People with hearing loss have trouble hearing when they are at a distance from the person speaking, when it is noisy, and in public places.
For children, this can be a particularly serious problem because children need to hear clear loud speech all of their waking hours if they are going to succeed in developing the auditory brain and learn language optimally.
While hearing aids, cochlear implants, and bone-anchored devices are designed to provide improved auditory access, they provide optimal access only when the talker and listener are close and when there is no competing noise. When there is competing noise or when the talker and listener are more than a few feet apart, additional technology is needed.
Types of group listening systems
The goal of group listening systems (hearing loops, FM systems, and infra-red technology) is to increase the size of the “Listening Bubble” for people with hearing loss. In other words, additional technology helps a person with hearing loss hear things that are out of the range of their hearing aids, or in situations when speech is affected, or to hear better in noise.
Several different listening systems are available to improve hearing in noise. FM systems are the most common systems used in schools because they provide good quality signals with a broad frequency response. In the US, FM systems are the most common group technology and, while the quality is excellent, they require additional equipment to couple to hearing aids.
Hearing loops are common in Europe but less so in the US, although the movement to increase loop technology is growing quickly. Infra-red systems can also provide assistance in group situations, but they are not commonly used.
How do hearing loops work?
Hearing loops transmit an audio signal using magnetic energy to the telecoil in a hearing aid or cochlear implant. The loop can be set up in any room by stringing looped wires around the room. They can be used in large places like theaters, auditoriums, churches and temples, or in smaller places like meeting rooms, taxis or the checkout counter at a supermarket.
A loop can also be used to aid in hearing TV. The user has to turn on the T-coil on their hearing aid, cochlear implant or BAHA. The audiologist needs to be sure that the T-coil is activated, and the user needs to know how to turn it on and off. The signal from the group amplification system will be transmitted through the loop to the T-coil on the personal listening system.
Telecoil vs Mic-telecoil position
If the hearing aid or cochlear implant is set to “T” only, the user will hear only sound going into the group microphone. She will not hear people sitting close by and will not hear her own voice. I am not excited about putting kids in a position where they cannot hear those around them. However, if the hearing aid or cochlear implant is set to the “M-T” position, the child should be able to hear from the microphone of his own device as well as from the group microphone. It means that parent, teacher and child can communicate while listening to a program.
Frequency response of hearing loops
Hearing loops are great devices and important contributions to improve listening. However, loop systems have some limitations. The telecoil of hearing aids do not have as wide a frequency response as the hearing aid or cochlear implant microphones. Telecoils have a reduced high-frequency response. That means that a child may not hear high frequency phonemes, missing sibilant, fricatives, pluralization, etc.
Hearing loops for kids
Hearing loops are great for theaters, movies, auditoriums, museums, etc., but they should not be used in classrooms and in other situations in which listening is critical for learning. High frequencies are critical and need to be maximized. So make use of hearing loops for many listening situations that will improve socialization and activities.However, loops are not a substitute for the kind of high quality listening that is required in school.
I have great news regarding your recent blog about Hearing Loops for Children in Classrooms. While indeed in certain circumstances there can be reasons that a hearing loop may not be the most appropriate assistive listening system, lack of high frequency response is no longer one of them.
The latest IEC 60118-4 Induction Hearing Loop Standard requires that the magnetic frequency response extends from 100 to 5000 Hz +/- minus 3dB which is the same as the audio standard for professional sound systems. On the receiving end of this system the hearing aid manufacturers must (as per the latest S3.22 ANSI standard) provide data documenting the Microphone response to the Telecoil response – in the “as worn” or vertical position – the so called SPLIV test.
I can provide you any number of HA specs that show that these responses are typically identical, particularly in the high frequencies. I refer you to an educational AudiologyOnline article by Mike Valente, PhD which shows multiple Mic vs. telecoil test graphs: https://www.audiologyonline.com/articles/telecoil-lonely-transducer-that-big-11911
What is most exciting is the anecdotal evidence that has blown so many hearing loop advocates away. Comments that will bring tears to your eyes: https://www.loopwisconsin.com/PDFFiles/Own%20_words.pdf, children who report phenomenal speech understanding in TV rooms https://loopwisconsin.wordpress.com/2014/09/23/especially-children-can-be-especially-helped-by-hearing-loops/ and perhaps the most telling ones where users, with looks of amazement on their faces, comment they heard the breaking of the host, the pouring of the wine and even the whispered instructions of priests to altar boys.
Juliëtte Sterkens, AuD
HLAA Hearing Loop Advocate
Jane Madell’s informative article on listening systems underscores the need for assistive technology to help people with hearing loss of all ages to hear and participate as fully as possible in school, at work and at leisure. However, she needs to revise her view of the wonders of today’s induction loop technology and t-coil listening.
Induction (or hearing) loops are increasingly being installed in college lecture halls and auditoriums, confirming their value in learning environments. And while FM devices remain popular for students to hear teachers, the installation of induction loops in new school, library and auditorium construction is providing hearing access to young people with t-coils in their hearing devices at far less cost per pupil. And as Ms. Madell notes, the option of the “M plus T” setting enables them to hear conversation close by as well as instruction from up front.
