Mid States Laboratories Inc.


The three types of vents are:

Parallel: The sound channel and the vent are parallel. This is
the most popular form of venting because feedback is minimized.

Diagonal: This version intercepts the sound bore at an angle and can result in a greater tendency to feedback.

External (or Surface): They’re grooved on the outside of the earmold.


Vents can be bored to specific sizes within the limitations of the space available. Conventional sizes recommended by Mid-States are:

  • Pressure .031
  • Small .040
  • Medium .062
  • Large .093
  • IROS .125

A word of caution, not all vent sizes are created equal! When working with a new earmold lab, compare sizes or visit with us about your order.


These insert vents provide the hearing instrument dispenser with the flexibility of changing sizes by merely changing the plug-in vent, no tools necessary. With the friction fit S.A.V., the insert may be cemented in place once the final choice has been made.

When ordering an S.A.V., indicate regular or small style and the size of vent plug desired. The entire set of plugs will be included with each order at no extra charge.

Regular S.A.V. Vent Size:

  • .156
  • .125
  • .093
  • .062
  • .031
  • PLUG

Small S.A.V. Vent Size

  • .093
  • .062
  • .031
  • PLUG

Effects and Uses:

There is a predictable acoustic change occurring when an earmold is modified with a vent. The size of the ear can alter the magnitude of the change. Individual hearing impairments will also react differently to various acoustic modifications. When the hearing aid dispenser combines the appropriate hearing aid with the correct earmold modification, a client will receive a satisfactory fitting.

The following are general comments on the effects of venting.

  • .031 vent is primarily a pressure vent and it will have very little effect on the frequency response above 400Hz.
  • .062 to .093 vents will tend to increase the acoustic energy in the frequency range between 500 Hz and 1000 Hz. The acoustic energy increases as the vent diameter increases.
  • .125 to .150 vents will also increase the acoustic energy in the frequency range between 500 Hz and 1000 Hz. However, at some point, the vent may become large enough, in a small ear, to shunt the energy below 1000 Hz. Large vents may result in feedback more frequently with ear level hearing aids with an SSPL90 in excess of 115db.
  • For high frequency emphasis a short canal and a large vent is a common choice. Also consider using a bell bore.

The following are general comments regarding the use of venting.

  • A pressure vent is recommended for all molds where possible. This improves the comfort factor greatly.
  • If feedback does occur with a vent, the vent can be easily plugged. However, to drill a vent where none exists is quite difficult.
  • In general there is less chance of feedback with a parallel vent than with a diagonal vent.

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