Real Ear Measurement

I recently became curious about the current state of the art in headphone correction and HRTF measurement and have been trying out 3D audio software. As part of this I have tried to find a cheap real ear measurement system similar to those used for fitting hearing aids but was unsuccessful. The Mini DSP system doesn’t have the correct adapters. I wanted a personal linear phase measurement system for headphone correction and playing with HRTF filters.

If I was a premium headphone company I would consider offering this service to my customers just like all the Audiologists on the high street do for hearing aid fitment.

Does anyone know if this is happening as it is quite easy to set up with a PC a couple of ADCs and DACs and some speakers and some probe tubing on a microphone? You would have to use MATLAB or similar as a one off but a simple app would be usable by non tech types in a shop or at home for custom fitment. It could even be built in to some IEMs.There is also the option on using an in situ audiometry target - can see no sign of that anywhere at the moment?

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Found this on another site. https://www.audiosciencereview.com/forum/index.php?threads/measuring-hrtf-for-headphone-use.3962/

Of course this will require a cost to read unless you know a AES member. https://www.aes.org/e-lib/browse.cfm?elib=20781

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Those articles look encouraging. I think there will be an occlusion effect with the in ear microphone though and additional resonance with the technique shown. In general a thin tube probe tip is required to go into the deep canal within 5-7mm of the tympanic membrane to get the true levels based on current guidance for hearing aid fitment. Otherwise it will be out by a few dB. In general you would use a pseudo random spread spectrum sound input from a speaker in a sound treated room and then take the FFT of the epoch averaged canal probe output and compare against the target, the aid is reprogrammed so the output matches the target and output is rechecked. The probe is calibrated against a monitor microphone which which samples simultaneously outside the ear. With a bit of extra processing you can also get a linear phase transfer function out of this system for a HRTF or accurate headphone correction although hearing aid targets just use scalar rms amplitude.

You can buy all this stuff but it is tens of thousands of dollars for a turnkey system and it is designed to program hearing aids against a prescribed fitment target based on the subjects audiogram not a headphone target - it would need to output the predicted headphone target FIR filter to feed the Convolver in your audio software or hardware and I don’t think the current real ear systems will do that or even put out the raw data for you to process yourself.

Maybe the paper describes something similar, I will get my hands on it and have a look.

I bet someone is doing this on the cheap somewhere.

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Is this what you are looking for? (Same guy does AutoEQ btw)

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This looks like a good start, not using standard probe tube microphones still but that is just a detail. I will have a good look, its being done slightly differently to how I had expected but amounts to the same sort of thing I think. The room ambience is done via sampling the actual room by the looks of it. Can’t see any head tracking though which you do need for a realistic experience.

Thanks for the help.

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Well hope you can figure it out on the cheap. It would be interesting to see the results.

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I’m very interested in your results but you should probably also read the audiosciencereview.com link that @cpp posted completely if you haven’t already. There are definitely some issues with the accuracy of measuring frequency response using this type of relatively cheap in-ear microphones. Two things that come to mind are that the microphones are not calibrated and that they will not have the proper acoustic impedance.

I don’t think the Impulcifer project calibrates the microphone but that asr.com thread suggests some ways you could do so. As for acoustic impedance, it may not be a big issue as long as you are using headphones with a flat enough frequency response.

Also take all of the above with a grain of salt because I am not a professional, nor do I play one on TV. :slight_smile:

I’ll bet that’s because you’d have to make many measurements at various angles and then interpolate them somehow during playback as you turn your head.

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Had a look at the AES thread. They seem a bit disconnected from current practice in audiology for example https://www.aes.org/images/e-lib/thumbnails/3/9/3994_full.png deprecates real ear measurement but as I said it is on every Main Street at least in the UK for hearing aid fitment and it is in all the national guidance - https://www.audiologyonline.com/articles/real-ear-measurement-basic-terminology-1229
There is a lot of talk about personal transfer functions - you can spend $500 on a predicted HRTF based on a video of your head for example. I think a good start would just be accurate linear phase headphone correction then the 3D aspects would come later. I think you could pop down to the local audiologist with your headphones to have it done then load it up and off you go.

This is interesting - it uses in situ audiometry as I suggested at the
start of the topic. There are preset profiles for different headphones and then additional personal preference eq which they call “Sound ID”.

There is no real ear measurement but in theory this is less of a issue if in in situ audiometry is used to help predict a headphone correction filter using reference headphone model data as a baseline. It won’t be linear phase though and you can’t use it for HRTF.

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I played around with SoundID:

It’s pretty cool but even Sonarworks states the hearing test feature is not a very accurate measurement of your hearing. I got subjectively much better results using headphone correction EQ from oratory1990: https://www.reddit.com/r/oratory1990/wiki/index

You should consider posting your thoughts on real ear measurements on his Reddit community as he knows far more about accurately measuring headphone frequency response: https://www.reddit.com/r/oratory1990/

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Check this out:

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Surprising that this isn’t built into more vr media players, some games have it, I’m still using waves NX 3D audio driver app for windows which I like and you don’t need a vr rig. That costs about the same if you buy the Bluetooth headphone tracker.
Will have a look on the Reddit for headphone correction.
It’s surprising how many different groups are doing the same sort of stuff. Wouldn’t be surprised if Apple or google make it part of the OS or the AirPods (they already have head tracking and 3D audio but don’t support stereo media at the moment only 5.1 encoded or better.)

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