Beck: Thanks again for joining me Mead. Let's start with the topic of Musicians Ear Plugs. I know Etymotic has pioneered many of these products, and the three primary, custom-made products are the ER-9, ER-15, and ER-25, is that correct?
Killion: That's correct.
Beck: Can you discuss the differences in these products and their origin?
Killion: Certainly. The ER-15, the first truly high-fidelity earplug, was designed by Elmer Carlson while he was chief engineer at Knowles Electronics. The ER-9 and ER-25 produce 9 dB and 25 dB of attenuation, respectively. But may I start by telling you how Carlson succeeded in making something that nearly everyone assumed was impossible?
Killion: Carlson designed a small button containing a thin plastic diaphragm and an acoustic resistance. The compliance (springiness) of the diaphragm was selected to produce the desired 15 dB of attenuation at low frequencies (that was the easy part of the design and the hard part of producing the ER-15). The problem is more complicated at high frequencies however, because the normal open ear produces a boost of about 15 dB at 2.7 kHz (four white keys down from the highest note on the piano). Thus, strangely enough, 15 dB of protection at 2.7 kHz requires 0 dB of attenuation through the earplug at that frequency!
Beck: If the sound field SPL is 100 dB, and the eardrum receives the same 100 dB SPL through the earplug at 2.7 kHz, that is 15 dB less than the 115 dB that it would normally receive with the ear open?
In order to produce 0 dB of attenuation at 2.7 kHz, Carlson specified the dimensions of the sound channel in the earmold so that the acoustic mass of the air in that channel would resonate with the diaphragm compliance and form a peak at 2.7 kHz.
Beck: So earplugs without this resonance must muffle the highs by at least 15 dB?
Killion: Exactly. Also, most earplugs have more attenuation at high frequencies than low frequencies, so the net effect is a treble deficiency of 20 or 30 dB.
Beck: What does the acoustic resistance do?
Killion: By itself, the resonance peak Carlson introduced would be sharp and unpleasant sounding. He added the resistance to smooth the peak the necessary amount.
Beck: How well did Carlson achieve his goal?
Killion: Carlson's early KEMAR measurements showed a flat response from below 100 to above 10 kHz, except for a problem (too little attenuation) at 6 kHz. Rather than accepting almost perfect, he modified the design to include another resonance chamber in the button itself, to increase the attenuation in the region around 6 kHz. When we put the ER-15 into production, we asked Elliott Berger of E-A-R corporation (now AEARO) to perform REAT (Real Ear Attenuation at Threshold) testing. His tests on 12 real ears showed an almost perfectly flat response from 80 Hz to 16 kHz. It was a great design. When Elmer Carlson died this year, I lost one of my engineering heroes.
Beck: What was it that finally brought hearing protection for musicians to the forefront?
Killion: Arguably it was Berlioz. There had been discussions for some time about the possible risks to the viola and bassoon players sitting in front of the brass section, but it was a concert that included Berlioz' Damnation of Faust in 1989 that brought things to a head. In that performance, 283 musicians were crowded onto the old Orchestra Hall stage (it is larger now): 104 members of the full orchestra and 175 members of the Chicago Symphony Chorus (CSO). In addition, there were 4 soloists in front of the orchestra, further crowding the stage. The CSO string players rotate through the seats behind the principals. During those performances, Bob Swan, one of CSO violists, ended up at the back, right in front of the world's most powerful brass section with -- because of the crowding --his head virtually in the bell of a trombone. At the end of the concert he was deaf: He could not hear conversations or normal music. He saw an otolaryngologist, who put him on a course of steroids, and his hearing returned completely ( like a teenager's ) in a couple of weeks. Nonetheless, it was clear that others might not be so lucky.
Beck: What happened then?
Killion: A Sound Level Committee of the Chicago Symphony was formed. Subsequently I was invited to be a consultant to a nation-wide meeting of ICSOM (International Conference of Symphony and Opera Musicians) representatives and the Orchestra Managers of several prominent orchestras. Henry Fogel, manager of the Chicago Symphony Orchestra at the time, did an analysis of the types of music the CSO played over the decades, compared to what they typically play today. While Mozart and other relatively quiet early symphonies are still often played today, the mixture has changed in favor of the more recent symphonies by Mahler and Bruckner and others that may have 115 musicians on stage instead of 25. Moreover, musicians today are often asked to play at markings of triple forte (fortississimo, very very loud) and even quadruple forte (impossibly loud). When there are over a hundred highly skilled musicians making as much sound as they can on stage, it can become a hearing-loss risk issue.
