New Research on Audibility of Distortion in Headphones

[amirm]

To be presented at Bay Area AES section:

From presentation intro:

Hilton Garden Inn CupertinoCupertino, CA
Wednesday, April 29 • 5 PM - 7:30 PM
Overview

Join us for an evening of Headphone Distortion Audibility with industry experts, Dr. Sean Olive and Steve Temme.
Sean Olive will present ‘The Perception and Measurement of Nonlinear Distortion in Headphones’ a paper co-written with Pierre Lelièvre, Rtings.com. In this paper, they measure the thresholds and perceived sound quality from real distortion recorded in headphones and use 5 different distortion measurements to determine how well they predict audibility and quality. The results provide evidence that audible distortion in headphones with music occurs at significantly higher playback levels (104 to 112 dBA SPL) than what is considered typical and safe. Results are discussed, along with their practical implementations on future headphone design, testing and measurement.

Steve Temme will present ‘Measuring Distortion the way we Hear It’. Here, he explains the psychoacoustics of distortion, masking curves, and why it’s important to separate linear and non-linear distortion. He outlines the Normalized THD measurement metric that he introduced over 30 years ago, and demonstrates its correlation to audibility. He will also present a recent paper ‘Active noise cancellation distortions in headphones’ in which he investigates the trade-offs of active noise cancellation (ANC) in headphones, demonstrating how signal processing can reduce traditional distortion while simultaneously degrading perceived sound quality. He evaluates three high-end over-ear headphones using a variety of metrics to identify which best correlate with listener impressions.

Unfortunately it is not planned to be recorded. Hopefully we can get a copy of the presentation from @Sean Olive.

For now, it is nice to know that distortion *is* audible.

 

[amirm]

From this tiny graph:

It seems that the headphone was excited with a 100 Hz tone and its high order harmonics went above threshold of hearing from 3 to 5 kHz. If the vertical scale is calibrated, it is at 90 dBSPL. That doesn't seem too loud to me for momentary upper bass peaks. We can see that threshold of audibility is at 60 dBSPL, leaving us just 30 dB of actual signal at 100 Hz.

 

[Matias]

Yes, same as discussed here.

RTINGS study on distortion audibility in headphones and IEMs

Dear community of addicted to fun and learning, I just found this on RTINGS.com and thought it could be of general interest: The Surprising Truth About Headphone Distortion

www.audiosciencereview.com

 

[milosz]

One needs to be very careful when papping the conclusions of this study onto expectations for the experience of listening to the complex signal of actual music. Distortion of pure tones is certainly audible, as shown here, but what threshold of distortion is required for audibility of distortion when reproducing music cannot simply be inferred from these results. Certainly distortion is not what is desired in Hi Fi, and so Hi Fi goals are more likely achieved when pure tone distortion is minimized by the design, but this result doesn't tell us everything we need to know in transducer characterization when it comes to distortion.

 

[Mr. Swordfish]

The results provide evidence that audible distortion in headphones with music occurs at significantly higher playback levels (104 to 112 dBA SPL) than what is considered typical and safe.

I think what the author is trying to say is that audible distortion doesn't appear until higher playback levels than what is considered safe. i.e. you're likely damaging your hearing at levels that are still not distorted.

It's not at all surprising that distortion in headphones occurs at high levels. I think most headphones would distort at 130 db SPL.

 

[amirm]

Distortion of pure tones is certainly audible, as shown here, but what threshold of distortion is required for audibility of distortion when reproducing music cannot simply be inferred from these results.

Well, no. First, there are people who are disputing any level of audibility of distortion.

Second, we need to opt for transparency of the system. This is because we can't test everyone and every piece of music. If we can make the system transparent with said tones, then we have achieved that.

Now, if that transparency was not possible, that is one thing. But it is. At least for those who strive for excellence.

 

[amirm]

I think what the author is trying to say is that audible distortion doesn't appear until higher playback levels than what is considered safe. i.e. you're likely damaging your hearing at levels that are still not distorted.

