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

[David Rieben]

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 at gmail.com>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
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
>> MA, USA