Audiological Bulletin no. 31

Transcription

1 Auiological Bulletin no. 31 The effect - an introuction News from Auiological Research an Communication / 05-07

2 Introuction Venting in earmouls has been use for many years to control the soun pressure level in the ear canal at low frequencies an to make the hearing ai more comfortable to wear. Vents often reuce the occlusion effect an the buil-up of moisture in the ear canal as they permit the passage of air into the ear canal. The effect of ifferent sizes (iameters) has been escribe in litterature an many ifferent styles an shapes of s are available to make controlle ajustments to the soun characteristics of hearing ais in the ear canal. In most moern hearing ais the soun pressure level can be controlle effectively with high frequency resolution in the hearing ai amplifier itself. This avance technology reuces the nee for controlling the soun pressure level using a special esign. This leaves relief of the occlusion effect an improvement of wearer comfort as the main reasons for making s in earmouls or shells. The hearing ai works to control the soun pressure level in the ear an provie the correct soun pressure level at the earrum. But how can the hearing professional ensure that the soun pressure in the ear is as they prescribe? Hearing ai manufacturers measure an set up the hearing ais to prouce a specific soun pressure level base on an average hearing ai user s ear canal an the iniviual user s hearing loss - using a close earmoul. But as the real-life hearing ai may be fitte with an earmoul that is not close but equippe with a or an open fitting, the expecte soun pressure at low frequencies is reuce accoring to the egree of openness. The amplification provie by the hearing ai has to compensate for this iniviualisation of the earmoul or shell. For this to be possible, the fitting software nees to know the actual effect of the in the iniviual real-life earmoul.

3 The effect When creating a, which is basically making a hole through a close earmoul to be place in the ear of a hearing ai user, the soun pressure level in the ear canal changes. How big this change is epens mainly on the imensions of the hole an the resiual volume of the iniviual ear, that is the air-fille space between the earmoul an the earrum. The change in soun pressure level also epens on the source of the soun. This means that the change in soun pressure for soun coming from the hearing ai is ifferent from that for soun coming from, for example, the hearing ai user s own voice. When we talk about the effect, it is the effect of the on the soun generate by the hearing ai. Figure 1 shows the effect for a close earmoul (0 mm) an three ifferent iameters in an average ear. With an increasing iameter, the soun pressure falls for the low frequencies. In this example the amping of the soun from the hearing ai ue to a 3mm is 23B at 200Hz. The figure also shows that the leas to an increase in the hearing ai gain at some frequencies. The 3mm in this average ear will increase the soun level from the hearing ai by about 7B at 800Hz. 10 Vent effect [B] = 0 mm = 1 mm = 2 mm Ø Ø Ø = 3 mm Ø k k Frequency [Hz] Figure 1: Shows the effect for ifferent iameters. This means that the fitting software for the hearing ai shoul be able to hanle the effect of the to be able to calculate the correct amplification of the hearing ai for the iniviual user an hearing loss.

4 Vent imensions: Diameter an length In general terms, what efines the effect is the cavity in the ear an the opening to the external environment mae by the. At this stage we enter the worl of basic physics. The an the resiual volume of the ear canal can be seen as a close cavity with an opening in one en, like a bottle with a bottleneck. The imensions of the bottle etermine the frequency at which amping of the low frequency content in the cavity will start to occur. This frequency is calle the resonance frequency. This is the frequency of the tone you will hear if you blow over the top of such a bottle. In physical terms, a system consisting of a cavity an an opening of a certain length an iameter is calle a Helmholtz resonator. You see this in musical instruments as well as in louspeakers an room acoustics. The calculation of such a system s resonance frequency has been escribe by the physicist Helmholtz (1860) an is therefore calle the Helmholz frequency.

5 The Helmholtz resonance The Helmholtz resonance escribes the phenomenon of air resonance in a cavity, here a bottle. The resonance frequency of a bottle is efine as: f = k 2 L V L V Figure 2: A bottle as an example of a Helmholtz resonator. where k is a constant 1, is the iameter of the opening, L is the length of the opening an V is the volume of the bottle cavity. If the iameter, length or volume (, L or V) changes, the resonance frequency of the bottle changes too. If the iameter of the bottle () is mae larger, the resonance frequency will be higher. A longer bottle neck (L) will lower the resonance frequency. A bigger bottle cavity (V) will lower the resonance frequency. How oes this compare to the situation of a hearing ai an an earmoul place in an ear? The in the earmoul can be seen as the neck of the bottle an the resiual volume of the ear canal as the cavity in the bottle. = Figure 3: An ear canal with an earmoul or shell is also a Helmholtz resonator. c k = where c is equal to the spee of soun 2 π

6 Let us look at what this means for two cases: A larger resiual volume (V in this example) leas to a lower resonance frequency an thus a ecrease effect. A longer (L) also leas to a lower resonance frequency an thus a ecrease effect. A larger iameter of the () will lea to an increase effect. You can see the effect in Figure 4, where the effect of a 2mm on an average ear is shown for the lengths of a stanar earmoul (approx. 18 mm) an a custom élan earmoul (approx. 2 mm) to the left an ear sizes (large, meium, small) to the right Vent effect [B] normal earmoul shell only with Vent effect [B] Large ear Meium ear Small ear k k Frequency [Hz] k k Frequency [Hz] Figure 4: Vent effect for long an short s an for small, meium an large ears. The effect in this figure is shown for ifferent lengths on the left an for ifferent resiual cavity sizes with a stanar earmoul on the right. The size is 2 mm an all other parameters are average. Summary The iameter alone is not enough to preict how the soun pressure level in the iniviual ear canal iffers from the expecte soun pressure in an average ear. Intene an unintene leakages an further parameters / factors may have a large effect on the actual soun pressure in the ear canal that the fitting software has to take into account for the hearing ai to provie the gain neee in orer to perform optimally for the iniviual user. Therefore it is essential for a precise fitting that the fitting software can assess correctly the actual acoustic situation in the ear of the user.