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Author Topic: What Is Impedence In A Speaker ?  (Read 5751 times)

neilep

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What Is Impedence In A Speaker ?
« on: 01/09/2009 16:16:13 »
Dearest Impedenceologists,

As a sheepy I of course luff muzak. Muzak is my all time favourite thing that instruments make when played together to form a melody or tune.

I prefer to listen to muzak through a thing called a speaker....some people like little speakers that they wear on their head but I find that if I wear those then I can't pick the wax out of my ears and eat it.

look here's a speaker :

A Speaker Not Speaking

Nice eh ?

Being delivered next Tuesday.

This speaker has an impedance of 6 ohms !..........that's nice !!

What on earth does that mean anyway ?..what is Impedance in a speaker ? and why do some speakers have different impedance and what is the thing in the speaker that causes it to have the impedance that it does ?

Ewe see...i just do not know !...no..really...I don't !

I think, if we all get together for some huggable answerfest luff then it's quite possible that some time soon I will know YAYYYYY !!..cos knowing stuff is like...well kewl !

Hugs & Shmishes

mwah mwah mwah !!

neil
I Believe A Real Load That This Site Does Have Credence
It'll Help Me Know The Nature Of Impedance
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graham.d

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What Is Impedence In A Speaker ?
« Reply #1 on: 01/09/2009 16:40:00 »
If you put an AC voltage of certain value (this is usually measured as a root mean square, or rms, value) then the impedance will tell you how much rms current will be consumed. For speakers this is usually specified at a frequency of 1kHz. It is just Ohms law: Current = Voltage/Impedance. It gives a user some idea of what sort of amplifier is needed. The standards for speakers are typically in the range 4 to 15 Ohms. If the voltage available in the amplifier is not high (if run from batteries for example) it is best to use a low impedance because otherwise you won't get much power out (P = V^2/R). If you are driving headphones you don't need much power and it can be simpler to use quite a high impedance "speaker" (<30 ohms for example). 6 ohms is a bit unusual; 8 ohms would be more common for Hi-Fi type speakers, though I'm a bit out of touch.

Pumblechook

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What Is Impedence In A Speaker ?
« Reply #2 on: 01/09/2009 16:43:49 »
It is a dynamic impedance and may vary with the power applied.  The DC coil resistance will be a fraction of an Ohm without checking one.

Geezer

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What Is Impedence In A Speaker ?
« Reply #3 on: 01/09/2009 19:34:41 »
Dearest Impedenceologists,

What on earth does that mean anyway ?..what is Impedance in a speaker ? and why do some speakers have different impedance and what is the thing in the speaker that causes it to have the impedance that it does ?

They call them speakers because they have a voice coil in 'em  (they do - have a voice coil that is)

From a sheepy perspective, it's a measure of how much the speaker impedes the flow of alternating current. Bit like resistance, but impedance varies with frequency.

The amplifier (that's the box with the cool knobs and dials ) is designed to drive speakers of a particular impedance, so to get the best sound quality (and maximum power output), you should, ideally, match the speakers to the amplifier.

There are probably many reasons for different impedance values. Some are to do with the technology used in the amplifier (transistor, valves, FETs ) and the particular design of the amplifier. There are lots and lots of different design approaches. It's possible to match any design of amplifier to any speaker impedance with a transformer, but transformers are expensive and heavy, so designers tend to avoid them.

For high power situations, the amplifier can deliver a lot of power into a speaker with a low impedance at a lower voltage. As the impedance goes up, the amp is required to produce greater voltages to deliver the same amount of power. At high power levels, the voltages involved can be considerable. So low impedance is preferred, except that, when the speaker is some distance from the amp, the high currents involved in driving low impedance speakers result in significant power losses in the cables connecting the amp to the speaker! That can be reduced with thicker cables, but they are more expensive because of all the copper in them, not to mention heavy. Watt's a poor engineer to do?

The thing that creates the impedance is the voice coil. It's a coil of wire (copper is the all time fave.) attached to the back of the speaker cone. The coil is inserted into a stationary magnet. Current from the amplifier flows through the coil and interacts with the magnetic field produced by the magnet causing the coil to push and pull the speaker cone thereby producing mellifluous sheep like sounds. BAAAch's "Sheep May Safely Graze" might be an all time fave.

(Not quite done yet!) However, as the voice coil moves hither and yon in the magnetic field, a voltage is also induced in the voice coil. This voltage tends to oppose the voltage supplied by the amplifier, and therefore, the flow of current from the amplifier is impeded. Pretty cool, eh?

The impedance of the coil is a function of the number of turns in the coil (among other things). More turns, more impedance.  Soooo, if you want to change the impedance of your new speakers when they arrive, all you have to do is rip the voice coils out and replace them with different ones. Easy!

« Last Edit: 02/09/2009 00:52:27 by Geezer »

techmind

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What Is Impedence In A Speaker ?
« Reply #4 on: 01/09/2009 23:12:40 »
"Impedance" is a more generalised form of resistance.

A pure resistor has the same resistance at all frequencies, while an inductor (or inductor/resistor/capacitor combo) has an effective resistance (known as impedance) which is different at different frequencies. Impedance is always quoted at a stated (or implied) frequency, eg 1kHz for speakers.