That is what they are saying. But there is no such single number metric in reality. OSHA safety requirements are for continuous noise, not impulsive music. I have done a video on this:

Peak SPLs have been captured from many live concerts that are far in excess of what is stated and these are in "normal" seats. There are no warnings saying you go deaf if you go to such concerts.

 

[daniboun]

Hey, this is a good opportunity to share this article again. It's super easy to understand.

Audibility thresholds of amp and DAC measurements

Introduction A recurring theme on ASR is whether or not the various measured qualities of the devices are audible. In this post, I'll present some clear and visual thresholds for when those imperfections can be considered a potentially audible concern. I will not explain the basics of amp/DAC...

www.audiosciencereview.com

 

[Ype]

Well, no. First, there are people who are disputing any level of audibility of distortion.

Second, we need to opt for transparency of the system. This is because we can't test everyone and every piece of music. If we can make the system transparent with said tones, then we have achieved that.

Now, if that transparency was not possible, that is one thing. But it is. At least for those who strive for excellence.

I've seen such conversations online.
Defending cheap devices with more distortion is one thing, as one has to make compromises, makes sense.
But orders of magnitude more expensive headphones where one apparently is paying for more distortion and sometimes even worse frequency response was the norm.

I am happy more research is being done, especially for optimizing cheaper designs.
We are so close to that with some of the great and cheap IEMs that my gut tells me it is no longer an audible concern, but I cannot say for certain.
As you said, it can be done and in my opinion, for more expensive devices the target should be zero distortion.

 

[PJR918BMW]

Hey, this is a good opportunity to share this article again. It's super easy to understand.

Audibility thresholds of amp and DAC measurements

Introduction A recurring theme on ASR is whether or not the various measured qualities of the devices are audible. In this post, I'll present some clear and visual thresholds for when those imperfections can be considered a potentially audible concern. I will not explain the basics of amp/DAC...

www.audiosciencereview.com

Just had a look at that. I know its ancient history but I do still wonder about NWavGuy.. I still have 2 of his ODAC's in my desk. Can't bring myself to chuck them

 

[Postlan]

The claim by Sean Olive that, once frequency response is equalized, differences caused by distortion are difficult for humans to hear is something he has repeated for many years.

Therefore, it is not at all surprising that, under the specific conditions of this test that he designed, the results align with that claim.

 

[Koo]

It would be nice to get some vindication finally, for a long time I have thought it is easy to hear how much punchier/faster low distortion planar magnetic bass sounds, especially in electronic music or signal closer to square waves. Even when comparing to other very high end dynamic headphones with level matched Harman EQ, the bass difference is very audible for me at normal listening levels. But a lot of people think it is nonsense.

 

[Juhazi]

The claim by Sean Olive that, once frequency response is equalized, differences caused by distortion are difficult for humans to hear is something he has repeated for many years.

Therefore, it is not at all surprising that, under the specific conditions of this test that he designed, the results align with that claim.

It is not a claim, it is a verified result of specific studies. Audibility of harmonic (nonlinear) distortion has been studied a lot and we have no reason to be worried about dacs, cd players or amps. (There are some grave exceptions like low output tube amps or mains hum in specific miserable cases) Loudspeakers do have audible distortion at high spl, but not in everyday use. Now it is clear that modern reasonably well designed headphones are clean of audible distortion. Frequency response deviations are easily audible and so are most mechanical resonance noises which are not rare.

Nonlinear Distortion

Nonlinear Distortion

www.klippel.de

Listening Test

www.klippel.de

Modern measurement technology makes it possible to reveal and quantisize many inaudible issues, but ambient noise and music/soundtrack as source for listening masks those totally.

 

[amirm]

Now it is clear that modern reasonably well designed headphones are clean of audible distortion.

That's not what the abstract says: "The results provide evidence that audible distortion in headphones with music occurs at significantly higher playback levels (104 to 112 dBA SPL) than what is considered typical and safe."

The bit about what is "typical" or "safe" is a judgement. The real bit is that they are saying distortion does rise to level of audibility.

You also don't know what headphones they tested nor what "well designed headphone" means.

 

[amirm]

Modern measurement technology makes it possible to reveal and quantisize many inaudible issues, but ambient noise and music/soundtrack as source for listening masks those totally.

