Psychoacoustic/Ear-Related IMD

[pozz]

First interesting point in the presentation below: The first presenter and founder of Etymotic, Mead Killion, claims that the ear canal's inherent IMD is around 1%, but IMD inside the cochlea is around 7% due to the action of mechanical amplification. The produced distortion components are known as otoacoustic emissions or OAEs. At around 11min:34sec, you can listen to a recording of the ear playing Bach.

Second interesting point: Jonathan Siegel claims (skip to 26min) that it is possible to provide accurate, low variability measurements of OAEs by compensating for the resonances of the ear canal. This is done by using the Etymotic ER10X to distinguish the initial pressure wave from reflections. More detail in this paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5848844/

 

[JohnYang1997]

1% is almost correct. 7% i don't know. There is website that you can test for yourself. Mine can go -40db.
https://www.klippel.de/listeningtest/lt/

 

[pozz]

1% is almost correct. 7% i don't know. There is website that you can test for yourself. Mine can go -40db.
https://www.klippel.de/listeningtest/lt/

These are different areas.

Otoacoustic emissions are sounds generated by the ear itself—the links you posted tests for the audibility of distortion artificially added to the source signal.

When we usually talk about IMD/HD, we mean the distortion created by the recording/rendering/playback chain. What I find interesting about the 1%/7% statistics above is that they characterize the distortion produced by the human listening mechanism itself.

 

[JohnYang1997]

These are different areas.

Otoacoustic emissions are sounds generated by the ear itself—the links you posted tests for the audibility of distortion artificially added to the source signal.

When we usually talk about IMD/HD, we mean the distortion created by the recording/rendering/playback chain. What I find interesting about the 1%/7% statistics above is that they characterize the distortion produced by the human listening mechanism itself.

So that leads to what type of imd.
Seriously 7% is just a typical number. And there is no other testing methods. You can see the distribution of the results. Many people are indeed in the -20+db region, but that doesn't mean all of us or human have imd of that level. All I said was idk. I didn't say it was wrong. So you can check for yourself. To me I can certainly hear the imd of my ear when playing pure two tones after the test using other generators. And in the test it's getting harder and harder to distinguish the distortion of the source or the ear. You should definitely try for yourself.

 

[pozz]

Please read the attached article and listen to the whole presentation. I've skipped ahead and jumped to conclusions plenty of times myself, so no hard feelings, and here's the short version.

The testing method is to use probes in the ear cavity. The breakthrough is that the level of the ear's IMD components can be reliably measured by using the method Siegel describes.

If you are unsure what I mean by "the ear's IMD components", the very beginning of the presentation by Killion gives an example: the seismologist David Kemp discovered otoacoustic emissions in 1978 when he played clicks into the ear of a subject and recorded clicks coming back at delays that are not compatible with the ear's dimensions (so they can't be echoes).

These IMD components are different from the instrument-related sum/difference combination tones known as Tartini tones, wherein if you play two notes you can generate a clear third (and softer others) if those two notes are at certain intervals.

The very interesting thing for me is that it means that whatever you are listening to, the ear system has already added a good portion of distortion in addition to its masking properties.

 

[Blumlein 88]

Excellent info at 31:40 in the video.

Why does the threshold of hearing shoot up so steeply just above 16 khz in the best of listeners? Because the basilar membrane has no where for hair cells to resonate with the traveling wave on the membrane for higher frequencies.

I've pointed this out so many times when people insist on how important ultrasonic frequencies are to our hearing. That membrane has no where to respond. There are no more hair cells. Those hair cells are what create the signal for the brain. Therefore, no you can't hear what difference response at 50 khz makes.

 

[Josq]

These IMD components are different from the instrument-related sum/difference combination tones known as Tartini tones, wherein if you play two notes you can generate a clear third (and softer others) if those two notes are at certain intervals.

The very interesting thing for me is that it means that whatever you are listening to, the ear system has already added a good portion of distortion in addition to its masking properties.

This appears to be the only post on this forum referring to Tartini tones.

I'm not sure why you call them instrument-related, in my understanding Tartini tones are a psychoachoustic IMD phenomenon (and therefore not measurable with a normal mic - yes there are things that we can hear clearly, but that we cannot see in conventional measurements!) .

But debates about definitions aside, I agree this is an extremely interesting subject. How much distortion is created inside our heads, depending on frequency, loudness, amount of polyphony, transients and other factors? Sometimes I think it is way more than I would like to consider.

Above what levels do distortion figures actually matter?

 

[pozz]

This appears to be the only post on this forum referring to Tartini tones.

I'm not sure why you call them instrument-related, in my understanding Tartini tones are a psychoachoustic IMD phenomenon (and therefore not measurable with a normal mic - yes there are things that we can hear clearly, but that we cannot see in conventional measurements!) .

But debates about definitions aside, I agree this is an extremely interesting subject. How much distortion is created inside our heads, depending on frequency, loudness, amount of polyphony, transients and other factors? Sometimes I think it is way more than I would like to consider.

Above what levels do distortion figures actually matter?

