix processing for loudspeaker outputs.
Claim overlap: Relates to improving auditorium acoustics with distributed loudspeakers and signal processing, but not to a wireless personal augmentation unit, not to a low-output body-worn transducer, and not to delaying a broadcast signal to match main-loudspeaker arrival at a listener. §102 verdict: no anticipation; background art on multi-channel auditorium delay/processing.

References 1, 2, 3, 5, 6, 9, 10, 11, 12 — secondary/background art

For these I have title + dates only (no full text retrieved), so my descriptions intentionally stay at the level the face permits:

• US 2,567,431 (Halstead, 1947/1951) — "Communications system of restricted-range type." Restricted-range (inductive/limited-field) communication; classic hearing-assistance lineage. Likely teaches localized wireless delivery to a personal receiver. No anticipation (no delay-synchronization to a main loudspeaker).
• US 3,235,804 (Mcintosh, 1959/1966) — "Receiver for lecture broadcasting system." Patron/lecture-assist receiver. No anticipation.
• FR 2,006,116 (Matsushita, 1968/1969) — French counterpart family for a headphone-type receiver (per later citation context). No anticipation on the face.
• JP S55-77295 (Matsushita, 1978/1980) — "Acoustic reproducing device." No anticipation on the face.
• US 4,610,024 (Sony, 1979/1986) — "Audio apparatus." No anticipation on the face.
• US 4,618,987 (Deutsche Post/RFZ, 1983/1986) — "Large-area acoustic radiation system." Distributed large-area sound reproduction; likely includes delay for speaker arrays, but not a wireless personal unit. No anticipation on the face.
• US 4,899,388 (Koss, 1988/1990) — "Infrared stereo speaker system." Wireless (IR) audio transmission to speakers/headphones — relevant to the wireless leg only. No anticipation.
• US 4,993,074 (Carroll, 1988/1991) — "Earphone spacer." Mechanical headphone accessory — not pertinent to any claim limitation.
• WO 1992/005673 (Nouveaux Studios Merjithur, 1989/1992) — "Polyphonic room with homogeneous sound." Room-acoustics/homogeneous-sound installation. No anticipation on the face.

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Non-patent citation

Dan Popescu, "Sound Reinforcement Systems in Early Danish Churches," Journal of the Audio Engineering Society, vol. 28, no. 10, Oct. 1980, pp. 713–717. The patent's own Background cites this for distributed loudspeaker systems using remotely located loudspeakers and delay equipment to synchronize amplified sound with the direct (live) sound — i.e., exactly the propagation-time-delay principle, applied to fixed wired loudspeakers. As a printed publication dated 1980 it qualifies under §102(b) relative to the 1992 priority date. §102 verdict: no anticipation of claims 1/4/6 (no wireless personal unit, no low-output body-worn transducer), but it is a strong §103 reference against the "delay to match arrival of the direct sound" element and is the reference the specification itself concedes.

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Consolidated §102 assessment

Bottom line: The cited art establishes that every sub-component of the '884 claims was known — wireless broadcast of a program to portable/patron receivers (Corderman '487; Koss '388; Saunders '500; Halstead '431), headphone delivery of a concert (JPS57-202138), and delaying a distributed loudspeaker's signal to match acoustic propagation time (Temmer '169; Popescu 1980; Deutsche Post '987). What no cited reference supplies — and what evidently carried the '884 claims to allowance — is the specific combination of a single-channel wireless transmission into a personal, low-maximum-output transducer whose delay is applied in the unit to match the main-loudspeaker arrival time. That is why I assess these references as §103 material, not §102 anticipators.

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Caveats and recommended verification

1. Ten of fourteen references were not read in full. My "no anticipation" calls on references 1, 2, 3, 5, 6, 7, 9, 10, 11, 12 rest on titles/dates plus the fact that the patent issued over them — not on verified claim-by-claim mapping. The single highest-value document to pull is JP S57-202138 ("Headphone concert system"), which by title is the nearest concert-headphone art and the only cited reference whose full disclosure could conceivably move a §102 analysis on claims 1/6.
2. Claim-4 "automatic adjustment" vs. the sibling's "detecting means." As flagged in the earlier summary, US 5,668,884 claim 4 recites an "automatic adjustment means" by function, whereas a third-party aggregator (RPX) reports the parent US 5,432,858 claim 1 as reciting a "detecting means for detecting an air-propagated signal." None of the 14 cited references is a microphone/air-signal-detection auto-sync reference, so the auto-sync feature was not met by the cited art under either wording. Verify claim text against the USPTO Patent Center PDF before relying on the distinction.
3. Reference 3 (FR 2,006,116) — one of my search results associated this Matsushita family with "Headphone type FM stereo receiver" (US 3,906,160 era). I did not verify the FR document itself; treat the description as unconfirmed.
4. Litigation/citation relationship ≠ prior art. As noted in the litigation section, several later patents cite '884 (e.g., US 7,110,552; US 2012/0087507 by Clair Brothers Audio Enterprises). Those are downstream, so they are not prior art to '884 and were correctly excluded here.
5. Governing law. Because the '884 priority date (1992-07-30) is pre-AIA, §102(a)/(b)/(e) analysis applies as described. I did not attempt to fix an invention date, so I have not asserted §102(a)-versus-(b) precisely for each reference.