Two further points should be mentioned:
1) Induction loops work far better for people with severe or profound loss than do IR or FM systems when headsets cannot be work and neckloops are needed to transmit sound signals to the t-coils in their hearing aids and cochlear implants.
2) The marketing by competing manufacturers of their costly branded assistive listening products has kept many people with hearing loss from understanding the value and simplicity of listening in all looped facilities, from the TV room at home to airports and concert halls, via the t-coils available in most of today’s hearing devices at little or no additional cost. And it is far more convenient to tune in art the touch of a button on one’s hearing device(s) than to have to carry around, or rent, and wear additional equipment.
Am just reading your article and yes you are 100% correct induction loops can be used all over but in Europe we do not recommend to use it in a class room. Simply as using the T switch the kids will not be able to hear each other (beside the frequency part). They will feel isolated this will later cause all kind of mental problems. We would recommend to improve the acoustics of the class room so that it becomes easier for the kids to hear what the teacher tells them. In general it is important that the kids can hear the key notes in a clear way. Additional to the acoustic improvement we should always take care that each classroom has a basic sound setting as the teacher is not able to cover the complete classroom with its own voice so using systems like classroom sound would be a start.
What my surprise still is is the cost of these products in the US also induction loops are still sold at your end as if it is rocket science. Counter loop are sold for 250 USD a realistic cost should not be more as 100 US and even this is high. We are specialized in sound in Europe and produce 1000x of inductions loops a year. We feel that this technology like classroom sound should be available for all people, this is possible!
Lets create an accessible world for all of us
Dimitri P. Lasscher
Y-sense
Excellent comments & citations! We will put these citations on our website. Thanks for your continuing education & advocacy! 🙂 Linda Heller, HLAA-Delaware
Dear Dr. Madell,
Thank you for your article on the Listening Bubble. I now use your analogy of a listening bubble when explaining the difficulty individuals with hearing loss face on a daily basis with my graduate and undergraduate students.
I also wanted to add some information about T coils. T Coils can be programmed have an excellent frequency response even in the high frequencies.
If a vertical T coil is not providing adequate high frequency gain in a hearing loop that meets the IEC standard, I would consider adjusting the T coil program so the T coil gain and frequency response matches the microphone gain and frequency response. Test box measures are helpful to verify a T coil’s performance and are highly recommended so individuals who live with hearing loss can benefit from hearing access via a hearing loop at home, school and public facilities via their T coils.
I too like the listening bubble imagery. It’s helpful, but the definition is incomplete. I’ve posted a comment on the listening bubble post.
@Karen: Just to clarify on the issue of high frequency rolloff on the telecoil program of most hearing aids, it seems to be an issue of the factory defaults when the responses to each of the programs are built by the fitting software, even when you select NAL-NL2 as opposed to the proprietary fitting prescription. Go ahead and create a dummy patient in your fitting software and plug in a typical moderate-severe sloping audiogram, and you’ll probably have a face-palm moment when you see the results.
I have No Idea why the manufacturers do this — Perhaps feedback reduction, or to compensate for the high frequency emphasis on POTS lines? — but after building the Master program I simply use the copy-paste function (it’s built into ReSound Aventa & Beltone’s software — YMMV) to transfer the gain settings into the T-coil program.
In addition — And this is Very Important — starting at 2kHz I add an *extra* 3, 4, maybe even 6dB/octave gain if the user will be availing themselves of baseband induction loops: As it turns out here in the United States, we use much more steel reinforced concrete in our buildings due to strict building codes because of hurricanes, tornadoes, earthquakes, and especially fires; and due to monotonically increasing hysteresis losses in the ferrous metals as the frequency is increased, the magnetic flux density of the signal becomes attenuated. In a properly installed and tested room loop system this rolloff is compensated for; however here in the US many sound contractors don’t bother to compensate for this well-documented effect (if they even know about it at all).
Another “Gotcha” I’ve seen is when the loop installer uses an inexpensive conventional audio amplifier as the loop driver: Conventional audio amps have as close to a Thevenin equivalent output impedance of zero ohms (i.e. ideal constant voltage source) for loudspeaker voice coil damping; while loop driver amplifiers have as close to a Thevenin equivalent output impedance of infinite ohms (ideal constant current source) to compensate for the -6dB/octave rolloff due to the amplifier output terminals looking into a mostly inductive load.
Can you use an inexpensive PA amplifier as a loop driver? YES! But, you’ll need an equalizer to add back in the 6dB/octave rolloff; and also to prevent output voltage swings from hitting the power supply rails and causing IM, TIM and harmonic distortion due to the clipping, use a 70.1 volt line transformer (if it’s not already built into the amp) to step up the output voltage in between the amplifier output terminals and the loop.
Dan Schwartz,
Editor, The Hearing Blog
“A hearing loop system may be useful in the auditorium, but it will not provide as clear a signal as an FM system because the telecoil of the hearing aid usually has a reduced frequency range.”
