In last week’s part of this two-part blog series, the one benefit of having a long mold made for in-ear monitor impressions was discussed. It had everything to do with minimizing the occlusion effect and absolutely nothing to do with ensuring that higher frequency sounds are properly transduced to the eardrum.
In this week’s part, we discuss Dr. Robert Oliveira’s finding that the ear impression should be made with the mouth open… well, not the audiologist! The client!
Robert Oliveira, president of Hearing Components, Inc. (www.HearingComponents.com) did some great work in the early 1990s that was published in Ear and Hearing and later Hearing Review. Dr. Oliveira essentially showed using radiographic scans that the human ear canal was dynamic and the front-back dimension could increase by 2-3 mm while other dimensions (e.g., the top-bottom dimension) would not change at all.
In retrospect one didn’t need to perform an MRI or CT scan to know this. Simply place your finger in your ear canal and open and close your mouth. You can easily see that the front-back dimension changes substantially.
This has to do with the anatomy of the back part of the jaw (the condyle) being adjacent to the front part of the ear canal. As the mouth is opened the jaw slides forward allowing the ear canal to have a larger front-back (or as the literature says, “anterior-posterior”) dimension.
A frequent complaint among wearers of in-ear monitors (and hearing aids for more significant hearing losses) is that there is non-sufficient bass response. Music may sound “thin”, or even “tinny”. The appropriate bass response provides music with a sensation of a “full” response. Some of this (and perhaps most of this) derives from an in-ear monitor that allows acoustic leakage along the front portion of the earcanal where the in-ear monitor does not sufficiently change shape with the anterior part of the moving earcanal.
Having the client open their mouth during the making of an earmold impression will allow the cast to take in to account the larger front-back dimension on the ultimate in-ear monitor. This would help create a better seal and a better low frequency bass response.
But after almost 25 years of doing this I am still not convinced that the clients’ mouth needs to be propped open by a piece of polystyrene that his held between the teeth. This device to prop open the mouth is available from some manufacturers (for example www.Starkey.com) for those who would like to try it.
After perhaps over a thousand earmold impressions taken over the years, and I have tried both methods (simply having one’s mouth slightly open or propping it open with a mouth block) I am not convinced that the propping of the mouth is any different. I know that many tour managers believe that this is the case and for them I do use it for their musicians (and security staff); being a parent, I know when to pick my battles, but I am not convinced that it makes any real difference.
This does have ramifications for other areas of our field. For example, when I instruct an industrial worker on how to insert their foam one-size-fits-all earplugs, I do tell them to open their mouths during the insertion. This allows them to get the earplugs deeper in the ear canal with (hopefully) better real world attenuation while on the job.
This can also be useful when inserting any earphone that goes into the ear canal – these may have a foam tip (and it was Dr. Robert Olivera who patented this idea in 1986) or perhaps a several pronged plastic tip that sits in the ear canal. In either case, having your mouth open during insertion can allow a deeper insertion and also one that may remain stable in the earcanal over many hours.
Perhaps the most important aspect of opening one’s mouth in my normal clinical practice, is when I perform admittance (impedance for those of us over the age of 55) testing. With some clients, placing a probe into their ear canal just won’t work without a leak. Having them open their mouth while inserting the probe usually does the trick – the front part of the ear canal moves forward thereby making the ear canal more circular in cross section, just like the circular probe.
And if you can’t get a good view of the tympanic membrane with your otoscope, have the client open their mouth; the ear canal becomes wider and may allow you to see around bends that were previously obstructing your view.