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

["Lau, Gary" <Gary.Lau@xxxxxx>]

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.  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.  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.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] 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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