August 14, 2012

Loudness perception and bass

We've all seen the equal loudness contour:

It tells us quite a few things, many of them missed in casual observation. The first thing we might notice, is that our hearing is most sensitive to midrange from about 200 Hz - 14 kHz. Bass is subjectively "turned down" indicating that it's actually not very important in the functional sense. Bass doesn't tend to occur in nature in isolation of much higher frequency content. We also note that our ears are especially sensitive around 3.5 kHz.

This was made clear to me when as a teenager, I measured my first system. I found a huge peak around 3.5k and playing test tones and measuring with an SPL meter outside the science lab in high school, those 107 dB peaks were just plain nasty! No doubt that was a big part of what made little 15W speakers sound loud to my teenage ears.

This 3.5k peak in sensitivity does have implications. Many midrange drivers have cone breakup around this region, and many crossovers are not far removed from this point. A very large part of our internal hearing mechanism is dedicated to 1 - 7 kHz primarily, this region is critical.

SPL and tonal balance

Notice how things change with level. At a typical quiet background noise level of about 26 dB, we need over 90 dB at 20 Hz before we perceive the bass as simply matching in loudness. Many subwoofers fail to even register as a quiet background noise level! Now if we listen at a moderate level of 60 dB, we need 110 dB @ 20 Hz or about 94 dB @ 40 Hz. We probably need about 25 - 30 dB increase in bass level to get it to sound about the same in level across the bass spectrum.

If we boost the level to 80 dB, we need about 20 dB increase in bass level. The bass is starting to sound a bit more solid. If we increase the level to 100 dB, we are now in the very loud range, and the bass only needs about 12 dB boost to match the level.

So why don't we all boost our bass by these crazy amounts? These effects are partially allowed for in the recording process, so you don't want to invert these curves and use them for EQ. But these curves reveal that the tonal balance changes with level and in this regard, it is the bass that changes the most.

I had previously thought that the equal loudness contours explained the harshness of some speakers when turned up loud. The contours don't support that view. Perception of treble stays about the same at high levels. In reality this might be explained in terms of system stress where distortion causes the treble to sound louder, especially where the amp is clipping.

What do we learn?

  • 10 dB in the midrange sounds twice as loud
  • 6 dB in the bass range sounds twice as loud - this number decreases at high SPL levels
  • Maximum sensitivity - 200 - 7k
  • Peak sensitivity 3.5k
  • A dip in sensitivity occurs around 10k

1 comment:

  1. Hi I think sudden onset distortion when turning things up is usually Doppler distortion caused by drivers acting as velocity sources due to generating treble while also moving with high velocity due to large bass transients. This distortion is perceived as a sudden harshness as the ear has limited frequency discrimination (move a tone by 0.1Hz at 1KHz and it sounds the same). Doppler distortions result is that original signal is FM modulated.

    So ways to avoid this is make lf drivers not produce hf (ie. avoid the traditional dome tweeter 6" midbass 2 way or full range drivers) or to make the lf driver move with lower velocity by making it larger (such as the constant directivity designs on this blog). Subs help alot as excursion increases with decreasing frequency usually.

    Clipping from my experience is very very noticeable (actual click noises) and happens at ludicrously loud volumes.


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