Libby Horn

Damping elements and acoustic horn systems improve frequency response

E.R. Libby
Associated Hearing Instruments
Upper Darby
PA 19082




Ear-mould modification by special bores and dampers particularly in relation to the    mid and high frequency range

It should be noted that these modifications are relatively independent of one another and can be reformed separately or together. Their actions, however, are limited to their own particular frequency region. For example, the effect of a horn can be achieved without venting or damping can be achieved without horn action.  Figure-1
  Fig. 1:  It should be noted that these  modifications independent of one another and can be performed separately or together. Their main actions are limited to their own particular frequency region. Reproduced with permission of the publisher.

Damper effects; smoothing the hearing-aid response
There has been continuing progress towards hearing-aid components with smoother and wider frequency response characteristics. The introduction of the electret microphone in the early 1970’s and the newly introduced wideband receivers have both wideband and flat characteristics but the resonances in the ear-mould tubing, which conduct sound to the ear, remain the main source of irregularity in the frequency response of a hearing aid. These peaks and troughs must be eliminated if an overall smooth frequency response is to be achieved. Damping techniques have been used to reduce the resonance in the sound transmission channel.   A new fused mesh damping element has been developed by Knowles Electronics known as the BF series dampers. Although these dampers were designed to fit tightly into 2 mm earmold tubing, they can be used at several other places along the transmission line such as at the earhook or inside the hearing aid itself. The effect of damping on the gain and output is quite dramatic. Nearly all behind-the-ear hearing aids have a maximum output and maximum gain of 1000Hz in the middle of the region of maximum sensitivity to the ear. As a result, the peak at 1000Hz may exceed the threshold, of discomfort, while the rest of the curve may be comfortably below the discomfort zone. The use of a damping element can change the frequency characteristic into a smoothly rising response which shifts the peak form 1000 Hz to approximately 2700 Hz. The results in a response curve which is similar to the natural transfer characteristic of the ear. Many patients will acknowledge this desirable frequency to be very natural, pleasing in quality and clear. The damping element affects the peaks and not the troughs on the responses unlike control trimmers which  effect the whole frequency response.    The most  practical location for a damper is at the tip of the earhook. Earhooks are available with various coded dampers already inserted.
The values and colors of the dampers:


ohm red

Fig.2: Frequency response of wideband aid with ear-mould damping.


Figure 2 shows the effects of these dampers on the frequency response of a wideband aid. It is worth noting that the author damps 90% of his cases. The only exceptions to this are high gain aids which are used in cases of severe and profound hearing loss. The author places the damper into the tip of the earmold (not into the earmold).

Fig.3: A Conventional ear-mould with a constant diameter bore can reduce the effective band width. A stepped diameter ear-mould, on the other hand can preserve and extend the high frequency emphasis to the limit of the transducer’s capabilities.  A typical ear canal response is shown on the lower portion of the Figure for comparison.


The effect of a horn: extension of the high frequency response
A conventional coupling system with a constant bore diameter earmold significantly limits the high frequency response of a wideband hearing aid. A hearing aid with a potential band width of 6500Hz could be limited to 4000 Hz, 3000 Hz or less when constant bore ear-mould tubing is used (Fig 3).   Killion  (1976) developed an innovative earmold coupling system which he called “earmold plumbing.” The system consisted of vents, predictable damping elements and an acoustic horn. Libby (1979) modified the Killion acoustic horn by developing a one piece, internally tapered, stepped bore horn known as the “Libby Horns.” (Fig 4).

Fig. 4: The Libby Horn. One piece tapered internal  stepped bore horn with Killion 8CR configuration.  


The Libby Horns preserves and extends the high frequency response to the limits of the transducer. The horn can be used in almost any form or earmold construction: occluded or unoccluded earmolds, high frequency earmolds or simply as a tube in the ear. It must be emphasized that the transmission line is the important element and not the type of earmold that supports the tubing. The horn can be cut if necessary in order to provide greater comfort.

     The Libby Horn can provide a smooth wideband response with the appropriate damping elements in the tip of the earhook (Fig 5). It can be used to obtain a maximum response at 2.7 Hz in order to compensate for the loss of canal resonance. It an occluded earmold is used, the last 11 mm of the horn can be cut away and the last part of the earmold can serve as the last portion of the horn.

Commercially Available Horns
Libby Horns are available in 3 and 4 mm sizes. The 3 mm horn will give approximately an 8-10 dB high frequency emphasis while the 4mm horn gives an emphasis of 10-12-dB.  Horn tubing tends to collect more moisture than regular constant bore tubing. A new earmold tubing has been developed which will eliminate the moisture problem to a great extent. The new material (Stay-Dry) is available in all horn sizes and also in 2 mm tubing.  

Final remarks
In general the stepped bore, damped horn systems will have 3 effects:
- greater high frequency band width to the limit of the transducer:
- elimination of the peak at 1000 Hz for a behind-the-ear hearing aid; and
- preservation of the open canal resonance peak at 2.7 kHz.
Significant improvements in word recognition, functional gain and sound quality judgments have been reported with the new earmold systems.

1.)  Carlson, E.V. (1974) Smoothing the Hearing Aid Frequency Response. J. Audio Eng  Soc., 22,426

2.)  Killion, M.C. (1979) Evaluation of High Fidelity Hearing Aids. University of Microfilms International, Ann Arbor, MI. USA

3.)  Knowles, H.S. and Killion, M.C. (1978) Frequency Characteristics of Recent Broadband Receivers, J. Audiol Tech, 17, 86,136

4.)  Libby, E.R. (1979) The Importance of Smoothness of Hearing Aid Frequency Response. Hear. Instrum. 4,20

5.)  Libby, E. R. (1980) Smooth Wideband Hearing Aid Responses-the New Frontier. Hear. Instrum.4,12

6.)  Libby, EA.R. Johnson J.H. and Longwell, T.F. (1981) Innovative Earmold Coupling Systems, Zenetron, Chicago, IL. USA


Fig. 5:  Occluded ear-mould with Libby Horn and damped earhook.