Beck: I recall seeing clear plastic barriers placed here and there on the stage, and I never really understood how they were supposed to block enough sound to protect the hearing of the fellow in front of the barrier, while not ruining or changing the sound of the ensemble, for the conductor and the folks sitting in the front.
Killion: In the early days, meaning the late 1980s or so, the only available solution was the Plexiglas screen. The actual impact on the sound in the audience was relatively small, although it did affect the sound for the players. We had a chance to make dosimeter measurements during actual performance on the CSO stage. One of the interesting findings was that the Plexiglas plates reduced the level at the nearby violas by about 3 dB but increased the level at the bassoon player's ear (on the other side of the barrier) by about 3 dB. So it was a mixed bag. I think one of the results from all the work with hearing protection is they don't use Plexiglass barriers anymore.
Beck: What did the group decide?
Killion: They decided that a hearing study in a major symphony was the first step. Chicago was handy, and Julia and Larry Royster agreed to help make measurements. The high point of my career as a conductor was standing at the podium on stage at Orchestra Hall for three minutes with the entire symphony before me. Unfortunately, I only had permission to talk: To explain a) the need for the study, b) the fact that many superb musicians had a loss of sensitivity for quiet sounds but it did not affect their playing ability because even pianissimo playing with the ear next to the instrument was loud compared to what was heard in the hall, and c) the need to place dosimeters on some of them during rehearsal and performances. We obtained thresholds on 65 (almost exactly half) of the regular CSO musicians, which was a pleasant surprise. (At one time, no orchestra musician would submit to testing for fear of losing their job if the results showed any loss.)
Beck: What did the musician's audiograms reveal?
Killion: The interesting thing from the Chicago Symphony study with reference to hearing impairment, was that all of the symphony musicians, on average, had slightly better hearing than their age adjusted, non-musician, non-noise-exposed counterparts. Even at 4 kHz their average hearing was somewhat better than the general population, but the musicians tended to have a slight but visible noise induced notch at 4 kHz. Perhaps I should say music induced. The violin players had a notch in their left ear with normal hearing in their right ear. The flute players typically had a notch in their right ear and presented as normal in the left ear. So it became quite clear that there was a problem.
Beck: Did you find any exceptions?
Killion: Yes. The most interesting was the concertmaster, Ruben Gonzalez. He had been a virtuoso soloist on a powerful Bergonzi violin for 30 years. When we obtained 11.5 hours of dosimeter readings on him during rehearsal, practice at home, and playing during a concert, we found a daily weighted average of 110 dB! Not surprisingly, perhaps, he had a violin-induced threshold notch in his left ear of 55 dB at 4 kHz; his right ear showed normal hearing. During our testing period he was practicing intensely for an upcoming performance of the Brahms Violin Concerto. Assuming his lifetime exposure was closer to 105 dB, we found that his hearing loss almost exactly matched that predicted by the published Damage Risk Criteria.
Beck: Don't I recall seeing you quoted in print saying that God must protect musicians, otherwise they would all be deaf?
Killion: Yes, I thought I was quoting something Dix Ward said, but he later said it wasn't he. In any case, that rash statement was made before we had the CSO data. The conclusion from that study was that -- just as in Ruben Gonzalez's case -- the dosimeter readings predicted the (generally small) hearing losses in an orchestra just as if the players had been exposed to industrial noise instead of music.
During the study, one of the musicians I got to know was the CSO's contrabassonist, Burl Lane, who sits in front of the brass section. He later told me a nice story about hearing protection before high fidelity earplugs were available. He had been in a car accident and the head trauma left him with tinnitus and extreme sensitivity to loud sounds. (Fortunately, his ear for playing was not affected: He was and is a superb jazz saxophonist as well as bassoonist). Burl took to putting in earplugs when an extremely loud passage was coming up. In one such rehearsal -- probably Bruckner or Mahler -- when Maestro Claudio Abbado was directing, Abbado saw him put the earplugs in and out and thought he was being a wise guy. He dressed him down in front of the orchestra. At intermission, Burl explained his problem and showed him a letter from his otolaryngologist stating that continued levels of loud exposure risked complete hearing loss. After intermission, Abbado rearranged the brass section behind him to reduce his exposure.
