34 hearingreview.com FEBRUARY 2006

How Do Vents Affect Hearing Aid Performance?A tutorial on venting, and its impact on the occlusion effect

F I T T I N G T I P S

Sounds Leaving the EarLow frequency output. The

effect 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 made

with 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. 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.

A vent is atube. As such, it issubject to tubingresonance. Figure2 also shows thata change in ventdiameter leads toa shift in the vent-associated reso-

W e typically consider sounds atthe eardrum to be a function ofthe output of the hearing aid

moderated 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. 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 sounds.1 Inan open-fitting situation, sounds enteringdirectly through the vent opening willhave a larger contribution to the SPL atthe eardrum.

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, NY.

By Francis Kuk, PhD, and Denise Keenan, MA

Open fittings may be a mixed

blessing. On one hand, more

people with a high frequency

hearing loss will agree to

wear hearing aids that are

almost totally free of

occlusion, and the fit is instant

and easy. On the other hand,

the indiscriminant use of

open fittings can compromise

the integrity of fittings,

especially audibility in the

important high frequencies.

Because open fitting, to a

large extent, is similar to the

use of a vent with an

extremely large diameter, this

article reviews the acoustic

effects of vent dimensions.

FIGURE 1. Sources of sound at the eardrum.

36 hearingreview.com FEBRUARY 2006

Vents and Hearing Aid Performance

nance. 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 used.

Figure 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. Second, thelonger vent is less effective than the shortervent in reducing low frequency output.

In 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 length increases, gain reduction inthe low frequency decreases and the fre-quency at which vent-associated reso-nance occurs decreases.

Maximum gain before feedback.Vent 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). The data were based on 10subjects with primarily a high-frequencysensorineural hearing loss when theactive feedback cancellation algorithm onthe hearing aid was deactivated.

in 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 (4.5 mmdiameter). The limited available gain with thelarger vent diameter may be one reason forthe decrease in performance.

Advantages 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 means

Figure 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. As expected,when the vent diameter increas-es, the available gain decreases.The 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 region.

Maximum gain on the ITEshows a similar trend: gaindecreases as vent diameter

increases. 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 measurements.2

The information on the maximumavailable gain before feedback has signifi-cant implications in the choice of ventdiameter and our clinical practice on theuse of open-fittings.

Open-fittings reduce high frequen-cy gain. 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 not the 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 formed.

Compromises 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. Figure 4 shows the wordrecognition scores as a function ofvent diameter (in a Diva SD-9XITC) when a group of mildlysloping high frequency hearingloss subjects were tested withCASPA3 words in quiet at a 30dBHL level. A systematic decrease

FIGURE 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).

FIGURE 4. Word recognition score in quiet (30 dB HL presentation level)as a function of vent diameter in the Senso Diva SD-9X ITC hearing aid.

FIGURE 3a-b. Maximum gain before feedback forthe 15-channel Diva hearing aid in the BTE model(top, 3a) and ITE model (bottom, 3b).

A

B

38 hearingreview.com FEBRUARY 2006

Vents and Hearing Aid Performance

to ensure the availability of as much gain aspossible to minimize intelligibility loss.

The use of an active feedback cancella-tion algorithm may be the only solution.Figure 5 shows the increase in available gainbefore feedback with the Diva active feed-back cancellation algorithm. Different ventdiameters, including an IROS vent, are used.One can see that the advantage of an activefeedback algorithm is an increase in theavailable gain beyond 1000 Hz. The effectincreases as the vent diameter increases. Forexample, it is only about 5 dB up to 3000 Hzwith a closed earmold, compared to asmuch as 10 dB up to 7000 Hz in the IROSvent. The need for an active feedback can-cellation algorithm in a hearing aid increas-es as the required vent diameter increases.

Bone-Conducted Sounds(Occlusion Effect)

Because a vent (or acoustic leakage)

provides a direct link between the wearers’acoustic environments and their ear-canals, one would expect similar venteffects on bone conducted sounds (or theocclusion effect) and the amplified soundsfrom a hearing aid.

Objective OE ratings. Figure 6 showsthe average occlusion effect as the ventdiameter is changed in a BTE (Figure 6a)and in an ITC (Figure 6b) hearing aid. TheOE is measured as the difference betweenthe real-ear occluded response (REORv)and the real-ear unaided response(REURv) during vocalization of /i/. Forboth styles of hearing aids, the average OEhas a peak frequency around 300-400 Hzwith a peak amplitude of about 20 dB. Onaverage, the OE decreases by about 4 dBfor every 1 mm increase in the vent diam-eter. For a 3 mm vent diameter, the averageOE is about 6-8 dB. Although tube fitting(in the BTE only) results in virtually noOE, the IROS vent (which has an averagediameter of 4.5 mm) in the ITC stillshowed almost 5 dB of OE around the 400-500 Hz region.

