FITTING TIPS How Do Vents Affect Hearing Aid Performance?

1 2006 How Do Vents Affect Hearing Aid Performance? A tutorial on venting, and its impact on the occlusion effect FITTING TIPSS ounds Leaving the EarLow frequency output. Theeffect of venting on the acoustic output of ahearing aid is well documented. Figure 2shows the effect of vent diameter and ventlength on the output frequency response. Astraight line at 0 would suggest no changeto the output relative to measurement madewith an occluding earmold; data above 0 suggest a gain increase (from resonance)while that below 0 suggest gain reductionwith the specific vent dimension (lengthand diameter). The solid line shows the result of a 6 mm-long vent, while the dotted line shows that ofa 22 mm-long vent.

2 For both vent lengths,one sees more low-frequency gain reductionas the vent diameter increases. For example,one sees that the output at 200 Hz is reducedby 7-8 dB with a 1 mm vent diameter, but asmuch as 28 dB reduction with a 3 mm ventdiameter. Thus, an increase in vent diameterleads to a reduction in low frequency outputbelow 1000 Hz. Avent is atube. As such, it issubject to tubingresonance. Figure2 also shows thata change in ventdiameter leads toa shift in the vent-associated reso-We typically consider sounds atthe eardrum to be a function ofthe output of the Hearing aidmoderated by the residual volume betweenthe tip of the Hearing aid/earmold and theeardrum. To a large extent, this is true foran occluding Hearing aid(onewithout any Vents or leakage)and when the wearer listens tosounds from their environ-ments.

3 On the other hand, witha vented Hearing aid and whenthe wearer talks, the overallsound pressure level at theeardrum also includes directsounds that enter (or leave)through the Vents (and anyunintentional leakage) andbone-conducted sounds gener-ated from the wearer s voice. The contribu-tion of each source varies depending on thestate of the wearer (speaking versus listen-ing) and the size of the leakage (or vent-ing), in addition to the gain settings on thehearing aid. Figure 1 shows a simplifieddiagram of the three sources of sound. In the extreme case of someone with ahigh frequency Hearing loss who is speak-ing while wearing a closed earmold, thelow frequency SPL at the eardrum is dom-inated by the bone-conducted open- FITTING situation, sounds enteringdirectly through the vent opening willhave a larger contribution to the SPL atthe eardrum.

4 Francis Kuk, PhD, is the director ofaudiology, and Denise Keenan, MA,is a research audiologist at theWidex Office of Research in ClinicalAmplification (ORCA) located inLisle, Ill, which is a division of WidexHearing Aid Co, Long Island City, Francis Kuk, PhD, and Denise Keenan, MAOpen fittings may be a mixedblessing. On one hand, morepeople with a high frequencyhearing loss will agree towear Hearing aids that arealmost totally free ofocclusion, and the fit is instantand easy. On the other hand,the indiscriminantuse ofopen fittings can compromisethe integrity of fittings,especially audibility in theimportant high open FITTING , to alarge extent, is similar to theuse of a vent with anextremely large diameter, thisarticle reviews the acousticeffects of vent 1.

5 Sources of sound at the eardrum. 2006 Vents and Hearing Aid Performancenance. For the 6mm-long vent, the reso-nance peak occurs at around 400 Hz whenthe vent diameter is 1 mm. It becomes 800Hz and 1200 Hz when the diameter is 2mm and 3 mm, respectively. The real-earSPL is higher than the coupler responsemeasured at the same frequencies when avent is 2 also shows the effect of ventlength on the low frequency output. Thelonger vent (eg, 22 mm) differs from theshorter one (eg, 6 mm) in two aspects. First,the longer vent has the vent-associated res-onance at a lower frequency. In this case, theresonance is at 300 Hz for the longer ventand 400 Hz for the shorter vent when bothhave a 1 mm vent diameter.

6 Second, thelonger vent is less effective than the shortervent in reducing low frequency summary, as vent diameter increases,real-ear low frequency output decreases,and the frequency at which vent-associat-ed resonance occurs increases. In contrast,as vent lengthincreases, gain reduction inthe low frequency decreases and the fre-quency at which vent-associated reso-nance occurs decreases. Maximum gain before diameter also affects the real-earhigh frequency output by limiting itsmaximum gain before feedback. Figure 3ashows the average maximum gain of a 15-channel, moderate-gain behind-the-ear(BTE) Hearing aid (Diva SD-9) when dif-ferent vent diameters are used; Figure 3bshows the same for an ITE Hearing aid(Diva SD-X).

7 The data were based on 10subjects with primarily a high-frequencysensorineural Hearing loss when theactive feedback cancellation algorithm onthe Hearing aid was word recognition score was observed asthe vent diameter was increased beyond 1mm. Almost 20% decrease in speech recogni-tion score was observed between a 1 mmvent diameter and the IROS vent ( mmdiameter). The limited available gain with thelarger vent diameter may be one reason forthe decrease in of active feedbackcancellation. The limited gain beforefeedback and its effect on speech intelligi-bility suggests the need to be conservativein venting when speech intelligibility is themain concern. On the other hand, when itis necessary to use a large vent, such asopen FITTING to maximize comfort (eg, min-imize occlusion), one should secure meansFigure 3a shows that, with aclosed earmold (blue curve), asmuch as 60 dB of gain is avail-able in the low frequencies butonly 50 dB is available in thehigh frequencies.

8 As expected,when the vent diameter increas-es, the available gain decrease is more rapid inthe high frequencies than in thelow frequencies. Indeed, notmuch gain decrease is observedbelow 500 Hz. When the ear-mold is replaced with a tube fit-ting, only 20 dB of maximumgain before feedback is availablein the 2-3 kHz gain on the ITEshows a similar trend: gaindecreases as vent diameterincreases. However, there is less availablegain in the high frequency region for theITE than for the BTE at the same vent diam-eter. This is due to the closer proximitybetween the microphone and the receiver inthe ITE than in the BTE. These values (withthe active feedback cancellation off) are sim-ilar to Dillon s information on the maximumavailable gain before feedback has signifi-cant implications in the choice of ventdiameter and our clinical practice on theuse of reduce high frequen-cy gain.

9 Open fittings (or larger ventdiameters), for the most part, have beenused for people with a high frequencyhearing loss. It should be clear from theabove observations that the rationalebehind this practice is to maximize com-fort with one s own voice and notthe audi-bility of high frequency sounds. Indeed, anopen FITTING typically results in poorer highfrequency audibility. The clinicians and thewearers must understand the objectives(and limitations) of open- FITTING so realisticexpectations can be on speechintelligibility. The reduction inhigh frequency gain would limitthe amount of speech cues avail-able to Hearing instrument wear-ers. This may Affect speech intelli-gibility.

10 Figure 4 shows the wordrecognition scores as a function ofvent diameter (in a Diva SD-9 XITC) when a group of mildlysloping high frequency hearingloss subjects were tested withCASPA3words in quiet at a 30dBHL level. A systematic decreaseFIGURE 2. Effect of vent length on low frequency output for three ventdiameters (1 mm in blue, 2 mm in green, and 3 mm in red). The solid lineshows the result of a 6 mm-long vent, while the dotted line shows thatof a 22 mm-long vent. A straight line at 0 would suggest no change tothe output measured with an occluding earmold; data above 0 suggesta gain increase (from resonance) while that below 0 suggest gain reduc-tion with the specific vent dimension (length and diameter).