In the mid-1960s, Stephen Ambrose invented what is recognized as the world's first wireless in-ear monitor technology (IEM).

A professional musician from the age of 12, he began modifying swimmer's earplugs with speakers and clay and completed his first IEM in 1965. The device marked one of the first times that full-spectrum high-fidelity sound was delivered within a fully sealed ear canal by an IEM.

Touring for decades with performers that included Stevie Wonder, Simon & Garfunkel, Diana Ross, Rush, Steve Miller, Kiss, and others, Ambrose perfected and commercialized his IEM designs and was the sole provider of in-ear monitors to the professional market for more than a decade.

He became concerned, however, over the reported 77% increased risk of hearing loss due to the use of personal listening devices. He began research with grants from the National Science Foundation and National Institutes of Health, and discovered the mechanism by which people were destroying their hearing.

To solve the problem, he invented and patented a "second eardrum" called the Ambrose Diaphonic Ear Lens (ADEL). The device absorbs harmful in-ear pressures, thereby preserving the health of the human ear, and allowing people to more safely hear sound.

To bring his invention to market, he founded Colorado-based Asius Technologies. The firm was chosen as a Wall Street Journal Startup of the Year Finalist, an Auto Desk Inventor of the Year, and Winner of the Denver Challenge Cup in healthcare innovation.

Engineering360 contributing editor Mike Farish talks to Stephen Ambrose about how his work to develop ear monitors for use by stage performers can now help people with impaired hearing.

MF: You grew up in a musical environment.

Stephen Ambrose, Asius Technologies SA: I grew up in Nashville where my father was a professor who taught church music and classical music at some of the colleges there and would not let any other type of music be played at home. But the Quadraphonic Studios were just across the road and I met Joan Baez there. She used to call me a “university brat.”

MF: So how did you get into other types of music?

SA: My mother bought me a guitar and although I was just in my teens, I was heard playing in a coffee house by a producer and invited to a studio to record some music.

MF: That is when you also first encountered headphones.

SA: Absolutely. I sneaked into a studio where Johnny Cash was recording and was allowed to listen to him through some headphones: it was amazing.

MF: That also gave you an idea of how you could play “forbidden” music at home.

SA: Because my dad didn't let me play the guitar or sing country at home, I had to find a way to be able to play very softly and yet hear myself, so I fashioned some earphones from some low-fidelity transistor radio earpieces that I stuck through holes. They were the first in-ear monitors.

That was the mid-‘60s and by the mid-‘70s I was an established performer - guitar and vocals – and had performed with some of my heroes such as Brian Wilson from the Beach Boys.

MF: You had also started making earphones as well.

Ambrose with Stevie Wonder, whose acute sense of hearing has helped recent technology developments. Image source: @StephenDAmbrose SA: I started making in-ear monitors in a small workshop in Hollywood. Apart from using them myself, they also began to be used by other performers. I showed them off at a music industry show in 1976. They were wireless devices and their purpose was to allow performers to hear themselves, and, if necessary, for their off-stage crew to communicate with them while they were performing.

MF: Some big names got interested in them.

SA: Absolutely. It was the early ‘80s and I had started working with Simon & Garfunkel, and Art Garfunkel had tried out my products. But there were some attempts to undermine what I was doing by spreading rumors that if there was any feedback while people were wearing them, there could be really damaging effects: nosebleeds and worse. Art was concerned and made it clear pretty forcefully when we had a recording session in New York that he would not be happy if that occurred.

MF: What did you do to stop that from happening?

SA: I took a dozen really top-of-the-line compressors – DBX160s – and put one on every single input to the board and one on the output to the transmitter. Art tried the system but he sang just a few bars before he pulled the earphones out and shouted at me, “This is horrible!”

MF: But someone else came to the rescue.

SA: Yes, a legendary figure from the music business, the engineer and producer Roy Halee – he engineered Bridge Over Troubled Water – was there. He came over and started pulling out all these patch boards. I thought they were going to throw me out. But, instead, in just about 15 seconds he dialed up a mix and told Art to the put the phones back on.

MF: How did Art react then?

Roy Halee with Paul Simon (l) and Art Garfunkel (r) at Columbia Studios in front of a console used for 8-track recording. Image source: HistoryofRecording.com SA: He said: “That sounds incredible.”

MF: And Roy Halee?

SA: Roy pointed at me and said, “He did it.” If Roy hadn't done that, in-ear monitors would never have made it out of the gate.

MF: In fact, Roy Halee had a continuing influence.

SA: We went on to tour the world for the next two-and-a-half years and Roy taught me how to engineer things. I don't have any formal technical training.

MF: But another great performer has also been an influence.

SA: Stevie Wonder's support team had realized that the ability to communicate with him on-stage would be more than useful because he is blind and could be in danger of falling when he got up and danced.

The ADEL G1 Auto Modules. Image source: Asius TechnologiesHe has given me lots of feedback. I can credit him with helping me with my more recent developments. That’s because his sense of hearing is so acute he could ask me for things and describe things in a way that you would simply never get from a book on acoustics. He sees the world acoustically.

MF: But then you began to get concerns that in-ear speakers could damage people's hearing.

