Most subwoofers are not mounted in free space. Attenuation could be closer to 3~4dB per doubling of distance. And that only applies to if the subwoofer is mounted on a flat featureless plane, which probably describes 0.01% of installations
Subwoofers still drop off at 6 dB per doubling, they just start out 6 dB higher into half space than they would radiating into full space (4 pi). High frequencies also drop off at 6 dB per doubling of distance due to the increasing area of the surface of the sphere (or half, quarter, etc. sphere) through which the acoustic energy passes, with the loss due to absorption added in.
However, the loss due to absorption is not proportional (on a dB scale) to doublings of distance, but to distance itself. As an example, at 10 kHz in 20% relative humidity air, the loss is 2.8 dB per 10 m.
If a source is measured at 110 dB at 1 m, it will be 90 dB - 0.9*2.8 dB = 87.5 dB at 10 m and would need almost twice the power at 10 kHz to achieve flat response.
At 100 m, it will be 70 dB - 9.9*2.8 dB = 42.3 dB and would need almost 600 times as much power to achieve flat response. At 10 m, compensating is reasonable, but at 100 m, you just can't get much 10 kHz in dry air.
The absorption loss is strong function of frequency and a weaker function of humidity. At 1 kHz and the same conditions, the absorption coefficient is only 0.065 dB / 10 m.
Table of absorption (second table on page)
Marc
Hmm, seems like I was misquoting Everest ("Master Handbook of Acoustics").
Let me give the direct quote:
"Hemispherical propagation"
True spherical divergence implies no reflecting surfaces at all. Tied to the earth's surface as we are, how about hemispherical sound propagation over the surface of this planet? Estimates made by the very convenient "6dB per distance double" rule are only rough approximations.
Reflections from the surface of the earth outdoors usually tend to make the sound level with distance something less than indicated by the 6dB per distance double. The reflective efficiency of the earth's surface varies from place to place. Note the sound level of a sound at 10ft and again at 20ft from the source. The distance between the two will probably be closer to 4dB than 6dB. For such outdoor measurements the distance law must be taken at "XdB (4?, 6?) per distance double". There is also the effect of general environmental noise that can influence the measurement of specific sound sources"
"Master Handbook of Acoustics, p71"
Thank you for great explanation.
Therefore, a close-mic'ed recording will alway sound different (and in you-are-there terms, wrong*) compared to a kunstkopf or direct-to-disc recording.
B.
*but for some perceptual reasons, right
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I don't think you misinterpreted it. I think it was misstated in the Handbook. There is an attenuation effect due to propogation along an absorbing surface. That would cause the level to be lower than predicted by 6 dB per doubling, as Everest said. But that is more, not less than 6 dB drop per doubling. Anything lower than 6 dB drop with doubling on a spherical wavefront means the wave is gaining energy from something.
I can't tell you what the magnitude is, but I think I have seen it in "Acoustics" by Kinsler and Frey. I can look tomorrow.
Marc
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