Flying the Accessible Skies...a New ALS Takes Off!
by Norman Lederman, M.S. & Charles Laszlo, Ph.D.
The high noise environment in airplanes is adverse to spoken communication regardless of one's hearing ability. Presently an estimated 10% of the population in the U.S.A. have a significant hearing loss and can benefit from the use of hearing aids and assistive listening systems (ALS) in difficult listening environments. With airline traffic exceeding 40 million passengers per year, addressing the needs of the growing market of hearing impaired travelers is significant to the airline industry and hearing health care professionals. Many hard of hearing travelers have affirmed that on board listening conditions often result in miscommunications, social isolation and potentially dangerous circumstances. Comments received in a recent survey also highlight the fact that hard of hearing travelers are not able to enjoy the same in-flight movies, entertainment channels and pilot announcements that are readily available to their fellow passengers. Click here for survey results.
With support from the National Institutes of Health's National Institute of Deafness and other Communication Disorders, the Transportation Development Center of the Department of Transportation Canada, Self Help for Hard of Hearing People, Inc., and United Airlines, an effective and user friendly assistive listening system specifically designed for use on airplanes was designed, tested and manufactured during a 3 year research and development period commencing in 1995.
Spurred on by the Americans with Disabilities Act (ADA), assistive listening devices and systems have proven to be an economical and highly effective means for providing accessibility to activities at school, work, houses of worship, in other public gathering places and at home. Frequency modulation (FM), induction loop and infrared ALS technologies are being employed in these areas and welcomed by many hard of hearing people. Unfortunately, these existing forms of ALS technology cannot be used on airplanes. Airlines prohibit the use of any RF (radio frequency) generating devices that are not part of the airplane's existing communication systems including wireless microphones and cellular phones. This prohibition is based on legitimate concerns about such equipment interfering with aircraft radio and electronic control systems. Concern has even been expressed about the potential interference created by passengers' lap top computers and CD players, consequently their use is restricted. Induction loops cannot be used because of the interference generated in hearing aid telecoil circuits by the airplane's electrical systems. Infrared systems which utilize light- beam technology offered the most promising potential for our project, but significant departures from traditional design were required.
Named FlightSound(TM) , the technology resulting from this project represents an innovative adaptation, refinement and integration of audio amplification, signal processing, infrared and noise cancelling microphone design, packaged into a portable battery powered ALS. The project also represents a unique alliance between two ALS companies: ALDS, Inc. in Richmond, British Columbia and Oval Window Audio in Nederland, Colorado. Coincidentally, both companies received grant awards in their respective countries to study the problem of hearing accessibility on board airplanes. Rather than developing two different (and competing) technologies, the two companies chose to work together, combining their expertise and resources, in the interest of achieving a common goal...to create the first ALS optimized for airplane travel.
All hearing assistance technologies operate on the premise that the higher the signal to noise ratio, the better chance hearing impaired people have of understanding what is being said. +20 dB is cited as a desirable level for speech comprehension by hard of hearing people. The authors of this proposal have made signal to noise measurements ranging from only +1 dB to +5 dB for announcements and conversation on board various passenger airplanes, with smaller planes registering worse than larger aircraft. These figures are corroborated by Archer and Teder.
With the technical restrictions placed on an airplane ALS, a hybrid infrared-direct audio input design approach was taken and the resulting system was tested by an independent electromagnetics/radio frequency lab to ensure interference-free compatibility with airplane electronics and standards. FlightSound consists of a rechargeable battery operated infrared transmitter and an infrared receiver that will also accept a direct audio input signal from the armrest headphone jack that carries all announcements and audio entertainment channels. The transmitter, designed for one- on-one communications at a distance of no greater than 8 feet, contains a close- proximity noise-cancelling microphone and external jack for head worn microphone. In addition to an audio compression circuit that levels out the varying microphone signal, the transmitter has a battery saver feature that places the unit in a low current drain mode when there is no signal presented to the microphone. The receiver, in addition to accessing the transmitter's infrared signal will also accept a direct input signal from the armrest headphone jack. These external signals are compressed in the FlightSound receiver in order to smooth out the typically extreme level changes that occur in audio signals originating from the pilots, flight attendants and audio entertainment channels. A maximum sound pressure level of 130 dBA and frequency response of 100 Hz to 10 KHz +/- 3 dB is presented through dual universal fit noise-blocking earphones. A tone control provides an adjustable low frequency roll off to compensate for the widely varying frequency responses of the one-on-one and direct audio input signals. A cochlear implant processor patch cord may also be used with the system. An automatic mute circuit silences the receiver if the infrared beam is blocked or out of range of the transmitter. Rechargeable battery life for both the receiver and transmitter averages 6 to 8 hours depending on usage. Back up rechargeable batteries and standard alkaline batteries provide the longer running time that may be required for international travel.
