How do you work out Ohms of a speaker?

[montburnsy]

I have a tech. question, it may not be the right place, but I was wondering how the Ohms of a speaker comes around when making one?
Example, if I was to make my own speaker(highly unlikely) where would the Ohms come into it? Is it the amount of copper wire inside the magnet? Is it the weight of the speaker? (these are just examples of guesses, 'cos I know the weight would have nothing to do with it! Or does it?)
Thnx in advance for all who have any imput on the answer.


Burnsy.

[Bob W]

I has to do with the gage of the wire, the length, and the amount of turns/pitch of the wire.

[MOVIEGEEK]

It depends on the speaker drivers(eg. woofer,midrange,tweeter) most home amps. are 8ohms,auto amps. are 4ohms.

[sLiMpAuLy]

Ohms are the amount of resistance between the positive and negative ends of the voice coil.

See Here

[johns0]

Ohms is simply the resistance in the wires,depending usually on length and diameter.

[redwudz]

Actually, it's the impedance, not the resistance. Impedance is more akin to the resistance vs frequency of a voice coil. Try measuring the resistance of a voice coil, you won't get the rated ohms. It's more of a dynamic measurement than a passive one such as resistance. Though, to make it simple, match the amplifier impedance to the speaker impedance to get the most efficient transfer of power.

[dicktheprick]

Here is a table of resistance of copper wire

AWG: 24, # of Strands: Solid, Dia per strand: 0.0201, resistance per 1000ft (ohms): 28.4
AWG: 24, # of Strands: 7, Dia per strand: 0.0080, resistance per 1000ft (ohms): 28.4
AWG: 22, # of Strands: Solid, Dia per strand: 0.0254, resistance per 1000ft (ohms): 18.0
AWG: 22, # of Strands: 7, Dia per strand: 0.0100, resistance per 1000ft (ohms): 19.0
AWG: 20, # of Strands: Solid, Dia per strand: 0.0320, resistance per 1000ft (ohms): 11.3
AWG: 20, # of Strands: 7, Dia per strand: 0.0126, resistance per 1000ft (ohms): 11.9
AWG: 18, # of Strands: Solid, Dia per strand: 0.0403, resistance per 1000ft (ohms): 7.2
AWG: 18, # of Strands: 7, Dia per strand: 0.0159, resistance per 1000ft (ohms): 7.5
AWG: 16, # of Strands: Solid, Dia per strand: 0.0508, resistance per 1000ft (ohms): 4.5
AWG: 16, # of Strands: 19, Dia per strand: 0.0113, resistance per 1000ft (ohms): 4.7


But you have to be careful in your calculations because the resistances are a vectors of impedence and if you add inductors or capacitors it changes the impedence of the circuit. and to get the most amount of rms power out of your "Load" you want to match the impedence of the "load", such as the speaker with that of the Amplifier. Thats why some amps have a 8 ohm and 4 ohm switch. If you're output is to 2 speakers @ 8 ohms each in parallel, the impedence is only 4 ohms.

[thor300]

Usually a speakers impedance is meassured at 1khz frequency. No matter how one meassure, it has nothing to do with resistance which would be almost 0.0 for a powerful 4 ohm subwoofer. You could say resistance is for DC current and impedance is for AC.

[johns0]

An 8 ohm speaker and 4 ohm speaker still measure 8 and 4 ohms by standard dvm meters.

[junkmalle]

I was wondering how the Ohms of a speaker comes around when making one?... Is it the amount of copper wire inside the magnet? Is it the weight of the speaker?

Redwudz is correct -- the Ohm rating of speakers is a measure of impedance not resistance. Impedance is the resistance to AC not DC. All of the things you mention come into play as well as the flexibility of the surround and any crossover components. Although the impedance is reported as a single value, it varies with frequency.

[Perro Grande]

Okay --- I've got to chime in on this one...

There are three factors that, in effect, result in loss in an electrical signal

The first one is Resistance. Resistance is what you measure with a simple DVM. DC resistance is, essentially "friction" against the motion of electrons (ie electrical current). It is present in virtually ALL conductors. When DC or Alternating Current (AC) passes through a conductor that is resistive, a voltage drop is produced (ie. loss). In the case of AC, the voltage drop is IN PHASE with the current.

The second factor is REACTANCE. Reactance exists ONLY when current alternates. Reactance (still measured in Ohms, incidentally) measures the effect of current either leading or lagging the changing voltage as the result of it passing through a reactive component -- either a capacitor or a coil of wire. Some definitions will relate reactance to "inertia." In reactance, the voltage drop is NOT in phase with the current.

