The Ear Mold
In this column, I'll cover a variety of topics related to hearing loss and hearing help. In an attempt to balance the technical and practical, I'll alternate between hearing aids and hearing science.
This month we will discuss an underestimated component of
the hearing Behind-The-Ear (BTE) hearing aids:
The earmold.
The Basics
An earmold is a custom-molded device that connects the tone hook of a
BTE to the ear. The basic components are labeled in figure 1. The tubing (A)
connects the earmold to the hearing aid. The Continuous Flow Adapter
(B) is an option we'll talk about a bit later. The sound bore (C) is the most
critical part of the earmold, since that's where the sound actually
travels. Depending on your hearing loss, your earmold may have a vent (D).
In theory, the earmold will act primarily as a conduit for the amplified
sound coming from the hearing aid. Several factors can affect the
extent to which the earmold interacts and changes the sound. These changes may be
intentional and beneficial to the fitting, but more often, they are
accidental and detrimental.
Let's start with the tubing. Earmold tubing is made of a type of nylon
called Tygon, and is defined by its inside and outside diameter. The
most common earmold tubing size is #13, which always had an inside diameter
of 0.076 inches. Three different thickness of #13 tubing are available.
#13 Medium, Thick and Super Heavy have the same inside diameter but their
outside diameters are 0.122", 0.130", and 0.142" respectively. The more
severe your hearing loss, the thicker your tubing should be. Tubing can
be connected to the earmold in several ways.
Often the choice of tubing connection is related to the material the mold is made of rather than
the acoustic behavior of that tubing system. The historical standard for
attaching tubing to an earmold is to apply an acetone-based cement to
both the tube and the inside of the sound bore, which partially melts both
surfaces. The tubing is then slid through the bore, usually all the way
to the tip, and the melted surfaces fuse together. This works well, and
initially, has very little effect on the hearing aid's sound. A few
caveats here. If the tubing is twisted when it is inserted, or if a the new
tube is thicker than the original one, a reduction of the hearing aid's high
frequency response will occur. Also, after a few months, the Tygon
tubing reacts with the cement and shrinks. This also reduces high frequencies.
I have seen as much as 15 dB reductions after just a few months.
Another way to attach tubing to very soft materials is using a brass ring called a
Tube Lock. This grabs the tube tightly and has a flange that digs into the
soft sides of the sound bore. This reduces the inside diameter and also
reduces high frequencies. A solution to this problem is the Continuous Flow
Adapter, or CFA. This system attaches a hard plastic elbow to a #13
tube. This elbow maintains the inside diameter of the tubing. This is then
snapped into a plastic seating ring inside the earmold. The inside
diameter cannot be reduced which maintains the high frequency response of the
hearing aid. If the hearing aid's response is not adequate for the loss, the
CFA can be combined with modifications to the shape and size of the sound
bore to add more output at different frequencies. Another way to add high
frequency output is a special tube called a Libby Horn that has a larger
diameter at the tip than at the hearing aid end. This works well, but
has the same shrinkage problem over time as standard #13 tubing.
If your hearing for low pitched sounds is better than for high pitches,
you may use a vent in your earmold to improve the quality of the sound
coming from the hearing aid. My rule of thumb is that if the hearing loss at
250, 500 and 1000 Hz averages to less than 40 dB, a vent should be used.
Vents allow low-pitched sounds to escape the hearing aid. The larger the
vent, the more low frequencies are reduced. The danger of venting is that if
you need a lot of power for high frequencies, a large vent may increase the
risk of feedback.
We'll look at some additional aspects of earmolds in a future column, so
keep your eyes peeled. Next time, we'll talk about the outer and middle
ear, as well as the physics of sound.
Brad Ingrao is the audiologist with Tobias & Battite, Inc. Hearing
Healthcare Centers of Boston
www.tobias-battite.com
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