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IEM and HRTF
gaillard
post Sep 28 2006, 18:47
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Hey guys,

Ok heres my dillema, i hope you don't mind helping.

I am getting my HRTFs measured and therefor when i use them i need to use canal phones that have a frequency response that is flat at my ear drum, as this is where the hrtfs will be measured from. I noticed the etymotic er4b are designed from the er1's that are equalized to the "soundfield." there er2's are equalized to the ear canal. So why did they develope the er4b from the er1's if you want to use it for binaural recordings??

what does equalizing it to the soundfield mean anyway? Is this diffuse field equalizing? in this case isn't the headphone equalized to a random soundfilled room?

My second concern is this. because of highend loudspeakers frequency response, CD's have a high end frequency rise. the er4s etymotics compensate for that, do the er4b? and how am i going to find flat responding (at the ear drum) canalphones with that compensation?

Also, wouldn't all canalphones be equalized for flat since they are in the ear canal? except for the cd higher frequency compensation? Or is it that manufacturers choose to modify the sound a little to what people will like?

I think the er4s must be flat "at the ear drum" except for a little roll off for the cd compensation. This is what i want, so the er4b is the same without the cd compensation then right?


thanks for the help guys and sorry for my ramble but those are my thoughts.



ps. i would rather not equalize my hd650's as there is good chance of making it worse than flat due to the difficult to equalize spectral qualities, i've been told by the hrtf system designers.

This post has been edited by gaillard: Sep 28 2006, 21:45
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jlohl
post Sep 28 2006, 21:17
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QUOTE
CD's, because of highen loudspeakers frequency response, have a high end frequency rise

The frequency response of loudspeakers measured at the ears in a normal room is generally slowly dropping above 2kHz or so because of two combined factors : directivity index of loudspeakers is increasing at high frequencies and room absorption is generally increasing. This gives a normal balanced sound field : a flat high frequency response at the ear would be much to bright ! This phenomem is not related to the CD format.
I suppose that the dropping high freq frequency response of headphones and IEM is to mimic this natural curve.

QUOTE
what does equalizing it to the soundfield mean anyway? Is this diffuse field equalizing? in this case isn't the headphone equalized to a random soundfilled room?

Maybe the so-called soundfield equalisation is something between free field and diffuse field equalisation (something like Head Acoustics "direction independant equalisation", see their equalisation brochure)

QUOTE
i need to use canal phones that have a frequency response that is flat at my ear drum

Apart from the HF drop, IEM equalised to the ear canal should be fine. But how do you equalise an IEM to your ear canal ?

This post has been edited by jlohl: Sep 28 2006, 21:37


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gaillard
post Sep 28 2006, 21:54
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QUOTE (jlohl @ Sep 28 2006, 16:17) *
But how do you equalise an IEM to your ear canal ?


exactly. The er-1 is a canal phone and etymotic says they equalized it to the soundfield... the er-4b is a descendent of this apparently.

but looking at headrooms frequency response measurement, it looks like the binaural version of the er4 is just the same except without the High Frequency drop, which would make sense, since the recording is meant to be played on headphones not loudspeakers?

If you look at the frequency response of the ER2 on the etymotic site (900 bucks) it is almost competely flat!
wouldn't that be what you want for binaural recordings!? so why is the etymotic er4b from the er1???? In other words whats with the hump in the frequency responses of any of the etymotics?? If it is in the ear canal what is there to compensate for, unlike full sized headphones..?


im confused.
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gaillard
post Sep 29 2006, 19:09
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Anyone know what the hump in the etymotics frequency response is? If its in your ear drum why the hump ? shouldn't it be flat with just a high frequency roll off?
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jlohl
post Sep 29 2006, 20:28
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what the hump in the etymotics frequency response is? If its in your ear drum why the hump ?

My understanding : when you mesure the frequency response at the eardrum of a head in a flat diffuse field, it shows a hump at 2.5-3kHz that is due to the head/pina/concha. With an IEM, the signal is brought directly to you eardrum so you don't have this hump. So it seems that Etymotic tried to have a frequency response that follows this curve and add this hump.
But for binaural recordings on a real or dummy head, I think that you should have a flat response IEM instead because the recording itself has already the hump.


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mkeroppi
post Nov 13 2006, 08:24
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Diffuse sound field is a field where sound comes from all directions equally, as opposed to a free sound field where sounds comes from one or more localized points. A real world example that's close to the definition is a good reverberant hall for diffuse and very open space with no reflections for free.

Diffuse field equalized headphones work with sounds picked up from diffuse field equalized microphones. Microphones with pick-up patterns such as omni, which are usually used as overheads; they pick up any sounds from all directions, including the the "room". Dummy heads+these microphones will pick up HRTF information also for binaural recordings.

Unfortunately, studios recordings are unlike live recordings. Sounds are picked up mostly in the free field; that's like something screaming into your ear. That's fine because speakers do not play into your ears. Headphones and IEMs are going to have this problem. It has nothing to do with HRTF at this point.

The 5dB drop @10kHz for the ER-4S is really to compensate for a similar boost to most recordings because of a similar drop from speakers to ear. It has nothing to do with the above problem.

You can also check a summary I did on Head-Fi a few years back on the ER-4B.

QUOTE (gaillard @ Sep 29 2006, 10:09) *
Anyone know what the hump in the etymotics frequency response is? If its in your ear drum why the hump ? shouldn't it be flat with just a high frequency roll off?


This is to compensate for the internal resonance of the ear canal when you plug it shut (with the IEM).
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gaillard
post Nov 21 2006, 23:16
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QUOTE (mkeroppi @ Nov 13 2006, 02:24) *
Diffuse sound field is a field where sound comes from all directions equally, as opposed to a free sound field where sounds comes from one or more localized points. A real world example that's close to the definition is a good reverberant hall for diffuse and very open space with no reflections for free.

Diffuse field equalized headphones work with sounds picked up from diffuse field equalized microphones. Microphones with pick-up patterns such as omni, which are usually used as overheads; they pick up any sounds from all directions, including the the "room". Dummy heads+these microphones will pick up HRTF information also for binaural recordings.

Unfortunately, studios recordings are unlike live recordings. Sounds are picked up mostly in the free field; that's like something screaming into your ear. That's fine because speakers do not play into your ears. Headphones and IEMs are going to have this problem. It has nothing to do with HRTF at this point.

The 5dB drop @10kHz for the ER-4S is really to compensate for a similar boost to most recordings because of a similar drop from speakers to ear. It has nothing to do with the above problem.

You can also check a summary I did on Head-Fi a few years back on the ER-4B.

QUOTE (gaillard @ Sep 29 2006, 10:09) *

Anyone know what the hump in the etymotics frequency response is? If its in your ear drum why the hump ? shouldn't it be flat with just a high frequency roll off?


This is to compensate for the internal resonance of the ear canal when you plug it shut (with the IEM).


Thanks for the replys guys, I've finally got this all worked out. I do wish there was a standard to audio recording... wouldn't that help ALL of our lives...

Thanks again!
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