Infrasonic squirms

Some experimental types have been working out the effect of infrasonic sound on people’s emotional states. The recipe: take a seven metre long pipe, stick a ‘long throw’ bass driver (woofer) therein a third of the way down, then drive it with a 17 hertz signal. Insert into live performances from time to time, then have the audience complete a questionnaire. Compare their feelings about the music when the infrasonic sound was on with their feelings when it was off. Apparently it had some effect.

I am interested in their infrasonic production apparatus. Not very much information is provided. Presumably the pipe is tuned to resonate at 17 hertz, and I imagine it would be designed to reduce harmonics to the minimum possible. The problem with producing pure bass is the harmonic distortion that is almost always generated. Let us say that the second harmonic is at, say, five per cent of the fundamental frequency (measure some subwoofers and you’ll find that this isn’t an unusual number!) That equates to 26 decibels quieter. But the sensitivity of the human ear drops off very sharply in the extreme bass regions. The Fletcher Munson curves (which attempt to equate the sensitivity of the human ear across the frequency spectrum) suggest a fall-off in sensitivity of as much as twelve decibels per octave in the bass (ie. between 40 and 80 hertz) and this drop-off likely becomes steeper the further into bass one descends. So high levels of distortion can result in audibility of an ‘infrasonic’ tone.

Thanks to my brother Mark for the heads-up. Go visit his Website Images of Canberra to see a stack of wonderful photos of Canberra and environs.

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