| The Acoustic Center: How it applies to Loudspeaker Measurements [message #68754] |
Wed, 27 July 2011 10:44  |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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I've seen a lot of misinformation spread around the internet, so I thought it might be helpful to post some links to useful information here. Hopefully this will dispel some myths.
To obtain accurate SPL measurements, one usually needs to measure at a distance and then calculate back to find an effective 1W/1M figure. If you measure too close, the value is usually way off. Also, the distance to a point source determines its delay reaching the listener, because of the speed of sound. So the acoustic center is important for that reason too.
Here are some links with more information on the subject:
Also, three AES papers on the subject:
Applications of the Acoustic Centre:
"This paper focuses on uses for the acoustic centre concept, which in this paper represents a particular point for a transducer that acts as the origin of its low-frequency radiation or reception. The concept, although new to loudspeakers, has long been employed for microphones when accurate acoustic pressure calibration is required. A theoretical justification of the concept is presented and several calculation methods are discussed. We first apply the concept to subwoofers, for which the acoustic centre is essentially a cabinet dimension away from the centre of the cabinet."
There are so many papers like this, written on the subject of the acoustic center, it's nearly impossible to reference them all. Many of them are on the differences between different cabinet shapes, like rectangular boxes, rounded-edge boxes, spheres, etc. Those shift the acoustic center by diffraction, some putting it pretty far out in front of the speaker. Other configurations put the acoustic center far behind the cabinet. All are frequency dependent, meaning the acoustic center moves with frequency.
I encourage anyone reading this to search for academic papers on the subject. Even a quick Google search turns up a lot of useful information:
The take-away from all this is the mouth or face of a loudspeaker should not be assumed to be its acoustic center. Since precision of the acoustic center position is required for close measurements, it makes sense to measure at a distance so that deviations in the position of the acoustic center have less influence.
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