Ambient noise is a non-issue in many headphones that block such. As to recording, that can be a factor but it is not universal truth.

 

[amirm]

BTW, the most authoritative and recently published paper on this is:

Perceptual Assessment of Headphone Distortion
Louis D Fielder
Dolby Laboratories Inc., 1275 Market Street, San Francisco, California, USA
Correspondence should be addressed to Louis D. Fielder (ldf @ dolby.com)

ABSTRACT
A perceptually-driven distortion metric for headphones is proposed that is based on a critical-band spectral
comparison of the distortion and noise to an appropriate masked threshold when the headphone is excited by a
sinewave signal. Additionally, new headphone-based masking curves for 20, 50, 100, 200, 315, 400, and 500 Hz
sine-waves are derived by subjective tests using bands of narrow-band noise being masked by a sinewave signal.
The ratios of measured distortion and noise levels in critical bands over the appropriate masking curve values are
compared, with the critical bands starting at the second harmonic. Once this is done the audibility of all these
contributions are combined into a single audibility value. Extension to loudspeaker measurements is briefly
discussed.

Here is a sample of his finding:

"Figure 16 shows the performance of a high-quality
circumaural headphone "A" that was shown to have
a problem producing 20 Hz at a 104 dB fundamental
drive level since it had a distortion audibility of 20.9
dB. It showed a very audible 3rd harmonic and other
audible harmonics up to the 15th harmonic. No
audible high-frequency noise was indicated."

Another:

"Figure 21 shows the performance of an insert-ear
headphone "D". In this case the headphone produced
50 Hz modulated audible chuffing noises, resulting
in a distortion audibility of 13.3 dB. The distortion
assessment process showed that the audible
distortion artifacts were noise-like sounds and not
lower-order harmonics. Although not shown here,
another insert headphone had a similar tendency to
create higher frequency distortion impairments
rather than low-order harmonics."

Here is how wide the variations are based on their audibility metric ("DA(dB"):

So let's not go around and say distortion doesn't matter. There are broad set of products out there, many of which have audible distortions especially in bass.

 

[nonloso]

Steve Temme will present ‘Measuring Distortion the way we Hear It’. Here, he explains the psychoacoustics of distortion, masking curves, and why it’s important to separate linear and non-linear distortion. He outlines the Normalized THD measurement metric that he introduced over 30 years ago, and demonstrates its correlation to audibility. He will also present a recent paper ‘Active noise cancellation distortions in headphones’ in which he investigates the trade-offs of active noise cancellation (ANC) in headphones, demonstrating how signal processing can reduce traditional distortion while simultaneously degrading perceived sound quality. He evaluates three high-end over-ear headphones using a variety of metrics to identify which best correlate with listener impressions.

Linear distortion?

 

[peniku8]

If we can make the system transparent with said tones, then we have achieved that.

Begs the question if a system could exhibit a sufficiently low (read: inaudible) harmonic distortion profile at any frequency while producing noticable IMD, which would not be caught by pure singular tone based testing.
I really hope they'll publish a paper based on the research they show at the presentation. Exciting stuff!

 

[Buckchester]

The claim by Sean Olive that, once frequency response is equalized, differences caused by distortion are difficult for humans to hear is something he has repeated for many years.

Therefore, it is not at all surprising that, under the specific conditions of this test that he designed, the results align with that claim.

Are you for real? So you think he is specifically designing tests that confirm his pre-conceived notions? Give me a break.

 

[Buckchester]

I've seen such conversations online.
Defending cheap devices with more distortion is one thing, as one has to make compromises, makes sense.
But orders of magnitude more expensive headphones where one apparently is paying for more distortion and sometimes even worse frequency response was the norm.

I am happy more research is being done, especially for optimizing cheaper designs.
We are so close to that with some of the great and cheap IEMs that my gut tells me it is no longer an audible concern, but I cannot say for certain.
As you said, it can be done and in my opinion, for more expensive devices the target should be zero distortion.

Zero distortion or inaudible distortion? I would argue the latter.