Speaker Resistance

[Dobber1978]

Got into an argument the other night about speaker resistance and what it really means.

I was fighting that this was an impedance that takes into account both the resistance and inductive properties of the speaker, basically the Z at a certian frequency.

The other guy was saying this was a purly resistive measurement that he could take with his multimeter.

Anyone care to clarify,

Thanks
Dob

 

[VEBill]

Speaker impedance varies with frequency, and not just due to inductance (the box has an effect as well). His only 'out' is that if he's talking about pure 'resistance', then he's right (but still wrong in the larger picture).

 

[LionelHutz]

Speaker impedance is always a impedance vs frequency curve.

Also, A speaker will generally say 4 ohms or 8 ohms or 6.5 ohms or something like that but you will almost never measure this rated resistance or impendance with an ohmmeter.

Go to Parts Express. There's lots of technical info with speaker impendace curves connected to their different drivers. If nothing else, look up the large Dayton woofers because I know they have lots of info attached to them. you'll find some impendance vs frequency curves for the

 

[OperaHouse]

If any of you are old enough, you may remember the Magnaplanar speaker. This was a thin wire glues on a sheet of mylar in a weak magnetic field. Terribly inefficient! This was almost pure resistance and almost any amplifier sounded good on it. Only problem was the glue used would eat into the aluminum wire and they would stop working. Think of an inductor in series with a resistor driven by an amplifier. Keeping that resistance low is important for amplifier control. I just started testing some current transformers with an old Marantz 32 with a single turn and a 4 ohm resistance in series to determine frequency response. Hey, I'm just too cheap to go out and buy a nice Pearson. Anyway, you see some strange things happening with garden variety CTs.

 

[VisiGoth]

You are entirely correct. However, you may be far more advanced technically than your friend so it is incumbent on you to educate with respect.

OperaHouse,
I am not OLD and I think magnapans are a great modern invention! Also, competing against a Pearson with home made toroids and matching networks to make nice pulse shapes was great fun when a few decades ago when I got to do that sort of thing. Thanks for bringing back the memories.

 

[itsmoked]

Dobber1978; My experience is that with a pile of speakers you can easily tell which are 4 ohm which are 8 ohm and which are 16 ohm with an ohm meter. Whatever you read will be near one of these standard values and so you you can declare which ohmage the speaker is.

In operation these things are inductive/resistance so you ARE talking "impedance".

 

[zeitghost]

Not only do you have the self inductance and resistance of the coil, you have a back emf generated by the coil moving in the field of the magnet.

Which complicates things a little more... impedancewise.

 

[ScottyUK]

Over here in the UK most 8[Ω] drivers measure about 6[&Omega] as pure resistance. As has bee stated before the overall impedance is a complex one, particularly if the design is a multi-way type using separate drivers for LF, mid and HF. Then factor in the mechanical resonances of each driver / enclosure combination and you have a really tricky analysis problem. You might want to look up the work of Thiele & Small who did a lot of work to de-mystify loudspeaker behaviour. Google finds plenty hits.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[zeitghost]

My el cheapo digital LCR meter came up with a negative inductance on an 8 ohm speaker I measured.

A proper analogue style Marconi LCR bridge came up with some fairly peculiar readings, didn't want to balance particularly, but there was a null of sorts at about 20uH.

The AC across the speaker was enough to produce quite a reasonable volume at 1kHz.

Twiddling the D control produced a better balance, but it was still weird.

Trying to measure the resistance on AC was pretty odd too..

 

[OperaHouse]

I have a CLARKE HESS 258 VAW METER that calculates true power. It is often used to measure things like core losses in inductors. That might be fun to drive a speaker with a 1W nominal signal at a couple of frequencies.

 

[RudyL54]

Your friend is correct. If the specs show resistance is 4 ohms or 8 ohms, it is purely resistive in value. Ohm is a unit of resistance. However, the real value of speaker is measured in impedance not only resistance, the value of which varies with frequency.

Speakers are constructed with coils & permanent magnet. Coils have resistance component, inductance & small capacitive reactance. Combined, they are called impedance.

If the speaker specs is 4-ohm speaker, that value is resistance value. This has to match with the output amplifier for maximum transfer.

 

[ScottyUK]

Rudy,

Inductive and capacitive reactances are measured in Ohms, and when they are combined with resistance to form a complex impedance, the resulting impedance is also measured in Ohms.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[BrianG]

Rudy,
To pick up on your point "If the speaker specs is 4-ohm speaker, that value is resistance value. This has to match with the output amplifier for maximum transfer" - with modern solid state amplifiers the output impedance is extremely low compared to the impedance of the speaker. The amplifier may state "xx Watts ouput power into z Ohms", but this is more a case of ensuring that there is sufficient voltage swing to provide the rated power, whilst at the same time keeping the load current (and hence output stage device current) within design limits.

 

[VEBill]

"...output impedance is extremely low..."

Exactly right! Impedance matching is not typically used in audio amplifiers; the output Z might be a small fraction of an ohm (the lower the better). Google 'damping factor' if you're interested. The concept is that the amplifier can control the speaker cone more like a piston, and less like a pendulum.

The DC resistance of a speaker is almost a non-issue in the real world. Who listens to DC ???? Real world data would start at around 20 Hz (YMMV). If your amplifier is putting out any DC, that is normally a Very Bad Thing.

 

[zeitghost]

<If your amplifier is putting out any DC, that is normally a Very Bad Thing.>


As your bass drivers discover in short order...

 

[IRstuff]

While impedance matching is not required in the amplifier, there is usually a damping resistor in the output, at least, for Class B amplifiers, in the past. Something like 10 turns of 22 gauge wire wound on and across a 1 ohm resistor.

TTFN

 

[VEBill]

Although it may share the word 'damping', I believe that such a low value L//R 'damping resistor' is more to do with damping the little MHz oscillation (sharp corners) at Class B cross-over than with the 'damping factor' issues associated with speaker and amplifier impedances.

 

[zeitghost]

Or fixes stability issues with electrostatic speakers?