Speakers and impedance
- Thread starter lude1962
- Start date
A set of speakers with a 4 Ohm nominal impedance would be both unusual and a demanding load for a home cinema AV receiver. You should really steer clear of anything below 6 Ohms.
In speaker terms "Nominal" is another way of saying 'average'. A speaker's true impedance varies with frequency and phase. A 6 or 8 Ohm speaker could have a resistance plot that goes as high as 30-40 Ohms for some frequencies and as low as 3-4 Ohms for others. If you then start with a 4 Ohms nominal speaker it might drop as low as 1-2 Ohms. That can be a problem with an amp expecting a 6-8 Ohms nominal load. The speaker sucks current from the amp. The lower the impedance then the more current that the speaker will suck. Current makes wires hot to the point of melting. Draw enough current and you'll probably fry the voice coil and blow the output transistors on the amp at the same time.
As a general guide, AV amps are generally rated for an 8 Ohm load but will cope with a 6 Ohm nominal impedance speaker set. Those receivers with switchable 6-8 Ohm impedance should be used only with speakers rated at 6 Ohms nominal or more. Don't be tempted to use the satellites or tower speakers from an all-in-one kit unless they're 6 Ohms or more.
In speaker terms "Nominal" is another way of saying 'average'. A speaker's true impedance varies with frequency and phase. A 6 or 8 Ohm speaker could have a resistance plot that goes as high as 30-40 Ohms for some frequencies and as low as 3-4 Ohms for others. If you then start with a 4 Ohms nominal speaker it might drop as low as 1-2 Ohms. That can be a problem with an amp expecting a 6-8 Ohms nominal load. The speaker sucks current from the amp. The lower the impedance then the more current that the speaker will suck. Current makes wires hot to the point of melting. Draw enough current and you'll probably fry the voice coil and blow the output transistors on the amp at the same time.
As a general guide, AV amps are generally rated for an 8 Ohm load but will cope with a 6 Ohm nominal impedance speaker set. Those receivers with switchable 6-8 Ohm impedance should be used only with speakers rated at 6 Ohms nominal or more. Don't be tempted to use the satellites or tower speakers from an all-in-one kit unless they're 6 Ohms or more.
Ah, you have Mordaunt Shorts. That changes things slightly.
The idea of nominal being an average isn't strictly accurate, but trying to explain the complexities of how the impedance above the resonant frequency of the speaker is usually given as the nominal impedance doesn't really make much sense in a quick description.
I think what MS are trying to do is present their impedance figures in the best light they can. The lowest impedance for their speakers is 4 Ohms. This will probably coincide with the speaker's lowest frequency. As the frequency increases and goes through the speaker's mechanical resonant frequency the impedance will peak and then drop again. This lowest point after the mechanical resonance is the 8 Ohm nominal. As the frequency continues to rise so does the impedance. So that's where the 4-8 Ohm rating comes from. They're being honest with their ratings but it does cause some confusion when other manufacturers would quote 8 Ohms for the same impedance plot and presume that the amp could cope with a dip to 4 Ohms.
So, the question then remains, what do you set your amp on. Correct?
Partly that depends on how good the amp is at delivering current and whether you'll regularly try to max it out. A lower setting (4 Ohms or 6 Ohms) will limit the current delivery and rein in both the dynamic range and maximum volume. But you would probably only notice that if regularly running at high volume on the amp, say -10dB~+10dB. It also depends whether you decide to treat the speaker as if some other manufacturer measured it. They might have stuck an 8 Ohm Nominal label on the back. My gut feeling is to go with 8 Ohms as that's typical for other speakers that would dip to 4 Ohms too.
The idea of nominal being an average isn't strictly accurate, but trying to explain the complexities of how the impedance above the resonant frequency of the speaker is usually given as the nominal impedance doesn't really make much sense in a quick description.
I think what MS are trying to do is present their impedance figures in the best light they can. The lowest impedance for their speakers is 4 Ohms. This will probably coincide with the speaker's lowest frequency. As the frequency increases and goes through the speaker's mechanical resonant frequency the impedance will peak and then drop again. This lowest point after the mechanical resonance is the 8 Ohm nominal. As the frequency continues to rise so does the impedance. So that's where the 4-8 Ohm rating comes from. They're being honest with their ratings but it does cause some confusion when other manufacturers would quote 8 Ohms for the same impedance plot and presume that the amp could cope with a dip to 4 Ohms.
So, the question then remains, what do you set your amp on. Correct?
Partly that depends on how good the amp is at delivering current and whether you'll regularly try to max it out. A lower setting (4 Ohms or 6 Ohms) will limit the current delivery and rein in both the dynamic range and maximum volume. But you would probably only notice that if regularly running at high volume on the amp, say -10dB~+10dB. It also depends whether you decide to treat the speaker as if some other manufacturer measured it. They might have stuck an 8 Ohm Nominal label on the back. My gut feeling is to go with 8 Ohms as that's typical for other speakers that would dip to 4 Ohms too.