Headphone distortion from infrasonic sounds?

Please pardon my stream-of-consciousness word splatter, but this something that I thought would be worth discussing …

It’s generally accepted that humans can’t hear infrasonic frequencies (below 20 Hz). Given this, evaluation of headphones generally focuses on frequency response and distortion characteristics for frequencies above 20 Hz. However, it occurs to me that distortion products from infrasonic frequencies do enter the audible range. For example, for a fundamental at 15 Hz, the 2nd harmonic would be 30 Hz, the 3rd 45 Hz, 4th 60 Hz, and so on. Now generally, we tend not to worry about higher order harmonic distortion since it’s generally just the 2nd and 3rd that are the strongest and things tail off pretty quickly after that. However, we also know that headphone drivers tend to have increasing distortion at lower frequencies, so perhaps that extends into higher order harmonics?

Here’s the core question - can distortion from a headphone that’s trying to reproduce infrasonic frequencies audibly harm reproduction of audible frequencies?

This of course raises a whole range of other questions, like:

  1. Do recordings commonly contain sonic content below 20 Hz?
  2. Do certain lossy encodings discard information below 20 Hz and would that potentially make them sound better in some cases than full-spectrum lossless encodings?
  3. Can said distortion be effectively addressed by applying a steep low-pass or brick-wall filter in the playback chain?

It’s entirely possible that I’m barking up the wrong tree and that there’s nothing to see here, was just a thought that sprang to mind.

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It might be academically interesting, but practically non-issue in my opinion.

  1. For lower frequency, say <30hz, we seem to perceive sound rather differently. I remember there were several good studies about this issue published long ago – like this one. If interested, search for noise and human auditory studies in the literature.
  2. Speakers, specifically sub-woofers in this context, introduce way, way, way more harmonic distortions than headphone on sub-bass. Even at totl level. Room even adds additional harmonics substantially – overall, in-room measurements of speakers indicate a lot more (+20 ~ +40 db) harmonic distortions than headphones. Admittedly, music producers use neither anechoic chamber nor ideal transducer. Musics we enjoy are created from such “limited” environment.

I believe distortion and coloration are perceptibly rather meaningful in a relative way – which presume good reference points.

To sum, I agree that there could be some infrasonic distortions. But even if so, those distortions are likely to be a part of our “naturalness and sonic fidelity”.

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Looking at this from a recording point of view, I can say that yes, distortion below the audible frequency level will cause distortion in the harmonics.

As I have mentioned before, I am a bassist and I like to play 4, 5 and 6 string basses. The lowest I will tune my 5 and 6 stings is B0, which has a fundamental frequency of just below 31Hz. Now, I know this is not below the audible level of human hearing but I will explain why this is relevant…

I do a lot of solo playing, looping etc. and until a couple of years ago, my bass rig consisted of a 15" sub (sometimes an 18") and a couple of 6"+2" pa cabs on top. I ran from my preamp that was connected to a BSS DSP which split the relevant frequencies to each speaker cab. When playing a low B with too much bass boost, this would cause audible distortion through the rig. If I turned off the sub part of the rig (therefore eliminating the frequencies below 80Hz where my crossovers were set) the same distortion would still be present in the mids (although to a much lower extent).

This is the same when recording. Sometimes I will play the bass lines on a keyboard and go lower than the B0. If I record a track that is distorted in the low end, I cannot fix this by adding a HPF and removing the fundamental.

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Thanks for introducing me to this. Led to the below, which seems to be a pretty accepted standard for testing subs. 30% distortion in the first harmonic is considered a passing value!

http://www.brentbutterworth.com/cea-2010-measurement-manual.html

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