by Peter Berryman
Psychoacoustics 101: Superior Olive
Recently my musical partner Lou
and I played in a fine Grange Hall Coffeehouse in East Sangerville,
Maine, where I met a charming Christmas tree farmer interested
in finding Finnish logging songs from the Midwest for his cello/accordion
band. Back home, I found suggestions for him from many helpful
folks, most of whom mentioned the stellar 1986 two-LP set called
Accordions in the Cutover. The Project Director
for this recording was James Leary, Professor of Folklore &
Co-Director of the Center for the Study of Upper Midwestern Cultures
at the University of Wisconsin, who has helped us greatly in ways
too numerous to mention over the years.
I could ramble on for days about this outstanding compilation, but my rambler was carjacked by a quote -- in the accompanying 36 page booklet -- by John Kezele, born 1912, piano accordionist from Upper Michigan:
There are many ways distance can be an important component of art. What is it about a musical instrument far away that makes it more evocative than one close at hand? And as a matter of fact, how do we know it's far away, and not just nearby and quiet?
As long as we're at it, how do we know so MANY things about a sound, like what direction it's coming from? Googling, I found the jurisdiction of such questions is called Psychoacoustics. Here's some of what I learned:
There's a computer in the midbrain called
the Superior Olive that is able to compare and make sense
of the difference between the signals coming from our two ears.
Thus equipped, we can tell if a sound comes from our left or our
right, and in fact by (subconsciously) comparing ear volumes,
just how FAR to our left or right. (Dog at one o'clock; cat at
ten thirty). That's the main way two little earphones can represent
an orchestra's worth of locations. Psychoacoustically, this volume
discrepancy between ears is called INTERAURAL LEVEL
DIFFERENCE, or ILD.
One ILD problem: Since big table-sized low frequency sound waves
are more easily diffracted around our cabbage sized head than
wee high frequency waves, this effect is lost at low frequencies
because both ears hear approximately the same volume.
But we CAN tell where sounds are coming from even if they're quite low, because we are also able to detect a difference in "waveform phases" between the two ears, in other words, what part of a sound wave's valley or peak is hitting each ear at a given moment. This is called the INTERAURAL TIME DIFFERENCE (ITD). In a happy irony, this effect is easier for our brain to compute when the waves are bigger, and more difficult when they're tiny, just like it's easier for us to count ocean breakers than lake ripples. How very convenient that this works best on low frequency (big) waves, just as the ILD works best on high frequency (little) waves! Thanks to this two-pronged approach, one way or another we're able to tell where the whole spectrum is coming from.
But wait. There's one problem with both ITD and ILD: Neither can tell whether a sound source is directly in front or in back of us. Or directly over or under us. The sound arrives at the same volume in both ears in these cases, so there's no Interaural LEVEL Difference. The waveforms' peaks and valleys hit both ears simultaneously too, so there's no Interaural TIME Difference. We know it isn't coming from our left or our right, but it could very well be coming from our front, our back, from above or below, or from any point along that whole plane, according to ILD and ITD.
To the rescue: ANATOMICAL TRANSFER FUNCTION (ATF). Since our backs are not shaped the same as our fronts, frequencies coming at us from different directions are absorbed and reflected differently on their way to our eardrums. This gives our brain hints about whether the night train is going to mash us from the front or from the rear as we walk the dark tracks. For example, they have found that because of the body's shape, sounds coming from our front become louder in the range of 3000 Hz, those from our rear are boosted at about 1000 Hz and those from above at 7000 Hz. The main problem of this system is that it's sometimes difficult to tell if the 3000 Hz boost we hear is because the sound is coming from our front, or because that particular choo-choo has a big sound peak at 3000 Hz. Fortunately, when reverberations off of walls and so forth are taken into consideration and compared and computed by the Superior Olive, even an unfamiliar sound source can often be pinpointed by ATF.
But if the sound you're trying to locate has one single pitch, like a smoke alarm, ATF does you no good at all, because there is no frequency spectrum for your Olive to analyze. My wife Kristi and I almost lost our minds (the ninth time) when we were hounded by a smoke alarm's shriek even after taking all the batteries out of every one in the house. We were ultimately driven to rip the alarms off the walls and pile pillows on them, to no avail. Somehow we finally remembered the Carbon Monoxide detector that we had forgotten about entirely. There was NO WAY we could tell where this screech was coming from, Olive or no Olive.
No doubt there are many more intricacies of zeroing in on sound sources, but there was one unnamed technique mentioned that intrigued me. I've named it the HASTILY UNDULATED HEAD (HUH?). If you want to know whether a sound is coming directly from the front of you or the back, or from straight above or below, turn and tilt your head a bit. That way the sound will no longer be coming right at you, perpendicular to the line drawn from ear to ear, but from an angle, and suddenly the ITD and ILD methods, useless in those directly perpendicular situations, will work just fine. I would bet my bottom dollar (which is pretty close to my top dollar) this is why dogs, cats, and all such people cock and turn their heads when they hear a sound and they're not sure of its origin. Though this may not work with a smoke alarm, it helps locate a concertina, even a half mile away.
And how can you tell a concertina is a half mile away? That's either a joke waiting for a punchline, or the syllabus for Psychoacoustics 102.
Sources:
--On the web there is much info about psychoacoustics, fascinating but much of it repetitive. One of many sites I visited was Physics Today on the web, with an article about the subject at http://www.aip.org/pt/nov99/locsound.html. Google for pychoacoustics and you get 90,000 hits! And I'd never HEARD of it!
--Accordions in the Cutover, the LP set and booklet produced by Northland College of Ashland, WI, in 1986, with Project Director James Leary, is out of print, but may be reissued as a CD later this year.