Speaker Impedance

I have one test instruments to test with, a VOM.

I have several old speakers that I want to hook up to a amp I am trying to build.

How do I determine the impedance of the speakers with only a VOM ?

What value of resistance would I measure across the speaker coil for different speaker impedance of say 4 or 8 or ? and other ohm speaker impedance ?

Is there a table someplace ?

TIA

Reply to
OGee
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** The "nominal impedance" of a moving coil speaker is 20% higher than the coils's DC resistance.
** No need for one.

.... Phil

Reply to
Phil Allison

Resistance will always be a few ohms or less, WAY below "impedance" which is literally a complex function of frequency , amplitude, loading, and speaker environment.

One "preferred" measurement method was to use a 1KC signal; one point of thousands; totally misleading and almost useless.

Just connect each one in its own preferred environment (box, baffle, boffle, horn, etc) and listen to it where you would like to place it (living room, bedroom, car, concert all, etc.).

The ear is one of the better test instruments.

Reply to
Robert Baer

It isn't always way below the complex impedance. The resistive component at DC is usually as low as it ever gets and the frequency dependent reactive components add to it. It shouldn't depend on amplitude unless you are saturating the magnetic components or about to blow the fuse.

If you measure it on DC and round up to the next power of two in ohms you won't be that far off the nominal impedance. It does vary a lot with frequency depending on the crossover network and drivers used. The OP might find the Wiki entry on loudspeakers nominal impedance helpful:

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When you look and the |Z| and phase response of some hifi speakers it is surprising that they work at all. B&W 802D a case in point - nominally a

8 ohm speaker with excursions 20ohm around 2-3kHz crossover.

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Using a continuous tone sine wave at real power into a loudspeaker is a good way to kill it if you hit a resonance. Real speaker tests are done by sweeping sufficiently quickly not to allow a resonance to build up or more usually with narrow bandpass pink noise.

The ear is surprisingly easily tricked.

An oscilloscope or spectrum analyser is much less easily fooled.

--
Regards, 
Martin Brown
Reply to
Martin Brown

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( snip some metal patient's garbage)

** You need to explain the "next power of two" bit - cos it reads like pure gibberish.

The correct procedures are to find the lowest, resistive impedance with a sine wave test in the mid frequency band OR just add 20% to the DC reading.

This for a cone loudspeaker driver tested *on its own* - NOT a system or some other contraption like a horn driver.

** You have posted this drivel before, it is still wrong and always will be.

.... Phil

Reply to
Phil Allison

At resonance, impedance rises quite a lot, much reducing power draw. Speakers don't have enough in-band resonance to destroy themselves, or they'd not be reliable or acceptable sounding in use. Excluding outliers like 1920s moving iron speakers etc.

NT

Reply to
tabbypurr

I've got two speakers in my lab. small '32 ohm' Resistance = 30 ohms. Car size speaker, '8 ohms', R = 7.3 ohms.

I know little of audio stuff, but I'm thinking Phil A's 'rule of thumb' of adding ~20% to the resistance measurement is sound advice. :^)

George H.

Reply to
George Herold

What is "car size"? 4000lbs? ;-) Car speakers aren't 8-ohms, rather

2-ohm (that V^2/R thing), usually.

Reply to
krw

Looks fairly tame. It's moving iron speakers your statement does apply to, but even they can produce fairly clear speech.

NT

Reply to
tabbypurr

Grin... It says 5210MWG midrange on it.

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That makes sense, I said I know little of audio. :^)

GH

Reply to
George Herold

Why do you care? Stated another way, "What are you gonna do with the information when you get it?"

Isn't the impedance going to be critically dependent on the enclosure? And to a lesser degree, the acoustics of the room? 40 years ago, I experimented with trying to get a decent sounding speaker in a pickup truck and ended up using an amplifier with negative output impedance.

Reply to
mike

With a "12 V" car battery voltage you can get about 4 Vrns output voltage from a half bridge, i.e. 0.5 W into 32 ohms or 8 W into 2 ohms. Some full bridge ICs claim 22 W into 4 ohms, requiring 14.4 V battery voltage (alternator maximum output voltage).

With a DC/DC inverter power supply, the voltage swing and speaker impedance can be more freely selected.

At least the DIN standard specifies that the actual impedance at any frequency should be at least 80 % of the nominal impedance, agreeing well with Phil's rule of thumb.

Reply to
upsidedown

Martin is speaking perfect english. Has to do with powers of 2.

Low-ish impedance speakers have typically been rated at an impedance that happens to be a power of 2... Like 2, 4, 8, 16, 32 Ohms.

boB

Reply to
boB

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** Wellll blow me down !!!

That trick works even for 1 ohm speakers, cos 2exp0 = 1

But is purest sophistry.

Nominal 6 and 12 ohm speakers are common too, it don't work for them.

The "add 20%" rule is based on loudspeaker physics and always works for cone drivers that have impedance minima in the band from 250 to 400Hz.

..... Phil

Reply to
Phil Allison

5 ohm used to be very common fwiw. And before that 2k-ish.

NT

Reply to
tabbypurr

I used to see 3.2 ohm speakers, as well. That was back when the US still used ninnyfarads.

Reply to
Michael A Terrell

one drivers that have impedance minima in the band from 250 to 400Hz. "

Sounds like a relatively crude approximation.

Also, correct me if I am wrong, that impedance minima is still higher than the DC resistance I assume. I would find it very hard to believe there is e nough EMF to make it go lower, but stranger things have happened. Like the tunnel diode that should not work.

Reply to
jurb6006

The impedance of any device must be (equal to or) higher than it's DC resistance or it's a battery.

Why should a tunnel diode not work? Both the tunnel diode and speaker will have "negative (incremental) resistance region(s)" but there is nothing untoward about that.

Reply to
krw

OK, you got the arithmetic right(ish). The typical car amplifier is more like 50W into a 2-ohm speaker.

Obviously. What do you think a 400W boost regulator costs? What does it add to the cost of the car. What's the market? ...just to "freely select" speaker impedance.

If more power is needed, 1-ohm speakers or dual voice-coil speakers (using two amplifier channels driven from the same source) are a lot cheaper.

Reply to
krw

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** A comment about "jurb" and not the physics of loudspeakers.

** Believe me - YOU are wrong.

.... Phil

Reply to
Phil Allison

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