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[TCML] Speakers - was: Terry filters (speaker/motor load modeling)

Hi Bill, Gary,

I can see what you're saying. A woofer does move air at generally <100 hz
albeit a pretty inefficient "motor". I'm sure most of us have actually felt the air
move while standing in front of a powerful stereo speaker when the music is
really cranking, so air is obviously being moved. When you started talking
about several thousand watts of amplification, the output sound will actually visiblly blow your hair, like standing in front of a fan. BTW, does anyone recall
the Bose 901 series speakers? They were pretty popular back in the 1980's.
I assume that they were the standard 8 ohm impedance type and it seems like
I heard someone say that as part of Bose's advertisement campaign for these
speakers that they actually connected the input leads of a Bose 901 directly to a 120 volt outlet and the speaker did not blow. Assuming the 8 ohms to be the correct resistance that the 120 volts, 60 Hz AC saw, that means that the speak-
er would have been processing 1875 volt-amps (not sure what the power factor
would be in this situation) of sound power! Sorry to get a little OT here, but has
anyone else heard this "story"?

David Rieben

----- Original Message ----- From: "Bill Lemieux" <gomezaddams@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, January 02, 2008 11:35 AM
Subject: Re: [TCML] Terry filters (speaker/motor load modeling)

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

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