Why do "in tune" bass lines sometimes seem sharp to me?

Question

I've noticed that certain bass and baritone register sounds often seem sharp to me, even when they are ostensibly in tune- the notes are checked with a tuner or generated by a digital synth that won't go out of tune. I've also noticed that some sounds seem to go even sharper at high volumes. What is causing this discrepancy between perceived pitch and the "actual" pitch of the instrument?

I've read unsourced claims that at high volumes, high frequencies sound sharper and low frequencies sound flatter. I haven't found good reputable or detailed sources for this.

I was reminded of this phenomenon by this question.

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I've tried to recreate the sharp-sounding garageband synth, but I can't recreate the effect of it going sharp at high volumes. So here is the original synth. (I made this song as a joke.) I also just found that the pitch misperception is stronger (and happens at lower volumes) through headphones/eq with more bass.

Answer 1

You are right about loud bass notes sounding different to what they are. Whenever I tuned the bottom E by ear prior to a gig (off the 3 treble strings ADG) I would check it by tuner with disappointment that 'my ear was deficient'. What the tuner says is correct pitch (41.2 Hz) always sounded sharp to me, and what my ear said was in tune the technology says is flat. I think it has something to do with sound pressure and the effect on the human ear drum - and partials creating inharmonicity. On a piano the bass notes are tuned flatter and the treble notes sharper - stretching the octaves aka the Railsback Curve

Answer 2

[This would probably better fit as a comment as I do not have any references to support it, like you I am looking forward for other answers…]

All these bass sounds are not pure sine tones thus, they will generate harmonics according to the harmonic series: the first one will at the octave, the second one at the octave plus a fifth plus roughly 2 cents compared to 12TET. Usually the volumes of these harmonics will be lower than the fundamental tone.

That said, the human ear is more sensible to mid range sound than to bass sounds. That means that the perception of the harmonics will be boosted compared to the fundamental tone as these harmonics are at higher frequencies. The second harmonic, which is a bit sharper than the fifth obtained from equal temperament will thus be boosted compared to the fundamental for bass sounds. Maybe that is what is driving this perception phenomenon?

Answer 3

For what it's worth, the headphones or speakers you're using can change a lot of these characteristics.

I remember when the foam padding on my headphones wore off and I replaced them with padding made by a third-party manufacturer which was too thick. Low frequencies leak out a lot due to the distance between the speaker and ear, amounting to an inherent treble-boost which I usually fix with a treble reducer equalizer setting.

My experience so far (somewhat subjective) is that treble and midrange hold better over a long range, while low frequencies (with sufficient amplitude) are better at going through walls (you can confirm this by standing outside an overly noisy club with speakers belting out boosted EDM basslines).

Answer 4

Many bass instruments with primary resonators have disharmonicity affecting the tone: partial vibration modes that end up having higher frequency than a proper harmonic. Those blend better with higher pitches when those sharp overtones match the other pitches than when the fundamental is "correct".

Prime suspects here are string instruments with comparatively thick strings, like bass guitars. However, if the prime source of overtones is by distortion of the fundamental rather than by overamplification of physical overtones, disharmonicity should not come into play. So it's more a "funky" or "twangy" bass that will be affected than a "hard rock" bass sound.

This theme of "distortion of the fundamental" is also applicable to wind instruments operating on interrupted air streams, like reed and brass instruments (so they should now show disharmonicity) but not flutes. Flutes, however, tend to have rather few overtones and thus are not all that helpful for establishing a harmonic base.

The lower bass notes of a piano tend to require tuning lower than "appropriate" (stretch tuning), particularly of upright pianos which have shorter and thicker strings compared to grand pianos.

Bowed string instruments are rather complex since their overtones to a good degree come from the sticking action of the bow, but the bowing action itself causes pitch shifts in the attack phase as well as loudness dependent pitch shifts. This is partly compensated by today's bowed string instruments almost universally being fretless and thus amenable to on-the-fly corrections by the player.

Generally continuous-tone music instruments facilitate oscillators that are fed from some energy source. More energy is fed into the oscillators for louder notes, requiring more of a phase-lead and possibly resulting in a higher frequency for higher volumes. However, the tap-off of energy into sound can counterbalance this again, so the net result really depends on multiple balancing factors, and some aspects of musical instrument design may focus on making pitches reliable across different loudness by design or during the process of intonation.

TLDR: can of worms.

Answer 5

It may be that your perception is "correcting" the equal temperament physical pitch to something that makes more sense to your brain, e.g. just intonation where the ratios between the pitches are more in line with the actual overtones present.

("more in line" because of inharmonicity.)