Beck: Sometime after that study, Etymotic introduced the Musicians Earplugs. Did the musicians wear them?
Killion: The Chicago Symphony provided those earplugs for any musician who wanted them. As I recall, over half of them obtained them and used them at least occasionally. Not all of them needed earplugs. In our CSO study we found daily-weighted-average exposures ranging from 79 dB up to 99 dB(A). The group 1 musicians -- bass, cello, harp, and piano -- had relatively mild exposures. The musicians near the percussion instruments such as the xylophone and the musicians near the brass section had the highest exposures. Across all tested positions, the maximum rms levels ranged from 96 to 116 dB(A) SPL. The instantaneous peaks at various locations ranged from 112 to 144 dB(A) SPL.
Beck: Which hearing protector do they prefer, Mead? The ER-9, the ER-15, or the ER-25?
Killion: The choice is determined more by the music than the musician, although the ER-9 was developed because some of the viola players reported they really liked the ER-15 earplugs but wondered if we could produce something with a little less attenuation. The ER-9 was the result: It has 9 dB attenuation at low frequencies and 14-15 dB at high frequencies.
Beck: Do I recall that the typical 4 kHz noise-induced notch is probably caused mostly by energy at 2.8 kHz?
Killion: Yes. Most animal experiments using intense pure tones indicate that damage occurs a half octave above the stimulus. This seems especially plausible regarding the 4 kHz notch we typically find in humans, since the resonance peak of the external ear is at 2.7 kHz.
Beck: So the ER-9 gives good audibility over most of the musical scale and good protection for the higher frequency sounds.
Killion: Yes. The ER-9 provides about 15 db of protection at 2.7 kHz. Fortunately, musicians have judged its overall frequency response to be adequately flat.
Beck: And the ER-25?
Killion: That was developed mostly for drummers and musicians in high-intensity rock groups. Drummers reported that the ER-15 earplugs were perfect when they played jazz, but they needed more protection when they played in some rock bands.
Beck: Mead, I know the foam earplugs can be pretty good when used properly. What do you see as the main differences between the over-the-counter foam plugs, and the ER-15s?
Killion: There are two important differences: The foam plugs are usually too much of a good thing as they may be providing 30 to 40 dB of attenuation, and on top of that, they really muffle high frequency sounds. According to Marshall Chasin of the Musicians Hearing Clinics of Canada, drummers using foam plugs end up with wrist problems! While wearing foam plugs, they keep trying harder and harder to hear the snap they are used to hearing when the stick hits the drumhead, and they just can't hear it. Switching to ER-15's or ER-25's allows them to hear the snap and their wrist problems tend to clear up.
Beck: I remember when he reported that too. I found that surprising, but I think he's probably correct.
Killion: I'd like to add that we've been talking about ear safety, but in my experience the benefits of ear comfort and hearing clearer can be just as important. On occasion, I've had the chance to play piano in a dance band or jazz combo (even got paid once or twice). In some cases the levels are so high that putting in Musicians Earplugs is a big relief.
Beck: What do you mean by clear sound? Don't earplugs always make the sound at least slightly less clear?
Killion: Maybe when things are quiet, but not when the levels are 100 dB and above. At those levels, first studied for the Navy by Erwin Pollack in World War II, the ear distorts so badly it can be hard to understand speech. James Lombardo told us he has fit most of the NFL defensive teams with ER-15 earplugs because each team's statistics show they reduced offside penalties. In some stadiums when the crowd is screaming, the levels can reach 105 dB down on the field. Reducing the levels reaching the player's ears not only allows them to hear more clearly, but also reduces the distraction factor.
Beck: Does that happen to musicians as well?
Killion: Absolutely. Lynn Gudmundsen, Gail Gudmundsen's sister, is a professional classical violinist and Country and Western (C&W) fiddle player. We went to hear her play in a C&W band one night. The band was so loud Gail and I couldn't understand each other even when we screamed. Lynn reported that at times like that, the sound from the band comes up like a wall behind her and she has trouble distinguishing instruments. With Musicians Earplugs, she can hear each instrument clearly.
