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12th July 2007, 11:01 PM  #1 
Wizard of Kelts
diyAudio Moderator Emeritus

Which dB @1M/1Watt Constitutes 100% Efficiency?
Friend has a pair of old Cerwin Vega PD3 PA speakers which need midrangesnot just diaphragms, there is a hole where the midranges used to be. Found out the midrange on this model is the Cerwin Vega H25. Looking at the frequency response, which I found here, I noticed that this unit seems to be putting out 112 dB @ 1M/1Watt. Chart is reproduced below. Win ISD will not allow simulations above 106 dB at 1M/1 Watt. Small's chart for bass response tops out at 112 dB at 1M/1 Watt. Seems to me this chart shows the Cerwin Vega, through part of it's band, as being either 100% efficient or 400% efficient, 6 dB meaning 4 times the power. Even 100% efficiency seems far fetched, 50% efficiency is superb. Any help understanding this would be appreciated.
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12th July 2007, 11:11 PM  #2 
Speakerholic
diyAudio Moderator

KW,
According to this calculator 112dB at 1 watt translates to an efficiency rating of 1 or 100% http://www.sengpielaudio.com/calculatorefficiency.htm 
12th July 2007, 11:17 PM  #3 
diyAudio Member
Join Date: Jan 2004
Location: Toronto, ON, Canada

Omnipolar radiation?

12th July 2007, 11:45 PM  #4 
diyAudio Member
Join Date: Oct 2004
Location: Vancouver, BC

that's impossible, nothing is 100% efficient. I didn't think speakers were even close...?

12th July 2007, 11:59 PM  #5 
diyAudio Member
Join Date: Dec 2001
Location: Melbourne, Australia

Wiki. Theoretically, if you have a driver with Fs of 116Hz, Vas of 403 litres and Qes of 0.6, then you'll have a 100% efficient driver (ie. 112dB/1w/1m).
But, nature will conspire against you to ensure that this (or other) set(s) of combinations for your driver will not occur.
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13th July 2007, 12:08 AM  #6  
Speakerholic
diyAudio Moderator

Quote:


13th July 2007, 12:13 AM  #7 
diyAudio Member

This is dependent upon the radiation pattern of the device. Remember, when you measure 112 dB you are measuring the intensity (power per unit area) at that point, not the power being radiated by the device. The reference intensity I_o is 10^12 watts/m^2, and SPL is given by
SPL = 10*log(I/I_o) = 120 + 10*log(I) so if you have 1 watt of full space radiation you get SPL = 120 + 10*log( 1w / 4pi ) = 109 or in half space you get SPL = 120 + 10*log( 1w / 2pi ) = 112 If, for a rough approximation, you treat the device as if it has uniform axisymmetric radiation with an exact beamwidth of x degrees, then the area radiated over at 1 m is 2*pi*(1  cos(x/2)) so the SPL for 1 watt of radiation is: SPL = 120 + 10*log(1 / 2*pi*(1  cos(x/2)) ) = 120  10log(2pi)  10*log(1  cos(x/2)) = 112  10*log(1  cos(x/2)) A couple sample values: BW SPL 360 109 180 112 120 115 90 117.3 60 120.7 I hope this helps Nolen 
13th July 2007, 04:05 AM  #8 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

Rybaudio I started to do the same calc, but got lost right at the beginning. How do you get from the 0dB level of 0.0002 dynes/cm^2 to watts/cm^2? There's something really simple I'm missing!

13th July 2007, 05:42 AM  #9  
diyAudio Member
Join Date: Jul 2005
Location: 65N 25E

Quote:
I guess that Rybaudio was using approxmate value of 10^12 W/m2 right away as it is commonly used in calculations like this. I found following http://en.wikipedia.org/wiki/Sound_power P=(A*p^2)/Z Z is the acoustic impedance of air, at +25c about 410 (Pa*s/m) SPL 0db reference level of 20uPascal translates then to 20u*20u*1m2/410 =0.976pW/m2 which rounds up to easy 10^12 W/m2 

13th July 2007, 08:19 AM  #10 
diyAudio Member

I got that value from a reference textbook a while back and have used it so many times that I just remember it. I think it was in Kinsler and Frey's Fundamentals of Acoustics, but I'm not sure and right now most of my books are packed away in a move.

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