Lastly comes IMPEDANCE. Impedance (also in Ohms) is a measurement of the total loss components (opposition to electrical flow) in an object. It includes BOTH reactive and resistive components, and is the best overall measurement to consider when dealing with AC circuits.

Ideally, a resistor will have no reactance, and a perfect coil will have no resistance... In the real world, this is not the case, so impedance is the most useful of all measurements.

Of course, a simple speaker is fundamentally a coil of wire (an inductor). As such, it will have both resistance (due to the wire itself) and reactance (because it is a coil). At DC, which is what your DVM uses to measure resistance, it will typically read 2, 4, 8, or 16 ohms. However, at varying audible AC frequencies, this resistance value is less meaningful than the reactive component (which varies as a function of frequency).

The impedance value given for a speaker is an aggregate average across the frequency response range of that speaker.

From a simple OHM's law (V=IR) perspective, voltage drops, current, etc, can be computed based on reactance, resistance, or impedance. Thus, calculating the impedance of speakers in parallel and in series works exactly the same as resistors in parallel and/or series.

So, in speaker design, you must consider the final impedance of ALL the actual speakers (drivers) in a design. If you produce an impedance rating lower than your amplifier is capable of supporting, you risk overloading your amp. (The lower the impedance, the higher the flow of current, thus the overload potential).

However, to make matters more complex, most speaker designs involve some form of a crossover. This component is ALSO a reactive component (essentially a filter), so this, too, gets in to the equation. This is how you can have a "speaker" that is rated at 8 ohms made up of several drivers and a crossover. You have to take ALL components into account when computing the final impedance.

Making your own speaker is a really cool, but non-trivial project. I usually recommend that people doing this invest in a quality crossover first. They are typically designed for a specific number of drivers at a specific impedance per driver, yielding a specific impedance for the entire speaker unit (typically 8 ohms). From there, you can select drivers to use based on size, frequency response, efficiency, etc. Lastly, you can design an appropriate enclosure to match your drivers...

[thor300]

Very detailed by Perro Grande, i THINK he know his stuff I find crossovers to be the hardest part. Early in the 1990'ies i had competition level car stereo, never got satisfied with the crossovers, it was a never ending job to improve it. Ended up with all active setup with Soundstream amps and active x-overs, and still not good enough. Digital x-overs was insane expensive that time, never got to that level.

[FOO]

It's a difficult problem.
The impedance varies with everything, including air pressure.

An ideal speaker would have zero DC resistance.

Having said that , it's surprising to me that the magnitude of the
impedance of an 8 ohm speaker is fairly constant with frequency.
I would have suspected wild variation

[junkmalle]

Perro Grande,

Well said!

I do have one question. You said:

The impedance value given for a speaker is an aggregate average across the frequency response range of that speaker.

I was under the impression that there was a lot of variation in exactly how manufacturers determine their impedance values. Some may use pink noise, some a 1 KHz sine wave, other some other weighting. I didn't think there was a standard for this. Is there?

[vcdforme]

I heard that Saddam was pondering this very question
when they caught him. Short on materials, he was trying
to repair a blown tweeter.

the true answer may be imaginary.

[mike1061]

Perro Grande
Thanks. It is good to see such a detailed, clear answer.
Mike.

[KBeee]

It's a difficult problem.
The impedance varies with everything, including air pressure.

An ideal speaker would have zero DC resistance.

Having said that , it's surprising to me that the magnitude of the
impedance of an 8 ohm speaker is fairly constant with frequency.
I would have suspected wild variation

The trouble with having your "perfect" 0 ohm speaker is that it'd blow your amplifier because basically you're putting a dead short across the terminals.
Also, the 3 factors making up the impedence of a load are capacitance, reactance, and resistance. Capacitance/reactance (from coils, capacitors etc.), make the current lead/lag the voltage, and can cancel each other out in a balanced load. When they are calculated or measured, and not balanced, the lead or lag will give a Power Factor, which is basically a measure of wasted energy. A PF of 0.9 would mean that although you are supplying (for instance) 10V and 10A to a load, only 90w are utilised instead of 100W.

[FOO]

You are incorrect about a number of things.

The zero resistance speaker would not necessarily blow the amp
because the impedance would be non-zero at everything but DC ,
and the amp is supposed to have zero DC out.

Power Factor, which is basically a measure of wasted energy

Completely incorrect . A non unity PF wastes zero energy.