Hearing aids

Many people today wear glasses (visible) and contact lenses (invisible) and do not think much of it. Hearing aids, on the other hand, like to be hidden. An entire industry thrives on miniaturizing hearing aids. No one would flaunt the hearing aid as a piece of jewelry. A hearing aid with diamonds from Van Cleef & Arpels or Tiffany? That's pie in the sky at best. Here's a look at a growing industry.

There are two types of hearing aids: analog and digital. Analog devices consist of a microphone, an amplifier and one or two earphones.
A digital device consists of a microphone, an analog-to-digital converter, a control device. a digital-to-analog converter, an amplifier and an earphone.

The analog device is in principle superior to the digital in sound quality because it is simpler (even though the hearing aid industry likes to claim otherwise). Here a user experience:

Direct comparison of the two hearing aids showed me that in simple listening situations, with two people or just listening to the radio, I hear much more clearly with the analog device.
I then went to the acoustician and asked him if he could adjust the characteristic curve of the digital device for me in the same way as the analog device. This was not possible!!!! The slope of the individual channels of the digital device is so low that adjustment is not possible.

Or a musician's blog (quoted by Michael Fremer):  

“I’ve had analog hearing aids for years and my banjo sounded great. They broke recently and I am now trying my fifth set of digital hearing aids. They distort the sound of my music and nothing that the audiologist does seems to help.”

Larger analog devices offer bass and treble controls whose slopes are no worse than those of digital devices. However, even the analog device is far from the ideal of a straight wire with gain.

First problem: the microphone.

All hearing aids - analog and digital - use tiny microphones with sound holes usually the size of a pinhead. These are electret, piezo-electret or MEMS*) microphones. The fact that these tiny transducers can deliver a realistic signal at all (and at a very low price) is amazing, but defies any high-fidelity standard of at most 1 percent distortion tolerance.

When it comes to the amplifier of the analog devices, the manufacturer can qualify. Mostly, however, economies are made (in terms of money and space): Some devices deliver a reasonable sound pressure level only at maximum volume.

When it comes to earphones, the user usually has a free hand. He or she can choose between wired and bluetooth transducers. In terms of sound quality and power consumption, wired phones are superior to bluetooth. Therefore, wired earphones are currently experiencing a revival. By purchasing high-quality phones, users can significantly improve listening quality and volume compared to the bypack or factory installed phones.

However, most analog devices still available today are integrated: Microphone, amplifier, and earphone are combined in one single component: an earbud or behind-the-ear device with a tube-coupled earphone. 

The situation is obvious: if you are looking for top audio quality, you should look for analog devices. The problem: there is no qualified advice available. The digital industry has largely chased analog equipment from the market. Most acousticians sell only digital devices, on whose delivery, physical and digital fitting they earn. That the result is not always satisfactory for users is confirmed by a German ENT doctor who says: "Most hearing aids are lying in a drawer because patients no longer use them."

What problems exist with digital devices? In addition to the shortcomings of analog devices, there are others that are specific:

Digital hearing aids don’t have unlimited power available to drive the A/D convertors. Hearing aids need to operate at 1 to 1.3 volts and must keep the battery current consumption in check to have reasonable battery life for the wearer of the device. With limited power available, the operating range of the hearing instrument A/D convertor is also limited.

The input analog-to-digital converter almost always operates at 16-bit level, which means it can't convert higher volumes and distorts.

Modern digital hearing aids usually sport a 16-bit A/D converter, which at best aims for a dynamic range of about 96 dB (but usually it is a bit below 96 dB, more like 85 dB).

While a “less than full” dynamic range is sufficient for most speech inputs, instrumental music can easily exceed 105 dB SPL and thus present a signal that is beyond the upper limits of the dynamic operating range of the hearing aid A/D convertor. Instrumental music, having both an overall higher intensity and greater crest factor, tends to overdrive the front end of the hearing aid because of the limited available dynamic range on the given A/D converters.

Therefore it is advisable to change to 18bit conversion. But which manufacturer actually uses 18bit conversion? Silence. Information is hard to come by. Instead, manufacturers indulge in flowery descriptions of the alleged top performance of their devices.

The acoustician's art of manipulating a hearing aid on the computer may be impressive to the layman, but appears rather limited to the technician and does not seem to justify the high cost of digital devices.

Another problem with digital devices is time delay, latency. Since the signal is processed digitally in the control unit, time passes and the signal arrives in the headphones with a small delay compared to the original sound. This delay means that two identical signals (direct sound and headphone sound) reach the ear with a delay. If the time difference is very short, it can sound like an additional "space" surrounding the sound source. If the difference is greater, the headphone signal sounds like an echo or reverberation of the original sound. 