The peak frequency of occlusion increas-es as the vent diameter increases. Theseresults show that OE is a function of the ventcharacteristics and not a function of the hear-ing aid style. It suggests the possibility thatthe magnitude of the OE may be predictableif the vent dimensions are known.

To further explore that possibility, weplotted the acoustic mass of the vent againstthe measured occlusion effect for each indi-

vidual subject.4 The acousticmass is directly proportional tothe length of the vent andinversely proportional to thesquare of the vent diameter.2 Theregression lines in Figure 7 showthat the objective OE is a func-tion of the acoustic mass of thevent. The larger the acousticmass (or the smaller vent diame-ter or longer vent length), thehigher the OE. In other words,the OE is a physical quantity thatcan be estimated based on thedimensions of the vent system.The observed differences amongsubjects are most likely related tothe individual’s middle ear char-acteristics and their interactionwith the vent configurations.

Subjective OE ratings.While the objective OE is meas-urable and predictable from thevent dimensions, the subjectiveocclusion effect or occlusion

FIGURE 6a-b. Occlusion effect for different vent diameters in a BTE(top, 6a) and ITC (bottom, 6b) Diva hearing aid.

FIGURE 5. Changes in maximum available gain fordifferent vent diameters in an ITC hearing aid.

A

B

40 hearingreview.com FEBRUARY 2006

Vents and Hearing Aid Performance

rating may not be easily predicted. Figure8 shows the individual subjective occlu-sion ratings (with a rating of 1 being “ownvoice very hollow” and 10 being “ownvoice very natural, no hollowness”) as afunction of vent dimensions. The medianratings are connected by the solid line. Nochange in subjective rating is seen as thevent diameter increases from 0 mm to 1mm. The most significant change occurswhen the vent diameter increases from 1mm to 2 mm. Further increases in ventdiameters do not improve subjectiveocclusion ratings.

This suggests that the relationshipbetween subjective OE and objective OE isnot a simple 1-to-1 relationship. Beyond a2 mm vent diameter, subjective OE is notlikely to improve simply with a larger vent

decreased at the eardrum depending on thevent diameter. With a completely closedearmold, the input is decreased by almost20 dB in the high frequencies. Less attenu-ation is noted as the vent diameter increas-es. Furthermore, at a vent diameter of 3mm, the input is enhanced by almost 3 dBacross the frequencies up to 4000 Hz. It isexpected that a larger vent diameter mayenhance the input to a level between thatprovided by the open ear and the 3 mmvent conditions. This naturally enhancedsound is a main source of interaction withthe directly amplified sounds.

When the natural sounds and theamplified sounds around 2000-3000 Hzare similar in magnitude and phase charac-teristics, they add to result in an output atthe eardrum that is 3-6 dB higher than

either of the input alone. On the otherhand, if these two sounds are of the samemagnitude but out of phase, cancellationwill occur to result in a lower output andeven negative gain. This phase cancellationmay occur around 3000 Hz (from the earcanal resonance) and in the lower frequen-cies (from the vent associated resonance)when the gain provided by the hearing aidis similar in magnitude but opposite inphase to the resonant frequencies. This willresult in irregular “dips” being displayed inthe measured real-ear responses.

The perceptual consequence of phasecancellation is poor sound quality (rougher,harsher sound) and—depending on the fre-quencies where phase cancellation occurs—speech understanding may be affected. Itmay also be a reason why, even with an opentube fitting, subjective OE was still not aperfect “10” (discussed earlier). To furtherimprove the sound quality of a hearing aid,these frequencies where phase cancellationwill likely occur must be accounted for inthe design stage in order to minimize its

diameter (even though the objec-tive OE is lowered). This high-lights the complexity of the sub-jective occlusion rating and sug-gests that additional mechanismis necessary to yield an acceptablerating of one’s own voice.