If only it was as easy as this to point out one kind of product: this is the best!
Conventional FM systems:
The small FM – equipments we know from MAF of HAT (Hearing Assistive Technology) – Phonak’s Inspiro and Roger, Oticon’s Amigo and so on – have in fact very week transmitters and thus also short range and in the peripheral areas the signal becomes weaker and weaker, resulting in increasing noise in the receiver.
How big the range is, depends on the antenna condition.
The transmitter’s antenna is in many systems part of the microphone cord, and it is therefore vital that the microphone cord is stretched out in its full length. If, for instance, winded around the transmitter the transmitting range decreases dramatically – might be down to 4 – 6 feet, and the noise increases correspondingly or result in no transmission of the signal at all.
At the other end – the receiver.
The receiver does naturally also need an antenna to receive the radio signal. Because all MAF’s want their product to be appealing to the user, the buzzword has been smaller is better. But from a technical point of view this isn’t always correct – especially not concerning the receiver’s antenna.
Many FM systems work in either 170 MHz band = 175 cm Wavelength or 34 MHz band = 88o cm Wavelength (Wavelength (Lambda) = Wave Velocity (v) / Frequency (f)).
Antennas shall always be half, quarter, eighth (and so on) of the wavelength, and to have a good or fair antenna you in fact need an antenna that has a length of a ½ or a ¼ of the Wavelength. Fair would then be a ¼ antenna or 43 cm.
Now my guess: no one has seen such antenna length in HAT. Shorter antennas = poorer receiving. And you might imagine the problems in mini FM receivers which are directly mounted onto the HA/CI.
Another factor not always paid attention to is reflected radio signals in antiphase. That also causes weak radio signals and increasing noise in the receiver.
Antiphase are radio signals from the transmitter that are reflected by metal parts in the building. Could be iron web in the concrete construction or big heating radiators or even from metal panels used in the ceiling or blinds. The signal from the transmitter will hit the metal object and be reflected and be received in the receiver together with the direct transmitted radio signal. But because the reflected signal has a longer way of travelling it arrives slightly later. Now depending on the radio wave length and the traveling length it may arrive more or less in antiphase to the direct radio signal. When the two radio signals are added it can lead to a weaker resulting signal.
The problem with those reflecting radio signals are: You can’t predict where they are – you can never know where the “weak points” occurs. They are – so to say – dynamic and are moving accordingly to the moving of the transmitter worn by the teacher or speaker.
These spots with sudden noise is of course not enhancing to the understanding of speech. Furthermore, the sudden noise in itself can be much disturbing and even lead to minor shock and insecurity in the child or the child deselects the use of the equipment.
Conclusion is: big risk of unpredictable noise and drop outs due to weak transmitter and too short antennas.
Digital Audio Broadcasting (DAB):
The new trends have been going away from conventional FM to digital radio transmitting including using higher radio frequencies (800 Mhz band) (a.o. Phonak’s Roger and Comfort audio’s Digisystem)
The transmitters are not sending stronger signals – that is regulated by law. But with using DAB systems the problems with the noisy grey peripheral areas are reduced to a very minimum. In fact the experience normally is either you can hear or you can’t.
Using the higher transmitting frequency leads to better antenna conditions. The same antenna lengths have a better reception for high band (800 MHz) than for low band (170 MHz). so the transmission becomes more stable.
Digital radio transmitting however takes time for coding and decoding the signal. This could be a problem in some systems. Bluetooth is for instance too slow for HAT – the sound and the lip moves will not be synchronous, but with modern encoding algorithm it’s no more an issue.
Some benefits with digital radio transmitting are the possibility to transmit additional information parallel to transmitting the speech. This is used in different ways to be able to overcome the “walking around” microphone and replace it with systems with more microphones for the students in the classroom – leading to a more fluent conversation.
Another benefit is the ability to of encrypting the signal so the conversation cannot be overheard from outside the room. A facility needed in examens or in courtrooms.
Also for digital radio transmitting systems we see mini receivers which are directly mounted onto the HA/CI.
Hearing loops:
Talking about Hearing loops one often picture a little electric cord following the skirtings in the room and connected to an amplifier. Simple as that? No, there is much more to Hearing loops. First of all find out how big area the hearing loop shall cover. Then find an amplifier that must be of good quality and be able to support the loop with sufficient and stable current, have a full frequency response and high dynamic. If possible use a Super Loop System (ref 11) and use an adequate loop cord – not too thin and be careful how to setup the loop. Then when setting up the system make sure to meet the requirements in the international IEC 60118-4 standard, that tells about Frequency response, dynamic, magnetic field. Done this you’ll find a hearing loop that’ll fulfill all requirements to good listening quality.
It is then important to look at the HA and the telecoil. For HA manufacturers supplying the Nordic countries there is a standard – EN60118-1 – from 1995 to follow. It is telling about the sensitivity of the telecoil implicating that the telecoil response matches the microphone response.
