On 2022-01-30, Sampo Syreeni wrote:
There's at least one paper somewhere which systematically tested how binaural (then called dichotic) localisation accuracy depended on the bandwidth (and so inverse time localisation) of brief pulses. In it, it was shown that localisation accuracy decreased until the bandwidth of the signal was something like a megahertz.
By the way, this particular experiment was a bit of a tour de force as I recall. I think it bears a reminder.
Because, if you want anybody to hear anything in a psychoacoustical experiment, you need to deliver enough energy for them to hear stuff. This is rather easy if you use sinusoidal signals: just hum long enough and the energy will be there, to be picked up by the resonant machinery of the inner ear.
Wideband impulses are a different animal altogether. There, it's still not difficult to produce an acoustical pulse of 1MHz bandwidth. But when you think about what it means to concentrate enough electrical (to be transformed into acoustic) energy for the pulse to be *heard*, into microsecond scale, suddenly you run into a problem.
That's because of impedance and time scale. Mostly inductance, but at this range, not even just that. The thing is, you're suddenly dealing with low radio frequencies, so that you cannot deal with your speakers or headphones, or indeed even their cables, via the DC approximation. Suddenly even in audio work you have to deal with the cables as transmission lines, and since you have to deliver a wideband impulse in order to make your measurement, the speaker coils ought to be matched for best energy transfer.
Which as first order elements they naturally aren't. Wideband matching is an infinite order problem, and we don't have an infinite number of well-thought-out elements to work with, in them sixties. So what do you do?
Well, you damp the fuck out of your circuit, and go to the usual voltage controlled audio setup: low input impedance, high on the other end. That way you control what is on the other end, at the cost of energetic efficiency. This is for example how Ethernet signaling works: you terminate the line resistively, and so dissipate unnecessary power, in order to get a clean signal. Much the same in mic lines and the like: low-to-high impedance.
Except that most electrodynamical transducers are bulky, and by the way, induction-heavy. And the wires aren't really transparent at megahertz ranges. And you can't go to matching anyways, because matching depends on frequency, and so is a narrowband construct relying on electrical resonance. In order for us to deliver a well-defined pulse from DC upto 1MHz, we need something else.
So we rewind our transducers for lots of extremely thin wire, and go from permanent magnets as the stator to high amperage electromagnets. In the whole Tesla range, not Gauss. Still, the mechanical range isn't enough: too much mass and tensile strength, so that the speaker/phone vibrates at too-high-Q, and doesn't respond adequately outside its natural range. What is there to do?
Well, let's elevate the driving force/voltage, then, and shorten the cables to where negative feedback regains control authority to within less than a quarter of a cycle @1MHz.
I seem to remember this led to there being something like 10A going through the headphones' stators, and upwards of 5000V through a finely wound moving coil. Per ear.
Because it's *insanely* hard to pack enough energy into a microsecond acoustical pulse in order for it to be heard. You're fast approaching a Dirac impulse, which means whatever energy you have to expend, you have to do it at millions of times the instantaneous power at microsecond scale as you would have done it at the normal seconds. And if you want to retain the pulse-like waveform, you can't rely on any resonant phenomena to amplify the amplitude of your signal.
Accordingly later experiments went on to utilize dielectric breakdown. Basically sparks/arcs, which can more easily be loaded with a high current low inductance source. In high density neutral gas discharge tubes, you can even get the timing right, and then by geometry can focus the pulse into a clean outwards propagating acoustical wavefront.
-- Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front +358-40-3751464, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
