Impedance with ohm meter

[Boss Daddy]

Hi. I want to know if you measure the impedance across your speaker terminals is that the correct impedance. I.e most hifi loudspeakers are considered 8ohm, but if you measure with ohm meter you might get 6.5ohm for example. Reason being if i want to connect two same speakers per side in parallel that make a difference on amp in ohm range. So two 8 ohms will load amp down to 4ohm, but two 6.5 measured will result in 3.1ohm….correct? , and that could cause a problem if out of the amps range.

 

[j_garcia]

Not exactly, that is why the spec is "Nominal 8 Ohm" meaning average across its band. Impedance varies with frequency, so it refers to which "common" category a speaker can be classified as: typically 8 or 4 Ohm. 8 being relatively easy to drive by most receivers and 4 Ohm obviously being more difficult to drive. It isn't that simple, since there are more factors, but that's a rule of thumb.

 

[Verdinut]

Hi. I want to know if you measure the impedance across your speaker terminals is that the correct impedance. I.e most hifi loudspeakers are considered 8ohm, but if you measure with ohm meter you might get 6.5ohm for example. Reason being if i want to connect two same speakers per side in parallel that make a difference on amp in ohm range. So two 8 ohms will load amp down to 4ohm, but two 6.5 measured will result in 3.1ohm….correct? , and that could cause a problem if out of the amps range.

What you measure with a multimeter is the DC resistance of the voice coil of the driver. The nominal impedance is not the same, and it's usually at least 30% higher than the DC resistance. Also, that impedance varies with the frequency fed to the speaker.

 

[Swerd]

As j_garcia and Verdinut said, impedance varies with the frequency of the audio signal, which is an AC signal. A reading from a typical volt-ohm meter will give you a single resistance value, which makes sense only if you measure a DC signal. Any audio signal is AC.

Below is an example of a speaker that the manufacturer says is 'nominally 8 ohms'. Much of the range from 100 to 1,000 Hz is under 8 ohms. This is fairly typical of many 2-way loudspeakers.

 

[Pogre]

Also, that impedance varies with the frequency fed to the speaker.

Yup, and just to illustrate this is what a fairly typical speaker's impedance curve can look like.

You can see it's up and down all over the place depending on frequency. Then there are the phase angles too, of which I have little understanding but it's definitely a factor for how difficult a particular speaker may be to drive.

 

[Pogre]

As j_garcia said, impedance varies with the frequency of the audio signal, which is an AC signal. A reading from a typical volt-ohm meter will give you a single resistance value, which makes sense only if you measure a DC signal. Any audio signal is AC.

Below is an example of a speaker that the manufacturer says is 'nominally 8 ohms'. Much of the range from 100 to 1,000 Hz is under 8 ohms. This is fairly typical of many 2-way loudspeakers.
View attachment 54176

Ha! We both posted a chart at the same time! Lol

 

[Swerd]

In the chart you posted, is the black line impedance, and the green line impedance phase angle? It might help the OP if you point that out.

 

[Pogre]

In the chart you posted, is the black line impedance, and the green line impedance phase angle? It might help the OP if you point that out.

I was just scratching my head wondering that myself... lol. I just grabbed the first one I saw and didn't see it wasn't specified. Let's assume the green line is phase and black is impedance. What does the phase angle represent? I know that steep phase angles aren't good but I've never really understood what it is.

 

[Swerd]

I was just scratching my head wondering that myself... lol. I just grabbed the first one I saw and didn't see it wasn't specified. Let's assume the green line is phase and black is impedance. What does the phase angle represent? I know that steep phase angles aren't good but I've never really understood what it is.

I hoped you wouldn't ask that. It's something I can understand, without being able to explain well. Words fail, but math works.

Audio signals are electrical waves. That means the signal voltage and current are waves – they have both magnitude as well as direction or angle.

Both the voltage and current are waves, but their phases are different. Imagine voltage is red and current is blue. At any one point in time, their sine waves are not in phase with each other.

Maybe it helps to see this animated illustration:

https://en.wikipedia.org/wiki/File:phase_shifter_using_IQ_modulator.gif

Electrical characteristics of dynamic loudspeakers - Wikipedia

en.wikipedia.org

Phase (waves) - Wikipedia

en.wikipedia.org

I'll quit while I'm ahead.

 

[Pogre]

I hoped you wouldn't ask that. It's something I can understand, without being able to explain well.

Audio signals are electrical waves. That means the signal voltage and current are waves – they have both magnitude as well as direction or angle. (I'll quit while I'm ahead.)
View attachment 54179

Electrical characteristics of dynamic loudspeakers - Wikipedia

en.wikipedia.org

Surprisingly, I think I understand a little better now so this helps!

 

[Verdinut]

As j_garcia and Verdinut said, impedance varies with the frequency of the audio signal, which is an AC signal. A reading from a typical volt-ohm meter will give you a single resistance value, which makes sense only if you measure a DC signal. Any audio signal is AC.

Below is an example of a speaker that the manufacturer says is 'nominally 8 ohms'. Much of the range from 100 to 1,000 Hz is under 8 ohms. This is fairly typical of many 2-way loudspeakers.
View attachment 54176

The impedance curve above is that of a 6 ohm driver, it is only 6 ohms around 50 Hz and below 5 ohms at frequencies surrounding 175 Hz. But the manufacturer dares pushing it on the market as an 8 ohm speaker, to sell more to the AVR owners.

 

[lovinthehd]

This two part article may help the OP too https://www.audioholics.com/loudspeaker-design/understanding-loudspeaker-measurements

 

[Alex2507]

Surprisingly, I think I understand a little better now so this helps!

Yeah, it’s the cosign of the tangent. I get it too.

 

[vader540is]

I use this

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