I'll verify the key prior-art references from the patent's citation list before building the §103 combinations.

Now let me verify the delay-based and large-area radiation references.

Obviousness Analysis — U.S. Patent 5,668,884 under 35 U.S.C. § 103

Building on the claim breakdown and bibliographic sections generated earlier. I do not repeat them here except where a cross-reference is needed.

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1. Legal framework and the reference set I am permitted to use

Applicable law. Because the effective priority date is 1992-07-30, pre-AIA § 103(a) governs. The critical date for § 102(b) art is therefore 1991-07-30; later-filed but earlier-published U.S. patents and applications can still qualify under § 102(e)/(a).

The analysis proceeds under Graham v. John Deere Co., 383 U.S. 1 (1966) (scope and content of the prior art; differences; level of ordinary skill; secondary considerations), as refined by KSR Int'l Co. v. Teleflex Inc., 550 U.S. 398 (2007) — no rigid teaching–suggestion–motivation requirement; a combination of known elements with predictable results is obvious where a PHOSITA would have had a "reasonable expectation of success," and "a technique used to improve one device" may be applied to improve a similar device "in the same way" absent some teaching away.

Level of ordinary skill (my proposed definition). A degreed electrical/acoustical engineer (or equivalent) with ~2–4 years of experience in sound-reinforcement/auditorium audio, familiar with (a) delay lines used to time-align remote loudspeakers, (b) FM/AM/induction-loop and IR wireless program-distribution systems for audiences, and (c) portable receivers/headphones. This mirrors the actual experience of the named inventors and of the ordinary artisan of the early 1990s. The subject matter is a predictable art (component substitution and rearrangement of known audio elements), which lowers the bar for combinability.

Prior-art set used. Per the task, I rely on the Prior Art section of the patent page — the 14 "Citations" references plus the Popescu NPL — at https://patents.google.com/patent/[US5668884](/patent/US5668884)/en. I verified the four most load-bearing references against primary PDFs (Section 3). I flag below which references I could only characterise by title/abstract.

Additional reference I surfaced by search that is not on the face of '884 (flagging explicitly): US 1,642,040 (McCutchen, AT&T, 1927‑09‑13) — a public-address system in which, where "an acoustic delay in the air path will cause a bad blurring or echo effect," "the delay network … will be inserted in the circuit … to compensate for this delay. … the listener will not receive the speech from the horn in advance of the direct speech … but will receive the speech from the horn in the same phase with and at the same time as the direct speech." PDF: https://patentimages.storage.googleapis.com/b3/d3/e1/a155a4cd241e1a/US1642040.pdf. This is the cleanest single-teaching of the delay-compensation principle and should be added by any challenger, even though it is absent from Google's citation list.

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2. The limitations that carry the case

Distilling the claim set (see the earlier section for the full text), only five limitations need to be met by art; everything else is admitted or trivial:

Any invalidity theory must therefore be: wireless personal-receiver distribution (L2, L3) + air-path-compensating delay (L4) + placement/automation of that delay (L5a/L5b) — a two-reference combination at minimum, and the motivation is supplied on the face of the patent itself.

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3. Combination A — the primary § 103 theory (claims 1 and 6)

References: Corderman (US 4,165,487) as primary; Halstead (US 2,567,431) and/or McIntosh (US 3,235,804) for the personal-transducer aspect; Popescu (NPL) and/or McCutchen (US 1,642,040) for the delay; plus the applicant's own admitted prior art.

Element-by-element:

• L1 + L2 + L3 (Corderman): Corderman discloses a hall with a PA system driving main loudspeakers; the apparatus "receives an audio signal as an input … from a public address sound system" and rebroadcasts it on an AM carrier via a loop/strip antenna to portable AM receivers carried by patrons (see § 1 citation, esp. claims 1 and 12 of US 4,165,487). That is literally "converting said first electrical signal into a transmission signal and wirelessly transmitting" it, with the receiver in the hands/on the person of an audience member.
• L3 (Halstead/McIntosh): For the "auxiliary transducer" the combination adds the auditorium headphone of Halstead (headphone unit 42 associated with each chair/worn receiver) or the hearing-aid-type earpiece hung from the ear of McIntosh. Both are personal, low-output, and (in McIntosh's case) expressly described as isolating the ear from ambient noise — the very acoustical premise of the "several feet … in ambient sound" language.
• L4 (Popescu and/or McCutchen and/or US 4,618,987): The specification itself admits the technique: remote loudspeakers whose "input signals [are] delayed in time with respect to the signals provided to the main loudspeaker(s)/cluster(s) to synchronize their acoustic output with that arriving from the main loudspeaker(s)," citing Popescu. McCutchen adds the express rationale: prevent the reinforced sound from arriving before the direct sound, which produces "a bad blurring or echo effect."
• L5a (placement + single channel): Corderman already broadcasts one program (no multi-channel zoning needed for a single-language program), and the placement of the delay in the listener's receiver rather than upstream is a rearrangement of parts — see § 5.

