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Earmold Design News
DETERMINING EARMOLD VENTING & CANAL LENGTH
By Gregory Frazer, Au.D., Ph.D. & Marni Novick, M.A.
We’ve been dispensing hearing aids in a private practice setting for 27 years combined. Over the
years, we’ve been asked how to determine the appropriate earmold vent size and canal length.
One way to get around having to select a specific vent size, is to order a Select-A-Vent (SAV) on
every patient. That way, you can determine the appropriate vent size with the earmold and/or
hearing aid in the patient’s ear. However, you still have to know what canal length to order, as
well as which SAV to use based on the patients acoustic needs. In addition, we don’t find SAVs
cosmetically appealing, they can fall out, sometimes they don’t fit, and they’re not cost-effective
(they add $2.50 to the cost of each mold, while a vent is free at EDI). Nevertheless, the question
is how do you determine the correct vent size and canal length? Believe it or not, this is a very
simple question that is complicated to answer. We will try to make our explanation as succinct
as possible.
Advantages and Disadvantages to Venting
There are at least 5 advantages to venting an earmold: 1) to reduce unnecessary low frequency
gain and output from the ear canal/eardrum, 2) to allow unamplified sound to enter the ear
canal, 3) to reduce the occlusion effect, 4) to relieve the feeling of pressure in the ear, and, 5) to
reduce moisture build-up in the ear canal. One disadvantage to venting is that the volume of the
vent combines with the ear canal cavity to produce a Helmholtz resonator (increased gain in the
250-1000 Hz region), sometimes resulting in an echo or barrel effect when the patient speaks.
Another disadvantage of venting is, of course, feedback. As vent size increases, the amount of
acoustic leakage increases, and therefore the probability of feedback increases.
Parallel vs. Diagonal Vents
Parallel vents are generally used, because not only do diagonal vents decrease low frequency
gain like parallel vents, diagonal vents also decrease mid frequency (630-1600 Hz) gain by up to
10 dB. Therefore, unless you want to decrease mid frequency gain, parallel venting should be
used.
The Effect of Parallel Vent Diameter/Canal Length
The rule of thumb is that as the parallel vent size increases (1 to 3mm), low frequency gain
decreases (up to 30dB) at 500 Hz and below, and mid frequency gain 630-1600 Hz increases
(up to 10 dB). As canal length increases (6 to 22 mm), the effect of venting is reduced. Thus, low
frequency gain decreases are less (15 dB versus 30 dB) with long canals, and mid frequency
gain will decrease as well (in fact, there may be no increase in mid frequency gain at all). So,
what does this mean to you? If you want to increase the mid frequencies, and decrease the low
frequencies, you should order a short canal (6mm) and a large vent (3mm). A long canal
(22mm) with a large vent (3mm) will not reduce the low frequencies as much as a short canal
(by as much as 10 dB), and the mid frequencies will be reduced more with a long canal than a
short canal. Pressure equalization vents (.06-.8mm) on the other hand, no matter what the canal
length is, generally only decrease gain up to 6 dB below 250 Hz. (See Table 6-5)
A Venting Selection Guide
Figure 1. is a rough guide for selecting the appropriate venting based on low frequency hearing
loss. In general, if the hearing loss below 1000 Hz is 25 dB or less, an open mold (large vent
(3.0 mm) or acoustic modifier) is recommended. If the hearing loss is 30-45 dB in the low
frequencies, an acoustic modifier or medium vent (2.0mm) is recommended. If the hearing loss
is 50-60 dB in the low frequencies, a small vent (1.0 mm) or pressure vent (.06-.88 mm) is
recommended. When the loss exceeds 60 dB, we usually do not put in a pressure vent, unless
the patient is occluded or their voice bothers them. When in doubt, put in a pressure vent, and
plug it if it causes problems with feedback. The “Rule of Thumb” is that it is easier to plug a vent
than it is to drill a vent. We use cushion grip, a denture adhesive, to plug vents. However, before
plugging the vent with cushion grip, which is hard to get out of a vent, plug the vent with Fun Tak
or clay to make sure the patient is not bothered by a plugged vent. If plugging the vent does not
bother the patient (occlusion, resonance in his voice, etc.), then plug the vent with cushion grip.
Note, we generally plug the lateral portion of the vent, since patients feel less occluded than
when we plug the medial portion of the vent. However, if feedback is still an issue after plugging
the lateral portion of the vent, try plugging the medial portion of the vent at the tip of the canal.
Generally, feedback is less when the medial portion of the vent is plugged. If you need to remove
cushion grip from the vent, just heat it with a hair dryer. Cushion grip can be obtained at most
drug stores.
EDI Venting
Pressure Vent =
Small = .031”/ 1.0 mm
Medium = .062”/ 1.66 mm
Large = .125” /
SAV =
In closing, we want to add that we generally employ the smallest vent possible for every patient.