So the Telecoil, for most hearing aids (Remember Denmark has a leading role in producing hearing aids – Oticon, Widex and GN Resound), is related to how the microphone is set up, but audiologists may have to fine tune the telecoil response so it matches the microphone response.
If it is auto-correlated and you change the microphone response, the Telecoil response will change in the same manner as the change to the microphone response.
Microphones:
A most vital part of all HAT’s – no matter if Hearing loop, Conventional FM transmitters, Digital radio transmitters or infrared – is indeed the microphone but not often paid enough attention.
Many of the HAT systems on the market do suffer from good quality microphones.
Try to connect different HAT mic’s to a HiFi amplifier with a good loudspeaker system, it is obvious how poor they can be.
Imagine these those microphones used in professional radio or television broadcasting.
Broadcasting stations would never accept microphones that don’t have a flat full frequency characteristic with a poor dynamic range, nor would they accept mic clips that are unstable and makes noise at every movement.
Lapel mic’s have to be placed somewhere on the speakers clothes, but when the speaker turns the head left and right the sound will vary – be fluctuant, which can be most disturbing to the listener.
The sound technicians will mostly prefer a boom mic to keep a stable nonfluctuant sound.
Coming to the issue whether FM systems are better than Telecoil and Loop systems in classrooms there are more issues to consider.
Frequency response – signal (speech) dynamic – signal stability – eases of usage – and standards – and not to forget: The users acceptance of using the system.
Frequency response:
Provided the Telecoil in the HA is related to how the microphone is set up, the telecoil response will be the same as the microphone response.
In technical sheets for hearing aids from a.o. Widex, Phonak and Oticon we typically read of frequency responses from 200 Hz to 7500 Hz – some say up to 10000 Hz. And according to HA manufacturing practise, the frequency response for the Telecoil shall be the same as for the microphone in the HA. So that’s the responses we have to meet.
Conventional FM systems (a.o. Phonak, Oticon, Connevans) have often been emphasized for their frequency responses in fact the responses generally lies in the range 100 – 7.500 Hz – some goes higher and some not so high. But mini receivers with frequency responses going from 100 Hz to 6000 Hz will limit the perception of high frequency sound such as /t/ /th/.
However it is not quite clear if those frequency responses mentioned on the data sheets include the microphones or only the transmitters or the all over systems or they only count that specific part of the system on the data sheet.
(see list below).
Well designed hearing loop systems with a well designed and well selected loop amplifier are fully able to match frequency response and signal dynamic as we meet in FM- and DAB systems – in fact some are even better.
From manufacturer of hearing loop amplifiers Bo Edin in Sweden, you find frequency responses in the range 70 -10.000 Hz and even up to 16.000 Hz using a special flat copper folio loop cord.
(see list below).
Signal (speech) dynamic:
Not many manufacturers of Hearing Assistive Technology (HAT) do inform of their system’s dynamic range, which might lead to the conclusion, that this is an issue they’re not proud of. This is a pity because sound dynamic is a very important subject in understanding the meaning of speech. So it ought to be a must for HAT manufacturers to inform this on data sheets.
(don’t get confused by Phonak’s Dynamic FM because it doesn’t refer to the dynamic in the sound but more to the ability to adapt sound with different microphone conditions).
Signal stability:
The stability of a good hearing loop can be considerable higher than conventional FM systems concerning dropouts. There might be spots with electromagnetic disturbance from old fluorescent lights, electric installations or other electronic equipment such as old fashioned PC screens. But those disturbances are permanent predictable giving the user the possibility to know where they are and to learn how to avoid them. For real advanced hearing loop systems it is even possible to reduce the influence of electromagnetic disturbance if the loop signal is digital coded. Unfortunately this technique isn’t used much because of the lack of a standard for this technique.
Ease of usage:
Using a hearing loop doesn’t require mounting mini receivers to the HA, wearing a receiver or having cables between receiver and HA. You simply – with a remote control or using the toggle button on the HA – change the program in the HA.
You need no extra wearings, which can be an issue to especially teens who are shy exhibiting their hearing loss and to toddlers with weak ears and tendency to put everything into their mouth.
Standards:
For hearing loops in public room such as theatres and churches there is a standard to follow: the international IEC 60118-4 standard. And for telecoil there is the international standard IEC 60118-1 Edition 3.1 from 1999-01
No such kind of standards are found within the FM system area.
The advantages of a standard are obvious: everyone can join in without changing receiver frequencies or transmitting codex. You have one system for all.
Imagine how two or more students in the same classroom with different type of FM systems. See the teacher equipped with a necklace of microphones and transmitters.
Users acceptance of using the system:
From a pedagogical point of view we have found it worth asking the pupils with hearing loss in Denmark what they prefer.
From a survey done in 2009 (Ref 12) it is obvious, that the majority prefer using the Telecoil and Loop.
This survey was carried out at a special school for deaf students in 2009. We are repeating this survey spring 2015 including pupils with hearing loss from local school settings.
