The LF Problem: Low Frequency Expansion in Our Hearing

One of the key problems revealed here is the low frequency expansion that exists in our hearing. This is a very serious, essentially unsolvable, problem that explains why we have such trouble with low frequency decisions in mixing and mastering. Take a look at Figure 9.

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Figure 9 shows the actual amplitude change that occurs, on average, for us to perceive a 10 dB change. The big problem is that this amplitude varies as a function of frequency. Further, the difference between percveived loudness and real amplitude change is quite signficant. So, small changes in actual amplitude result in comparatively large perceived changes in low frequency.

By the same token, low frequency anomalies, peaks, dips and the like caused by the speaker design and its placement in the room are multiplied. Think of our hearing as a low frequency expander. See Figure 10.

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	Shows the expansion ratios for the bottom eight 1/3 octave bands. 1.6:1 is a heap ‘o expansion! No wonder we can’t get the bottom end right!

It gets worse when we think about the range of end-user systems. It’s bad enough that we mix on a quirky, ported monitor in a pretty odd-sounding room, but then our mix is supposed to “travel” to thousands of different quirky speakers in zillions of different rooms, all while our hearing multiplies the errors!

Some limitations to keep in mind

Before you let this news completely ruin your day, there are a couple of things to keep in mind. First, the Equal Loudness research was done with sine waves, and it may be that the actual complex perceived spectrum at these various levels isn’t quite as perversely different as these graphs suggest.

Second, we’ve been living with these curves in our collective heads for a long time now, and we’ve managed to survive. Our perception of these response changes is tempered by the accompanying level change and the masking effect of consciousness, so it isn’t quite as frustrating as the graphs look.

Nonetheless, there are some things to think about here, obviously.

NEXT> Implications for Recording

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COMMENTS

Singapore Feb 21, 2009 10:11 PM

Hi Dave,
I enjoyed reading your article even though I am not in the recording business. At least I know how complex it is and the science is really good to understand. I stumbled on your article as I was doing some general research on hearing as we are in the digital signal processing business. We use complex near real-time algorithms to process inputs from bluetooth headset devices, handsfree carkits, multiple microphone arrays, etc to reduce the noise, establish direction of sound arrival, cancel out echoes, etc. We have developed a handsfree carkit the size of a mobile phone which can output 95dB without the acoustic feedback even though the mic is on the same box. I learned a lot, although not fully understanding everything but believe that there's a lot more to understand to make a great product, as with the skills required in making great recordings.
Cheers and thanks!

Philip

Philip Tan

Groton, MA Feb 22, 2009 10:17 AM

Hi Philip!

Thanks for your commnts and kind thoughts about my article on Equal Loudness Contours. It sounds like your are hard at work on some really interesting (and probably quite important) stuff.

Good luck, and keep up the good work.

Best regards,

Dave Moulton

Dave Moulton

Ireland Sep 19, 2011 05:38 PM

Hello Dave, by any chance would you have the original 1950s 1/3 octave equal loudness curves in an Excel Spread Sheet format by any chance?

Diarmuid Keaney

The LF Problem: Low Frequency Expansion in Our Hearing

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Shows the expansion ratios for the bottom eight 1/3 octave bands. 1.6:1 is a heap ‘o expansion! No wonder we can’t get the bottom end right!

Some limitations to keep in mind

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