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In Reply to: follow up Q while you are here Paul... posted by Ryder on January 11, 2004 at 11:21:12:
Everyone leaves this out because it's complicated to explain, and you end up with pretty much the same result.In a room, you can separate the sound field into two parts, the direct field and the reverberant field. The reverberand field is all the energy that has bounced at least once off a wall. It's very chaotic, but averaged over a region of space or frequency the mean level is uniform in the room - at least, if the room is not too weird.
The direct field falls off with an inverse square law, just as you said.
There is a "critical distance" at which the direct field equals the mean reverberant field. Beyond that distance, the averaged sound level does not vary. In an average living room, with speakers of average directivity, that critical distance happens to be approximately one meter - an extremely lucky, or at least convenient, coincidence. In that case the one meter anechoic SPL is equal to the reverberant level that most listeners would hear if they are more than a critical distance from the speaker.
Clearly, this is a big part of how specific conditions may cause systems to behave differently. A lively room has a smaller critical distance, while a more directional speaker will have a larger critical distance. At a guess, critical distance is between 0.5m and 2m for 70% of speakers/rooms.
Most of this information is in the PA literature, not in the audio/hifi literature that I have seen.
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Follow Ups
- Good point! - Paul Joppa 01/11/0413:52:37 01/11/04 (0)
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