However, since the introduction of digital radio transmitting we have succeeded much better with assistive listening devices for school children and even found that pupils at an age of 5 years old are able to state their preferences, CI-users or HA-users. They receive via loop – either stationary loop in the classroom or a neckloop reciever. The latter enables the pupil to adjust volume as well.
(CI. Cochlear Implant. HA: Hearing Aid).
Furthermore, we have found that using FM in school settings also has it’s limits since there are only a certain number of frequencies available – otherwise the pupils will overhear each others lectures from one class to another. With increasing inclusion in local schools of pupils with hearing loss, this is a matter that needs to be looked into. The pupils don’t always know what the right sound should sound like.
These findings have made us look specifically into how we introduce the audiological treatments and the use of sound and pleasure of listening to sounds to children with hearing loss. We realize, how important it is to be at ease with the sound you hear and feel to also have the effort to understand spoken language.
This is quite a challenge but much worth the effort.
Finally it is difficult to have more than one child with a hearing loss using FM-system in the same group. With a Telecoil and Hearing loop system we are able to have more children with hearing loss using the same system, which also enables the children to match each other on tempo, listening strategies etc, and as adults if ever we find public access in cinemas, churches etc, it will be Hearing loops.
Telecoils tend to be cheaper than FM-receivers and so far Telecoils have not been proven of worse quality than FM-receivers. This means we can help more children because it is cheaper.
In Associations for people with hearing loss a Loop is the only thing that enables us to have meetings where everyone can have access, provided we have Telecoils in our HA/CI.
All together there are a number of things to take into consideration still when choosing between Telecoil / Hearing loop and FM-systems.
Aïda Regel Poulsen
Hearing Consultant
for pupils with hearing loss included in local school settings
Finn Brocher Petersen
Audio and Hearing Technician
Manufacture
Product
Type
Range
Dynamic
Bo Edin
Univox® PLS-X5 (Ref 1)
Hearing loop
75 – 6800 Hz
50-70 dB (+1,5 dB)
Bo Edin
UniVox® SLS-300 XF (Ref 2)
Hearing loop
70 – 10,000Hz
>70 dB
Phonak
Roger-Pen (Ref 3)
Digital Radio
100 – 7300 Hz
Not said
Phonak
Inspiro (Ref 4)
Analog FM
100 – 6000 Hz
Not said
Phonak
MLxi receiver (Ref 5)
Analog FM
100 – 6000 Hz
Not said
Oticon
Amigo (Ref 6)
Analog FM
100 – 8500 Hz
Not said
Oticon
Micro Receiver R1 / R2 (Ref 7)
Analog FM
100 – 7500 Hz
Not said
Comfort Audio
Microphone DM10 (Ref 8)
Digital Radio
100 ‐ 7000 Hz
60 dB
Comfort Audio
Micro Receiver DT20 (Ref 9)
Digital Radio
100 ‐ 7000 Hz
60 dB
Connevans
fmGenie (Ref 10)
Analog FM
70 – 11000 Hz
Not said
Ref 1: https://www.univox.eu/k/p/PLS_X5_BrGb_140616.pdf
Ref 2: https://www.univoxaudio.co.uk/catalogue/products/pdfs/SLS-300_XF_BrGb_mw.pdf
Ref 3: https://www.phonak.com/content/dam/phonak/gc_nl/Roger/Datasheet_Roger_Pen_210x297_GB_V1.00.pdf
Ref 4: https://www.phonakpro.com/content/dam/phonak/b2b/C_M_tools/FM/Transmitters/Brochures/inspiro-Technical-Data.pdf
Ref 5: https://www.phonakpro.com/content/dam/phonak/b2b/C_M_tools/FM/Receivers/datasheets/MLxi-Technical-Data.pdf
Ref 6: https://www.oticon.com.br/~asset/cache.ashx?id=10589&type=14&format=web
Ref 7: https://www.pro.oticonusa.com/~asset/cache.ashx?id=18310&type=14&format=web
Ref 8: https://www.comfortaudio.com/dk/wp-content/blogs.dir/9/files_mf/tddm10_eu20130530.pdf
Ref 9: https://www.comfortaudio.com/dk/wp-content/blogs.dir/9/files_mf/tdsdt20_20130311.pdf
Ref 10: https://www.connevans.info/image/connevans/fmgenie_specw.pdf
Ref 11: Super Loop System SLS is a balanced uncorrelated induction loop system. It consists of two separate loop systems (Master and Slave) with a separate current controlled amplifiers connected to each loop. SLS systems have several advantages compared to old conventional loop system.
Ref 12: https://www.cfh.dk/(informationsmateriale/materialer/Do%20school%20children%20with%20hearing%20loss.pdf
Legend
CI = Cochlear Implant
FM = Frequency modulation
HA = Hearing aid
HAT = Hearing Assistive Technology
MAF = Manufacturers
DAB = Digital Audio Broadcasting
Finn, I don’t know when or where you learned about antenna engineering, but among other things physical length is almost totally unrelated to electrical length when loading coils are used for impedance matching, or continuous helical coil antennas which are used in some 27 mHz (11 meter) CB antennas for cars & trucks and “rubber duckie” flexible antennas used on VHF handy-talkies and scanners.
