Effects of Earmold/Earshell Venting

1. Earmold Acoustics
-Effects of Earmold/Earshell Venting-
CSD 861
Spring 2014

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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.

8. Venting
Ma = 1500*l/d2
where...
Ma equals acoustic mass in Henrys
l equals vent length in millimeters
d equals vent diameter in millimeters

9. 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.

10. 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.

11. Venting

12. 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

13. 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

14. Venting
-Effects on HA gain and MPO-
• Side Note on Filters:
High Cut or
Low Cut or

15. Venting
-Effects on HA gain and MPO-

16. 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.

17. Venting
-Effects on HA gain and MPO-

18. Venting
-Effects on HA gain and MPO-

19. 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

20. Venting
-Effects on HA gain and MPO-

21. Venting
-Effects on HA gain and MPO-

22. Venting
-Effects on HA gain and MPO-

23. Venting
-Proprietary HA Technology addressing mixing of sound pathways-

24. Venting
-Effects on venting on feedback-
• Feedback occurs when HA gain > attenuation
of feedback pathway

25. Venting
-Effects on venting on feedback-

26. Venting
-Effects on venting on feedback-

27. 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

28. 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.

29. 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)

30. 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.

31. Venting
-Parallel vs. Y (diagonal) vents-

32. 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!!!