Large area assistive listening systems (ALS): review and…

Summary

Reviewed loop, FM and infrared systems, held focus groups with users, manufacturers and installers, and tested what signal quality listeners with hearing loss accept, to recommend performance standards to the U.S. Access Board.

Key findings

Section 2.1.4 (printed p. 8) explains that an M/T position keeps both microphone and telecoil operating so a listener can also hear surrounding sounds, giving nearby comments during a lecture or stage performance as an example.
Sections 2.1.3 and 2.1.4 describe selecting a hearing aid's telecoil position to receive a loop and an M/T position with both microphone and telecoil active. Section 2.2.3 describes neckloop and silhouette couplers for telecoil users and direct audio input as a wired connection from an FM or infrared receiver to a hearing aid that accepts it. These are descriptions of coupling paths, not current model availability.
49 adults with sensorineural hearing loss, in six audiometric groups, and 10 with normal hearing rated recorded sentences under reverberation and noise, induction-loop noise and peak clipping. Ratings were similar across degrees and shapes of hearing loss, so the authors made one set of recommendations for all, based on the 75th percentile of ratings from listeners with hearing loss.
Recommended, measured at the earphones, a Speech Transmission Index of at least 0.84, a signal-to-noise ratio of at least 18 dB, peak clipping no more than 18 dB below peak, and receiver output of 110 to 118 dB SPL with at least 50 dB of volume range.
Recommended at least one neckloop for every four headphone-type receivers, 3.5 mm receiver outputs, low-battery warnings, one trained person to issue, check in and maintain receivers, and training for installers, whose competence was a recurring concern in all focus groups.
Recommended signs saying exactly where receivers can be obtained, and posting the FM frequencies and infrared subcarriers in use for people who bring their own receivers.
Reports that a proposal to standardise FM and infrared transmission was not taken up for fear of stifling innovation, and calls instead for a "universal" receiver covering FM, infrared and loops.
Its review of earlier studies found that loop, FM and infrared systems all improved speech scores over public-address listening, with small differences between them, and that listeners with normal hearing also benefited in large auditoriums.
Reported that only about 30% of hearing aids in the United States then had a telecoil.
Described induction loops as the oldest and least used of the three systems in the US (used much more often in Europe), rarely in direct competition with infrared and FM for new installations, and said this was unlikely to change unless more people with hearing loss, or their hearing aid dispensers, chose hearing aids with telecoils.
Said infrared systems from different companies are not always compatible, even on the same subcarrier, because they may differ in pre-processing and receiver characteristics, and that complete compatibility may be questionable when one maker's FM receiver is used with another maker's transmitter.
Listed the duties of the trained person who lends receivers: putting rechargeable batteries on charge after each performance, checking disposable batteries before each performance, a listening test on every receiver before lending it, helping listeners choose headphones, neckloop or earbuds, and hygiene after return, such as wiping foam cushions with an antiseptic solution and replacing disposable ones.
On infrared: 95 kHz was the most used subcarrier, by informal agreement among makers rather than a standard; 250 kHz was added for stereo, and 300 kHz, 2.3 MHz and 2.8 MHz were coming into use, partly because new fluorescent lamp ballasts interfered at 95 kHz.
No standard set the infrared level needed at seats, and makers traded emitter power against receiver sensitivity; adding emitters overcomes all but the most unfavourable light, such as direct sunlight, but reception "may be uncertain and has to be empirically determined in each venue".
The common infrared receiver was the under-chin stethoset, whose tubing adds resonant peaks; some under-chin jacks did not take standard neckloop plugs, body receivers with a front lens suited an overhead emitter poorly, and no infrared receiver had a low-battery light.
A few infrared transmitters were reportedly usable at some outdoor venues, though not in direct sunlight.
Recommends that the same trained person, at the same place in the venue, both checks receivers out and checks them back in, and can operate and troubleshoot them.
Section 5.2, Distributing Receivers (printed page 27), also recommends briefly instructing users in receiver operation and supplying a simple instruction card when necessary, alongside help choosing the coupling arrangement.
Explains that differences in modulation, filters and emitter power help explain why different infrared receivers perform differently with a given system.
Says turning one's back to the emitter, or putting an infrared receiver in a pocket, can eliminate or distort the audio, and that sunlight and incandescent light contain a great deal of infrared energy.
Says any number of infrared systems can be used in adjacent auditoriums without interference or spillover.
Says that with an incompatible receiver a signal may be received, and even understood, but with reduced clarity (section 2.3).
Section 2.3 (printed p. 16) explains the analogue infrared path: the audio frequency-modulates an RF subcarrier, which then amplitude-modulates the infrared light emitted by the diodes.
Section 2.3 (printed p. 16) distinguishes the infrared light carrier from the RF subcarrier carrying the audio and describes using several subcarriers for separate audio channels. This supports the subcarrier/channel distinction without treating the report's historical frequencies as current recommendations.

Limits

Analogue-era systems and listening tests from 1999; the criteria rest on ratings of recorded sentences, not venue measurements. These are recommendations to the Access Board, not an enforceable standard, and the report predates Bluetooth streaming.

Read the original

www.access-board.gov/files/research/assistive-listening-systems.pdf

Link checked 14 October 2026. We link to the original and do not host copies.

Cited in

Cite this source

Bakke MH, Levitt H, Ross M, Erickson F. Large area assistive listening systems (ALS): review and recommendations. Final report to the U.S. Access Board; Lexington School for the Deaf, Rehabilitation Engineering Research Center on Hearing Enhancement. December 1999. https://www.access-board.gov/files/research/assistive-listening-systems.pdf (accessed 14 October 2026).

The accessed date is when this library last checked the link; use the date you read it. To point readers to this summary instead, link to https://assistivelisteningknowledgebase.org/evidence/bakke-1999/

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