Inventor Stephen Ambrose Claims Development of Safer Ear Tip
Ambrose claims to have invented the IEM in the mid-70s as a musician and a sound engineer. Working with everybody from Stevie Wonder to Mel Torme, he developed the first on-stage wireless IEM for touring groups, and displayed the first commercially available device of this kind at the 1978 NAMM convention under the name “SoundSight MicroMonitors.”
While his invention solved the problem of being able to hear what’s going on on-stage, it unfortunately did not solve the problem of listening fatigue for performers. Artists continued to complain after performances, and Stephen continued to work on the problem. Now, some 30 years later, he thinks he has the solution.
A New Look at the Problem
Ambrose believes when an IEM is inserted and sealed in an ear canal, the acoustics of trapped volume of air act very differently than the acoustics of the open ear canal. The trapped volume of air essentially becomes a fluid coupler allowing the driver to over-pressurize the eardrum, causing excursions of the tympanic membrane 100-1000 times greater than normal. He has made a movie of the eardrum under these conditions apparently showing this phenomenon. His term for the slug of air moving back and forth between the driver and the eardrum is an “oscillating static pressure.”
He has coined the term “trapped volume insertion gain” (TVIG), and claims it is responsible for triggering the Stapedius reflex (tightening of the muscles of the middle ear to reduce the amount of sound reaching the inner ear), which lowers the perceived volume, but may be the cause of listening fatigue.
To decouple the driver diaphragm from the eardrum, Ambrose invented a small, flexible diaphragm in the inner wall of the eartip that permits relief of excess pressure in the ear canal. He calls this invention the "Ambrose Diaphonic Ear Lens" (ADEL). In one video, he claims that an open vent would cause the loss of bass response, but that an area of compliant material acts like the airbag a stunt man falls into, having cushion without rebound, and keeps the ear canal sealed while still providing pressure relief. Another way of looking at it is that it creates an acoustic impedance matcher such that the ear canal is operating more like it would under normal conditions --- without the static oscillating trapped volume.
Ambrose offers reams of data in his AES paper; I would like to focus on two of his findings.
Trapped Volume Insertion Gain --- Here’s the abstract from the paper: “ Sound Reproduction within a Closed Ear Canal: Acoustical and Physiological Effects”
- “When a sound producing device such as insert earphones or a hearing aid is sealed in the ear canal, the fact that only a tiny segment of the sound wave can exist in this small volume at any given instant, produces an oscillation of the static pressure in the ear canal. This effect can greatly boosts the SPL in the ear canal, especially at low frequencies, a phenomena which we call Trapped Volume Insertion Gain (TVIG). In this study the TVIG has been found by numerical modeling as well as direct measurements using a Zwislocki coupler and the ear of a human subject, to be as much as 50dB greater than sound pressures typically generated while listening to sounds in an open environment. Even at moderate listening volumes, the TVIG can increase the low frequency SPL in the ear canal to levels where they produce excursions of the tympanic membrane that are 100 to 1000 times greater than in normal open-ear hearing. Additionally, the high SPL at low frequencies in the trapped volume of the ear canal, can easily exceed the threshold necessary to trigger the Stapedius reflex, a stiffing response of the middle ear, which reduces its sensitivity, and may lead to audio fatigue. The addition of a compliant membrane covered vent in the sound tube of an insert ear tip was found to reduce the TVIG by up to 20 dB, such that the Stapedius reflex would likely not be triggered.”
Here he demonstrates the difference in volume between an IEM positioned where it normally would be both with and without an eartip.
Fig 1. Upper pictures show IEM positioned normally in a Zwislocki coupler both with and without and eartip. Lower graph shows the frequency response of a sealed and unsealed IEM, and the noise floor of the measurement system.
I have some problems with this evidence. I can only take from the bolded statement in the abstract above that he believes the uncoupled IEM is equivalent to “to sounds in an open environment.” This, it seems to me, is patently untrue. The volume of air able to be moved by the driver in an unsealed IEM as it radiates essentially into free space creates an extremely poor bass response. Putting an unsealed IEM next to the ear is not like “sounds in an open environment.” It seems to me his claims of 50dB greater sound pressure level with TVIG comes simply from putting the IEM into its designed acoustic environment.
Reduced Low Frequency Content from TVIG with ADEL --- Ambrose has demonstrated that his device does indeed reduce the low frequency response of IEMs in a controlled manner.
Fig 2. shows the frequency response of Skullcandy Titans with and without the device.
I am very familiar with the bass hump in many IEMs, and to think it may come with an over-pressurization of the eardrum is a rather frightening thought. I would say the fully 2/3 of the IEMs I measure show evidence of an over-productive bass response. I also note that Etymotic IEMs (a company who’s technical prowess I respect greatly) do not exhibit this response.
Fig 3. shows the frequency response of a number of common headphones exhibiting the humped bass response that might be evidence of TVIG, and an Etymotic ER4PT which doesn’t show the hump.
Note that the frequency response curve of the IEMs in Fig 2. with his device in place is very good. Headphones known to have great bass response (Audez’e LCD-2, HiFiMAN HE-500) are somewhat similarly flat, and not artificially humped up in the bass like those IEMs in the graph. In one of his videos, Ambrose suggests that way we hear IEMs psychologically is dramatically different than free-field listening: because the driver diaphragm is fluid coupled (the air in the ear canal can be considered a fluid directly coupling the driver motion to the eardrum motion) the apparent “speaker” in the audio chain is now the eardrum itself. This places the perceived sound source at the eardrum, which is roughly behind the eyeballs, and perceptibly narrows the resulting audio image.
I have clearly experienced the perception of a narrowed audio image with IEMs. If you watch his video (link in resources at end of article), Ambrose discusses some of the subjective acoustic changes that occur with IEMs. I find myself surprisingly intrigued by his ideas.
Well … I’m nowhere near coming to any conclusions, yet. I do see a problem with Ambrose's quantification of TVIG using an unsealed IEM as a reference for “sounds in an open environment,” but I also see his argument that the acoustics of this trapped volume and the oscillating static pressure could very well be troublesome. For example, under this condition the driver can exert all its horsepower almost directly on the eardrum, which, it seems to me, could easily be more powerful than normal sound pressure waves.
The idea of an airbag (essentially) to decouple the driver from the eardrum is a tantalizing thought; I can imagine the driver radiating sound into the ear canal without feeling the back-pressure from the eardrum, and how this might sound more natural. But I also know that headphones, as a class and not just IEMs, are more acoustic coupler than a speaker radiating sound into the free field, so my imagination may just be making things up.
I did talk to Stephen on the phone at length, and will do so again to talk about some of the queries I have. I will report back; this is a story that needs following, methinks.
One thing I do know with certainty, I really want to hear and measure a pair of his eartips. I’ll let you know what happens. In the meantime, here are some links to follow so you can begin to make some judgments for yourself.
Stephen Ambrose's Asius Technologies ADEL information page.
Official press release on Asius ADEL technology. A lengthy but very interesting video on the ADEL produced by the National Science Foundation, which has awarded grants to Ambrose for his work. (Patience please, large file.) AES Convention Paper "Sound Reproduction within a Closed Ear Canal: Acoustical and Psychological Effects"
1982 BAM article crediting Ambrose with the invention of IEM.
Patent for Diaphonic Acoustic Transduction Coupler and Ear Bud.