Motivation (this is the strong part). The '884 specification supplies the motivation in its own words, and admissions in a specification are prior-art admissions:

• "as the sound produced by the loudspeakers travel over distance, distortion … naturally occur[s] … the perceived loudness and the sound pressure level decreases with increasing distance"; the audience "want[s] … the sound quality delivered to them by public address speaker systems approaching recording studio quality."
• The specification's stated design goal is to place the transducer as close to the listener as possible: "the location of the remote receiver/amplifier unit and its associated transducer device is preferably made as close as possible to the listener."

A PHOSITA seeking to deliver studio-quality sound to seats 100–300 ft from stage, faced with Halstead/Corderman's already-known technique of beaming the program to a personal receiver at the listener's ear, and with Popescu/McCutchen's already-known technique of delaying the reinforcement so it does not precede or smear the direct arrival, has every incentive to do exactly what claim 1 recites. The result — an augmentation arriving in phase with the direct sound — is the predictable, stated goal of both references. KSR forecloses any argument that the combination is non-obvious merely because these references are from different decades or different sub-fields (auditorium reinforcement vs. assistive listening). They are all in the same field of endeavor: audio distribution to seated audiences.

Reasonable expectation of success. Setting the delay is arithmetic: distance ÷ ~1130 ft/s (e.g., ~133 ms for a seat 150 ft from stage). Delay lines were commercially routine — the specification itself names the T.C. Electronics Model 1280DDL digital delay — and both Popescu (1980) and US 4,618,987 (1986) describe doing it in exactly this context. There is no unpredictability to shelter behind.

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4. Combination B — an alternative theory (claims 1, 6, and useful against 4)

References: US 4,618,987 (Deutsche Post) in view of Corderman and/or Halstead, and optionally US 5,131,051 (Yamaha).

US 4,618,987 discloses a system that, on its face, contains both halves of the concept in one environment: a distributed array of radiators in the reception area, each fed through delay and amplification devices whose control inputs are driven by a comparison-and-control device so that "the acoustic radiators each radiate only after movement past of the wave fronts of the original sound source … and the acoustic radiators which are closer to the source, and the time lags or amplifications are differentiated according to the capacity and class of the sources." Critically, the reference also teaches automation: "the approximately sound-path proportional control is executed automatically directly from the change of the microphone location" — i.e., a control loop that measures/deduces the source-to-listener relationship and sets the delay accordingly.

Combining US 4,618,987's automatic, propagation-time-proportional delay control with Corderman's or Halstead's wireless personal receiver plainly renders claim 4 obvious: the only difference is the substitution of a personal, wearable transducer for the reference's fixed radiator, and the substitution of the listener's position for the radiator position in the control algorithm — a predictable substitution with a known advantage (the "hard of hearing / distant patron" problem that Corderman, Halstead and McIntosh all address).

Note on claim 4's breadth. As printed (per the earlier section, and consistent with the reissue text), claim 4 does not require microphone-based sensing, does not require a single channel, and does not require the delay to be inside the personal unit. Its only distinguishing feature over claim 1 is the adverb "automatically." That makes claim 4 the weakest independent claim, and US 4,618,987's automatic control is directly on point.

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5. L5a in detail: why "single predetermined channel" and "delay in the unit" add nothing

Three independent reasons, all KSR-supported:

1. Rearrangement of parts / placement of a component. The specification itself discloses the same delay located upstream of the transmitter (FIG. 1, delay unit 36) and inside the receiver unit (FIGS. 2–3, elements 108/208). A patent applicant's own disclosure of both placements is a concession that the choice is a design option; moving a delay line from the transmit chain to the receive chain is the paradigm of an obvious change in arrangement of parts.
2. Functional trade-off. The specification explains the trade-off that drives the choice: more zones/channels buy finer time alignment but cost more transmitters; in the manual (FIG. 2) and self-synchronizing (FIG. 3) schemes "the entire audience is covered by a single transmitter zone." Choosing the single-channel/receiver-delay design to reduce transmitter count, spectrum use and cost is the ordinary engineering response to a stated cost/complexity trade-off — precisely the "design incentive … market forces" KSR says may render a variation obvious.
3. Purely conventional signalling. Transmitting program audio on "a single predetermined channel" is what Corderman (one AM carrier for one program) and McIntosh (each loop carries one program) already do. Halstead's multi-channel switchable receiver — the more sophisticated arrangement — does not make the simpler single-channel arrangement patentable; the direction of simplification is disclosed by the same art.

