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IEMs, Singers and Occlusion
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<blockquote data-quote="George Friedman-Jimenez" data-source="post: 108553" data-attributes="member: 1115"><p>Re: IEMs, Singers and Occlusion</p><p></p><p>This is a fascinating discussion, thank you Andy and others for starting and continuing it. The author has much credibility, he is a very senior and well respected audiology researcher working for many years with a multi-institutional team on multiple academic audiology research projects funded by the National Institutes of Health. The journal is, however, a lay publication not a peer reviewed scientific journal and thus leaves out some important scientific details and in my view oversimplifies some concepts.</p><p></p><p>For example, he speaks of "vibrations escaping through the open ear canal" rather than the reduction of pressure amplitudes in an enclosed space when the space is vented. That issue aside, I think the concepts are pretty well presented and make physical sense. By way of my background, I am a musician, I do sound for our group, and am also a physician and was a physics major undergrad. Although I am not either an ENT specialist or a monitor engineer, I do have a personal interest in both IEMs and hearing loss. I do not have any financial stake in the IEM or hearing aid business and my comments reflect only my own thoughts and end user interest in this topic.</p><p></p><p>The obvious difference between the current forum discussion and Dr. Ross' article is that he is discussing hearing aids not IEMs. This is important because the difficulty with venting the ear canal for a hearing aid is that it makes control of feedback from the transducer inside the ear canal to the microphone embedded in the hearing aid outside the ear canal much more difficult to control. This is not an issue with most IEMs that have no embedded microphone, so the venting solution becomes much more viable.</p><p></p><p>I have not seen vented IEMs, however I have a pair of passive musician's earplugs with a -10dB filter that is vented. I have had major problems with those as the vent seems to open and close almost randomly for unknown reasons, greatly affecting the attenuation from minute to minute. I will work with the manufacturer to try to resolve that problem but for now, I have stopped using the vented -10dB filters and use only the unvented -17dB filters.</p><p></p><p>I agree with Dr. Ross that making the earmold go deeper, so it ends past the second bend in the external ear canal, is one potentially effective approach. This can be demonstrated, as one forum member posted, by the reduction in occlusion effect when you press the IEM into the ear canal so it goes deep enough. This little experiment, is potentially very risky and could possibly lead to damage or rupture of the eardrum if the end of the IEM contacts the eardrum, so I strongly recommend against people trying it. Trust me, it works, as some of us have found out by carefully, recklessly or thoughtlessly trying it. </p><p></p><p>I would state the mechanism of the occlusion effect a bit differently than Dr. Ross stated it. The walls of the ear canal are cartilage covered with skin in the parts of the canal closest to the outside, and are bone covered with skin deeper in the canal, beyond where the canal makes its second bend. When you speak or sing, the vibration from the larynx is transmitted throughout the skull and the cartilage vibrates with much more amplitude than the bone. So if you occlude the ear canal with your finger, or use an IEM that ends before reaching the bony part, the larger vibrations of the cartilage are in contact with the air in the enclosed ear canal, are effectively transmitted to the eardrum, and you can hear the sounds very loudly. Dr. Ross reports around 20 dB louder sound than that you hear via external transmission through the air to your open ears. He did not mention the very large variation of this amplification over different frequencies, so that the sound heard is mainly lows and mids and not suitable for pitch control or optimal control of singing tone. </p><p></p><p>If you occlude the ear canal deeper than the second bend, and the walls in contact with the air are the bony part that has a much smaller amplitude of vibration, there is much less sound transmitted to the air inside the sealed ear canal and you don't hear the sounds nearly as loudly. Incidentally, in this setting, the loudness is not governed by the inverse square relationship, which applies to freefield not nearfield or enclosed spaces, but rather how much air is moved by the walls of the ear canal and whether the canal is completely sealed or vented.</p><p></p><p>The difficulty with the deep earmold approach is that it becomes much more difficult and risky to make earmolds as you go deeper. A physical block is placed in the ear canal to prevent the injected earmold material from contacting the eardrum. Placing this physical block safely without contacting or rupturing the eardrum becomes more difficult as you get closer to the eardrum, so most audiologists do it very cautiously. This often leads to earmolds that do not go quite deep enough, thus the common problem of the occlusion effect. Plus you don't want an earmold that, if touched or pressed on the outside, will contact the eardrum and possibly rupture it. I am hoping the venting approach works for IEMs so that would alleviate the problem in a way that would provide a greater margin of safety than the deep earmold approach.</p></blockquote><p></p>
