speaker ohms

[1tube_ampnut]

Hello, im wondering if anyone knows why im reading 23 ohms on my speaker, is it a 24 ohm speaker, 32 ohm speaker.  It is out of a 4x10 cab one speaker is dead and want to start looking for a replacement...4x10 series. 
Thank you

[stingray_65]

What do the other speakers read?

What was the impedance of the cab?

and what caused the speaker failure?

are you measuring the speaker in circuit?

and exactly what make and model cab do you have?

quite a few things could be at play here, without a few concise details all I could do is throw out a few guesses at best (I'm not a good guesser by the way)

OH! and what kind of meter are you using?

Ray

[HotBluePlates]

Hello, im wondering if anyone knows why im reading 23 ohms on my speaker, is it a 24 ohm speaker, 32 ohm speaker.  It is out of a 4x10 cab one speaker is dead and want to start looking for a replacement...4x10 series. 
Thank you

If the speaker says "32 ohms" then you're close enough. Unless you have a reason to think the speaker is damaged, your reading is perfectly reasonable.

It my understanding that the resistance on a speaker will read about 70% if read with VOM. 

No, it will read something less than the marked impedance, and that's about as close as we can reasonably guess.

Impedance (measured in ohms) is the combination of resistance (measured in ohms) and reactance (also measured in ohm, but with a phase angle). Pure reactance is 90-degrees out of phase with pure resistance, either +90 or -90 degrees.

When you combine resistance and reactance, the individual quantities represent 2 legs of a right triangle, and the resulting impedance is the hypotenuse. Now you get to recall the Pythagorean Theorem from grade school, and take the square root of the sum of the squares of the two sides.

You can combine a lot of different pairs of numbers, and still arrive at the same total impedance.
7Ω resistance and 3.87Ω reactance -> 8Ω impedance
6Ω resistance and 5.29Ω reactance -> 8Ω impedance
5Ω resistance and 6.24Ω reactance -> 8Ω impedance
4Ω resistance and 6.92Ω reactance -> 8Ω impedance
3Ω resistance and 7.41Ω reactance -> 8Ω impedance

This said, we can guess the resistance might be more than half the total impedance, only because the speaker's coil is made of wire which racks up resistance pretty quickly compared to how fast its  inductive reactance increases.

[1tube_ampnut]

Hi Guys, the speaker is out of an acoustic 134 combo amp,  I've read that these amps were rated at 4ohms. I'm using a cheap vom but will check the other speakers with a good fluke meter.   If I remember correctly the other speakers were reading 21-23 ohms individually.   The amp has no ohm rating on it so I can't awnser that question.  Thank you

[PRR]

> Impedance (measured in ohms) is the combination of resistance (measured in ohms) and reactance (also measured in ohm, but with a phase angle).

What is that reactance?

In fact there are three simple reactances and an acoustic impedance which is not a simple reactance.

And the way speaker impedance is usually understood, it is nearly a no-reactance problem.

Coming up from DC to low-low-frequency, cone motion is restrained by stiffness, there's no back-EMF, the impedance is essentially the DC resistance.

At a few dozen Hz the shorter excursion means stiffness has less effect, back-EMF rises. This is reflected as an inductance.

Above 100Hz we run into speaker mass. This reflects as a capacitance.

In between there is a resonance limited by mechanical Q which may be 5 or 10. Resonant impedance is that much higher than either L or C reactance.

Above 147Hz the mass capacitance falls to less than DC resistance.

Above ~~500Hz there is some component of Acoustic Impedance reflected back through the motor. For most cone speakers this is nearly zero. In the highest-efficiency horn drivers this may be reflected as a series resistance similar to DC resistance.

Way up above 2KHz (where music power is declining) the voice coil self-inductance makes impedance rise. This is a pretty crappy inductor so it does not rise double-per-octave like an ideal inductor.

The graph is all over.

So where to give a single-number impedance spec? The usual rule is the middle of the middle valley, typically around 400Hz. Here the mass reactance is small and partially nulled by coil reactance over a couple octaves. It is nearly pure resistance. And the impedance will not drop below this value over the useful range of the speaker.

Here (below) is a 12" Eminence. DC resistance is given as 6.45 ohms, just off the bottom of the chart. Resonance at 105Hz. Inductance given as 0.6mH but this may not include iron-loss.

It is 8r at 400Hz or 500Hz, 7.5r to 9r from 200hz to 600Hz, 7.5r to 14r from 160Hz to 1.6KHz. It varies only 2:1 over 10:1 (3 octave) of frequency, opposed to 10:1 for a pure reactance. It isn't very reactive.

The high impedance at resonance and treble are intrinsic. The only way to reduce them is to add losses and reduce efficiency. It is left to the amplifier designer to accommodate these. There is a whole chapter in Radiotron 3rd. The peak at resonance may be beneficial, it reduces the strain on the amplifier.

For cone-type speakers, the valley impedance will generally be DCR plus 10%+10% small overlapping reactances plus few-% acoustic loading. Multiply DCR by 1.2 or 1.3 and assume the nearest standard impedance.

[1tube_ampnut]

I found these numbers 67 7503, which i think are eminence made in 1975.  the other numbers are 13 15