# At what distance in a room should SPL readings start dropping off?

Discussion in 'Archived Threads 2001-2004' started by AaronNWilson, Oct 24, 2001.

1. ### AaronNWilson Second Unit

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Is there different ways that one can work out how loud a sound that is 120 dB 2 feet from the speaker should be 35 feet from the speaker?
Do you have to take into account thinks such as wall surfaces/reflectivity etc?
Also I have noticed that when I run frequency sweeps at a certain frequency the walls vibrate, can you tell me exactly what it is that i'm hearing on these occasions? ie the whole wall moving as one, or the plasterboard trying to break away from the studs?
Thanks,
Aaron

2. ### Patrick Sun Moderator Moderator

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You're hearing resonating frequencies combined witht he walls and the materials. There's no way to predict when the walls will start vibrating at a frequency to become audible.
And unless you only listen to frequency sweeps, the vibrating walls won't be a big problem when listening to normal stuff, music/DVDs.
I think SPLs is inversely proportional to the square of the distance measured. I think it's something like 6dB for each doubling of the distance.
1m 0dB
2m -6dB
4m -12dB
8m -18dB
(But I could be wrong on the heuristic above).
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3. ### DanWiggins Second Unit

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For loss with range:
dB Loss = 20 * logR
Where
R = range in meters
log = base 10 logarithm
So, for 10m away (about 33 feet), you'll have
dB Loss = 20 * log(10)
= 20 * 1
= 20 dB
Assiming you had 120 dB SPL at 1 meter (you actually 114.12 dB SPL if you measured 120 dB SPL @ 2 feet), you should have 100 dB SPL @ 10 meters (or, in the case you gave, 93.56 dB SPL).
Dan Wiggins

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