Beck: You must have a lot of musician stories?
Killion: Yes, some of them sad. One drummer who had been working for years without ear protection called saying that his tinnitus was now so bad that even when he wore an earmuff over foam E-A-R plugs for an evening playing with his group he had roaring tinnitus for the next week. He was thinking of becoming a stockbroker.
Beck: So foam plugs are good for something when you need maximum protection?
Killion: Absolutely. E-A-R foam earplugs inserted really deeply (so you have to use your fingernails to remove them) give a fairly high-fidelity response, with an average attenuation of 40 dB. This is sometimes a good thing. The British division of E-A-R company estimates that they have saved 40,000 marriages. Snoring can reach 100 dB SPL and awaken the sleeping partner. Deeply inserted plugs can reduce that to a more livable 60 dB SPL, and be comfortable enough to sleep in. Just as important, there are circumstances where even 40 dB of attenuation isn't enough, such on aircraft carrier decks. The new Joint Fighter jets are reputed to produce 150 dB SPL at the locations on deck where the crew must stand to launch the jets.
Beck: Let's return to milder earplugs. Just to clarify, the ER series are all custom-made ear plugs. Can you please tell me a little about the process of manufacturing these devices?
Killion: Sure. We've talked about the critical plastic diaphragm. These are formed with heat and pressure to produce an acoustical compliance of 0.21 cgs acoustical microfarads (for the technically inclined). After assembly into the button with the acoustic resistance, they're sent to the earmold lab. The earmold lab has the burden of not only making the earmold seal and fit comfortably, but they make the acoustic mass of the sound channel equal to 0.02 cgs acoustical Henries. We supply them with a meter that makes it possible for them to measure the acoustic mass.
Beck: So the person wanting such earplugs goes to a hearing professional to obtain them?
Killion: Yes, the professional will take a deep ear impression in most cases, in order to minimize the occlusion effect. I say in most cases, because anyone playing a reed or brass instrument will find the vibration on the lips can create a very loud sound in the earcanal unless the earmold seals deeply in the earcanal -- at or inside the bony portion. Some singers, however, like a little occlusion effect to help them hear themselves. I recall one of the BeeGees singers, and Art Garfunkel (from Simon and Garfunkel) often put a finger in his ear in order to hear himself better.
Beck: What is the difference in the production of the ER-9, 15, and 25 that allows the different attenuation? Is the bore different, or is it the density of the material that changes the attenuation?
Killion: No, we change the acoustic compliance: Decreasing the compliance increases the attenuation
Beck: I see. So it's actually a sound-attenuating diaphragm that accounts for the various attenuation factors. In other words, the diaphragm varies and changes the stiffness so the sound approaching the eardrum is attenuated by a specific factor.
Killion: That's true. We also change the resistor to keep the response shape the same, as nearly as possible, as we change the diaphragm.
Beck: As a musician, if I had my audiologist send in a set of ear impressions and told you I'll need ER-9s for an acoustic set and I also want ER-15s for an electric set, would you make one set of plugs with two interchangeable diaphragms, or two totally separate sets of ear plugs?
Killion: The ear mold laboratory would send one pair of finished ear shells with two sets of buttons, the ER-9 and ER-15. This works because we also modify the size of the sound outlet in the button as we change the diaphragm. This way the same earmold works with all three buttons.
Beck: How long do the shells last?
Killion: If it's made out of silicon, it lasts as long as you don't lose it or tear it. If it's made out of vinyl, it depends entirely on the earcanal chemistry of the user (just as it does with vinyl BTE hearing aid earmolds).
Beck: Where did the ER-20 come in?
Killion: With the success of the ER-15, we realized there was a need for a much less expensive, over-the-counter high-fidelity earplug for everyday use. Jonathan Stewart of Etymotic Research and I obtained the help of Bob Falco and Elliott Berger of EAR (now AEARO) corporation, and the ER-20 was born. In the ER-20, we used horn action and a length resonance at 2.7 kHz and a small-cavity resonance to obtain a high-fidelity response. The addition of a one-size-fits-most triple-flange eartip completed the design.
