Basic speaker Q

[Phil Allison]

Hi to all,


everyone knows the nominal impedance of a cone speaker is slightly greater
than the DC resistance of the wire used in the voice coil. An nominal 8 ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

Seems from recent postings on other NGs that most folk have this one
wrong.



........... Phil

 

[Lord Garth]

Hi to all,


everyone knows the nominal impedance of a cone speaker is slightly greater
than the DC resistance of the wire used in the voice coil. An nominal 8 ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

Z=(R² + (2*PI*F*L)²)**½

Z is the impedance in Ohms
R is the DC coil resistance in Ohms
F is the frequency in Hertz
L is the inductance in Henries

You can see the impedance depends upon frequency.

 

[Phil Allison]

Z=(R² + (2*PI*F*L)²)**½

Z is the impedance in Ohms
R is the DC coil resistance in Ohms
F is the frequency in Hertz
L is the inductance in Henries

You can see the impedance depends upon frequency.

** Unfortunately, that is the popular but wrong answer.



.......... Phil

 

[Grog]

Hi to all,


everyone knows the nominal impedance of a cone speaker is slightly greater
than the DC resistance of the wire used in the voice coil. An nominal 8 ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

Seems from recent postings on other NGs that most folk have this one
wrong.



.......... Phil

I'll have a go!

Coil in a magentice field, jam some volts on afformentioned coil and it
moves.
But moving coil within said magnetic field makes some volts of it's own.
If jammed in volts is outa phase with generated volts then :
Speaker volts = jammed in volts - generated volts.
This makes it look like a higher resistance to the voltage jammer in device.

So it would seem that the speaker impedance is not a function of the coil
inductance but more a function of the number of turns of the coil, the
strength of the magnetic field and how fast the coil is moving.
Add in a bunch of other factors like cone excursion, magnetic saturation
and a few other power transformer terminologies which are beyond the
scope of this document and call it "nominal" impedance.

Sounds good to me..
Greg the Grog.

 

[Mark Hathaway]

well, are we going to assume a frequency independent or dependant model?


mark hathaway

 

[Phil Allison]

"Mark Hathaway"

well, are we going to assume a frequency independent or dependant model?

* The nominal impedance of a cone speaker is measured in the mid band -
usually at 250Hz or 400 Hz.

I am not concerned with fake nominal values that are not measurements.

If you test a speaker at 250 - 400 Hz you get a value that is about 20 %
higher than the DC ohms - why ?



............ Phil

 

[Mark Hathaway]

reactance of the assembly

"Mark Hathaway"




* The nominal impedance of a cone speaker is measured in the mid band -
usually at 250Hz or 400 Hz.

I am not concerned with fake nominal values that are not measurements.

If you test a speaker at 250 - 400 Hz you get a value that is about 20 %
higher than the DC ohms - why ?



........... Phil

 

[Phil Allison]

"Mark Hathaway" <

reactance of the assembly

** Huh ?

Is this like a hostile senate ?




........ Phil

 

[Alan Rutlidge]

** Unfortunately, that is the popular but wrong answer.



......... Phil

Thanks Phil for enlightening us that it was the "wrong" answer, so how about
sharing your pearls of wisdom and telling us the correct one?

So far the formula above implies that the impedance will vary with frequency
which sounds pretty plausible, but I guess from your comment some or all of
the formula is incorrect. Your learned explanation and the correct formula
would be appreciated.

Cheers,
Alan

 

[Phil Allison]

"Alan Rutlidge" <

Thanks Phil for enlightening us that it was the "wrong" answer, so how about
sharing your pearls of wisdom and telling us the correct one?

So far the formula above implies that the impedance will vary with frequency
which sounds pretty plausible, but I guess from your comment some or all of
the formula is incorrect. Your learned explanation and the correct formula
would be appreciated.

** The real answer is simple and not a formula - someone will post it
soon.





......... Phil

 

[the swan]

Hi to all,


everyone knows the nominal impedance of a cone speaker is slightly greater
than the DC resistance of the wire used in the voice coil. An nominal 8 ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

electrical impedance + acoustic impedance

 

[Phil Allison]

electrical impedance + acoustic impedance

** The electrical impedance at 250 or 490 Hz is all we are able to
measure.

The DC resistance is part of it - the extra bit is due to what
exactly?





........... Phil

 

[Mark Hathaway]

not hostile at all.


the equation for Z is as follows


Z = (R^2 + X^2) ^ (1/2)

 

[Phil Allison]

not hostile at all.

the equation for Z is as follows

Z = (R^2 + X^2) ^ (1/2)

** Lord Garth beat you to that one hours ago.



........ Phil

 

[Mark Hathaway]

popular but wrong hey?

I'd love to see your source, or is this your own theory?


I checked my answer against a few publishments, I'd love to see you
contradict them.



Mark

 

[geo]

Hi to all,


everyone knows the nominal impedance of a cone speaker is slightly greater
than the DC resistance of the wire used in the voice coil. An nominal 8 ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

Seems from recent postings on other NGs that most folk have this one
wrong.

back emf?

ross

 

[Phil Allison]

"Mark Hathaway" <

I'd love to see your source, or is this your own theory?
I checked my answer against a few publishments, I'd love to see you
contradict them.

** Go ahead - post them.

Make my day......



.......... Phil

 

[Brian Goldsmith]

everyone knows the nominal impedance of a cone speaker is slightly
greater
than the DC resistance of the wire used in the voice coil. An nominal 8
ohm
speaker will have about 6 or 7 ohms of DC resistance for example.

Where does the extra 1 or 2 ohms come from ?

***** Skin effect,Brian Goldsmith.

 

[the swan]

** The electrical impedance at 250 or 490 Hz is all we are able to
measure.

The DC resistance is part of it - the extra bit is due to what
exactly?

I am intrigued as to how "nominal impedance" is measured?
I am only guessing that it is the impedance of the driver when mounted
on an "infinite" baffle with a generator (amplifier) output impedance of
0 ohms.
The total acoustic resistance is then:

Rat = Ras + (((B.l)^2)/(Re.Sd^2)

where: Ras = acoustic resistance of driver suspension
B = magnetic flux density in driver air gap
l = length of voice coil conductor in magnetic field of air gap
Re = DC resistance of voice coil (the "6 or 7 ohms")
Sd = effective projected surface area of driver diaphragm

But then again, it could all be bollox

 

[Phil Allison]

"the swan" <

Phil Allison wrote:

I am intrigued as to how "nominal impedance" is measured?

** At a frequency of 250 to 400 Hz one simply measures the ratio of amps
to volts with a sine wave. This can be by use of a series low ohms shunt
to allow the determination of amps *or* a voltage source fed through a
known series resistor and the speaker and then the ratio of voltages
determined by AC meter reading.

The ratio of impedances is the same as the voltage ratio.

As already said, I am not concerned with the fake nominals ( 8 or 16)
quoted by some speaker makers.




.......... Phil