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Say you were picking a speaker for a club or auditorium The speaker is rated at 90dbw meaning an input of one watt will produce 90db at a distance of one meter. What will be the likely decibel level at ten or twenty meters? I was thinking that for an omnidirectional radiator in free space the sound intensity would fall in inverse proportion to the radius cubed but not sure how to convert that to decibels. In a real-world space you have walls, floor and ceiling and reverberation and absorbent bodies and so on. Is there a rule of thumb?
It's never too late to turn back the clock.
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This may help;
http://www.prosoundweb.com/article/far-field_criteria_for_loudspeaker_measurement/
"A very small loudspeaker might possibly be measured at 1 meter, but for larger loudspeakers it's a different story.
For large devices, the beginning of the far-field must be determined, marking the minimum distance at radiation parameters can be measured.
The resultant data is then referenced back to the 1 meter reference distance (Figure 1) using the inverse-square law."
Thanks everyone for their help! If nearfield spl falls 6 db when you double the distance is that in meters? If you measure 90 db at 1 meter then at 2 meters you get 84db and at 4 meters you get 78 db? -- until the critical point where reverberation takes over?
It's never too late to turn back the clock.
then the farther away I get the LOUDER it seems to be!OK, perhaps it's by the square of the distance, but I've never measured it.
Edits: 06/13/16
Meters, feet, miles, if you double the distance SPL drops by 6dB.
For outdoor settings its Six dB reduction for every doubling of the distance. For line arrays for the freqs within the near field it drops 3dB for doubling of distance. Once those freqs are out in the far field you are in the six dB reduction for doubling distance again. You need long line array columns to be able to reproduce LFs in the near field.Advice: For Club use get Hi efficiency speakers with more output than 90db/W. In well designed rooms with non parallel walls, adeq absorption and limited reverberation Point source speakers will provide the most natural sound. Consider room treatment and directional speakers in problem rooms with poor speech intelligibility, echoes (long rev times). In these rooms horns and line arrays may be useful + room treatment
Luck
Rafaro
Edits: 06/11/16
The answer depends on the directivity index of the speaker and the room acoustics.
Basically, if you are far from the speaker then the reverberant field of the room predominates and the sound does not change with distance. Close to the speaker it falls with the inverse square of the distance. In a typical home environment, the transition from near to far is around 1 meter. If I recall correctly, it is more like 10-20 meters in a concert hall - but there is a LOT of variation between halls.
All the above calculations are different at different frequencies; it is quite common to do the calculations for each octave.
For most rooms the transition point is less than 20 foot.
Double the distance and SPL falls 6db.
True for most speakers but not for planars and line sources where the drop is basically linear, I believe.
Planars and line sources follow the same inverse square rule unless they're at least 3 wavelengths in dimension, and even then they will revert to the inverse square rule at a distance.
Hello Bill:
Yes 3 WL lengths line arrays to keep those freq in the near field but directional line arrays like the few horn and rear cancelling designs claim to be able to reduce their length and maintain lower than expected freqs in the near field. Never really been able to understand that.
Rafaro
The straight line array can have a reduced SPL fall off, even half of a point source but this only happens when the line is long compared to wavelength.
The weakness of the design is that this is very frequency dependent so most line arrays are curved to make them more like an astigmatic point source instead which reduces the spectral balance change with distance, while reduced, the interference pattern they produce imposes a limited working distance requiring "delay rings".
A true point source in a large horn can have much greater throw, have little or no distance related spectral change, sounds better, is more intelligible and is smaller, with the down side of being a smaller sale for the dealer.
+1
Point source horns over line arrays.
Hello Tom, Dennis and group:
Walking within the nearfield of a line array I can appreciate the stable sound pressure but always get the feeling of a synthetic nature to the sound. The chaotic nature of the sound reconstruction within it to me its audible and distracts from the performance. This is understandable considering that we evolved our hearing listening to point sources and as such find them sounding natural. The popularity of line arrays seems more to do with the ease of mounting and dismounting them and ease in providing SPL coverage but at the expense of a natural stable sound spectrum. Your designs Tom are based around your patents with innovative point source horns. I hold your designs in the highest regard. My interest in speaker design started after getting "dumped" the Job of setting up the sound system in addition to playing bass in the band. By necessity I rapidly evolved to high efficiency compact but expandable exponential horn designs. I have enjoyed the learning experience perfecting the designs and I am happy with results but I donīt play or even do Gigs anymore. Anecdotally it does seems that line arrays with directional speakers expo horns in my case can produce lower than predictable freqs using shorter columns.
Regards
Rafaro
" I believe."
hahax
(I think not)
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