My speakers say 4-8 ohms? What are they?

deanos63

Standard Member
Hi all,

All of my speakers say IMPENDANCE: 4 - 8 Ohms.

Now doing some research, I have found out that this means they can be used with an amp in this range ( I think, correct me if I'm wrong)

However, If I was to say add an extra front speaker, how would I go about calculating the setup if all i know is that the speaker I have is 4 - 8 Ohm?

Example, My amp says use speakers between 8 - 16 Ohm. My current centre speaker say 4 - 8 Ohm, now i want to add another centre from the same channel on the amp?

sorry if I am being thick.:clap:
 

scjgreen

Active Member
If you wire the 2 speakers on 1 channel in parallel then the load on the amp should be 8 ohms.
 

piaf

Standard Member
I might be making some confusion but I would expect that two speakers in parallel give you half the resistance? So two 8 ohm speakers would go down to 4 ohm.

If you have a 4-8 Speakers most likey the average usage is around 6 (like mentioned for the XTZ).
If your Amp is 8 to 16, then you are now lower than you should be.
 
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Regardless of Parallel or Series connection I am not understand the net gain.

Its not like the volume going to be doubled or quality increased.

If the current centre is not cutting the mustard replace with one better speaker.
 

BlueWizard

Distinguished Member
Hi all,

All of my speakers say IMPENDANCE: 4 - 8 Ohms.

...

sorry if I am being thick.:clap:
Any chance these are Mordaunt Short speakers?

I think this "4-8" designation means that the speakers are mostly 8 ohms, but occasionally 4 ohms.

The lowest a drivers impedance will go is the DC resistance of the voice coil, which is typically about 75% of the rated impedance; so about 6 ohms for an 8 ohm speaker. However, once you add the acoustic loading of the speaker box and the phase shifts in the crossover network, it is possible that the impedance might drop slightly below the DC resistance at certain specific frequencies.

The dips, or low points in a speakers impedance, are usually just above and below the speaker's resonance frequency, which is typcially about 30hz to 40hz. In the case of the dip below the resonance peak, this is out of the working range of the speaker. The band above the resonance point, is usually pretty wide, and gradually increase has the frequency goes up.

Here is a link to a graph of the impedance of a typical 6.5" woofer (Dayton). The Impedance is the bottom line on the bottom graphs on the right. Notice the peak at about 35 hz, and the dip on either side of that peak. The dip appears to bottom out at about 200hz.

http://www.parts-express.com/pdf/295-305s.pdf

The low dip on the low end appears to be about 4.8 ohms at about 2 hz. The dip above the resonance peak appears to be about 6.8 ohms at 200hz. This speaker itself is rated at a nominal 8 ohms.

For the record, the impedance right at the resonant peak is 71 ohms, and at 20khz, it is about 85 ohms. Though, in all likelihood, components have been added to the crossover to keep the impedance of the woofer closer to 8 ohms as the frequency rises.

I think for the most part, you can consider your speaker functionally an 8 ohm speaker. However, since the impedance does drop low at one particular frequency, you should not use more than one speaker per amp channel.

So, I think, as long as you are only using one speaker on an amp channel, these speakers should work fine with virtually any amp.

In short, assuming you have the most basic reasonable amp, don't worry about it.

Steve/bluewizard
 
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Mark.Yudkin

Distinguished Member
When a speaker is specifies as 4-8 Ohms, it means that the impedance, which is a function of frequency, varies between a nominal 4 Ohms and a nominal 8 Ohms. No statement is made as to the actual impedance at any specific frequency, or whether it's "more like" 8 Ohms or "more like" 4 Ohms "more of the time", or even whether it may drop (even significantly) below 4 Ohms or rise (significantly) above 8 Ohms. What it also means is that the manufacturer is being more precise than many, who just write "6 Ohms" regardless of frequency-dependent measurements.

Practically, such a speaker is fine when used on most equipment, and so can be used without concerns.

Running two speaker on the same amplification channel is generally a bad idea. You won't improve the sound, but typically will make it worse. Running two speakers in parallel will halve the impedance, running two speakers in series will double the impedance.
 

Hrodwulf

Novice Member
Hi guys. Just going to throw my hat in the ring here to try and scoop up some of your advice. SO I have a set of Mission MV6 (8ohms) and a set of Eltax concept 180 (4-8ohms) Im looking at an amp that says on the back "when running two sets of speakers simultaneously total impedance must be 8ohms or more". Am I going to have any problems?
 

BlueWizard

Distinguished Member
Hi guys. Just going to throw my hat in the ring here to try and scoop up some of your advice. SO I have a set of Mission MV6 (8ohms) and a set of Eltax concept 180 (4-8ohms) Im looking at an amp that says on the back "when running two sets of speakers simultaneously total impedance must be 8ohms or more". Am I going to have any problems?
Maybe ... maybe not. A lot of factors come into play.

- Specifically what is the Amp? High quality amp, less of a problem. Low quality amp, more of a problem.

- How well is the amp ventilated? Is it out in the open or crammed into a tight space?

- How loud do you play? The louder you play, the more demand you put on the amp.

Most speakers/drivers have a Resonance Frequency, typically around 30hz to 40hz. At this frequency all the Resistive, Capacitive, Inductive, and cabinet loading factors come together and create a peak in the impedance than can be 10 times the rated impedance.

However, on either side of this peak, the impedance drops down to its lowest values. This, below, is just a woofer, but it illustrates the point I am making -

http://www.parts-express.com/pedocs/specs/DC200-8 8_ Classic Woofer 8 Ohm Specification Sheet.pdf

Scroll down about half way until you see a graph on the right that has a Blue line. That blue line is the impedance of this 8" Dayton Classic Woofer. Notice where the blue line peaks, and notice that the lowest impedance is on either side of that peak.

If you look at the fs specification in the chart above the graph, you will see that the Resonance Peak is at 30.3hz. The lowest dip to the right of the peak is at about 150hz and is about 7 ohms. With cabinet loading the dips could be lower.

Likely that is what is happening with the 4-8 ohms speaker. Overall, about 8 ohms, but at the deepest dips perhaps 4 ohms or less. Many of the B&W speakers that are rated at 8 ohms have a deep dip that drops down to 3.2 ohms.

How well the speaker will work together will depend on how closely these dip in impedance line up. If they are perfectly in sync, likely you WILL have a problem. But if the dips occur at different frequencies, likely you WILL NOT have a problem.

Also, the limits of an amp are Current and Heat Dissipation. Lower impedance means more Current and that in turn means more Heat. If the amp can supply the Current but can't dissipate heat fast enough, you will run into problems at higher volumes. If the amp can supply the Current, and dissipate the Heat, you are probably home free. If the amp can NOT deliver the current demanded, then you probably don't have a very good amp and need a new one.

But then ... that's just my opinion.

Steve/bluewizard
 

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