I wasn't completely clear. Tartini tones refer mostly to those heard by musicians while playing instruments. They are a type of otoacoustic emission (a DPOAE) rather than a separate phenomenon. See this link and this one for a discussion of specifics and numbers, the range being around -40dB to -60dB below the primary stimulus (pure tones). During testing pure tones are played between 40dB SPL and 80dB SPL. The measurements are done with probe microphones placed very close to the eardrum.

Main thing to take away is that the level of the spectral content or distortion added by the ear itself will vary continuously but is always present. Whether or not it will be separately audible is entirely to do with the type of stimulus, and it seems to have a dual role, contributing to both masking and discrimination.

The main reason I posted this talk in the first place is because I was struck by the thought that even the sound of a pure tone is much richer than we would normally think. We can't hear the purity. There is always some complex spectral addition that adds to the timbral perception (called roughness in psychoacoustic literature). Maybe this is an obvious point. But I can't help but think that even after stripping away everything, using high-quality transducers, electronics, an anechoic chamber, careful level-setting for playback to ensure no auditory threshold shifts and careful signal generation, what we hear is a human version of that purity. A kind of bodily constant.

Maybe that aural complexity is necessary, the nonlinearity being some basic background for our cognitive processes against which we plot and map all other sonic events.

 

[Pio2001]

Hi,
I certainly hear a lot of intermodulation inside my ears. A test proving that distorsion occurs in the ear consists in playing two sweep tones at once, staying 1000 Hz apart from each other, from 8 kHz to 22 kHz (a classic IMD test).
Say that your hearing threshold is 15 kHz. If you hear a 1000 Hz ghost tone throughout all the sweep, then intermodulation is occuring in the reproduction device. But if the 1000 Hz ghost tone stops exactly when the highest sweep goes past 15 kHz and you can't hear it anymore, then it was in your ears.
Don't try this with speakers, though, playing high pitched pure tones may damage the tweeters.

However, I don't need this test to realize that I'm hearing IMD inside my ears, because listening with speakers, I can hear the IMD products located in one ear or the other, instead of them being located somewhere outside my head.

The funny thing that I've noticed is that I can hear more clearly my ears' distortion listening with speakers in a mid-field setup than listening with headphones.

Comparing these two songs below is interesting. The first one is properly recorded, while the second one has got a lot of intermodulation distortion in the recording.

The tricky part is the high pitched voice sung at 0:15 in the first video :

And at 2:25 in the second video :

In the first video, I can hear intermodulation in my ears while listening with speakers, but it seems less obvious with headphones (although definitely present too). The IMD product that I'm hearing is around 200 Hz.
In the second video, it is the opposite. I can hear the already present IMD products more clearly with headphones than with speakers. Its pitch is lower than the one occurring in my ears. It sounds rather around 100 Hz.

The same phenomenon occurs in this track. From 0:43 to 1:23, when the three recorders are playing together, there is a lot of intermodulation in my ears, both with speakers or headphones, but it sounds more annoying with speakers :

 

[Pio2001]

Reminder : the IMD, or Inter Modulation Distortion occurs when two frequencies f1 and f2 are played at once, and consists in the presence of two additional frequencies f1 + f2 and f1 - f2.

For example, while playing a 10000 Hz frequency together with a 12500 Hz frequency, IMD may produce two extra tones, the lower one of 12500 - 10000 = 2500 Hz, and the higher one of 10000 + 12500 = 22500 Hz.

It sounds like a dissonant interference sound playing together with the melody with the same rhythm, but with completely different notes.

Now that we have understood this, we can look at the spectral analysis of the three above audio examples.

First video, the part at 0:15 s (timestamps are not the same as in the video)

We can see nothing else than steady tones, noise, and harmonics H1 and H2 of the high singer. IMD would have produced variable tones different from H1 and H2. The detection threshold is -150 dB / frequency band.
There is no IMD in this recording. Any heard IMD comes either from the playback device, or from our hearing.

Here is the sonogram of the second video at 2:25 (timestamps are not the same as in the video) :

Here, IMD between the two singers is visible in the recording. I have indicated the f2 - f1 IMD that I can hear, near the bottom.
We can also see the f1 + f2 product as a copy of high singer H1, just below high singer H2, but with the same slope as H1.

Isolating a small part in the middle of the above sample, we can estimate the order of magnitude of the distortion.

The IMD that is present in this recording is somewhere around -53-(-23) and -53-(-30) dB, which is -30 and -23 dB, or, in percent, 3% and 7%.
Mind however that in a spectral view, dB are not the amplitude of the signal, but the amplitude in each frequency band. Let's say that we are somewhere between 1 % and 10%.
The IMD that I can hear in this recording is about the same as the IMD that I can hear in my own ears when I listen to the other recording that has no IMD itself. Thus it agrees with the 7% figure given in the beginning of the discussion for IMD in the human ear.

Here is the spectrogram of the last video at 1:03, with the three recorders playing at once (timestamps are not the same as in the video)

I chose a part where I can clearly hear IMD while one recorder is playing an ascending melody of three notes (the three ascending red steps), as I hear a descending pitched distortion (whose frequencies equal the decreasing gaps between the red lines).

Here, we can see that this IMD is not in the recording.
There are two suspect frequencies visible below 250 Hz (green lines), but they can't be IMD, because they would change as f2-f1 (the space between the red lines) changes.