This post explains some of the comonly used terms in audiology clinics. Effort has been made to keep the language simple and easily understandable for the members of public. Suggestions and comments are welcome.
Cochlea and hair cells
Human ear is formed of three parts. Outer ear that we can see, middle ear with ear drum and three little bones joined together to connect the ear drum to inner most part of the ear, which is housed in special snail like bony portion called Cochlea. Inside the cochlea, there are millions of hair like cells, that are called Cochlear Hair Cells. There are two types of hair cells, which derive their names from their relative positioning in the Cochlea, called Inner and Outer Hair Cells.
Audiogram is the name for the graph of hearing, which is obtained through a standard hearing test. This is obtained by measuring the minimum hearing level that one can hear across the speech frequency range. On the horizontal axis of this graph are frequencies/pitches/tones in the range of 250 to 8000Hz (as most of the human speech is contained in that range) and on the vertical axis are volume level of those frequencies measured in a unit called decibel hearing level or dBHL. Louder the number of decibel, louder the sound is. For example when we whisper, the average sound is around 40-45 dB, normal conversation from a meter away is around 65-70 dB and a loud voice will be around 80dB. Bigger the number in the frequency/pitch value, sharper the sound is so, 250Hz is a very deep sound (corresponding to bass in the music) whereas 8000Hz is a very sharp sound (corresponding to treble in the music).
A standard graph is made up of various symbols for example circles for the right ear and crosses for the left ear hearing responses. There are triangles to depict one’s hearing thresholds obtained through bone conduction (explained below)
Most sounds are a mixture of tones with one or more dominant tones. For example, if you clap your hands, sounds of several different tones will release from your hands with dominant low tones. However, audiogram is generally performed using single tones. That is why it is sometime termed as pure tone audiogram or PTA test of hearing.
Audiogram is generally considered the beginning of finding what is wrong with somebody’s hearing. However, it is just a starting point and a qualified audiologist will do several other tests to identify one’s actual hearing abilities and disruption of functionality due to poor hearing.
This is a special hearing test where instead of tones, the actual speech is used to test hearing. There are several variants of the test where speech samples could be sounds, words or sentences. Generally speaking, most important speech tests in an adult audiology clinics are those that are performed with a white noise in background, known as speech in noise tests.
Air conduction and bone conduction hearing tests
When the hearing is tested through a headphone on the ear or insert ear phone in the ear canal, it is called an air conduction hearing test.
When the hearing is tested through a special gadget placed on the bone behind the ear (known as mastoid bone), it is called a bone conduction hearing test.
In simplistic terms, in air conduction hearing we hear via the ear drum whereas the bone conduction hearing test device sends the sounds directly to the Cochlea.
In simplistic terms, we hear our others voices via air conduction hearing whereas we hearing our own voice via both air and bone conduction hearing mechanisms. That is why when you record your voice and play back to yourself, it sounds very alien. This is due to the fact that when you hear your voice through a speaker-you are mostly hearing it through air conduction mechanism only but when you hear it directly coming from your mouth, internally the sound is transferring via bone conduction and you also hear it through the air. The combination of hearing gives your voice a unique quality that is not replicated when it is heard only through air via the speaker.
Masking in hearing test
Most people have just the same hearing ability in both ears. When one loses hearing because of ageing process, generally they will develop a similar hearing loss in both ears. However, some people can hear better in one ear than the other. When a person can hear better in one ear, it is tricky to measure their hearing accurately. While testing the bad ear, sound could be so loud that it can transfer to the good ear as both ears are so close to each other inside the head. To avoid this, a special noise (which sounds like wind noise) is introduced into the good ear to keep it busy while testing the bad ear.
Hearing loss on audiogram
Normal hearing range is generally considered to be between 0-20 dBHL minimum levels of hearing across the pitch range tested on the audiogram. Anything below 20dBHL (so, a threshold with higher number than 20), is called hearing loss.
Once again the readers are advised that audiogram is only the very beginning of the quest in finding one’s hearing problems. It is often the first step, which an astute professional will use in combination of assessment technique to know about somebody’s hearing. This may include client’s history of hearing issues, other hearing tests procedures and research based questionnaires of hearing ability etc.