Sounds Entering into theEar (Direct Sounds)

One of the possible reasons forthe imperfect relationshipbetween subjective OE rating andobjective OE and the decrease inword recognition score with alarger vent diameter may be relat-ed to the interaction of the ampli-fied sounds with the direct soundsthat enter through the vent. Asdiscussed in Figure 1,the SPL at the eardrum isthe result of the interac-tions of the amplifiedsounds and the directsounds that enter

through the vent. Figure 9 showsthe change in SPL (or attenua-tion/gain) at the eardrum result-ing from using inserts of differentvent diameters when the referencesound is presented at the ear canalentrance. A value larger than “0”indicates that the SPL is higher atthe eardrum than at the ear canalentrance reference; a numbersmaller than “0” suggests that theinsert attenuates sounds so they are softerat the eardrum than at the entrance.

Figure 9 shows the typical ear canal res-onance around 3000 Hz, suggesting thatthe input sound is enhanced by almost 20dB around that frequency in an open-fittingsituation. With a vented, occluding ear-mold, the input sounds may be increased or

FIGURE 7. Relationship between occlusion effect (in dB) andacoustic mass (in log Henry). From Kuk et al 2005.4

FIGURE 8. Relationship between subjective occlusion rating andvent diameters.

FIGURE 9. Attenuation characteristics of inserts with differentvent diameters. The unaided response (REUR) is also includedfor comparison.

42 hearingreview.com FEBRUARY 2006

Vents and Hearing Aid Performance

occurrence. (For a discussion on hearingaid design concepts and phase cancella-tion, see Kuk et al’s article5).

So, What’s the Optimal Vent Diameter?

As shown above, open fitting mini-mizes subjective and objective occlusioneffects; however, it does so at a cost oflimiting audibility and possibly decreas-ing the benefits provided by a directionalmicrophone.5 Furthermore, there is thepotential degradation in sound qualityfrom the interaction between the directsounds and the amplified sounds.Logically, it will be beneficial to know theprecise vent diameter so one may mini-mize occlusion while preserving intelligi-bility and sound quality. A simpleapproach is to select a vent diameter thatis large enough to minimize as muchocclusion as possible but not so large thatthe required gain in the high frequenciesis compromised.

If one assumes that the average OE is 20dB and that each 1 mm increase in ventdiameter decreases the OE by about 4 dB,one would require a vent diameter of 5 mm

to totally eliminate all occlusion effect. Thisis a very large vent and may not be feasiblein most cases. On the other hand, that ventsize may not be necessary if the hearing aidwearer can be taught to accept some degreeof physical occlusion through counseling.6

Considering all the issues at hand, if theprimary purpose is to optimize own-voicequality, with speech intelligibility being a closesecondary objective, we would recommend:

n An open fitting for someone with amild hearing loss and for someonewith essentially normal hearing (lessthan 20 dB HL) at 500 Hz.

n Individuals with >20-30 dB HL at 500Hz would require a vent diameter thatis at least 3 mm wide.

n As the degree of hearing loss increases,the diameter of the needed vent decreas-es. In general, every 10 dB increase inhearing loss at 500 Hz would require a0.5 mm decrease in vent diameter.

This recommendation assumes the aver-age vent length (around second bend) andthat active feedback cancellation is avail-able on the hearing aid (decrease the ventdiameter by 1 mm when active feedbackcancellation is not available). w

References1. Kuk F, Keenan D, Peeters H. Ampclusion 103:

Managing high frequency hearing loss. The

Hearing Review. 2005;12(4):36-42.

2. Dillon H. Hearing aid earmolds, earshells and

coupling systems. Hearing Aids. New York:

Thieme Medical Publishers/Boomerang Press;

2001:117-157.

3. Mackersie C, Boothroyd A, Minniear D.

Evaluation of the Computer-assisted

Speech Perception Assessment Test

(CASPA). J Am Acad Audiol.

2001;12(8):390-396.

4. Kuk F, Keenan D, Lau C. Vent configurations on

subjective and objective occlusion effect. J

Am Acad Audiol. 2005;16(9):747-762.

5. Kuk F, Keenan D, Sonne M, Ludvigsen C.

Efficacy of an open fitting hearing aid. The

Hearing Review. 2005;12(2):26-32.

6. Kuk F, Ludvigsen C. Ampclusion 102: A 5-step

approach to remediation. The Hearing

Review. 2002;9(9):34-43.

Correspondence can be addressed to HRor Francis Kuk, Widex Office of Researchin Clinical Amplification, 2300 Cabot Dr,Ste 415, Lisle, IL 60532; email:fkuk@aol.com.

Reprinted with permission. “How Do Vents Affect Hearing Aid Performance?”,

Hearing Review, February 2006; Volume 13, Number 2: Pages 34, 36, 38, 40, & 42.