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Radiating Power at Low Frequencies

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Jim Reid


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Radiating Power at Low Frequencies

PostMon Oct 04, 2010 9:33 pm

One acoustic watt is a loud sound when radiated into any home room.
A 12" speaker (typically a 10" diameter moving surface) Must generate
an excursion of 0.06 inch (peak amplitude of 0.03 inch) to radiate one
acoustic watt. This is about the limit of excursion tolerable non-linear
distortion. I believe a good many speakers might be able do this down
to about 65 Hz. - 8' C or so. That would be a very loud sound!

At just a little lower in pitch, 60 Hz., the cone must move 0.7 inch, p-p ;
power radiated at our estimated distortion limit of 0.06 inch, 0.0072 watt.

At 50 Hz., 1.0 inch, p-p; power from .06 inch, 0.0036 watt.

At 40 Hz., 1.5 inch, p-p; power from 0.06 inch excursion, 0.0016 watt.

At 35 Hz., 2.0 inch, p-p; power from 0.06 excursion, 0.00085 watt.

So, the high compliance speaker (very "loose" suspension ring material) was
developed back in the 1950's (by Acoustic Research, and soon imitated by
KLH and others. Even Allen Organ Co. soon picked up on the design).

Also the reason the top performing subs by Definitive Tech have one to four
added "passive" radiators whose cone excursions add to the primary driven
cone(s). I have no idea the peak to peak excursions Definitive's design provides.
It must be amazingly large! Largest cone diameter used by Definitive is
14 inch; do not know the radiating surface diameter. The passive Definitive
are the same cone diameter as the driven cone.

However, I have yet to figure how the back wave compression of the driven
element causes the passive radiators to be "in phase" with the driven cone(s).
Jim Reid

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