Assistive Listening Knowledge Base

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What audio level should the stream carry: -35 dBFS or louder?

From https://assistivelisteningknowledgebase.org/questions/audio-level/ · updated 13 October 2026

  • Affects sound engineers, installers, listeners
  • 12 sources
  • Updated
  • First published

Technologies: Auracast

In brief

The Bluetooth SIG recommends -35 dBFS. A Swiss standards-committee expert recommends about -25 dBFS for theatres, and Pro Audito reports that neckloop tests with a 1 kHz tone reached the required field only with the transmitter fully driven and receiver volume usually at maximum. Receivers handle volume differently.

Why it matters

Pro Audito reports that its neckloop tests needed full transmitter drive and receiver volume usually at maximum to reach the required field strength1. Its theatre guidance recommends limiting peaks to avoid clipping and describes different reference levels for venue, TV and music content2.

What the evidence shows

  • The SIG reference. The SIG recommends a long-term audio level of -35 dBFS with a -3 dBFS maximum3.
  • A different recommendation for theatres. A standards-committee expert notes that manufacturers, the SIG and standards committees do not yet agree; that TV content is often streamed at about -27 to -23 dBFS and music at -16 to -14 dBFS; and recommends about -25 dBFS for theatres, with light compression (about 2:1) and a limiter, which the article suggests may work better than automatic gain control2.
  • Neckloops need a full signal. Pro Audito reports that the FM, infrared and Auracast systems it tested reached the required neckloop field strength in 1 kHz sine-tone tests only with the transmitter fully driven and receiver volume usually at maximum. It recommends an automatic gain control or fast peak limiter that holds the signal about 1 to 3 dB below clipping1.
  • Volume belongs to the listener. The SIG's 2026 transmitter design guide says transmitters need no volume control, because the specifications assume the level passes through unchanged and listeners set the volume on their own receivers; it warns that turning a transmitter down can lead to painfully loud audio when a listener switches streams4.
  • Receivers respond differently. A McMaster/SoundsGood Labs synopsis reports wide variation in measured output level between transmitter and earbud combinations, and different effects depending on which device's volume was changed; it gives neither a device count nor a funding statement5.
  • The codec barely changes the level. The SIG's LC3 characterization reports level changes typically below 0.1 dB for normal speech and music; these are codec measurements, without a listener sample for this result6.
  • US output limits date from 1999. US standards require assistive listening systems to deliver 110 to 118 dB SPL with a 50 dB volume range7,8, figures recommended in a US Department of Education-funded 1999 report to the US Access Board that drew on focus groups and on quality ratings by 49 listeners with hearing loss and 10 with normal hearing9. ICC A117.1 gives no test method for them8.
  • Who sets the level matters. A review notes that recommended Auracast level calibration mirrors the hearing-loop standard, but that simple transmitters are likely to be set up by non-experts rather than trained installers10. In Pro Audito's unpaid test of one transmitter and three clip receivers supplied free by the manufacturer, a transmitter without a signal-level meter made the correct level harder to set11.

Where sources disagree

-35 dBFS from the SIG3 versus about -25 dBFS for theatres2. Pro Audito's advice to drive the transmitter to about 1 to 3 dB below clipping for neckloop users1 concerns peaks rather than the long-term level, and none of the sources compares the approaches directly.

What would settle it

An agreed reference level in IEC 60118-17, which is to specify the average and peak audio signal levels to be transmitted for hearing aids12, or a SIG update, and measurements of how level differences affect listeners across hearing aids, earbuds and neckloop receivers.

Planning while it is open

  • Control dynamics with a limiter so the level stays near the chosen reference without clipping2,1.
  • Set the level at the source rather than turning the transmitter down; listeners set volume on their own receivers4.
  • With a digital (Dante or AES67) feed, the console's output level in dBFS can be used to check the stream level, provided the transmitter adds no gain2; check that the equipment provides a way to measure the level, since Pro Audito found that a missing signal-level display made setup harder11.

