[sdiy] Speaker Impedance

[Bob Weigel]

PS and to get another good diy point in, I fully agree you can't tell 
necessarily what the impedence is by taking a DC / ohmmeter 
measurement.  Unless..you know the manufacturer only issued speakers 
under certain impedences (4, 8 or 16 ohm let's say) NOW I am NOT SURE 
how precise these ratings really are across the board.  Some 
manufacturers I'm sure are fairly precise.  Others might generate a 
design that works well and is a compromise of some kind and they go 'eh, 
close enough. It's really 6.9ohms but we'll call it 8 ohm just to avoid 
confusing the marketplace.   I mean..seriously that might happen.  I 
have never tried taking speakers in the lab and really working the 
numbers to see how close various ones get to those ratings but, I 
haven't been able to find that an amp is consistently getting fried by 
using an alternate speaker or anything like that so I don't think it's a 
factor.  As I say, another benefit of being in Bozeman, MT.  If it 
breaks..they almost always bring it back to me.  There are no chronic 
problem amps out there that I've done speaker swaps on just fyi and I've 
done many.
       Then you absolutely in every case I have encountered ranging from 
home stereo speakers from dozens of mfgs (though I know only a few 
people probably made their speakers; Eminence, JBL, etc.) to pro PA 
cabinets (EAW, Peavey, EV, JBL, etc.) to guitar speakers by the 
masses.... you just put the probes on them.  If it measures around 4 
down to 2.5 ish then it's what they are calling a 4 ohm speaker.  If it 
measures from 8 down to I think like 4.8ohms it's what they call an 8 
ohm speaker.  15 down to 11ohms is a 16 ohm speaker in all the cases 
I've witnessed. 
      I'm so experienced with this because when someone brings in an 
combo amp or speaker for repair, you just immediately probe that speaker 
if it has any kind of problem with output or lack of signal.  For 
example.. If you blew outputs for instance in a combo amp, you want to 
check that speaker because it might have also gotten damaged in the 
event.  You check the voice coil excursion and listen carefully for 
roughness caused by bubbling of the enamel coating.  You measure the 
impedence to be sure it didn't go open or even start to short 
somewhere.  A keen technician knows within a couple tenths of an ohm 
what common speakers they work on will measure.  Every once in a while 
you'll see something that you haven't worked on and it may vary so far 
that you can't be sure if it's good or not from the DC measurment 
but..believe me in my line of work it's just really a useful thing to 
do. -Bob
   

harrybissell wrote:

>Hi Bob
>
>Ok let's try it another, another way. Essentially we agree except for
>the 'why' of it.
>
>Take the 8 ohms speaker. In this case I'll even say that the DC resistance and
>the impedance are both eight ohms.
>
>The resistor is a pure resistance, and is eight ohms.
>
>The resistor and speaker are receiving the same voltage (current is equal)
>
>The resistor (because it is essentially non-inductive in this case - think carbon
>composition) dissipates all the power as heat.
>
>The speaker, because of its design (as a coil) dissipates some power in making
>a magnetic field and moving off-center, and holding that position against the
>spring forces that would like to re-center it.  The rest is dissipated as heat.
>
>The speaker is doing 'some' of its job... otoh its kind of stupid because the
>function is to move air... and it is not doing that. And our hearing does not go to
>DC either.
>
>The "DC" resistance is not really that important to us, except that as you noted...
>we wish it was low or non-existant.  Its not... too bad for us.
>
>The "impedance" is the real object... because it expresses how much work we can
>get out of the speaker at some real (hopefully perceptable) frequency.  We will also
>
>be careful to band-limit our input so that we don't make waste heat for freqeuncies
>we can't / don't want to hear.
>
>The DC in the resistor produces only heat.  The DC in the coil produces heat
>and displacement.  Since we don't hear it (the frequency is infinitely low...)
>it produces no sound.
>
>Maybe we can agree, one is useless... and the other one might as well be.
>
>My comments about DC resistance are for a broad range of speakers, including
>tiny little things you would not drive with a guitar amp.  The point is that you
>can't
>take an ohmmeter reading and tell what the 'impedance' is.   If the speakers have
>different DC resistances, but the same impedance at a frequency, they will
>draw the same current and produce the same 'power'... how much displacement
>that causes is more mechanical efficiency that electrical.  At any rate, the
>impedance
>cannot be lower than the DC resistance.
>
>And I might add I've been tempted to hook up speakers to one of my
>inverters.  Do you have enough for 840KW ???   (loud, yes :^)
>1200A at 700V...  1KHz.  Rock and Roll ???   :^P
>
>H^) harry
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