United Airlines was invited to be the field test site. They accepted and offered that the test occur at the Denver International Airport (DIA) in Colorado, on board a new Boeing 777. Test subjects were drawn from a local chapter of Self Help for Hard of Hearing People, Inc. and the Denver Ear Institute.
On December 2, 1995, a special sound system was set up on board a Boeing 777 parked in a United Airlines hangar at DIA. Our plan was to reproduce the acoustical conditions encountered on board an airplane in flight without incurring the extravagant expense and liability of having to leave the ground. This was accomplished by digitally recording and reproducing actual in-flight sound samples through a custom sound system with a frequency response of 8 to 20,000 Hertz. Frequency spectra and sound pressure "A" weighted levels were matched with data provided by United Airlines and Boeing engineers.
Four hearing aid users and four cochlear implant users with a minimum 65% speech intelligibility participated in the evaluation. Each brought a partner to act as her/his in- flight neighbor. The evaluation proceeded as follows:
With airplane cruise altitude noise presented at a constant level of 74 dBA, the project's consulting audiologist presented four Speech Intelligibility Test (SIT) lists over the aircraft's public address system at a signal to noise ratio of +3 to +5 dB. This approximated real life listening conditions on board an in-flight Boeing 777. Each hard of hearing listener wrote down responses on an identical answer form. The listeners' "normally hearing" partners served as controls and also wrote down responses. The conditions of the testing were as follows:
List 1: Quiet (with only the air circulation system on). The "pre-flight" ambient noise of the aircraft was 58 dBA. The sound pressure level of the aircraft's P.A. system was adjusted to a typical level of 77 dBA. The ALS was not used. Personal hearing aids and cochlear implants were used alone.
List 2: Quiet with ALS. List was administered through the aircraft's P.A. and hearing aid listeners used the ALS alone. For the remainder of the test, cochlear implantees used a patch cord to interface with the ALS receiver.
List 3: Noise without ALS. With in-flight noise replicated at 74 dBA, list was administered through the aircraft's P.A. system creating a realistic signal to noise ratio of +3 to +5 dB. Personal hearing aids and cochlear implants were used alone.
List 4: Noise with ALS. With in-flight noise replicated at 74 dBA level, list was administered through the aircraft's P.A. system creating a realistic signal to noise level ratio of +3 to +5 dB. ALS was used without hearing aids. Cochlear implantees plugged their processors into the ALS receiver.
Partner Test: Two versions of this test were prepared. Half of the listeners received List 1 for the "without ALS" condition and List 2 for the "with ALS" condition. The remaining listeners were assigned the lists in reverse order. Facing the listener, the partner read the list of sentences. Both tests were administered with the replicated in- flight background noise. The listeners had the opportunity to lip read as well as hear their partners in order to simulate a realistic in-flight condition. The listener wrote down as much of each sentence as possible on the answer form provided. Scoring was based on percentage of correct key words and total words.
The most significant improvements could be seen for cochlear implant and hearing aid users during the phase of the test presented through the aircraft's P.A. system. The cochlear implant users' averaged score improved from 52% without the ALS to 79% with the ALS in noise. The hearing aid users averaged score improved from 83% to 94% for the same conditions. Looking at the individual scores for both groups, the benefit of the ALS varied widely across subjects, for example, one cochlear implant user's score jumped from 17% without the ALS to 71% with the ALS in noise.
The Partner Test did not demonstrate any significant improvements for any of the test subjects. This may be explained by the close proximity of the partners together with the fact that all of the subjects were good speech readers for the material we presented. However, the listeners' comments did mention how much easier and less stressful it was to carry on a conversation with the ALS, as compared to not using it.
Technical innovation and federal legislation have resulted in the development and acceptance of a wide range of assistive technologies that are improving the quality of life for many people. Auditory accessibility on board airplanes is now a reality...easing communication stress and hazards for millions of hard of hearing travelers.
Norman Lederman, M.S., is the Director of Research and Development at Oval Window Audio, 33 Wildflower Court, Nederland, CO 80466. He may be reached at 303-447-3607 or click on his name to e-mail him. Charles Laszlo, Ph.D., is Chairman of ALDS, Inc., in Richmond, B.C., Canada.
Acknowledgements: A special thank you to Charles Laszlo, Hans Roesler and Steve Unger at ALDS, Inc. for their engineering support, camaraderie and willingness to work together to actualize our common vision. The authors and project staff wish to thank the National Institutes of Health's National Institute of Deafness and Other Communication Disorders and the Department of Transportation Canada for their financial support of the FlightSoundproject. Thank you to Self Help for Hard of Hearing People, Inc. and United Airlines for data, facilities and helpful staff support.
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