Message-ID: <4C7F056A.4090000@hifidelity.com>
Date: Thu, 02 Sep 2010 02:01:14 +0000
From: Steve Dove <dsp@hifidelity.com>
User-Agent: Mozilla Thunderbird 1.0.6 (Windows/20050716)
MIME-Version: 1.0
To:  rsgb_lf_group@blacksheep.org
References: <000a01cb493d$fb8f3410$0401a8c0@xphd97xgq27nyf><4C7E2046.536.2C4F91@v.d.heide.on-line.de><BF4A524700075746A6467658DFC7102C63821D3C97@ICTS-S-EXC2-CA.luna.kuleuven.be><AANLkTikieMQyYw9jK8fuxyznoBtknevc0Lm1-YaFJrB8@mail.gmail.com><002501cb49f0$acabccb0$0401a8c0@xphd97xgq27nyf> <AANLkTikv=jPM3+dXJoTyOAW0fU3JozVjJj0Jhx1HRwVS@mail.gmail.com> <003b01cb49fc$89767860$0401a8c0@xphd97xgq27nyf>
In-Reply-To: <003b01cb49fc$89767860$0401a8c0@xphd97xgq27nyf>
Subject: Re: LF: VLF THOUGHT
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


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