Resistance of car sub-woofer

[audioguru]

My son bought a used car sub-woofer. It is a Pioneer 8" Bandpass one and on the web it is listed as 250 Watts Peak Music power and 80 watts Nominal. It is rated at 4 ohms. It is supposed to have a neoprene surround.

But its surround appears to be cheap foam. Neoprene foam?
Its cone is stamped Pioneer.
It measures 0.9 ohms DC on my good Fluke meter when my meter's lead resistance is subtracted. It measures 1.2 ohms DC on my cheap digital meter.
It moves in both directions when it is driven from a 1.2V battery cell.

I refuse to test it with my amplifier because its resistance measures way too low.

What do you think?

 

[microtexan]

What should be the DC resistance on the speaker?

 

[PatM]

The impedance is 4 ohms.
This is not a DC resistance.
2 different animals.
The actual impedance of a loudspeaker near resonance is usually much higher than its dc resistance.

 

[Noggin]

DC resistance is little too low. DC resistance of a subwoofer is usually a bit lower than their listed impedance, so I'd expect a 4 ohm sub to be about 3 ohms. Is it possible that its dual voice coil, 4 ohms each, and they're wired in parallel? Otherwise, the voice coil could have a short in it, not unheard of. Then again, maybe not. If you can get a spec sheet for it, it may actually list the DC resistance.

People generally get cheap low power speakers with high "music power" ratings and put them on crap amps and turn the gain way the heck up and clip the hell out of the music.

 

[kchriste]

I'd put a 4Ω resistor (Or a known good spkr) in series with it and see how it performs. Then try it direct to the amp if satisfied.

 

[audioguru]

For hundreds of "8 ohm impedance" speakers I have measured 7 ohms DC.
For many "4 ohm impedance" speakers I have measured 3.2 ohms DC.
But this "4 ohm impedance" speaker measures almost a dead short DC.

The voice coil of a speaker is a coil of wire that has a fairly high resistance. The impedance should be a little higher than the resistance. Or you could say that the resistance should measure a little less than the impedance. But not this low. I think something is wrong with this speaker.

 

[crutschow]

Some car subwoofers are 2 ohm to allow the amp to deliver more power from a 12V supply. Perhaps the unit you have is such a unit that's mismarked.

 

[tcmtech]

Just for curiosity sake I just measured a pair of 12 inch 4 ohm 300 watt pioneer speakers I have and they are by resistance measurement about 2.7 ohms.
I bought them new and have always had them ran off of good quality amplifiers.

I never knew that until now. I just assumed they were always around 4 ohms. Hmmm...

Yours could be a mislabeled 2 ohm unit or if used there is that fair likeliness that its partially shorted. Or the possibility its a foreign knock off of a pioneer and not actually a real pioneer speaker. All of my pioneer speakers have good quality rubber one them. I have never bought one that had anything less.

 

[audioguru]

About 15 years ago I made some speakers using Pioneer 8" woofers that had foam surrounds and cost $23.00 each. They were rated at 60W RMS. A few years ago the surrounds decomposed. It is commonly called, "foam surround rot".

 

[Nigel Goodwin]

Why not just measure the impedance of the speaker instead of the resistance?, this will clear it up.

I would fully agree that the resistance of a 4 ohm speaker is usually 3 ohms, and an 8 ohm 6-7 ohms.

As for Neoprene, it is a type of foam, used in wetsuits.

 

[grim]

of course it's not just the impedance and resistance to think of, what about the power?

if you take some 4 Ohm speakers, of different power ratings, they must have a different resistance, because a 100W speaker will be wound in much larger gauge wire than a 10W speaker.

if you are basing the resistnace figures from experience of 20W systems, then they aren't relevant to a 250W system.

 

[audioguru]

I have an 18" speaker rated for a very high power. Its impedance is spec'd at 8 ohms.
I have a little 3" low power speaker from a clock radio. Its impedance is also spec'd at 8 ohms.
Their resistance measures 7 ohms DC for each of them.
Of course the wire size for their voice coils is much different.
Their resistance is not just 2 ohms like the ratio for the sub-woofer speaker.

Maybe the sub-woofer speaker has a voice-coil in two layers and one layer is shorted to the other.

I like butyl rubber as a speaker surround. They seem to last forever.

Speaking of rubber, did you know that Chrysler fills car tires with nitrogen? The molecules are bigger than the oxygen in air which leaks out. While the oxygen in air passes rubber molecules it oxidizes them which makes them age and the tire cracks.
I had to replace a set of low wear but cracked tires a few years ago. I didn't know about nitrogen.

 

[chemelec]

I have an 18" speaker rated for a very high power. Its impedance is spec'd at 8 ohms.
I have a little 3" low power speaker from a clock radio. Its impedance is also spec'd at 8 ohms.
Their resistance measures 7 ohms DC for each of them.
Of course the wire size for their voice coils is much different.
Their resistance is not just 2 ohms like the ratio for the sub-woofer speaker.

