Stomata is an important component that affects the acoustic parameters of hearing aids.
In the pre-selection process of hearing aids, it is important to predict the size of the stomata required by the user to improve the accuracy and satisfaction of the hearing aid selection.
What is a vent?
The vent is equivalent to a cylindrical lumen.
The longer and narrower the lumen, the greater the acoustic quality within the pores, and vice versa.
As seen in the figure below, the stomata usually consist of lumens of different pore sizes. In general, the longer and wider the stomata, the more sound is leaking.
In the actual fitting of the hearing aid, the air hole mainly affects the frequency response of the low frequency part (0Hz-1KHz), and the specific frequency range depends on the size of the air hole and the gain of the hearing aid.
When the air hole acts on the low frequency sound, it also causes the overall gain and maximum output of the hearing aid to change.
Acoustic effect of stomata
1, reducing low frequency gain
The amplified sound through the hearing aid will enter the human ear through the sound hole.
The closer the sound hole is to the tympanic membrane, the smaller the volume of the residual ear canal, and the greater the sound pressure produced.
If there are air holes in the hearing aid body, the sound will leak a certain amount, so that the sound pressure reaching the eardrum is reduced.
In general, the obstruction of the hearing aid vents to sound increases as the frequency increases. Conversely, the obstruction of the sound of the residual ear canal becomes smaller as the frequency increases.
As such, the interaction of the stomata and the residual ear canal ultimately reduces the low frequency gain in the ear canal.
The reduced amount of gain is related to the size of the pores.
As seen in the figure below, different sizes of pores have different effects on the reduction of low frequency gain.
In general, the larger the inner diameter of the pores, the more effective the effect of lowering the low frequency gain.
2, sound propagation channel
The pores act as a natural sound transmission path while hindering low-frequency sound waves.
That is to say, a part of the sound will enter the ear canal directly through the air hole without being amplified by the hearing aid.
As seen in the following figure, the solid line indicates the amplified path of the hearing aid, and the broken line indicates the vent path. When the sound wave reaches the ear canal, it enters the ear canal through two paths, causing the tympanic membrane to vibrate.
However, during the actual listening process, the wearer does not hear two different sounds at the same time (the electronic sound amplified by the hearing aid and the natural sound transmitted through the vent hole), and the sound transmitted through the two channels will eventually converge to The residual ear canal produces a sound pressure together, causing the tympanic membrane to vibrate.
The figure below shows the experimentally measured true ear insertion gain value (REIG):
1), vent path:
When only the vent path is tested (the dotted line in the figure), the hearing aid is in the off state (equivalent to an earplug), so there is no amplification of the sound, and it will hinder the high-frequency sound waves entering the ear canal;
2),Zoom in path:
When the amplification path is tested (the solid red line in the figure), the hearing aid is turned on, the low frequency sound is attenuated, and the high frequency sound is amplified;
When a vent is used and the hearing aid is on (black solid line in the figure), its true ear insertion gain value (REIG) is substantially similar to the true ear insertion gain value (REIG) of the amplification path – affected by the venting hole, The curve differs only in the low frequency portion, while the high frequency portion does not change.
This shows that the sound transmission effect of the vent hole is not obvious during the actual wearing process. When the user wears the hearing aid with the vent hole, two different sounds are not simultaneously heard.
Practical use of vents
The air inside and outside the ear canal can be exchanged through the air holes, thereby avoiding excessive moisture in the ear canal.
For some hearing-impaired people with tympanic membrane perforation, if the perforation is not large, the vent hole can be selected when wearing the custom machine to avoid excessive moisture in the long-term boring environment of the ear canal, causing bacteria to breed and cause infection.
For hearing aid users with low-frequency hearing close to normal, the internal noise generated by the hearing aid may be an important factor affecting the wearer’s satisfaction. The vents not only allow the internal and external gases to circulate, but also reduce some of the internal noise.