L&SI Eurovision report: loud wedges and RF tricks

[Tom Baldwin]

While reading the L&SI write-up on this year's Eurovision, I came across a couple of things I just didn't understand...

 

On p.41, column 2, reference is made to the d&b M2 wedge's max SPL of 143dB... and I lack the imagination to see how you could use a wedge producing that sort of level! Is this simply headroom (cf modern cars which can cruise comfortably in excess of all speed limits), or is there some application where you'd actually need this much power from a single unit?

 

On p.43, final paragraph, the use of mechanical filters is mentioned as part of the RF planning - what are these, and how are they used?

 

Please, enlighten the lampy!

 

Cheers,

 

Tom

[paulears]

mechanical filters in the RF sense are tuned cavity devices. The basic feature is a small hollow device - the cavity resonates at a designed frequency, and has a very high Q - much higher than that available from the small, filters found inside receivers which tend to be simple tuned circuits, of the L/C style. The cavity is tuned, often with a small plunger type device. On the frequency, there is a small amount of loss passing through it, but away from the frequency, the cavity provides an awful lot of attenuation. Cavity filters tend to be too narrow for wide band channels normally, but they can be made to do the job. They can be set up for band stop, band pass, or pass below, or pass above. When lots of channels are in use, there are many problems - some we talk about, like intermod, which is the one that requires careful band planning so that a transmitter on channel X doesn't cause problems on receivers tuned to channel Z. By using mechanical filters it's possible to squeeze more channels into a given amount of bandwidth. Setting up such arrays of mechincal filters is not a job for beginners - when I have attempted such things, I've got a headache very quickly. without proper (and expensive) test kit, setting these things up properly is next to impossible - and a job for the real experts. RF kit of this kind resembles plumbing, not electronics!

[Dmills]

I have used short stubs of heliax as a quick and dirty filter for radio mic RX inputs before now, cut to roughly right then tuned with a small trimmer. It is slightly lower Q then a cavity, and so a bit more useable for wideband mics.

 

A network analyser (or a spectrum analyser with tracking gen) is a must for this sort of play.

 

Regards, Dan.

[Rob_Beech]

As for the first part of the question. Headroom is vital here so allowing for very loud monitors with headroom on top is necessary. A speaker tends to alter its response / the way it sounds when it is pushed to its limits, this is not good in FOH applications but if you are running at a continuous level you can compensate for this. With monitors however, it is a little more of a problem, as the response changes you'll find "hot spots" in the frequency range which aren't there when they are at a lower level. having alot of headroom on the monitors helps for this (as well as the obvious point about general system setup and headroom). You also have to remember how this is measured. 143dB is of course a peak figure, under normal rules we'd accept a figure 6dB lower as a continuous level. 137dB is extremely loud, but this is measured at 1m.stood back 1 metre from them, a tall person may be 3m away. On massive stages, there may be a 2m gap between the monitor and the performers feet, then the performers height, the diagonal of that could be 4m, already we've lost 12dB there. Given the fact we're possibly allowing for 12dB of headroom aswell, we are seeing figures of 113dB continuous to their ears.

 

yes, this is VERY LOUD, and what we'd call TOO LOUD, especially given that other sound from the stage will only add to this, but, at the end of the day, they DO have them loud.

 

If you take a smaller monitor, you would loose the system headroom, and perhaps not have the advantage of having a useful gap, in order to get the same level.

 

Having a pair gives you more headroom and the possibility of a better LF response. And can often be positioned better. - it can however cause coupling in the low mid section causing them to become rather boomy, this can be compensated using an eq or other dsp.

 

Rob