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Sunday, May 23, 2010

How deep do you insert your ER4?



This article has been approved by Etymotic Research.
This article has been (unofficially?) approved by Sensaphonics.
Don't worry- I am not, in any way, affiliated with above companies at all.



It is known that, to avoid the occlusion effect of IEMs(including custom-moulded ones) and hearing aids, you should fill up your external ear canal till the 2nd bend(approximately 1/2 of the external ear canal length), where the bony part is. Furthermore, that's very likely the depth you can achieve the tonal accuracy of ER4. (and freedom from lack of bass)




[Industry-standard occluded ear simulators]
[IEC 60318-4(formerly IEC 60711) & ANSI/ASA S3.25 ]

Manufacturers of insert earphones use above ear simulators to measure, and fine-tune their products. These simulators are even implemented in Head-And-Torso-Simulators(aka dummy heads) too. Insert earphones are measured as shown below:













With full insertion, the tip of insert earphones reaches the reference plane. This reference plane located at the opening of the simulators indicates the insertion depth(or volume), at which you can enjoy the reference response that the manufacturer of the earphones intended. It's just best to think, if earphones are made & calibrated this way, in order to meet the tonal accuracy manufacturers have guaranteed upon their products, you must insert your IEMs(or ER4) the exact same way.







"An essential concept is that of the reference plane. This is a plane, at right angles to the longitudinal axis of the ear canal, located at the point in the ear canal where the earmold or ear shell usually terminates (defined in the standards to be approximately 13mm from the ear drum). An ear simulator (and very approximately, a coupler) represents the acoustic impedance of the residual ear canal volume and middle ear from this point inward."


Dillon, H, Hearing Aids. Sydney, Australia: Boomerang Press, 2001.

According to IEC 60318-4 and ANSI/ASA S3.25, this residual volume is defined as 0.547cc and 0.558cc each. Considering *the average volume of an ear canal with ∅7mm x 26mm, this residual volume can be achieved by filling up about the half of the ear canal.
*1cc = 26 x pi x 3.5^2




As seen in this CT scan, the 2nd bend is just at the center of the ear canal, and it can be assumed that's the depth 0.5cc can be achieved approximately.









(ER-4B diffuse-field compensated: Full Insertion / Shallow Insertion)


Full insertion depth is a must for ER-4 (or any IEMs calibrated in a similar way), otherwise you'll end up with a linear distortion such as above. On IEC 60138-4 & ANSI/ASA S3.25 simulators, λ/2 resonance occurs at ~*13kHz, and this acoustic impedance 'pulls' the earphone response at the high frequency range. Shallow insertion causes this interaction to occur at lower frequencies, and consequently will cause a sibilance and tin-can like timbre, especially on Etymotic Research ER-4B and Shure E4C. Above graphs show the effect of full insertion & **insertion 4mm short.



*13,000Hz = speed of sound / ( 2 x 13mm )

**10,000Hz = speed of sound / ( 2 x 17mm )






[From 3M]


I actually built a DIY Zwislocki coupler sometime ago, and was able to further confirm the above. Therefore, I'd recommend:


Insert your ER4 upto the 2nd bend, or until you can not hear the resonance of your own voice.

(to be more precise, the residual canal volume of 0.5cc(Same principle also applies to other IEMs calibrated/manufactured in reference to 2cm^3)



On Nov. 18th, 2010, Mr. J. Stewart from Etymotic Research provided further insights:

"...Insertion to the second bend may be difficult to determine for some users. When fitting insert earphones with foam or flanged ear tips, I focus first on obtaining a good seal (eartip fully inserted in the canal). Minor adjustments in insertion depth (if needed) may then be made..."


On the phone, he also stated, the occlusion effect of an earphone is not much of a concern, since it's caused by the piston-like mechanical vibration between the eardrum and the earphone, rather than the feedback-induced vibration of an hearing aid.

28 comments:

  1. Great! I was wandering exactly where the sweet spot was. Thanks. I mailed Etymotic on this but they were too generic and basic... too much "101 guide"
    Ah! I was wondering also if the custom-fit was of the ideal length/depth... thanks for being my guinea pig! :)

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  2. Rin,

    Have you seen this?

    http://www.soundandvisionmag.com/article/through-diaphonic-lens

    It could change insertion depth/custom tips for everyone.