Beck: Why did you make it 20 dB rather than 15 dB?
Killion: Our friends at AEARO wanted an NRR of at least 12 dB, so they could sell them into the industrial market. Even today, in the industrial market there is little recognition among purchasing agents that more is not better. At least 75% of those needing hearing protection need less than 10 dB of attenuation, but a 29 dB earplug is looked on by purchasing agents as much better than a 12 dB earplug.
Beck: What is the best definition of NRR?
Killion: Roughly speaking, NRR (Noise Reduction Rating) is the weighted average attenuation of an earplug, minus two standard deviations of the REAT measurement variations, minus 3 dB. The result is that the ER-20 earplug, which gives a true average attenuation of 20 dB, is rated as 20 - 2*2.5 - 3 = 12 dB.
Beck: You sound annoyed.
Killion: Worse than annoyed. When you force people to wear ear protection with excessive attenuation, they drill holes in their earmuffs and put earplugs in only halfway. They do this because they want to be able to hear forklift trucks bearing down on them, and want to hear the sounds of their machines approaching blowup conditions. I once suggested that we should also publish a worst-case ARR (Audibility Reduction Rating) calculated exactly like the NRR: ARR = Attenuation + 2*std+3 dB. On the traditional foam earplug, the ARR would be 51 dB. Someone with a 40 dB hearing loss would have worst-case 90 dB thresholds!
Beck: Okay. Let's talk a little bit more about the ER-20. I think that's the one size fits most earplug?
Killion: That's right.
Beck: Where would somebody get the ER-20?
Killion: They can get them at retail places, guitar centers, and if I may, I would say at other progressive music stores. Of course the ER-20 is available through our website and other people's websites. The ER-20 was developed because the custom musician's earplugs are about $150, which is inexpensive compared to the cost of losing your hearing, but it's not cheap!
The ER-20 is quite a bit less expensive, and that makes it more practical for lots of people. We've sold a few hundred thousand units, so price is a real-world issue.
Beck: So to get the lesser price, the trade off is custom versus generic fit?
Killion: Yes, and although the ER-20 sound is very good, it's not quite as good as the custom made products acoustically.
Beck: What percentage of people would be able to use the ER-20s, based on the physical fit?
Killion: Probably some 90 to 95 percent.
Beck: If you don't mind, I'd like to switch topics to a related issue, the ER-4. Can you please tell me, what is an ER-4?
Killion: The ER-4 is a hi-fidelity earphone -- by our measurements the highest-fidelity earphone available regardless of style or price -- with the ability to exclude outside sounds by 34 dB (triple-flange eartip) to 42 dB (foam eartip), based on EARCAL REAT measurements. These values far exceed the average roughly 15 dB noise exclusion of any active-noise-reduction earphone we have tested, by the way.
Beck: Who would use the ER-4, and when?
Killion: Anyone in a noisy environment that wants to hear music as if they were in quiet at home. Air travelers have been our most enthusiastic customers. But the ER-4 earphones can also provide hearing protection. The singer on stage in front of a wall of loudspeakers needs a loudspeaker monitor in front of them, turned up loud enough to overcome the wall of loudspeakers, so they can hear what they're singing! That means they're probably at dangerous, or at least uncomfortable loudness levels and, it muddies up the mix of the vocalist because the sound comes back at them into their microphones. If you switch to insert earphones which attenuate outside sounds 20-25 dB, then you can turn the overall levels down from 105 to 90 or 95, something much more pleasant. Also you don't have the loudspeaker in front of you muddying up your microphone pick-up if you're a vocalist.
Beck: Mead, thanks for your time. I appreciate your help and I think I have a much better understanding of Musicians Earplugs.
Killion: You're welcome Doug. I'll look forward to the next time we speak.
Editors Note: An early version of this interview was released in 2003. This interview (above) has been updated and revised. Thanks to Dr. Killion for his time and knowledge!
---Douglas L. Beck, Au.D., Editor-In-Chief
Beck: Thanks again for joining me Mead. Let's start with the topic of Musicians Ear Plugs. I know Etymotic has pioneered many of these products, and the three primary, custom-made products are the ER-9, ER-15, and ER-25, is that correct?