In addition, although some gain may be desired for fixed base station vertically polarized antennas, in fact for portable receivers where the orientation angle as well as direction is unknown, you want as close to an isotropic (equal in all directions) radiation pattern as possible, which implies a physically short (in relation to wavelength λ) antenna: Mobile phone makers spend millions of dollars testing their devices in RF anechoic chambers specifically to insure as close to an isotropic response as possible; and in fact Starkey built one of these as part of their Halo MFi project:
https://twitter.com/jasongalster/status/453997904231821312/photo/1
Dan Schwartz,
Editor, The Hearing Blog;
RCA (Camden) Engineering alumnus
Sorry the last part might look a bit confusing. I wasn’t aware that the blog didn’t accept tab Tables.
For enlightening I’ve tried to put up the table in text string here:
Manufacture – Product – Type – Range – Dynamic
(Ref 1)
Bo Edin – Univox®PLS-X5 – Hearing loop – (75-6800Hz) – 50-70dB (+1,5dB)
(Ref 2)
Bo Edin – UniVox®SLS-300 XF – Hearing loop – (70-10000Hz) – >70dB
(Ref 3)
Phonak – Roger-Pen – Digital Radio – (100–7300Hz) – Not said
(Ref 4)
Phonak – Inspiro – Analog FM – (100-6000Hz) – Not said
(Ref 5)
Phonak – MLxi receiver – Analog FM – (100-6000Hz) – Not said
(Ref 6)
Oticon – Amigo – Analog FM – (100-8500Hz) – Not said
(Ref 7)
Oticon – Micro Receiver R1 / R2 – Analog FM – (100-7500Hz) – Not said
(Ref 8)
Comfort Audio – Microphone DM10 – Digital Radio – (100-7000Hz) – 60dB
(Ref 9)
Comfort Audio – Micro Receiver DT20 – Digital Radio – (100-7000Hz) – 60dB
(Ref 10)
Connevans – fmGenie – Analog FM – (70-11000Hz) – Not said
I particularly like your explanation of the Telecoil frequency response and the UniVox SLS300 hearing loop system with flat copper for foil frequency response reaching up to – in some cases -16,000 Hz. Your technical description confirms what I have observed in properly installed Hearing Loops with my clients, consumers all over the country, during the 3rd international hearing loop conference in Eastbourne the UK, and including children with hearing aids and cochlear implants.
I thank you both as i realize that this answer took a considerable amount of time, effort and research.
There so much confusion and misinformation out there by people who don’t have the facts. I for one did not realize the FM limitations you described. I’m glad that you both, trusted authorities in Denmark, explained the reality of the situation.
I look forward to hear of your findings of your planned 2015 survey into the preferences of hard of hearing children. I love that you are getting them involved in this discussion.
Very interesting reading for myself as a non-engineer. I was particularly interested in the discussion about the limitations of FM systems. DAB was also new to me, and I would like a reference to an explanation for non-engineers.
To Mr. D. Abell (a.o.)
DAB – Digital audio broadcasting.
Used in commercial radio broadcasting digital audio broadcasting, DAB, is the most fundamental advancement in radio technology since that introduction of FM stereo radio. It gives listeners interference – free reception of CD quality sound, easy to use radios, and the potential for wider listening choice through many additional stations and services.
DAB is a most reliable multi service digital broadcasting system for reception by mobile, portable and fixed receivers with a simple, non-directional antenna.
DAB system is a high spectrum and power efficient sound and data broadcasting system. It uses advanced digital audio compression techniques to achieve a spectrum efficiency equivalent to or higher than that of conventional FM radio.
DAB has been under development since 1981 of the Institute Fur Rundfunktechnik (IRT) in Munich, Germany. https://www.irt.de/en/home.html
To distinguish between electronic systems, we first of all have to realize the fact that every electronic audio devise makes a certain level of electronic background noise in itself, and thus the resulting outcome will have a signal to noise ratio (S/N).
One might compare that to the classroom where the teachers speech represent the signal and the noise from computer fans, fresh air vents and the students fiddling with papers, cuffing and such, represent the background noise.
Looking in data sheet only few manufactures of HAT systems discloses the signal to noise ratio of the system. This is almost always found for professional microphone systems (ref 1), and we only can guess why.
In a conventional analog (ref 2) FM system the audio information is transmitted as variations of the transmitting carrier frequency. The receiver identifies these variations as the audio signal.
Over distance FM radio signals become weaker. And to simplify the explanation let’s make an example where we scale the signal level from level 10 (full) to level 1 (weak) signal. We measure the signal in the distance of 6 feet from the transmitter is level 9 – and we measure the signal in the distance of 30 feet from the transmitter is level 2.
We know that the receivers we use are producing a certain background noise that we’ve measured to 1. Simplified the outcome at 6 feet distance from the transmitter will have a S/N 9/1 and at 30 feet a S/N 2/1 and any amplification will not change those S/N ratios.