Conductive and sensorineural hearing loss
If the hearing loss is due to a problem in outer ear or middle ear (ear drum and the air cavity behind it, which contains three tiny bones and some other important structures of hearing), it results in loss of conduction of sound to the inner ear and is therefore, termed as conductive loss. For example, wax, a hole in the ear drum or fluid behind the ear drum due to infection (known as otitis media) or due to mucous collection (known as Glue ear) may cause conductive loss
Hearing loss that mainly occurs due to fault in the inner ear or Cochlea and beyond that in the hearing system of the body, is called sensorineural hearing loss. For example, ageing process and certain medical/genetic conditions can cause sensorineural hearing loss.
How are conductive and sensorineural hearing loss diagnosed by audiogram
Most people have almost equal hearing from air and bone conduction testing. If the air and bone conduction However, in cases where there is conductive issue (as described above), air conduction hearing thresholds will be raised over and above bone conduction by a certain degree or more.
Auditory processing disorders
This is a term given to a set of problems in understanding speech in quiet and noisy environment, despite having normal or near normal hearing. Both children and adults can get affected with these issues.
A mention of these disorders also reinforces the limitations of audiogram where it has been suggested as far back as 1950 (Hirsh et al) that “a normal’ pure-tone audiogram defines normal hearing in a restricted sense only.” (Courtesy: Dr Kevin Munro, Manchester)
Any device that enhances ones hearing. This definition encompasses everything from ear trumpet to the most modern devices that can sit completely inside the ear to enhance the hearing. Essentially, the hearing aid has a tiny microphone that picks up the sounds, a body that processes the sounds and a speaker that delivers the amplified sounds into the ears.
Analogue and digital hearing aids
Analogue is mostly a thing of past as far as the hearing aids are concerned. Most modern hearing aids are based on digital technology with a small computer chip inside them. They lack any moving parts.
Analogue is akin to gramophones and tape-recorders whereas digital is more like the cd/dvd players versions of sound production system. Modern electronic devices such as phones, music players and even cars have embraced digital technology.
In digital hearing aid the audiologist can set or prescribe a volume at different tones, according to the hearing loss at each of the tones. There are few research based formulae, which can tell the audiologists how much volume a client with hearing loss will need at each of the tones amplified. These formulae are called prescription algorithms. Prescription algorithm will suggest a volume when the hearing loss of a client is fed into the hearing aid software. So, the computer generates a volume that is best suited for client’s hearing loss. This volume is called prescription gain. Gain, here, means how much of the sound is amplified or added to the original sound in other words, how much the sound is ‘gained’ by the user by the use of the hearing aid.
Digital hearing aid have ability to amplify different tones/frequencies differently. The number of frequencies that can be amplified independently are called frequency channels. Generally speaking, it is believed that higher the number of frequency channel, greater the hearing aid benefit can be due to the fact that hearing aid can be customised better. However, studies suggest that after a set number of channels is achieved, adding more channels may not have significant effect on speech understanding any further due to additional channels only. That magic number is reported to be slightly different in different studies. The safest thing to suggest here is that modern equipment are mostly equipped with sufficient number of channel for straight forward hearing loss cases. However, individual needs may vary and some individuals may benefit from a higher number of channels in their hearing aids than others.
Another feature of digital hearing aid is sound compression. This refers to the fact that hearing aids control the volume on the basis of how loud are the sounds getting into the hearing aid microphone. Loud sounds are not amplified as much as the soft sounds. Therefore, the compression system works as an automatic volume control in the hearing aids. Most commonly used compression system in the one known as wide dynamic range compression or WDRC
Digital noise management
Digital hearing aids have a noise management system that can manage the noise around them in more than one ways. Modern hearing aids are clever enough to distinguish environmental noise from the speech. Therefore, they continuously work to enhance the speech and reduce the noise a little bit.
The other clever system that they have is to automatically and digitally change the direction where the microphone is picking the sounds from. Hearing aid detects where the speech is coming from, whether front, back or the sides of the listeners and microphones are digitally zoomed in the direction of speech to give the listener a better speech understanding. For this system to work a pair of microphones work with each other within the same hearing instrument giving rise to a ‘directional’ microphone system. Many of modern hearing aids can wirelessly talk to each other to give even more enhanced directionality features to pick up more speech than noise.
Feedback is the name for whistling that originates from the hearing instrument. Basically, it is a result of amplified sound leaking out of the ear and feeding back to microphone, getting re-amplified again. This results in a howling sound same as if one puts a mic near the speaker. Modern hearing aids can detect feedback sounds and cancel them out digitally so, the annoying whistling sounds are not generated from them