SA: I had started to get some misgivings as early as the late ‘70s and developed a technique to mitigate the situation that was finally patented in 1989. It took the pressure from the back of the speaker and routed it through a port to the front of the speaker. But the better it works, the more it cancels the sound. That’s because you are taking 180-degree out-of-phase sound from the back and mixing it with sound at the front. It worked, but you had to tune it very carefully for individual users.

MF: What is the root cause of the problem?

SA: The problem is that if you seal the ear canal with an in-ear speaker, then it becomes effectively a sealed tube. You start to generate pneumatic pressures on the eardrum that are much more powerful than purely acoustic pressures. I realized that because, in the ‘90s, I started to find that after, say, three hours of wearing my own earbuds while mixing the sound for a concert my ears were ringing.

MF: What did you do?

SA: Initially, I decided to take a rest from touring and making the products until I could figure out how to solve the problem.

MF: Your understanding of the situation became much deeper.

SA: Yes. With a freefield speaker, the diaphragm moves backwards and forwards. That means that at its extremes, when it changes direction it is stationary. So it is generating the maximum pressure when it is in the middle of one of its excursions because when it is moving the fastest, it is velocity-based.

But when you seal it inside a closed environment, then the pressure is greatest at its point of maximum extension; it is displacement-based. So, if you go from an unsealed to a sealed ear, you raise the bass by 60 decibels (dB) but you also drop the mid-range by 10-20dB. That is something no one had noticed before.

MF: Other research helped confirm your findings.

SA: There was a study of in-ear hearing aids carried out by an audiologist in Australia. It showed that when you put one in you get what is called an “insertion gain” – the sound volume rises – but the loudness perception is reduced in the mid-range. That is because you are holding the eardrum still in the mid-range because it cannot vibrate without compressing that trapped air in the ear canal and it has not got enough energy to do that.

(Click to enlarge.) Diagram of the ADEL module. Image source: Asius Technologies You are making the ear less sensitive in the mid-range while overwhelming it in the bass. In addition, the extremes of the bass excursion tighten the eardrum to make it even less sensitive in the mid-range and the highs.

MF: You came up with a solution?

SA: About 18 months ago, we introduced the first of a new product range called the ADEL, which stands for Ambrose Diaphonic Ear Lens. The key feature is that it contains an ultra-thin membrane behind the speaker which acts in effect as a “second eardrum” to absorb pneumatic pressure and so reduce the pressures on the user's actual eardrums. Technically speaking, it is a “pneumatically compliant membrane surface.”

We are now having them manufactured by a company in Georgia with R&D in Colorado, where I am based. There are two versions. One has radio communications so it can be used by on-stage performers and one is a conventional wired version to plug into a personal stereo. Beyonce used them when she performed at the Super Bowl.

MF: Finding the right material was obviously crucial.

SA: Initially, I could not find a material that was suitable. Materials that were thin enough were gummy and sticky so they would have become covered in contaminants. That would not have been good enough.

MF: Then you had a “Eureka” moment.

SA: I found exactly what I needed from W.L. Gore. It is an ePTFE fluoropolymer material that is used in surgery to repair blood vessels. It is not a new material; it was originally developed by DuPont back in the ‘40s. It is immensely strong but so lightweight that a sheet of will hang in the air.

In the ADEL it is about a micron in thickness. Moreover, it actually works in two ways. It bends under very low pressures, but it also has a surface property known as physiosorbtion. That effectively means that it has a boundary lower that also acts in a compliant way even if the membrane itself is stationary.

MF: You are working on something even more exciting?

Brian Johnson of AC/DC had to stop performing in early 2016 due to hearing loss. A recent development from Asius could help him return to the stage. Image source: Wikimedia SA: We are working to develop a hybrid version of the ADEL that will contain not just the “eardrum” at the rear but also, at the front, a bubble made from the same material that is inflated by pneumatic pressures generated by the speaker. The air to do this is pulled in from the external atmosphere through a tiny orifice by means of a modified synthetic jet. It is a sort of acoustic rectifier.

The device then seals itself to the inside of the ear canal, and when it pulsates it conveys conducted sound very efficiently in a spectrum that can be as low as the bass can go and up to above 10,000 Hertz (Hz).

But a key point is that it does so in a manner that mimics the way people hear conducted sound naturally through soft tissues rather than just bone. That is something that conventional in-ear devices cannot do: they are just point sources of sound. That also accounts for the frequency range. By itself, bone conduction cannot get higher than about 4,000Hz. The device also has a small hole at its innermost point to allow sound from the speaker to reach the eardrum directly.

MF: It can do more than just let people with normal hearing listen to music.

SA: There are times when it can allow people with hearing impairments to hear again at very high fidelity. For instance, the mix of sound inputs would allow someone who was deaf in one ear to hear again in stereo.

MF: You are also working with a well-known performer to develop the concept.

SA: I am working with Brian Johnson of AC/DC, who had to stop performing earlier in 2016, to see if we can get him back on the stage. He has tried them out and was quoted in Rolling Stone as saying: “It works, it just totally works I was totally amazed to hear music again like I haven't for years I can't wait for it to be miniaturized so I can use it on stage.”

MF: Have modern design technologies helped?

SA: Absolutely. I use Autodesk design software and a Formlabs printer. Compared with my early days, the ability they give me to get my ideas embodied in physical prototypes very quickly in my office makes a huge difference.