    What are your thoughts?

    Thanks, Jim

    ReplyDelete
    Replies
    1. Yup, it is based on their research: http://www.aes.org/e-lib/browse.cfm?elib=15786 And I even bought the skullcandy titan to verify their claim! Not convinced at all.

      Although it might solve the air pressure mismatch in ears, it will not solve the insertion depth induced first-length resonance issue discussed in my article. FYI, with slightly vacumized (less air pressure than that of the outside) ear canal, ER4 actually performs better in the frequency region above 10 kHz; Knowles ED-9689 behaves much more linear.

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    2. Thanks for the link

      Jim

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  3. Great article! This blog has most of the research I've been looking for already on it and I seem to agree with what you write in general. Thank you.

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  4. It's well known that the Shure SE535 frequency response graph show a huge dip around the 10KHz frequency range. Do you think that Shure deliberately tuned it that way to deal with the resonance you mentioned?

    ReplyDelete
    Replies
    1. I wish I had a chance to analyze SE535; have never got hands on Shure and Westone at all..!

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    2. You have now ;)

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  5. "...until you can not hear the resonance of your own voice."

    I am having troubles understanding this statement. Do I have it inserted properly when I could hear my own voice as if there isn't anything blocking my ear canal?

    ReplyDelete
    Replies
    1. Your goal is to obtain a residual canal volume of 0.5cc. Insert your IEM until this is gone: http://en.wikipedia.org/wiki/Occlusion_effect It is a VERY rough estimation, but should work unless you are a really small/large person.

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  6. There's this bit I don't get about the resonance calculation up there, shouldn't it be f=v/(4L) instead of 2L since the outer ear now acts as an open-closed tube?

    ReplyDelete
    Replies
    1. The moment you insert an IEM into an external ear canal, the canal is open no more, right?

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    2. So you are saying a closed-closed tube has the same acoustical characteristics as an open-open one (or at least the same resonance calculation applies)? Guess I'm missing something here.

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    3. Yup. As this involves with the basic principle of the driver-pipe system, you might want to refer to relevant texts.

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  7. Quick question


    How do we identify the second bend inside the ear canal? When i put my finger in my ear with my mouth open i can reach a wall where my finger cannot go through but a penetrate it with a Q-tip.

    the second bend lies beyond this wall or behind? Just need to know where the second bend is actually situated. Thanks

    ReplyDelete
    Replies
    1. Unfortunately, only you can figure out exactly where it is. Here is a good indicator.. Insert an IEM until this effect is gone: http://en.wikipedia.org/wiki/Occlusion_effect

      And cross your finger that your other half of the ear canal is about the same size as the "average human", of which the ear simulator was modeled after. :p (Or go get yourself an ear impression, and see where the second bend is located)

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    2. Thanks. I havent really notice this effect that you mentioned with my IEMs even though they are not going really deep as I would like to (this is the reason that they offer average isolation). Maybe, because i can hear the outside noise, this effect is not noticeable because the echo-like waves can escape to the outside environment

      When i get my BA 200s with the double flange i will tell you about it.

      Delete
  8. Hi Rin, what do you think of something like this http://www.howtogeek.com/57481/how-to-make-custom-silicone-ear-molds-for-your-in-ear-monitors/ ?

    ReplyDelete
    Replies
    1. Bore acoustics will be seriously messed up along the way.. :(

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    2. It's not the same as silicone CIEM?

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    3. I am afraid bore acoustics is slightly more complicated than simply incising a hole with an xacto knife.

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  9. Is there any way to not mess it up with that method? using carefully estimated drill hole or something?

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    Replies
    1. My 2 cents is:

      First, calculate the optimum bore structure for the IEM.
      Second, calculate the optimum insertion depth of the IEM in relative to your ear canal geometry.
      Third, carefully drill the hole using a handpiece drill.

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    2. Okay thanks, as long as there are possibilities it's worth trying. Hope you can experiment with something like that someday. :)

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  10. Deep. Real deep. lol Rin, do you have an er4s measurements review?

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    Replies
    1. I might analyze it once I am done with my current loan of 20+ headphones LOL

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    2. Get on that Rin! :-) I want some er4s graphs with different tips and filters and such. :)

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