Re: [TCML] Terry filters

[FIFTYGUY@xxxxxxx]

In a message dated 1/1/08 9:51:48 A.M. Eastern Standard Time,  
dgoodfelo@xxxxxxxxxxx writes:

 
>In the woofer circuit, the load, the speaker, is an 8 Ohm resistor,  and 

>forms an integral part of a 1-pole R-L network.  There is some  inductance 
in 

>the woofer, but it's predominantly resistive; otherwise it wouldn't  consume 

>power.  The choke in series with a *resistive* load does  indeed form a 6 

>dB/octave low-pass filter.  My previous statement that a  capacitor is 

>necessary to form a low-pass filter network assumed that we do not  have a 

>resistive load.


    FWIW, a woofer has a significant inductance. An  easy check would be to 
read the DC resistance of a typical "8 Ohm" woofer - it's  usually around 3 
Ohms. 
    The woofer doesn't have to be mainly resistive to  consume power. It's a 
solenoid, and the work it does is consumed in moving air  (granted, with 
terrible overall efficiency). 
    Ever look at the impedance curve of a typical  woofer? Another lesson in 
resonance! The "8 Ohms" is a nominal rating only, only  sometimes close to the 
average value over a certain range. The impedance rises  significantly with 
increasing frequency, my guess due to the inductance. At the  woofer's physical 
resonant frequency, the impedance has a sharp peak. Below the  resonant 
frequency, the woofer looks more like a pure resistance. The resonance  and 
impedance curve of the woofer can change drastically depending on the type  of 
enclosure it's used in. Obviously, the woofer will have varied responses at  
different frequencies.
    Some manufacturers have long touted their addition  of components to 
flatten the impedance curve of their loudspeakers. Supposedly  this also reduces 
the demands on the amplifier driving the speakers and is  ultimately intended 
to have tangible acoustic benefits. 
 
    Here's a site I just found that covers this  well:
 
_http://www.geocities.com/f4ier/driver.htm_ 
(http://www.geocities.com/f4ier/driver.htm) 
 
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities




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