[TCML] Terry filters (speaker/motor load modeling)

[Bill Lemieux]

On Jan 2, 2008, at 8:08 AM, Lau, Gary wrote:

> Hi Bill,
>
> I don't wish to go too far off on a speaker-modeling tangent,  
> especially since this is not my area of expertise, and the full  
> answer is more than the bandwidth of this forum will tolerate.  But  
> since correctly modeling components and loads IS on-topic for this  
> forum, here goes.
>
> First - speakers don't "do" work; they consume work, or power. I'm  
> guessing that's what you really meant.

No, what I meant is that a speaker, like a motor, consumes electrical  
power to do mechanical work.
The work being performed, and the reason large amounts of power are  
required to drive a woofer, is because the woofer is moving air - lots  
of air.  A woofer is just a linear motor.  Even if the voice coil were  
wound with  superconducting wire, forcing the voice coil to move  
against MECHANICAL resistance would still require power.

>  If I were to model a motor or speaker UNDER LOAD in Spice, it would  
> need to be predominantly resistive.  Inductors don't consume power.

>  This is important to understand and why dummy loads for speakers  
> and antennas must be only resistive.
>
> Consider an amplifier driving an 8 Ohm resistive speaker load.  Now  
> place a perfectly lossless, non-saturating, superconducting 1:1  
> transformer between the speaker and the amplifier.  The load looks  
> no different to the amplifier, even though it is now driving a 0 DC- 
> Ohm primary inductor, and the combination of the transformer and 8  
> Ohm load will still be modeled as just an 8 Ohm resistor.  But if  
> the speaker was ideal and suddenly found itself operating in a  
> vacuum, the "work" of creating sound waves would cease and the 8 Ohm  
> load would cease, and the amplifier would see no load.
>
> My statement was simply that if one substituted a simple 0 DC-Ohm  
> inductor for a speaker, the inductor would consume no power - it  
> can't get hot.  With a real speaker, it's the vibrating air that  
> gets hot.  Only resistors consume power.

Your next statement contradicts that.

> A superconducting motor under a 1 HP load, even though it measures 0  
> DC Ohms and some significant inductance under static (no load)  
> conditions, must be modeled as predominantly resistive to reflect  
> the 1 HP load.  The resistor value would change depending on the  
> magnitude of the mechanical load.  If there were no load, after the  
> rotor accelerates, the resistive component would go away.

I think we understand each other.
Anyway, I'm done.

> Regards, Gary Lau
> MA, USA
>
>> -----Original Message-----
>> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On
>> Behalf Of Bill Lemieux
>> Sent: Tuesday, January 01, 2008 3:58 PM
>> To: Tesla Coil Mailing List
>> Subject: Re: [TCML] Terry filters
>>
>>> Hi Dave,
>>>
>>> Your analogy about using just an inductor for a woofer low pass
>>> network made me think hard for a moment about my post.  You are
>>> exactly right about the requirements for 6/12/18 dB/octave filters.
>>> But the woofer situation is very different than NST protection
>>> networks and I maintain my position. Allow me to explain.
>>>
>>> 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.
>>
>> What?!?  Hang on a minute.  The woofer is doing _work_!  Your
>> statement suggests that a motor wound with superconductor (all
>> inductance, no resistance) and delivering 1 HP to a load would  
>> consume
>> no power!
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