Hearing Aid Technology
The advancements in hearing aid technology in recent years have pushed the boundaries in terms of how much benefit you can receive from digital hearing aids.
Digital hearing aids can be expertly bespoke fitted to match your exact hearing loss and listening needs. To do this, digital hearing aids convert sounds entering the microphone into ‘digitized’ codes. For digital hearing aids to convert sounds into digitized codes they must analyse sounds at regular intervals. The more often a digital hearing aid does this, the more accurate the digitized codes will be for sounds. The number of times a digital hearing aid analyses sounds per second is called the ‘sampling rate’.
To provide even more precise digitized codes, digital hearing aids then break-up the digitized codes into ‘bits’. Without this process, two very similar (but not identical) sounds would end up having the same digitized code and therefore perceived as being the same sound. The greater the number of bits the digitized code can be broken up into, the more precise this process is.
Once sounds have been accurately and precisely digitized it enters the processing chip inside the digital hearing aid. Here, the digitized codes are split into different frequency ‘channels’. This allows the digital hearing aid to be fine-tuned at each frequency channel according to the individual’s hearing loss and listening needs. Other digital processing also occurs at this stage (see below). Afterwards, the digitized codes are recombined and brought back together as one.
The final stage is for the digital hearing aid to convert the processed digitized codes back into sounds and present it into somebody’s ear. It is the receiver (loudspeaker) that carries out this task.
Please select from the tabs below to learn more about digital hearing aids and the different technologies available.
Speech is made up of a range of frequencies. The lower frequencies are usually the vowels and provide the volume in speech. The higher frequencies are mainly the consonants and provide the clarity in speech. For the majority of hearing losses, the higher frequencies are more affected than the lower frequencies. This is why many people with hearing loss are still able to hear people speak but are not sure what they are saying.
Channels within a hearing aid can separate speech (and also all other sounds) into its different frequency regions. By doing so, the hearing aid can then provide the correct sound level (and compression level) for each frequency region based on somebody’s individual hearing test results and listening needs. There are approximately between 17-20 distinct frequency regions (‘critical bands’ based on the Bark scale) important to hearing speech within the organ of hearing (cochlea). Therefore the majority of hearing aid manufacturers deem this to be the optimum number of channels within a hearing aid.
The quietest sounds a person can hear and the loudest sounds a person can comfortably tolerate is known as the ‘dynamic range’. A mixed and sensorineural hearing loss typically leads to a reduced dynamic range. A reduced dynamic range is when quiet sounds cannot be heard by a person but they still perceive loud sounds at the same or similar level as somebody with normal hearing. This is a phenomenon known as ‘recruitment’.
If a hearing aid amplified quiet and loud sounds by the same level for somebody with a reduced dynamic range, the loud sounds would be perceived as being uncomfortably loud. Therefore digital hearing aids apply compression. Compression is when quiet sounds are amplified more than loud sounds. As a result, an individual can both hear quiet sounds and perceive loud sounds as being comfortable.
Many people with hearing loss struggle to hear speech in noisy situations. Even with hearing aids people can still continue to experience difficulties in some noisy situations, especially if they do not have adequate noise reduction.
Noise reduction within a hearing aid separates speech from noise (the more channels the better the separation), before reducing the sound level of channels where noise is more dominant than speech. To do this, the hearing aid must identify what is speech and what is noise. Hearing aids are able to do this quite accurately given that speech tends to fluctuate more in sound level and frequency than noise. The end result is that speech can be heard with less listening effort.
The external part of the ear (pinna) helps you to hear sounds arriving from the front and sides of you by shielding some of the sounds arriving from the back of you. This can be helpful when listening to speech in some noisy situations. However, the location of the microphone on some hearing aid styles, in particular the behind-the-ear (BTE) hearing aid, are not shielded by the external ear meaning it can no longer provide you with this benefit.
Hearing aids with two (dual) microphones are able to help. Sounds arriving more in front of you will enter the front microphone before the back microphone and vice-versa. This helps hearing aids determine the direction of sounds. Using this information the hearing aid can then focus on speech and away from noise (if they are arriving from different directions).
Fixed directionality is when the hearing aid only picks up all sounds directly in front of you. It will only help when noise is directly behind you and speech is directly in front of you.
Adaptive directionality is when the hearing aid continuously monitors the whole environment in search of speech before focusing in that direction and away from noise (again if they are arriving from different direction).
Zoom directionality follows the same principals as Adaptive directionality but has the added advantage of being able to narrow it’s focus to capture more of the speech and less of the noise.
When amplified sounds leak out of the ear and re-enter the hearing aid microphone, the hearing aid will re-amplify it back into the ear at a louder sound level. The re-amplified sounds will then leak out of the ear again and re-enter the microphone, causing it to be re-amplified back into the ear at an even louder sound level. If this cycle continues your hearing aid will begin to whistle. The cycle is known as a ‘feedback loop’.
Digital hearing aids are able to stop the feedback loop by generating and replicating the leaked amplified sounds in the opposite ‘phase’. Being in the opposite phase it will cancel out the leaked amplified sounds so it does not re-enter the hearing aid microphone.