For a speaker, the impedance tells you how much "power" the speaker will draw for a given amplitude of audio-frequency signal.

lyner

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What Is Impedence In A Speaker ?
« Reply #5 on: 02/09/2009 00:35:36 »
Impedance Z is defined as R +jX (a complex quantity)
R is the resistance
X is the reactance and is there because of capacitance or inductance. When you have an alternating voltage the reactance may be significant.
In a loudspeaker drive unit, the reactance is predominantly Inductive - because the speech coil is a coil and, at audio frequencies, will just be an inductance.  As the coil moves in and out of the magnet, there is a voltage, or back emf, induced (as in a generator) and this 'appears' as a significant contribution to the  inductance.
SO, if you measure the DC resistance of a speaker coil, it may not be the same as the 'impedance' quoted, which will be the square root of (R2 + X2).
This impedance, as graham-d says, has to be specified at a particular frequency.
It makes life very difficult for designers of crossover filters because the impedance will be specific to a particular drive unit (and even the enclosure, which affects the motion of the speaker cone and, hence, the movement of the coil).
There are not many speaker / amplifier / connecting lead combinations which truly take all this into account. Even those which cost a fortune. You have to listen with your wallet, very often.

Geezer

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What Is Impedence In A Speaker ?
« Reply #6 on: 02/09/2009 03:16:05 »
In a loudspeaker drive unit, the reactance is predominantly Inductive - because the speech coil is a coil and, at audio frequencies, will just be an inductance.  As the coil moves in and out of the magnet, there is a voltage, or back emf, induced (as in a generator) and this 'appears' as a significant contribution to the  inductance.

A minor clarification. The voice coil winding certainly does have some inductance, but not much. At frequencies below around 4 kHz, the reactance in a loudspeaker is not significant. The dominant current "impeder" is the induced voltage, or back EMF. As Sophiecentaur points out, it might "appear" that the loudspeaker has a large reactance, but the effect is perhaps more like the back EMF produced by an electric motor. The resistance of the voice coil is negligible, although it is possible to design a circuit that cancels out the effect of the resistive loss if desired.

The reactance produced by the voice coil inductance does become much more significant at higher frequencies.

Geezer

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What Is Impedence In A Speaker ?
« Reply #7 on: 02/09/2009 05:21:16 »
Neil,

Just checking, but perhaps you were referring to speaker impudence rather than impedance? Your spelling was slightly ambiguous, but we should cut you some slack because you are having to translate from Baaasque.

LeeE

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What Is Impedence In A Speaker ?
« Reply #8 on: 02/09/2009 14:34:03 »
I think Sophiecentaur has been the only person to mention crossovers so far, and it's really the impedance of the crossover that the amplifier sees, not the impedance of the loudspeaker voice coils (unless you're using an active crossover and multiple power amps).

lyner

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What Is Impedence In A Speaker ?
« Reply #9 on: 02/09/2009 18:10:21 »
There is really a series of transforms at work. The shape of the cabinet (volume, port or horn) affects how the drive unit 'matches to ' the air outside and how much acoustic energy can actually be launched. Any resonances (like the Helholtz resonator in a ported cabinet) can affect the amplitude of the coild movement and will modify the acoustic impedance that the speaker cone sees and affects the movement of the cone. This, in turn, may alter the loading of the crossover (i.e. the effective effective impedance it sees), affecting  its frequency response and its input impedance. Then, unless the amplifier has a genuine zero output resistance, this will affect the power that the amplifier delivers into this system at any particular frequency. (Hence the obsession with very expensive thick speaker leads - but to what end if the rest of the system doesn't warrant it?)

Edit poet. = port. Duh!
« Last Edit: 02/09/2009 22:56:26 by sophiecentaur »

Geezer

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What Is Impedence In A Speaker ?
« Reply #10 on: 02/09/2009 18:12:48 »
I think Sophiecentaur has been the only person to mention crossovers so far, and it's really the impedance of the crossover that the amplifier sees, not the impedance of the loudspeaker voice coils (unless you're using an active crossover and multiple power amps).

Well yes, and then again, no.

Assuming there is a passive crossover network in a speaker enclosure, it will have an effect on the impedance presented to the amplifier. The impedance will be a function of both the speaker(s) and the crossover network, so the amplifier isn't just "seeing" the crossover network. For example, at lower frequencies, the crossover network is designed to provide a low impedance path to the woofer. At low frequencies the impedance seen by the amplifier is mainly the impedance of the woofer itself (even although the term "impedance" in that situation may be something of a misnomer!) (Neil, don't be alarmed - it's not one of those woofers that like to bite sheep)

BTW, here's a link to a paper that goes into the subject in some depth, but it's also easy to read.

http://sound.westhost.com/lr-passive.htm#intro

lyner

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What Is Impedence In A Speaker ?
« Reply #11 on: 03/09/2009 09:47:52 »
Geezer:
Impedance.
It's fine to use the term for this. It just shows what you'd get if you measured V/I (Complex, of course).  The measuring equipment doesn't know what's downstream or upstream.

Geezer

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What Is Impedence In A Speaker ?
« Reply #12 on: 03/09/2009 20:28:12 »
Geezer:
Impedance.
It's fine to use the term for this. It just shows what you'd get if you measured V/I (Complex, of course).  The measuring equipment doesn't know what's downstream or upstream.

Agreed. No other way to describe it.

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What Is Impedence In A Speaker ?
« Reply #12 on: 03/09/2009 20:28:12 »