In-ear monitors need to be aimed at the eardrum: true or false?

I went to the vet for my regular check-up… actually it was for my two new kittens, Broccoli and Feather, not for me.  I still get the cats mixed up; they are both 4 months old, long haired female tabbies from the same litter, and to my untrained eye, look identical.  While sitting in the vet’s office, and while trying not to watch as the vet gave the last of a series of rabies shots, I noticed a great poster on the wall of the peripheral hearing anatomy of the cat.  Like ours, it started from the outer ear meatus and went medially towards the middle ear (complete with the three ossicles), but unlike ours, the ear canal dipped downwards and then medially inwards.

I was thinking that it would be quite difficult to make an in-ear monitor for my cats (in case they wanted to listen to their favorite cat songs such as Soft Kitty).  The earmold bore would have to dip down and then continue straight, almost parallel to the floor, towards the eardrum.  If I didn’t get the impression long enough there would be no chance that the receiver(s) in the in-ear monitors would point towards the eardrum.

When making ear mold impressions for in-ear monitors we want to accomplish three things:  (1) as long of a bore as possible to minimize the occlusion effect, (2) have the most medial part aiming at the eardrum to ensure that higher frequency sound energy is optimally transduced, and (3) having the mouth open so that the front-back diameter for the in-ear monitor would be large enough to ensure a good seal, even while purring in order to get a good bass response.  I am not sure that I could even get a cat to keep their mouth open long enough for an ear mold impression, but assuming that I could, I would want to get a good seal where the ultimate orientation of the in-ear monitor receiver(s) would point towards the eardrum.

After the vet appointment, I dropped off the cats at home and consulted with the world’s leading cat audiologist Snickers McFeline.  (I know that its hard to believe, but that really is his name!)  Dr. McFeline told me that I was completely wet behind the ears and must be off my feed.  An optimal in-ear monitor for a cat, and even for a human does not need to be aiming at the eardrum.  This is the result of incorrect human (and cat) logic.  Here is where I went wrong…

We know from a loudspeaker orientation in a room that low frequency sounds emanate from the loudspeaker in any direction and to say the least, are non-directional.  This is why the bass woofer loudspeaker can be placed anywhere in a room; under a couch, facing the wall, or facing upwards.  This is also why we call a home stereo system with 6 loudspeakers (5 normal ones and one large sub-woofer) a 5.1 system rather than a 6.0 system.  There are 6 loudspeakers but the sub-woofer low frequency loudspeaker is non-directional and to say that sound comes from 6 directions (from 6 loudspeakers) would be false.  The honest audio engineering field (they must be cat lovers) correctly identified this system as a 5 direction, and one other sub-woofer, or 5.1 system and not a 6.0 system, even though everyone knows that 6.0 is better than 5.1.

For higher frequency sounds, we do know that they are progressively more directional as the frequency increases.  For higher frequency sounds (from a tweeter loudspeaker) sounds tend to emanate almost like a laser beam.  If one moves slightly to the right or left of the main axis, the high frequency sound has a lower sound level.  Indeed at least one manufacturer (www.Hypersound.com) has used this fact to allow one person among several in a room to be able to listen to a TV without anything more than their ears.  This is also why trumpet players in an orchestra should be placed on risers- the high frequency (which are also the higher sound level) components of their trumpet will go over the heads of those other musicians downwind and thereby reduce their potential music exposure.  (Brass players also like to be on risers so that everyone can see them!)

So, given that lower frequency sounds are not directional and higher frequency sounds are very directional, it follows that for in-ear monitors, the receiver(s) should point directly at the eardrum in order to optimize the high frequency sound transmission.  Reading through the literature accompanying in-ear monitors from the manufacturer, this is always stated clearly.  And I must take, at least in part, much of the blame for this.

Back in the late 1980s when www.FutureSonics.com was among the first (if not the very first) in-ear monitor manufacturers, the president of the company asked me for ear mold impression directions that he could provide to his clients.  Knowing the work of Bob Oliveira of www.HearingComponents.com, I knew that the front-back diameter of an ear canal can change by several mm when the mouth is open, I suggested that an earmold impression be taken with the mouth open, or at least ajar.  I had also suggested that the receiver(s) be aimed towards the eardrum in order to ensure that the higher frequency sounds would be optimally transduced.  And since then, these earmold impression instructions are given by virtually every in-ear monitor manufacturer.

Even though the first comment is true (the front-back diameter does increase when the mouth is open), the second comment about needing to aim the sound at the eardrum, is false.

While it is true that higher frequency sounds are quite directional from loudspeakers in a room, this same reasoning does not follow for an occluded small volume space such as the ear canal.    In fact if we scale the room dimensions down to that of an ear canal, what is low-frequency (non-directional) sound in a room becomes “low frequency sound” relative to an ear canal at about 30,000 Hz.  Just open any introductory textbook on acoustics and the reasons are all there in black and white print.  The acoustics of small occluded spaces are too complex to go in to here, but what is observed in a room cannot be extrapolated to an occluded small ear canal.

It is an error to assume (and was my error until recently) that the in-ear monitor (or indeed any in-ear earphone that may be used for listening to music or speech) needs to be aimed at the ear drum.  There are many over the ear earphones (that can be used as monitors as well) that provide a wonderfully flat and broad band response without having to aim at the eardrum.

About Marshall Chasin

Marshall Chasin, AuD, is a clinical and research audiologist who has a special interest in the prevention of hearing loss for musicians, as well as the treatment of those who have hearing loss. I have other special interests such as clarinet and karate, but those may come out in the blog over time.