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Is there a "rule of thumb" as far as the volume of a given room/hall goes, and the distance from the speakers that listeners will be sitting, to help guesstimate what kind of 1meter SPLs (the only ones I usually see quoted re: speaker sensitivity) a speaker needs to be able to reach in order to generate a decent volume for the listeners (who are more likely at 10-20 meters)?
As an aside, what do you think is the max SPL at the listening position during a symphony orchestra concert for people sitting mid hall? Say during the big brusing climaxes, aka in Pines of Rome or other such "show" pieces (no need to get extreme by bringing the 1812 Overture into the discussion, please!).
Thanks --Mike
Follow Ups:
I have no rule of thumb, but an example might illustrate the important parameters.Assume an auditorium with around 500 seats and a volume of 2500 m^3.
Assume a reverberation time of 1.5 s, which corresponds to a 20% average sound absorption coefficient for the room surfaces. Let's say the speaker has a directivity gain of 3, corresponding to 10 dB.With the voume V, reverberation time T60, and speaker gain G, we can calculate the critical distance Xr from the speaker for which the direct SPL equals the reverberant SPL in the room.
Xr = 0.1 sqrt(G V /(pi T60)) = 4 m
If the speaker has a power sensitivity of 90 dB/1W at 1m, then the direct SPL will be 90 - 20log(Xr/1m) = 78 dB at 4 m, where it will be equal to the reverberant SPL anywhere in the room. So 1 W from this loudspeaker will get you 78 dB at 10 m or 20 m.
A 100 W amplifier will give 20 dB more, or a direct SPL of 110 dB at 1 m, 84 dB at 20 m. The reverberant SPL will be 110-12 = 98 dB, and at 20 m it will be 98-84 = 14 dB higher than the direct SPL.
You can find more detail under the link to my site and in modes1.xls .
SL
I had no feeling whatsoever for how powerful the reverberant field's contribution would be in a large hall. Thank you for posting this example!
Let's see...If we want 105 dB peaks at 20 meters, and if we assume a point source radiator in free space, that would require 131 dB at 1 meter.
My calculation goes like this: With a point source the sound pressure level falls off 6 dB for each doubling of distance, or 20 dB for each tenfold increase in distance. So, working backwards from the 105 dB peak level, we'd need 20 dB more to take it out to 10 meters, and another 6 dB to take it from there out to 20 meters. 105+20+6 = 131.
Let's see if we can whittle away at this figure. We'll have two speakers, not just one, so we gain back 3 dB. And, I would estimate, the room will give us back something like 6 dB from the reverberant field's contribution. So now we're down in the region of 122 dB per speaker output at 1 meter in order to get 105 dB at 20 meters, the exact figure depending on room acoustics.
I have never taken measurements in a room that large, so this is just my best guess as to what you'd need.
Remember also that you can't precisely extrapolate a speaker's maximum SPL from its efficiency and maximum power handling - most speakers suffer from dynamic compression, especially as they approach their power handling limits. So putting 1,000 watts into a 92 dB speaker won't necessarily give you 122 dB peaks at one meter - instead, you'll probably get something between 115 and 120 dB. Generally speaking, high efficiency speakers suffer less from dynamic compression than low efficiency speakers do.
Because I am an audio nut, I brough my slp meter to a live microtonal type music preformance by George Walker at school. At the loudest, I measured a 107db average when the band was going all out. Mind you that this was unamplified music with violins and the like. I was about 50-65 feet away from the stage. I had to write a paper on the show for a class so I wanted to gather as much info as I could about it. It was much louder than I thought it would be, but the average SLP was about 85Db or so. If you wanted to really see about sound reinforement on a large scale, check out JBL's website under the pro area. 20 meters away is quite far and if you could sit 20 meters away from the speakers, I would say that you are in a very large room indeed.
Still, the 20m was an outside figure - I am thinking 10m-20m is a good place to start as far as approximating the power and SPL needs.Thanks -
M
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