Audiological Bulletin no. 32

Transcription

1 Auiological Bulletin no. 32 Estimating real-ear acoustics News from Auiological Research an Communication / 05-07

2 Introuction When eveloping an testing hearing ais, the hearing professional nees a possibility to measure their output an function when use by a hearing ai user. But as it is not always practical or possible to make tests an measurements on the actual user, the professional nees a substitute that can make the hearing ai react as if it was in real use. One common substitute is an acoustic coupler, normally a 711-coupler or a 2cc-coupler. If the hea shaow effect is neee, one can use the Kemar mannequin for example. These are all mechanical ways to simulate the acoustics of a real ear, but as ears are ifferent, one such substitute cannot simulate all ears. Therefore the target for these evices is to get as close to an average hearing ai user as possible within the esign limitations of each evice. Another way to test an measure the function of a hearing ai in real life is by simulating the hearing ai esign on a computer. Simulating real life physical phenomena by computer is common toay. Every time meteorologists forecast the weather, they simulate how weather conitions are most likely to change in the ays ahea. In computer games you can fly a plane, rive a car, play tennis, golf, football or otherwise o things that resemble real life, but where your experience is base on computer simulations. Just as it is possible to calculate how a ball reacts to being hit an how it flies through the air, it is possible to calculate how soun will propagate through a hearing ai an into the ear canal. An in the same way as a computer simulate player can hit a ball har or soft an thereby change the way the ball flies, the simulate ear canal acoustics can be change to stuy the effect of factors such as the iameter or the size an shape of the ear canal. There is an important ifference between physical substitutes such as acoustic couplers an computer simulation. In couplers, it is very ifficult to change the parameters involve, whereas in computer simulation it is possible to exchange all the parameters (e.g. iameter, length, ear size, compliance) one by one an examine the effect on the soun pressure. Furthermore, as the entire setup is implemente as calculations in a computer, it is not only possible to check the soun pressure at the earrum, but also to look at the soun pressure at other places; for example, to estimate how much soun will return to the hearing ai microphone from the ear canal, or to estimate how much soun will propagate irectly into the ear canal through s an leakages.

3 What oes ear acoustics involve? What then is involve in a moel of ear acoustics? The soun has to come from somewhere. It is recore at the hearing ai microphone, sent through the hearing ai, an playe back at the receiver, which is connecte to the ear canal through various tubes of ifferent imensions an shapes. In the ear canal, the size an shape of the ear canal itself have to be efine. The earrum reacts to soun an the way it reacts also has an influence on the soun pressure in the ear canal. Therefore the earrum an the reaction of the mile ear cavity also play a role. As the soun in the ear canal can be altere ue to s or leakages, these must also be put into the simulation. Besies being playe back by the hearing ai, soun can also enter the ear canal irectly through the, earrum an leakages, an will be mixe with the soun from the hearing ai in the ear canal. As these ifferent elements have been escribe in literature on acoustics, the researcher can enter them as elements in the computer simulation. The efault imensions will, of course, be the average of a high amount of ata measure on hearing ai users for the calculations to fit as many people as possible. As more iniviual ata becomes available, iniviual imensions can be exchange to get a more precise simulation of the iniviual ear acoustics. By changing the imensions in the simulation, one can examine what happens in the real worl in an easy way.

4 An example of this is shown in the figure below. On the left, the ifference in soun pressure at the earrum between a close earmoul an earmouls with ifferent size s is simulate for an average person. On the right, the same situation is shown but this time the attenuation between the receiver output an the microphone input is calculate, that is, the amount of gain the hearing ai can provie before proucing auible feeback. Vent effect [B] k k Frequency [Hz] = 0 mm = 1 mm = 2 mm = 3 mm Max gain before feeback [B] = 0 mm = 1 mm 30 = 2 mm = 3 mm k k Frequency [Hz] Therefore it is possible to create a substitute for a real hearing ai an a real ear in a computer program to simulate soun in real life. Using this kin of simulation, it is also possible to simulate a change in some imensions (for example the iameter) an stuy what happens to the soun pressure at various places in the hearing ai an ear when oing so. Further use of simulation Going back to the comparison with the simulate ball player in a computer game, where the player can kick a ball har or soft an thereby affect the way the ball flies through the air, we will shortly look at another way of using computer simulations. If we look only at how far the ball flies without knowing anything about what the player has one, is it then possible to estimate how har it was kicke?? Yes - if the ball flies a long way, we may assume that the kick was har just as a short flight will be associate with a soft kick. In the same way, it is possible to measure the soun pressure in the ear an, by comparing the soun pressure for ifferent sizes, estimate how large a shoul be to give that soun pressure. Furthermore we can estimate the effect that fits best by looking at the maximum possible gain before auible feeback occurs.

5 The Wiex AISA concept In the fitting of the Integrate Signal Processing (ISP) series of hearing ais from Wiex, a complex simulation of real-ear acoustics ensures that the iniviual hearing ai user acquires the gain neee for the optimal benefit of the hearing ai. This part of the fitting is calle AISA (Assesment of In-Situ Acoustics) an it oes exactly what is escribe above. Base on measurements of parameters uring the fitting routine it estimates other parameters, for example the iniviual effect, which can be estimate by measuring the feeback gain uring the feeback test. This allows the fitting rationale to compensate for the actual, iniviual effect instea of using average ata. How the AISA ata is use in the compensation will be treate in the auiological bulletin Ensuring the correct in-situ gain. Litterature Further stuies of moelling ear acoustics can be foun in the following backgroun material: Brüel & Kjær, Ear simulator type 4157, Prouct ata sheet DP Egolf, BT Haley, HC Howell & VD Larson, A technique for simulating the amplifier-to-earrum transfer function of an in situ hearing ai, JASA 84 (1), pp. 1-10, 1988 EAG Shaw & MR Stinson, Network concepts an energy flow in the human mile ear, JASA, Suppl 1, 69, S43, Text of invite 101th ASA meeting, May, 1981 Kuk, F. an Norahn, M. Where an Accurate Fitting Begins: Assessment of In-Situ Acoustics (AISA) (2007 Jul.) The Hearing Review, July 2006 Lybarger S.F. (1985). Earmols. In Hanbook of Clinical Auiology. Thir eition, J. Katz (e), Baltimore: Williams an Wilkins MR Stinson & BW Lawton, Specification of the geometry of the human ear canal for the preiction of soun pressure level istribution, JASA, 85 (6), pp , 1989 Norahn, M. an Kuk, F. Assessment of in-situ acoustics (AISA) - Making an accurate fitting even more accurate (2006 Apr). Hearing Review, 13 supp. (4), 12-22