As mentioned not only the distance between transmitter and receiver is causing decreasing signals followed by decreasing S/N ratio. Signals can be damaged by simultaneous receiving direct and reflected signals which can’t be separated and consequently will be identified as one signal. And if the reflected signal that has been added is more or less fully in antiphase and have a higher or lower level, the receiver will identify it as a weakened signal leading to poor S/N ratio or even just noise.
In DAB systems the audio signal is (simplified) digital-coded into “ones and zeroes” and transmitted. In DAB systems we also have the issue of electronic background noise – that’s not the difference. But in the DAB receiver the “ones and zeroes” are reestablished as long as the receiver can identify them in the transmitted signal – in fact they are not reestablished, but new and clear “ones and zeroes” are build by the receiver at full level according to those identified by the receiver. So the S/N at outcome will stay 10/1 as long as the receiver can identify even the weakest “ones and zeroes”.
In other words:
As the noise level increases in an analog transmission system, the overall signal-to-noise ratio also increases linearly with a consequent decrease in quality. Analog systems tend toward graceful failures, whereas a digital transmission system suffering a similar increase of noise increase in the transmission system will suffer a sudden cataclysmic total loss of signal (if squelched) or high level crashing sounds (if not squelched). Digital systems are either “perfectly fine” or crash cataclysmically upon exceeding a certain threshold of Signal/noise ratio.
Given equal transmission considerations, the principle difference between analog and digital performance is that the digital system fails cataclysmically while the analog system fails gradually.
But the very best way to illustrate this would be if the professionals within the hearing care tested the HAT equipment on their own ears. Most systems have the option to connect a set of good headphones through which one can hear the quality of sound and what happen when transmitting conditions get poor, e.g. because of distance, not sufficient antennas and in buildings with iron construction. It’s important when testing and judging always to remember and keep a good reference in mind e.g. the home HiFi stereo set, and ask oneself “would I be pleased with this sound quality a whole day through?”
I once did the test in an auditorium. After being using the normal professional wireless microphone set connected to the loudspeaker system in the room for an hour. Then in the coffee break I switched over and connected an conventional HAT equipment to the loudspeaker system instead. The people were appalled by the experience.
Ref 1:
For Phonak Roger pen no S/N ratio is disclose.
https://www.phonak.com/content/dam/phonak/gc_nl/Roger/Datasheet_Roger_Pen_210x297_GB_V1.00.pdf
For Phonak inspire the S/N ratio is 45 dB – but only for the transmitter not the over all system.
https://www.phonakpro.com/content/dam/phonak/b2b/C_M_tools/FM/Transmitters/Brochures/inspiro-Technical-Data.pdf
For Oticon Amigo the S/N ratio is 50 dB
https://www.oticon.com.br/~asset/cache.ashx?id=10589&type=14&format=web
For comparing we can look at a professional wireless microphone system much use by TV and radio studios in Europe, the Sennheiser ew 300 G3 series, which disclose a S/N ratio even or bigger than 115 dB.
https://en-us.sennheiser.com/global-downloads/file/693/EW300G3_529661_0109_US_INT.pdf
Ref 2: In a conventional analog FM system the audio might be handled digitally before it’s transmitted, but the transmitting is still analog.
There’s a Very Big “But” that must be kept in mind: Baseband induction (T-coil-based) neck loops or room loops must NEVER be used on infants, children, and others who are unable to report they have a problem. In fact, improperly configured HAT systems have led some experts such as audiologist Holly Teagle at UNC and noted educational audiologist Cheryl DaConde Johnson to recommend holding off deploying this vitally-needed technology on infants & toddlers specifically because they can’t report a problem.
On the other hand, experts such as Erin Schafer at UNT and Jace Wolfe at Hearts for Hearing recommend HAT be deployed on ALL infants & children, with the proviso that they be robust digital systems such as the GNReSound (and licensee Cochlear) Unite system or the Phonak Roger system, where the 2.45 gHz digital audio is streamed directly into the headworn devices. For more on this subject, please see this article where I discuss robustness in depth:
“More On Our Recommendation For Remote Mic/FM Use For All Hearing Impaired Children”
https://thehearingblog.com/archives/4276
For much more on this, see chapter 7 of the first edition or chapter 9 of the second edition of “Programming Cochlear Implants” by Jace Wolfe at Hearts for Hearing and Erin Schafer at UNT — This chapter alone is worth the price of the book.
“Book Review: Programming Cochlear Implants, by Jace Wolfe PhD and Erin Schafer PhD”
https://thehearingblog.com/archives/197
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From a discussion with Aïda Regel Poulsen, Dana Mulvaney, Avi Blau and Juliëtte Sterkens in the IFHOH group on Facebook at:
https://www.facebook.com/groups/IFHOH/829779767091320/
I wrote:
Now, on to the Comfort Digisystem, and specifically the DT10 headworn receivers: They sent me a system including DM10 transmitter and DT05 programmer to evaluate for my blog; and I have them in front of me now.
