[sdiy] Speaker Impedance

[Bob Weigel]

harrybissell wrote:

>Bob Weigel wrote: <snip>
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>> NOW....with the
>>resistor you have the equivelent of an 8 ohm speaker with 7 ohms of DC
>>resistance that is only making heat..no sound. So now it's half as
>>potentially efficient roughly.
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>The "DC resistance" of the speaker is not just 'making heat'... the cone will
>be displaced by some amount from magentic field.
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Obviously...you aren't reading my grammar as intended.  If there was 
ONLY a DC resistance...no sound.  On the other hand, let's say we have 
this speaker with the 7 ohm dc voice coild and 8 ohm overal impedence 
rating with signal ok?  Now... let's build a very buildable coil which 
has exactly the same number of windings but less DC resistance by use of 
a more efficient winding style or using flat wire for instance like 
Peavey Black WIdow uses for instance.  Except..let's use the same 
dimensions.  OK?  SO...now you answer me and I'll see if you have the 
background to make this worthwhile to continue.  If you have TWO speakes 
with EXACTLY the same number of turns in the same spacing, but..one has 
less DC resistance than the other (because square cornered wire fills up 
the space with more charge carrying material...) AND the same diameter 
in the same magnetic field.....will or will they not both create the 
same amount of sounds if the same basket/suspension is attached????  
    Ok if you answered that they make the same amount of sound...then we 
can logically conclude that the DC resistance measured in the coil does 
NOT contribute to doing physical work that creates sound.  Only heat.  A 
perfect conductor if one existed would have no dc resistance but in the 
same configuration would make the same sound because ALL OF THE WORK 
BEING DONE IS A RESULT OF MAGNETIC FIELD INTERACTION.  And this is 
purely a result of the wire's orientation to the fixed magnetic 
structure around it. All that to say, exactly what I said before.  The 
DC component of the resistance only creates heat.  And it will 
regardless of the frequency of the signal up to the point where the 
conductivity properties of the wire start to shift because of the tiny 
amounts of distance that electrons need to be moving to constitute the 
same current at that frequency :-).  And we're talking some real high 
frequencies to start to see an actual reduction in any material I'm 
aware of.  Hence...for near audio frequencies the DC resistance is 
constantly an I^2*R loss TO HEAT/Radiation/MELTDOWN or however far it 
gets taken :-).  NO sound until it sizzles that results FROM THE 
PHYSICAL PROPERTY THAT WE ARE PERCEIVING WHEN WE MEASURE DC RESISTANCE.  
Is that more clear?   IE.  (I'll say it one more way just to be sure)   
TAKE the voice coil...ok?  Straighten it out. Put it in the same 
magnetic field but allign it so that the field is paralell to the wire!  
No force.  Charges only experience forces from perpendicular components 
via the 'right hand (or left for negative charges) rule'.  Right?  Yet 
the same DC resistance is there and it's a DC resistance that YOU said 
was MAKING MORE THAN HEAT!  :)  See it's not.  It's still there. The 
exact same DC resistance.  But it just makes heat.  Only the inductive 
impedence makes sound.  Ok man I have to explain things so thoroughly 
lately.  Hope something is making sense cuz..I'm dealing with these ebay 
people on the other side of the block here...

>Trouble is, the DC resistance of the 8 ohm speaker is WAY less than 8 ohms...
>it could be as low as 2-3 ohms.
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What speaker?  Dude I've been doing this work for a lot of years and 
I've never seen such a speaker.  I've designed many speaker systems and 
have tested hundreds and hundreds of speakers while comparing the DC 
reading to their ratings.  The most efficient speakers on the market in 
the woofer category measure around 5 ohms for an 8 ohm speaker.  It's a 
trick to do that and if you'd ever designed a voice coil you'd know why.

>Most people try to avoid providing DC for the two reasons... it makes the cones
>move off-center... sometimes so much so they leave the enclosure. 
>
Hhehe.  I try to make sure that i ONLY discharge 450V 7000uF capacitors 
through them to make sure they're up to the tas....JUUUUUST KIIDDDING!  
Man we're talking about probing the speaker with  micro or milliamps!  
You can't  even hear it..I don't know how we got on that but oh well. 
nuff time on this one.  -Bob

> The wattage
>of the DC resistance is WAY over what the speaker is designed to handle.
>
>I agree... half as efficient.  Two speakers in series is better. If you can
>afford to
>lose power, the series resistor will work OK as well.
>
>Another solution would be to take a speaker matching transformer (like a 70V
>line unit) that is good for the power... and connect the amp to 0 - 8 ohm taps
>and
>the speaker to the 0 - 4 ohm taps (autotransformer).   This will NOT waste any
>power
>... but the transformer is going to be quite expensive !!!    (maybe more than
>the
>right, or a second speaker...)
>
>H^) harry
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