Hi Mal, Warren,
You both have points, and misapprehensions, in about even measure.
In still, homogeneous air, the results are as Warren states.
They did after all measure all this stuff once upon a time.
On doing system design for Very Big rock shows we used those
figures as a basis for what went on low in the stacks for
close in to the stage, and in the upper levels for further out
in the crowd; very different configurations. The delay
towers a couple of hundred meters out had similar upper/lower
constraints. And all these were adjusted on-the-fly with
respect to temperature and humidity which both and in
combination radically affect air attenuation vs. frequency.
But the response from communities miles and miles away (as per
Jim) belied the theory totally! The complaints - often with
cassettes! - showed that given pretty average summer-evening
temperature inversion ducting the shows could be 'enjoyed'
widely. No real highs, but the vocals were understandable and
the tunes recognizable.
Most interesting on the tapes, though, was occasional
frequency-dependent dispersion or perhaps different paths /
modes; the bass and kick drum often seemed 'out-of-sync' with
the rest. I put this down to the bass-bins being effectively
omnidirectional, whilst everything else had the advantage of
stacking gain, directional cabinets, or horns; they could well
excite different modes. Alternatively the LF was arriving
through normal 'ground wave', and the rest through ducting.
Amplifier power ran typically 100-200kW overall over four
frequency bands, but speaker efficiencies meant that it
probably wasn't ever much more than a handful of acoustic kW,
and certainly not from a point source.
Sirens ARE very close to point source and save the frictional
losses are very mechanical-to-acoustically efficient; the
SPLs they can generate are fabulous and which easily rival or
excel those of a mondo festival PA. Albeit at one note.
High audio frequencies ARE difficult to make travel - I
remember having 15dB of gain at 15kHz on an already
small-horn-heavy upstairs array to hit the back of a stadium
still sounding decent. And the 'next-town-over' tapes
certainly had little above 2kHz on them.
The parallels with RF are striking in that DX is possible
beyond the dogmatically theoretical but only by odd
'propagation' modes. But, like high audio frequencies, all
this really isn't going very far, is it?
73
Steve
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