If this method is not quick or effective enough, some digital hearing aids also perform a calibration to calculate the level and frequency an amplified sound is required to be in order for it to leak out of the ear and create a feedback loop. With this information, the digital hearing aid can place a ‘cap’ so that it never amplifies a sound at that particular level and frequency into the ear.
Other digtial hearing aids use their internal memory to keep a record of all sounds that they have processed and amplified into the ear. Therefore if amplified sounds leak out of the ear and re-enter the hearing aid microphone, the digital hearing aid will be able to detect whether or not it has already been amplified into the ear. If so, the digital hearing aid will simply discard the already amplified sounds and not re-amplify them.
Some digital hearing aids also change the frequency of the amplified sounds ever so slightly compared to the original sound entering the microphone (‘frequency shifting’). This can potentially overcome the feedback loop since the same sound is never re-amplified back into ear.
Hearing aid manufacturers have all developed ‘wireless’ accessories to accompany their digital hearing aids. Wireless accessories can range from remote controls to be used by you to adjust and control your digital hearing aids, to remote microphones that can be clipped onto somebody either talking from a distance or in a noisy environment that streams their voice directly into your digital hearing aids. There are also wireless accessories available that can stream audio from your TV, laptop, iPod and conversation from your mobile phone directly into your digital hearing aids. More recently, mobile phone manufacturers have developed Apps that can be used similarly to a remote control to allow you to adjust and control your digital hearing aids.
Loop systems use a magnetic field to deliver sounds directly into your hearing aids. Sounds are picked up by an external microphone and converted into an electrical current which flows through a loop of wire. The loop of wire is either positioned around a room (e.g. perimeter of a bank or church), a chair (e.g. in a cinema or theatre) or worn by the individual themselves around the neck. The electromagnetic current is then detected by a coil of wire situated inside the hearing aid.
As discussed under compression, digital hearing aids amplify quiet sounds more than they do loud sounds. One potential drawback is that digital hearing aids amplify unwanted low level background noise, such as an air-conditioning unit or computer fan.
Expansion helps overcome this by reducing the amplification of such unwanted low level background noise but without affecting the amplification provided for speech. Expansion can also help with reducing internal hearing aid circuit noise, feedback (whistling) and hearing undesired conversations from distant tables in a restaurant for example.
A common complaint amongst hearing aids user’s is that they can hear wind noise excessively loud when outdoors. This may be due to their hearing aids not being bespoke fitted for them but also because of turbalent wind flow circulating inside the hearing aid microphone ports.
To reduce the effects of wind noise some digital hearing aids can detect and eliminate wind noise through comparing the sound level between the dual microphones in each channel. Unlike speech, there will be random and significant increased differences in sound level between the dual microphones for wind due to turbalence. This is picked up by digital hearing aids in each channel and the sound level is reduced back to normal speech levels.
A CROS system can help if you have no hearing whatsoever in one ear (single-sided deafness), or have a hearing loss in one ear but find a hearing aid doesn’t provide any benefit, or cannot wear a hearing aid in this ear for one reason or another (e.g. chronic ear infection).
A CROS system wirelessly transmits sounds from the side of the ‘poor’ ear to the ‘good’ ear. To do this, a transmitter in the style of a hearing aid is either fitted inside or behind the poor ear, also known as the ‘satellite’ ear. It transmits sounds arriving at the poor ear to a hearing aid worn in the good ear enabling you to hear them. It therefore removes the necessity of constantly turning your head to hear sounds from the poor side, and always having to position yourself to ensure the good ear is facing towards people you are talking to. Importantly, the hearing aid worn in the good ear doesn’t block the ear and so the natural hearing on this side remains unaffected.
A BiCROS system is similar to a CROS system but is designed for individuals who also have some degree of hearing loss in the good ear. Of course, the good ear is now known as the ‘better’ ear. The hearing aid worn in the better ear not only picks up sounds being wirelessly transmitted from the poor ear, but also amplifies sounds like a conventional hearing aid in the better ear to improve the hearing on this side at the same time.
Although a CROS/BiCROS system can help people hear sounds they previously couldn’t from their poor ear, they do not aim to provide ‘binaural benefits’. Binaural benefits are the added advantages of being able to hear with both ears as opposed to only one. They include improved clarity, ability to hear in groups and background noise, better sense of direction and awareness of environment, and reduced listening effort.
In some instances a CROS/BiCROS system can make hearing more difficult. For example, if there was interfering noise from the poor ear, this would now be audible in the good/better ear making it more difficult to hear somebody talking on this side. Fortunately, it is possible to deactivate the CROS/BiCROS system in such situations.
The latest generation of hearing aids have an extremely thin (invisible to the naked eye) water-repellent plastic layer applied to both the external and internal components.
Previously, moisture in and around the ears (e.g. sweat, raindrops) would infiltrate the battery contacts and internal electrical components causing corrosion and short-circuiting of the hearing aid. However, with nano-coating moisture alongside debris such as dead skin cells and ear wax simply roll off the hearing aid.