Earmold Acoustics-Effects of Earmold/Earshell Venting-

CSD 861Spring 2014

Venting

• A vent is often an intentional component of a an earmold/earshell

• simply a column of air which provides a channel between the air within the ear canal and the air external to the ear canal

Venting

• Used to allow low-frequency signals to escape– when hearing is normal in the lows, gain may

be too great so venting and decrease low-frequency output.

• Used to decrease occlusion effect– similarly, reduces low-frequency gain for

generated by cartilaginous vibration of ear canal walls secondary to own voice

Venting

• Used to allow low-frequency signals to enter unimpeded– when hearing is normal in the lows, sound

quality and localization may be improved if low-frequency sound enters unprocessed by the hearing aid

• Used to allow for pressure relief– if the mold is a tight fit, pressure may build up

when inserting the mold causing a sensation of aural fullness...a pressure vent can alleviate this issue

Venting

• Used to allow ear canal/middle ear aeration– for patients with lesser degrees of tympanic

membrane perforation or history of recurrent external otitis as an adult, a vent can decrease itching, irritation, infection, and drainage problems that would be present with an unvented mold.

Venting

• Want vent large enough to decrease occlusion effect but not so large that gain/output cannot reach target and/or feedback oscillation becomes problematic.

• There is also an interaction between venting and advanced DSP as well as compression

Venting

• A column of air has mass. Since we are concerned with its effects on sound transmission, we will discuss it in terms of its acoustic mass.

• The smaller the acoustic mass, the larger the venting effect

• For a uniform straight vent, we can calculate acoustic mass with a simple equation.

Venting

Ma = 1500*l/d2

where... Ma equals acoustic mass in Henrysl equals vent length in millimetersd equals vent diameter in millimeters

Venting

• The acoustic mass of a vent decreases as the vent length is shortened and as the vent diameter is increased.– This increases the venting effect.

• The acoustic mass of a vent increases as the vent length is lengthened and as the vent diameter is decreased.– This decreases the venting effect.

Venting• In practice, clinicians do not calculate

acoustic mass but it is important to understand the effects of vent length and diameter as clinicians may select vent size and alter vent length or diameter.

Venting

Venting-Effects on HA gain and MPO-

• Venting and the amplified sound path

– Recall that that for a given ear canal SPL, decreasing residual ear canal volume increases the SPL

– When we add a vent (an acoustic mass), a new path (outward) is added for sound to escape

Venting-Effects on HA gain and MPO-

• Venting and the amplified sound path

– Low-frequencies typically escape since the vent's acoustic impedance is lowest at low-frequencies and the ear's acoustic compliance is highest at low-frequencies

– This creates a low-cut filter with the exact nature of the filter determined by the acoustic mass of the vent

Venting-Effects on HA gain and MPO-

• Side Note on Filters:

High Cut or

Low Cut or

Venting-Effects on HA gain and MPO-

Venting-Effects on HA gain and MPO-

• Venting and the vent-transmitted (acoustic) sound path– The range of frequencies affected by venting

for the amplified sound path are the same frequencies that are affected by the vent-transmitted sound path.

– Attenuation of vent-transmitted sound occurs for frequencies greater than the resonance frequency of the vent.

Venting-Effects on HA gain and MPO-

Venting-Effects on HA gain and MPO-

Venting-Effects on HA gain and MPO-

• Venting and the combined amplified and vent-transmitted sound path– The hearing aid user is not aware of the two

distinct paths of amplified sound and vent-transmitted sound but it is important for the clinician to understand how they may interact.

– For a given frequency, when one path exceeds the other path by 10 dB or more, the effect of the weaker path is negligible

Venting-Effects on HA gain and MPO-

Venting-Effects on HA gain and MPO-

Venting-Effects on HA gain and MPO-

Venting-Proprietary HA Technology addressing mixing of sound pathways-

Venting-Effects on venting on feedback-

• Feedback occurs when HA gain > attenuation of feedback pathway

Venting-Effects on venting on feedback-

Venting-Effects on venting on feedback-

Venting and DSP-Effects of Vents on Mic Directivity-

• Directional microphones are the only self-contained HA technology that has been shown to improve speech-in-noise performance above and beyond restoration of audibility

• Directivity is most beneficial when the HA user is facing the talker of interest and noise is either diffuse or coming from behind the HA user

Venting and DSP-Effects of Vents on Mic Directivity-

• Directivity may be implemented across a wide bandwidth but HA directivity is may be abolished if the vent-transmitted path is within 5 dB of the amplified path

• Essentially, directivity works best when venting is minimized

• Open fittings minimize directional benefit.

Venting and DSP

• Adaptive noise reduction, also, only works when the amplified pathway is dominant. As the vent-transmitted path increases, noise reduction becomes less effective.

• On a positive note, the internal noise generated by the hearing aid is reduced when venting is applied (good for those with normal low-freq hearing)

Venting and DSP

• Compression is ineffective when the vent-transmitted path dominates

• Some manufacturers (e.g., Oticon) deactivate low-frequency channels when an open fitting is selected in order to reduce battery drain.

Venting-Parallel vs. Y (diagonal) vents-

Venting-Parallel vs. Y (diagonal) vents-

• Diagonal vents should only be used when space prohibits the manufacturing or a parallel vent.

• A diagonal vent serves as a high-cut filter and increases the likelihood of acoustic feedback....the worst of both worlds!!!