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6. Dependent claims 2, 3, 5, 7, 8

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7. § 102 adjacency: the reference I could not read

JPS 57‑202138 (Nippon Hoso Kyokai, published 1982‑12‑10), titled "Headphone concert system." On its title this is uncomfortably close to claims 1 and 6, and it is § 102(b) art (>1 year before 1992‑07‑30). I could not retrieve its disclosure and therefore do not assert what it contains. If its translated text discloses wireless headphone delivery of the concert program with a delay matched to the air path, it would convert Combination A from a § 103 case into a § 102 case for claim 1/6. Recommended action: obtain a JPO/J-PlatPat English abstract and full translation (and the same for JPS 55‑77295, US 5,058,169, US 5,131,051, US 4,829,500, US 4,899,388, US 4,610,024, FR 2,006,116 and WO 1992/005673) before finalising any opinion. Every reference I have marked "title only" above is a live gap in this analysis, not a null result.

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8. Secondary considerations (the patentee's side)

I found no evidenced nexus in the record I reviewed:

• No commercial-success or industry-praise evidence tied to the claim limitations (Clair Bros. Audio Enterprises' market position alone does not establish nexus).
• The long-felt need is articulated in the specification, but the solution (delay alignment for audience-distributed audio) had been published since at least 1927 (McCutchen), 1980 (Popescu) and 1986 (US 4,618,987) — which undercuts "failure of others."
• Cutting the other way: the family's "Cited By" record is large (353 third-party citations in Google's consolidated list), and the family-citing list includes US 5,619,582 (Oltman), "Enhanced concert audio process utilizing a synchronized headgear system," filed 1996‑01‑16 and granted 1997‑04‑08 — a near-simultaneous, independent arrival at the same solution. Independent invention this close in time is classic objective evidence that the claimed subject matter was within the ordinary artisan's grasp.

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9. Where the obviousness case is weakest (be candid with the client)

1. L5a's internal tension. Claim 1 requires both "delay means for delaying at least one of said first and said second electrical signals" and that the personal unit contain the delay means delaying the second signal — while claim 6's method requires single-channel transmission. If a court reads these as alternatives/inconsistent, the claim faces § 112(b) exposure (I flag this, but it is outside the § 103 brief). Conversely, a narrow construction of "single predetermined channel" plus "delay within the personal unit" could let a patentee argue that the FIG. 1 multi-channel zone art does not anticipate — which is why the Corderman/Halstead + Popescu/McCutchen/US 4,618,987 combination (rather than the FIG. 1 art) must be the lead theory.
2. L3's "low maximum output" limitation. It is a result/functional limitation. Under In re Swinehart, 439 F.2d 210 (CCPA 1971) and MPEP 2112, where a claim recites a characteristic that appears inherent in the prior-art device, the burden shifts to the applicant to show the prior art lacks it. A personal headphone/earpiece driven at conversational-audibility levels in concert ambient noise will generally satisfy "heard … only within a range of several feet," so this limitation likely adds little patentable weight — but it is not a guaranteed winner and depends on claim construction of "several feet" and "ambient sound."
3. References characterised by title only (§ 1 table). Combination A and Combination B do not depend on them, which is deliberate — I have built the case on references whose text I verified.

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10. Conclusion

The technical core of '884 — "send the program wirelessly to the audience's own ears and delay it so it lands with the direct sound, not before it" — is the union of two propositions that were each, individually, old and well documented by 1992: wireless distribution to personal receivers in a seated audience (Halstead 1951; McIntosh 1966; Corderman 1979) and air-path-compensating delay of the reinforcement (McCutchen 1927; Popescu 1980; Deutsche Post 1986). The only genuinely patent-worthy-looking differences — putting the delay in the receiver, using one channel, and letting the unit (or a control loop) set the delay — are placement, simplification and automation choices in a predictable art. In my view the claims would have been obvious under § 103(a), with the claims' one real vulnerability being whether a fact-finder accepts the "low maximum output sound level … several feet … in ambient sound" limitation as anything more than an inherent property of a personal ear-level transducer (see § 9.2).

Two open items must be closed before any final opinion is relied upon: (i) translations of the Japanese references — especially JPS 57‑202138 "Headphone concert system", which could elevate the case to § 102; and (ii) the claim text actually governing, given the RE38,405 reissue (filed 1999‑09‑16 — exactly at the two-year limit measured from the 1997‑09‑16 grant, which is itself consistent with a broadening reissue and therefore warrants a look at the reissue file history before relying on the printed '884 claim set).