[QUOTE="George Friedman-Jimenez, post: 108553, member: 1115"] Re: IEMs, Singers and Occlusion This is a fascinating discussion, thank you Andy and others for starting and continuing it. The author has much credibility, he is a very senior and well respected audiology researcher working for many years with a multi-institutional team on multiple academic audiology research projects funded by the National Institutes of Health. The journal is, however, a lay publication not a peer reviewed scientific journal and thus leaves out some important scientific details and in my view oversimplifies some concepts. For example, he speaks of "vibrations escaping through the open ear canal" rather than the reduction of pressure amplitudes in an enclosed space when the space is vented. That issue aside, I think the concepts are pretty well presented and make physical sense. By way of my background, I am a musician, I do sound for our group, and am also a physician and was a physics major undergrad. Although I am not either an ENT specialist or a monitor engineer, I do have a personal interest in both IEMs and hearing loss. I do not have any financial stake in the IEM or hearing aid business and my comments reflect only my own thoughts and end user interest in this topic. The obvious difference between the current forum discussion and Dr. Ross' article is that he is discussing hearing aids not IEMs. This is important because the difficulty with venting the ear canal for a hearing aid is that it makes control of feedback from the transducer inside the ear canal to the microphone embedded in the hearing aid outside the ear canal much more difficult to control. This is not an issue with most IEMs that have no embedded microphone, so the venting solution becomes much more viable. I have not seen vented IEMs, however I have a pair of passive musician's earplugs with a -10dB filter that is vented. I have had major problems with those as the vent seems to open and close almost randomly for unknown reasons, greatly affecting the attenuation from minute to minute. I will work with the manufacturer to try to resolve that problem but for now, I have stopped using the vented -10dB filters and use only the unvented -17dB filters. I agree with Dr. Ross that making the earmold go deeper, so it ends past the second bend in the external ear canal, is one potentially effective approach. This can be demonstrated, as one forum member posted, by the reduction in occlusion effect when you press the IEM into the ear canal so it goes deep enough. This little experiment, is potentially very risky and could possibly lead to damage or rupture of the eardrum if the end of the IEM contacts the eardrum, so I strongly recommend against people trying it. Trust me, it works, as some of us have found out by carefully, recklessly or thoughtlessly trying it. I would state the mechanism of the occlusion effect a bit differently than Dr. Ross stated it. The walls of the ear canal are cartilage covered with skin in the parts of the canal closest to the outside, and are bone covered with skin deeper in the canal, beyond where the canal makes its second bend. When you speak or sing, the vibration from the larynx is transmitted throughout the skull and the cartilage vibrates with much more amplitude than the bone. So if you occlude the ear canal with your finger, or use an IEM that ends before reaching the bony part, the larger vibrations of the cartilage are in contact with the air in the enclosed ear canal, are effectively transmitted to the eardrum, and you can hear the sounds very loudly. Dr. Ross reports around 20 dB louder sound than that you hear via external transmission through the air to your open ears. He did not mention the very large variation of this amplification over different frequencies, so that the sound heard is mainly lows and mids and not suitable for pitch control or optimal control of singing tone. If you occlude the ear canal deeper than the second bend, and the walls in contact with the air are the bony part that has a much smaller amplitude of vibration, there is much less sound transmitted to the air inside the sealed ear canal and you don't hear the sounds nearly as loudly. Incidentally, in this setting, the loudness is not governed by the inverse square relationship, which applies to freefield not nearfield or enclosed spaces, but rather how much air is moved by the walls of the ear canal and whether the canal is completely sealed or vented. The difficulty with the deep earmold approach is that it becomes much more difficult and risky to make earmolds as you go deeper. A physical block is placed in the ear canal to prevent the injected earmold material from contacting the eardrum. Placing this physical block safely without contacting or rupturing the eardrum becomes more difficult as you get closer to the eardrum, so most audiologists do it very cautiously. This often leads to earmolds that do not go quite deep enough, thus the common problem of the occlusion effect. Plus you don't want an earmold that, if touched or pressed on the outside, will contact the eardrum and possibly rupture it. I am hoping the venting approach works for IEMs so that would alleviate the problem in a way that would provide a greater margin of safety than the deep earmold approach. [/QUOTE]
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