The 900 mHz system has three showstopper flaws, any one of them making the system Not Acceptable; but when combined shows a miserable job by my communication systems engineering colleagues (yes, I was an RCA Engineer many years ago):
First and foremost, the receivers do not squelch during signal dropout, whether the teacher goes to the latrine or if there is interference. Any well-designed receiver since about the 1950’s has a squelch circuit; and the fact that this system leaves the user with an earful of static is a disgrace, especially since it’s a digital transmission system.
Second, the headworn DT10 micro-receivers draw 5 mA of current at 900 mHz, which when combined with the hearing aid drain rules out using on #13-fueled hearing aids as the load is too much, resulting in drop-outs due to oxygen exhaustion. This compares to the added 3 mA drawn by the ReSound/Cochlear and first generation Phonak Roger systems operating at 2.45 gHz; and as far as I can guess it’s due to the needless encryption of the signal. [Yes, there are a few *adult* needs for encrypted transmission, such as for military, intelligence, and courtrooms; but then these adults can easily use a pocket receiver — Power-hogging encryption doesn’t belong on headworn receivers worn by children.] The second generation Roger receivers which came out last spring are even better, cutting drain about 20% down to 2.4 mA.
Third, although the system operates in the overcrowded 900 mHz ISM band, the channels must be manually selected and are static, as the system lacks channel-hopping. In fact, it’s so bad, they actually have a (rather nice) RF spectrum analyzer in the DT05 so you can find a clear spot. However, since almost everything else operating at 900 mHz — cordless phones, baby monitors, factory automation systems, & such — uses frequency-hopping, there is no guarantee that in just seconds after you find a clear spot it will be occupied, filling the users’ ears full of static. On The Other Hand, the Phonak Roger, ReSound/Cochlear Unite, Williams Digiwave, and Bellman Audio Domino systems all use frequency hopping, which by the way is also baked into the IEEE 802.11 WiFi and 802.15 Bluetooth and Body Area Network standards.
In summary, the Comfort Audio Digisystem is a poorly conceived, poorly engineered, and poorly built system: It’s because of products like this, and also unreliable neckloop/room loop/T-coil systems, that led audiologist Holly Teagle and leading educational audiologist Cheryl DaConde Johnson to not recommend using FM on children too young to report there is a problem, denying them this vital technology. On The Other Hand, when a robust, bulletproof system such as Unite or Roger is used, then hearing assistance technology can — and must — be deployed.
Dan Schwartz,
Editor, The Hearing Blog
Two major questions appear in my mind:
Why is it that “Baseband induction (T-coil-based) neck loops or room loops must NEVER be used on infants, children, and others who are unable to report they have a problem.”?
If the kids are not able to report problems in loop systems, how are they able to report problems in radio systems?
From my experience radio systems – especially traditional FM systems – have even or more often problems with stability than loop systems. If the loop systems are made and installed professionally and not bought through any mail order company and installed by any randomly picked handyman.
If there are concerns about the stability or proper function in the systems, it’s much easier to control a loop system with a simple amplified telecoil receiver with headphones. This isn’t that easy for non professionals to do with radio systems i.e. parents and staff in kindergartens or schools.
Next issue is using mini receivers mounted on hearing aids to small kids. Battery consumption is one thing, but why increase the weight of a hearing system worn by small soft and weak ears by approximately 1,5 times because of mounting a receiver. Those ears are already loaded. And for teenagers they often are a plague because the receivers just underlines their hearing problems and that they might be different from their other mates.
The second question is rather pragmatic: If the Comfort Digisystem is that bad, why is it then, that Sonora (the company which owns Phonak and the Roger system) has bought the originated Swedish company and producer of the Digisystem – Comfort Audio? Of course the system has some qualities that Phonak will benefit of having access to. Here in Europe both products will be available through the same dealers and companies, and the marked in Scandinavia is widely dominated by the Digisystem for preschool and school children because of their qualities. Not only in connection stability, but because the overall sound quality is so much more and has so many more facet’s to it. And the kids who are using the systems know that, and they are able to make qualified choices IF they get opportunities to try out different systems and IF they are asked. So – as I saw a sign at a pizza take away: How can that many flies be wrong?”
As professional technicians we can talk about frequencies, bandwidth, dynamics, designs and whatever issue we might find to discuss, but it is generally the end users who is the experts if given the free choice.
Thank you Finn for your response. It is unfortunate that in the United States many hearing instruments are sold to users, including parents of children, that do not include Telecoils. Here the professionals are making the decision for them and it’s my experience that the MAJORITY of those professionals have never ever listened for themselves in an IEC standard hearing loop system. So how can they make an intelligent choice much less advise their clients and parents? Many also fail to educate these clients of the ADA (The Americans with Disabilities Act) or refer to the Hearing Loss Association of America (www.hearingloss.org) for information, education, support and advocacy. That hearing loops absolutely delight users is clear from the results in a study which involved over 800 hearing instrument users: https://www.hearingreview.com/2014/09/consumer-perceptions-impact-inductively-looped-venues-utility-hearing-devices/