"Everyone reading this understands comb filtering, time and phase coherence and the deleterious effects of time delay on sound quality. Comb filter effects produce, among other things, a bright, tinny sound and in worst case scenarios spatial distortions and/or just a “feeling” of discomfort." says Michael Fremer, veteran technical editor for Stereophile, in a hearing aid review.

"The comb-filter effect is particularly audible for users with mild-to-moderate hearing losses who are in turn likely to have the open and vented fittings where the problem is most prominent. The effect is most pronounced when the two sound contributions are of approximately equal amplitude, typically between 0.5 and 2 kHz, depending on the ear tip, ear canal, and hearing aid gain".... "This physical effect of delay has until now been an unavoidable price for the benefits of digital signal processing". ..."as more features are added to the hearing aid, the total group delay can increase (Herbig and Chalupper, 2010). For example, group delay can increase with a greater number of channels, more advanced digital noise reduction and feedback systems, etc." "....for isolated cases, an even shorter delay...might be required to achieve optimum sound quality. In such instances, we recommend disabling some of the adaptive signal processing features, which add to hearing aid delay. Switching to a device with fewer channels, and therefore fewer filters, would also be a reasonable solution."


Digital hearing aids offer a mix of advantages and disadvantages. Potential clients are most probably unaware of the problems and pitfalls of digital technology when they are shopping for a hearing aid.  As a result, they are easy prey for the audiologist next door, who may not be familiar with the evolving state of digital technology himself.  Massive advertising featuring smiling faces of older people is also not helpful. As a result, many, if not most, first encounters with digital technology are rather disappointing. As Joshua Alexander has discovered, the top models of top brands are showing latencies between a minimum level of 2 milliseconds and a maximum of 7-8 msec above which engineers not dare to go because of the consequences.

In general, there is agreement that delays below 10 ms are likely to be acceptable, although Stone et al indicated that delays below 5 or 6 ms may be acceptable, and Dillon et al showed a relationship between delay and preference values. 

The models examined by Alexander, however, are some of the best and most expensive digital hearing aids. What about all the others?

So what does a good analog device look like? In addition to in-ear and behind-ear devices, there exists a host of so-called hearing amplifiers that contain a microphone (in the casing or in the earphone cable) plus an amplifier and a plug for a wired earphone. One company (Axon) makes several types of such devices, priced at well under a hundred dollars and in sizes ranging from 5 to 10 centimeters. In terms of sound quality, these devices are similar, as they probably use similar mini-microphones and earphones.

If you want better sound quality, you will need a hearing amplifier with a microphone input to which an external quality microphone can be plugged. The bypack microphones supplied together with the amplifier often offer high sensitivity but limited sound quality. Among external plugin microphones, it is popular to use svivel models that can be bent by 90 degrees and pointed at an interlocutor, thus reducing ambient and background noise.

External microphones are usually much larger than the tiny transducers serving hearing aids. With size, the frequency range is also expanding: the sound becomes fuller, more natural.

In terms of amplifiers with an external microphone input, there are only about three or four models and one kit available, at different (but very moderate compared to digital devices) prices. The microphone input should conform to the 3.5-millimeter TRS standard. The TRRS standard, popular with smartphones and cameras, is almost always not suitable.

These amplifiers are not miniaturized and are accordingly clunky (about 10 cm side length) and heavy. But they do provide the user with a benchmark of what level of sound quality is achievable with a hearing aid. Ideally, every potential customer should buy (or borrow?) such an analog amplifier (including top quality wired earphones and a quality plugin microphone for less than 400 dollars total) in order to test a digital device before buying it. Would the acoustician play along?

*) MEMS or silicon microphones are often called digital microphones. An integrated A/D converter delivers the signal in PDM mode.



Clinical comparison of a digital and an analog hearing aid


"Digital signal processing in hearing instruments has brought new perspectives to the compensation of hearing impairment and may result in alleviation of the adverse effects of hearing problems. This study compares a commercially available digital signal processing hearing aid (HA) (Senso) with a modern analogue HA with programmable fitting (Logo). The HAs tested are identical in appearance and, in spite of a different mode of operation, the study design ensured blinding of the test subjects. Outcome parameters were: improvements in speech recognition score in noise (deltaSRSN) with the HAs; overall preference for HA; overall satisfaction; and various measures of HA performance evaluated by a self-assessment questionnaire. A total of 28 experienced HA users with sensorineural hearing impairment were included and 25 completed the trial. No significant differences were found in deltaSRSN between the two HAs. Eleven subjects indicated an overall preference for the digital HA, 10 preferred the analogue HA and 4 had no preference. Concerning overall satisfaction, 8 subjects rated the digital HA superior to the analogue one, whereas 7 indicated a superior rating for the analogue HA and 10 rated the HAs equal. Acceptability of noise from traffic was the only outcome parameter which gave a significant difference between the HAs in favour of the digital HA. It is concluded that there are no significant differences in outcome between the digital and analogue signal processing HAs tested by these experienced HA-users. outcome parameter where there was a significant difference between the HAs in favor of the digital HA.

