To: rsgb_lf_group@blacksheep.org
Date: Wed, 1 Sep 2010 23:14:39 -0400
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From: riese-k3djc@juno.com
Subject: Re: LF: VLF THOUGHT
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OK you have been to Burning Man fer sure

Bob K3DJC



On Thu, 02 Sep 2010 02:01:14 +0000 Steve Dove <dsp@hifidelity.com>
writes:
> 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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