If your 18 Inch Speaker has a 7 Ohm DC Resistance, It is Unlikly it actually Drives High Power. It Just Accepts High Power. So its probably a Low Efficiency output.

And DC Resistance has Nothing to do with Impedance.

Don't you have an Impedance Meter?
Connecting a Series Resistor to your Speaker and Using a Signal Generator at 1000Hz and Oscilloscope, You can determine Impedance.

 

[audioguru]

My 18" powerful speaker has a huge cone and a huge magnet so it is very sensitive. Its 4" voice coil has many turns of very thick wire. Of course its resistance is 7 ohms.

You cannot measure the impedance by itself. It is in series with the DC resistance of the voice coil. Since the DC resistance is too low for the sub-woofer then the measured impedance will also be too low. It will be too low to be driven by most amplifiers.

 

[chemelec]

If you have a BK 878, Universal LCR Meter, Set it to "Resistance".

It accurately Measures AC Resistance (Impedance) at either 1Khz or 120 Hz.

Great for checking Speaker Impedance.

 

[audioguru]

The inductance of the voice coil in a speaker causes its impedance to be high at 1khz.
Its low frequency resonance causes it to be high near its resonant frequency.
The enclosure tuning of the sub-woofer also affects the impedance.
Therfore the spec'd impedance of a speaker is shown as a graph at all frequencies in free air in its datasheet.
The Pioneer sub-woofer speaker system is so cheap ($265.00US) that it does not have a datasheet. They have another sub-woofer driver that sells for $1500.00US.

 

[Nigel Goodwin]

I like butyl rubber as a speaker surround. They seem to last forever.

Yes, it lasts far better than foam - foam falls to pieces after a few years.

Speaking of rubber, did you know that Chrysler fills car tires with nitrogen? The molecules are bigger than the oxygen in air which leaks out. While the oxygen in air passes rubber molecules it oxidizes them which makes them age and the tire cracks.
I had to replace a set of low wear but cracked tires a few years ago. I didn't know about nitrogen.

Don't know about manufacturers, but tyre centres in the UK commonly inflate using nitrogen after repairs or fitting new tyres. The technique came from Formula One racing cars, where they started doing it years ago to reduce flat tyres.

 

[audioguru]

I think Mercedes Benz is Dailmer and Dailmer owned Chrysler a few years ago. Maybe that is where the nitrogen in tires came from.
I wonder what the tiny little Fiat-Chrysler cars will be like in a couple of years?

 

[The Electrician]

It accurately Measures AC Resistance (Impedance) at either 1Khz or 120 Hz.

You have a serious misunderstanding here. Impedance is not just AC resistance. Impedance is the phasor combination of resistance and reactance.

The standard symbols for the relevant quantities are:

Z = impedance (at a particular frequency)
R = resistance (at a particular frequency)
X = reactance (at a particular frequency)

These quantities may vary with frequency, and the relationship among them is:

Z = R + jX

The AC resistance of a speaker is almost the same as the DC resistance below a few kilohertz, except for the region right around resonance. At higher frequencies the AC resistance increases due to losses in the magnetic circuit, and due to skin and proximity effect in the voice coil.

But impedance is not just AC resistance.

And DC Resistance has Nothing to do with Impedance.

It has a lot to do with impedance. Impedance varies with frequency because the components of impedance, resistance and reactance, vary with frequency. The AC resistance of the voice coil is composed of the DC resistance plus a component that increases with frequency. If you were to remove the DC resistance of the voice coil, the AC resistance would change a lot, because the DC resistance is a substantial part of the AC resistance.

I've attached 3 images showing the AC resistance, reactance and impedance of a raw driver, with a nominal impedance of 8Ω, as the frequency varies from 100 Hz to 20 kHz.

As you can see, at low frequencies the AC resistance is essentially the same as the DC resistance (which measures 6.8Ω with my Fluke meter).

The impedance of the driver reaches 8Ω somewhere between 3 and 4kHz. This behavior is typical of all drivers; the DC resistance is just a little less than the nominal impedance rating.

The web is full of information about speaker impedance; for example:

**broken link removed**

 

[The Electrician]

You cannot measure the impedance by itself. It is in series with the DC resistance of the voice coil.

This seems a little confused.

The DC resistance of the voice coil is a part of the impedance.

Measuring the impedance by itself is quite possible; you just use an LCR meter. See my other post for the measurement.

Did you mean to say that you can't measure the reactance? Or, perhaps you meant that you can't measure the part of the impedance which is exclusive of the DC resistance. But, of course, you can simply measure the AC resistance and then subtract the DC resistance if you want to know the part of the resistance which is due to the fact that the measurement is at a frequency above DC.