Thus, it can be concluded that there are no significant differences between the digital and analog signal processing HAs tested by these experienced HA users."


It can be assumed that strict technical comparability prevailed in the above study: same microphones, same headphones, since the gadgets under consideration were obviously different products of the same manufacturer. What weighs heavily is the fact that this clinical comparison was published with the prestige of the US government. Thus, this study stands in contrast to other studies with unclear backgrounds, which attest clear advantages for digital devices over analog ones. Usually,  these studies consider analog devices a historical predecessor of modern, digital technology.

It's evident that an industry is fighting for its market. Analog hearing aids are available on the Internet - without the need for an acoustician, starting at around 18 dollars. They should not be called hearing aids at all, but only hearing assistants, in order to protect the largely health insurance-financed hearing aid market, which is sealed off from the wild by requiring specialist prescriptions.

Most individuals who seek hearing help are offered a choice of only digital technology these days.

The customer, who gets a hearing aid almost entirely reimbursed by the insurance, doesn't really care how much the fun costs. The industry provides him or her the prestige of owning a high-tech device. Whether the piece of cutting-edge technology works better than an 18+ dollar device from AliExpress (or Ebay) that you have to wait two months for, she or he probably won't even want to test. When shopping for el cheapo gadgets, however, it is advisable to stick to analog models because digital ones usually sport high latencies.


Visible or invisible?

An Italian site explains:

""Invisible" hearing aids are those that are inserted so deeply into the ear canal that they are hardly noticed, except for a very small transparent handle the size of a pinhead that is used to pull the aid out.

These hearing aids prove to be the most popular with people with hearing loss, who do not want their deficit to be noticeable, but in fact provide far fewer functions than behind-the-ear hearing aids, which are technologically more advanced.

"Invisible" hearing aids have all the basic features that enable clear hearing in every situation, but because of their very small size they cannot take advantage of advanced features such as wireless connection with various devices, and they are not available in rechargeable mode, but only with disposable batteries. And the smaller the devices, the more frequently the batteries need to be replaced."


Replacing batteries is a nuisance and ecologically objectionable. But the rechargeable batteries of larger or costlier devices don't last forever either. Usually they can be recharged 1000 times. If recharged overnightly they last roughly three years. After that the expensive device is dead and can be trashed or kept as a souvenir. With some brands, the battery can be replaced, but this can cost "a few hundred dollars". 

Suggested for continued reading:

The Unintended Consequences of OTC Hearing Aids

Over-the-counter hearing aids will be available by October. They’ll benefit many but could lead to stigmatization, inadequate testing, and even hearing loss.

Special: Low frequencies and cheap digital devices

Cheap digital players (with and without Bluetooth) from the Far East are promising rich listening experiences for a few dozen dollars. As long as a music signal is supplied by wire, they work fabulously. However, when the acoustic signal comes via a microphone, latency spoils the listening pleasure.  Therefore, the signal provided by the microphone and the live sound must be strictly separated

This only occurs if the headphones are sealed and acoustically isolated from the ambient sound.
In reality, headphones or earphones behave in a semi-transparent manner, i.e., they allow some of the ambient sound to pass through - mainly the low tones. So the client gets the high and mid frequencies from the headphone, but the low frequencies from the environment. This acoustic mixing only works if the latency approaches zero.

But it offers three enormous advantages
- It corrects the typical problems of hearing impaired people who have lost their sensitivity to mid and high frequencies, but retained their ability to hear low frequencies. Because of the semi-transparent arrangement, the low tones are provided by the environment and the high tones are delivered by the hearing aid. By acting on the volume the listener can alter the acoustic mix.
- The fact the client does not need low frequencies to be amplified reduces the workload of the hearing aid because the low end can be filtered out of the signal.  The low tones would overload the weak electronics and consume power unnecessarily.
- If a microphone is used with a response tilted towards the high frequencies - typical of electret microphones -  a natural sounding mix can be obtained requiring only minimal electronic intervention, and minimal expense. This is true of analog hearing aids.

Because of the nasty latency problem, digital aids tend to be expensive. Since the U.S. has recently announced it will remove the obligatory prescription requirement and allow over-the-counter sales of hearing aids, new efforts can be expected to reduce the cost of digital devices. In fact, Sony already announced il will launch a lower cost hearing aid in cooperation with a major Danish group.  


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