Sources

  1. Can Bluetooth Auracast be used as an assistive listening system?. Pro Audito Schweiz. Pro Audito Schweiz (in German), 17 February 2025. Nonprofit advocacy (Pro Audito Schweiz, Swiss hearing-loss association) ↩︎ https://pro-audito.ch/news/kann-bluetooth-auracast-als-hoeranlage-eingesetzt-werden/
  2. Bluetooth Auracast in theaters and event venues. Graf B (Pro Audito Schweiz). Proscenium no. 3, pp. 10-11, Swiss Association of Technical Stage and Event Industry; English reprint by the Center for Hearing Access, September 2026. Nonprofit advocacy (Pro Audito Schweiz, Swiss hearing-loss association) ↩︎ https://centerforhearingaccess.org/wp-content/uploads/Proscenium-Technical-article-Bluetooth-Auracast-in-theaters-and-event-venues-1.pdf
  3. Auracast simple transmitter best practices guide, version 3. Bluetooth SIG, 17 October 2025. Standards body (Bluetooth SIG, an industry association funded by its member companies, which owns the Auracast trademark) ↩︎ https://www.bluetooth.com/wp-content/uploads/2022/10/Auracast-Transmitter_Recommendations.pdf
  4. How to build an Auracast transmitter: technical overview, version 3. Bluetooth SIG Market Development. Bluetooth SIG, 16 September 2026. Standards body (Bluetooth SIG, an industry association funded by its member companies, which owns the Auracast trademark) ↩︎ https://www.bluetooth.com/wp-content/uploads/2024/05/2403_How_To_Auracast_Transmitter.pdf
  5. Evaluation of the first generation of Auracast-compatible transmitters, hearing aids and earbuds. Bruce I, Clarke S, Armstrong S, Bosnyak D. IHCON 2026 (McMaster University; SoundsGood Labs), August 2026. Independent (McMaster University with SoundsGood Labs, which the synopsis does not describe; no funding statement in the synopsis) ↩︎ https://centerforhearingaccess.org/wp-content/uploads/IHCON-Synopsis-Bruce-Evaluation-of-the-first-generation-of-Auracast-compatible-transmitters-hearing-aids-and-Earbuds-August-2026.pdf
  6. Performance characterization of the Low Complexity Communication Codec. Bluetooth SIG Generic Audio Working Group. Bluetooth SIG, 13 June 2023. Standards body (Bluetooth SIG, an industry association funded by its member companies, which owns the Auracast trademark; laboratory tests by SenseLab of FORCE Technology and by Fraunhofer IIS) ↩︎ https://www.bluetooth.com/wp-content/uploads/2023/07/LC3Characterization_WP.pdf
  7. 2010 ADA Standards for Accessible Design. US Department of Justice. US Department of Justice (ada.gov), 15 September 2010. Government (US Department of Justice) ↩︎ https://www.ada.gov/law-and-regs/design-standards/2010-stds/
  8. ICC A117.1-2017 Accessible and Usable Buildings and Facilities, Section 706 Assistive Listening Systems. International Code Council. International Code Council (ICC Digital Codes), 2017. Standards body (International Code Council) ↩︎ https://codes.iccsafe.org/content/icca117-12017/chapter-7-communication-elements-and-features
  9. Large area assistive listening systems (ALS): review and recommendations. Bakke MH, Levitt H, Ross M, Erickson F. Final report to the U.S. Access Board; Lexington School for the Deaf, Rehabilitation Engineering Research Center on Hearing Enhancement, December 1999. Government (U.S. Department of Education, NIDRR grant H133E30015; final report to the U.S. Access Board) ↩︎ https://www.access-board.gov/files/research/assistive-listening-systems.pdf
  10. Opportunities and challenges for Bluetooth LE Audio assistive listening systems. Bruce IC, Armstrong S, Bosnyak DJ, Tawfik H. IEEE ICASSP 2025 (McMaster University), 12 March 2025. Independent (McMaster University); funding not stated in the abstract ↩︎ https://doi.org/10.1109/ICASSP49660.2025.10889430
  11. Tested for you: the Avantree Venucast Oasis Aura Auracast transmitter (in German). Pro Audito Schweiz, 16 July 2026. Independent (Pro Audito Schweiz, Swiss hearing-loss association; test units supplied free by the manufacturer, Pro Audito not paid) ↩︎ https://pro-audito.ch/news/fuer-sie-getestet-auracast-sender-avantree-venucast-oasis-aura/
  12. IEC 60118-17 (in preparation): assistive listening system for hearing aid users based on 2.4 GHz audio streaming. IEC TC 29 WG 13. International Electrotechnical Commission, undated (read 13 October 2026). Standards body (International Electrotechnical Commission) ↩︎ https://tc29.iec.ch/tc-activity/standards-under-preparation/

Change log

  • Published with 12 sources.
  • Added the neckloop test signal, receiver setting and study limitations.
  • Described test devices as supplied free without assuming a loan.

Cite this question

Equal Access Audio. “What audio level should the stream carry: -35 dBFS or louder?” Assistive Listening Knowledge Base. Updated 13 October 2026. https://assistivelisteningknowledgebase.org/questions/audio-level/ (accessed the date you read it).

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