We do this because we almost always fit patients with directional microphones, and the
effectiveness of noise reduction/cancellation decreases as the vent size increases. We will
address this issue in a future article
By Gregory Frazer, Au.D., Ph.D. & Marni Novick, M.A.
We’ve been dispensing hearing aids in a private practice setting for 27 years combined. Over the
years, we’ve been asked how to determine the appropriate earmold vent size and canal length.
One way to get around having to select a specific vent size, is to order a Select-A-Vent (SAV) on
every patient. That way, you can determine the appropriate vent size with the earmold and/or
hearing aid in the patient’s ear. However, you still have to know what canal length to order, as
well as which SAV to use based on the patients acoustic needs. In addition, we don’t find SAVs
cosmetically appealing, they can fall out, sometimes they don’t fit, and they’re not cost-effective
(they add $2.50 to the cost of each mold, while a vent is free at EDI). Nevertheless, the question
is how do you determine the correct vent size and canal length? Believe it or not, this is a very
simple question that is complicated to answer. We will try to make our explanation as succinct
as possible.
Advantages and Disadvantages to Venting
There are at least 5 advantages to venting an earmold: 1) to reduce unnecessary low frequency
gain and output from the ear canal/eardrum, 2) to allow unamplified sound to enter the ear
canal, 3) to reduce the occlusion effect, 4) to relieve the feeling of pressure in the ear, and, 5) to
reduce moisture build-up in the ear canal. One disadvantage to venting is that the volume of the
vent combines with the ear canal cavity to produce a Helmholtz resonator (increased gain in the
250-1000 Hz region), sometimes resulting in an echo or barrel effect when the patient speaks.
Another disadvantage of venting is, of course, feedback. As vent size increases, the amount of
acoustic leakage increases, and therefore the probability of feedback increases.
Parallel vs. Diagonal Vents
Parallel vents are generally used, because not only do diagonal vents decrease low frequency
gain like parallel vents, diagonal vents also decrease mid frequency (630-1600 Hz) gain by up to
10 dB. Therefore, unless you want to decrease mid frequency gain, parallel venting should be
used.
The Effect of Parallel Vent Diameter/Canal Length
The rule of thumb is that as the parallel vent size increases (1 to 3mm), low frequency gain
decreases (up to 30dB) at 500 Hz and below, and mid frequency gain 630-1600 Hz increases
(up to 10 dB). As canal length increases (6 to 22 mm), the effect of venting is reduced. Thus, low
frequency gain decreases are less (15 dB versus 30 dB) with long canals, and mid frequency
gain will decrease as well (in fact, there may be no increase in mid frequency gain at all). So,
what does this mean to you? If you want to increase the mid frequencies, and decrease the low
frequencies, you should order a short canal (6mm) and a large vent (3mm). A long canal
(22mm) with a large vent (3mm) will not reduce the low frequencies as much as a short canal
(by as much as 10 dB), and the mid frequencies will be reduced more with a long canal than a
short canal. Pressure equalization vents (.06-.8mm) on the other hand, no matter what the canal
length is, generally only decrease gain up to 6 dB below 250 Hz. (See Table 6-5)
A Venting Selection Guide
Figure 1. is a rough guide for selecting the appropriate venting based on low frequency hearing
loss. In general, if the hearing loss below 1000 Hz is 25 dB or less, an open mold (large vent
(3.0 mm) or acoustic modifier) is recommended. If the hearing loss is 30-45 dB in the low
frequencies, an acoustic modifier or medium vent (2.0mm) is recommended. If the hearing loss
is 50-60 dB in the low frequencies, a small vent (1.0 mm) or pressure vent (.06-.88 mm) is
recommended. When the loss exceeds 60 dB, we usually do not put in a pressure vent, unless
the patient is occluded or their voice bothers them. When in doubt, put in a pressure vent, and
plug it if it causes problems with feedback. The “Rule of Thumb” is that it is easier to plug a vent
than it is to drill a vent. We use cushion grip, a denture adhesive, to plug vents. However, before
plugging the vent with cushion grip, which is hard to get out of a vent, plug the vent with Fun Tak
or clay to make sure the patient is not bothered by a plugged vent. If plugging the vent does not
bother the patient (occlusion, resonance in his voice, etc.), then plug the vent with cushion grip.
Note, we generally plug the lateral portion of the vent, since patients feel less occluded than
when we plug the medial portion of the vent. However, if feedback is still an issue after plugging
the lateral portion of the vent, try plugging the medial portion of the vent at the tip of the canal.
Generally, feedback is less when the medial portion of the vent is plugged. If you need to remove
cushion grip from the vent, just heat it with a hair dryer. Cushion grip can be obtained at most
drug stores.
EDI Venting
Pressure Vent =
Small = .031”/ 1.0 mm
Medium = .062”/ 1.66 mm
Large = .125” /
SAV =
In closing, we want to add that we generally employ the smallest vent possible for every patient.
We do this because we almost always fit patients with directional microphones, and the
effectiveness of noise reduction/cancellation decreases as the vent size increases. We will
address this issue in a future article
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