Trotty
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Registered: 22nd Feb 01
Location: Bristol
User status: Offline
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SPEAKER IMPEDANCE
The impedance (resistance) of a speaker is measured in ohms. The resistance of the speaker is dependent on these factors:
• Voice coil's electrical impedance (resistance, inductance ect)
• Driver's mechanical impedance (stiffness, mass, damping)
• Driver's acoustic radiation impedance (resistance, reactance)
The impedance of a speaker will then determine how much power it can draw from an amplifier, and how much the amplifier can give out.
For example, many amplifiers (especially monoblock) can drop to as low as 1 ohm, and would give out for example 1200 watt RMS (2400 watt MAX). But connecting a 4 ohm speaker to it would not give this power output, because the resistance is much higher. It would give around a 3rd of the power. For example we will look at the ratings of the VIBE VP4:
450 watts RMS at 4 ohms
750 watts RMS at 2 ohms
1200 watts RMS at 1 ohm
Now as you can see the power differences are quite large, this is because when the resistance in the circuit that the amplifier is connected to is lower, the more power the amplifier can push through the circuit.
Speakers can be bought that have different impedance values. The most common are 4 ohm and 2 ohm speakers and 1 ohm speakers for ones that can handle huge amounts of power.
Also, speakers can be bought that have 2 voice coils inside the speaker. These are called ‘Dual voice coil’. You will typically see them described as a ‘Dual 4 ohm voice coil’ or a ‘Dual 2 ohm voice coil’. Also, they may say 2x4 ohms or 2x 2 ohms.
Single voice coil speaker
A speaker with a single voice coil can be connected to an amp in many ways. If it is a 4 ohm speaker, and the amplifier you have gives out the matching RMS value of the speaker at 4 ohms, then you can connect the speaker straight to the amplifier and away you go.
If you have an amplifier that can handle much more power at 2 ohms, say double the power for example, it would be a good idea to take advantage of this power, but surely you must say ‘I can only do this with a 2 ohm speaker?’ you are wrong.
If you have two 4 ohm speakers and their RMS values added together match the amplifiers output at 2 ohms, you connect both speakers in parallel to the amplifier, then the amplifier would see LESS resistance in the circuit. It would drop the resistance to 2 ohms so it would push out the power it is rated at 2 ohms. This is perfectly safe as long as your amplifier is classed as 2 ohm stable.
You can also do the same to two 2 ohm speakers, as long as power ratings match the amplifier ratings at 1 ohm, and that your amplifier is 1 ohm stable. Connecting two 2 ohms speakers in parallel would make the amplifier see less resistance once again, and would drop the resistance to 1 ohm.
Dual voice coil speaker
You can determine if a speaker has dual voice coils as I described earlier. If it has two 4 ohm coils, then you can wire both voice coils in parallel to give you a 2ohm load, and with a 2 ohm stable amp, will give you a much higher output than you would have had at 4 ohms.
If it has two 2 ohm voice coils, you can wire these in parallel to give you a 1 ohm load, which will once again give you a higher output from your amplifier if it is 1 ohm stable.
You DO NOT have to use both voice coils on the speaker, it is there if you want to get more power from your amplifier as long as it is stable.
Alternatively, if you can’t afford an amp that matches the power rating of the speaker when wired in parallel, you can use 2 amps.
Say the speaker is 1000RMS for example, and has two 4 ohm voice coils. You cant afford an amplifier that gives out 1000RMS at 2 ohms, but you can afford 2 amps that give 500 RMS at 4 ohms each (this is just a scenario ppl so dont slaughter me ), then you could wire two amps to the same speaker. You would connect each amp to one of the voice coils.
Can be the same again for the two 2 ohm coils. Say the speaker is 1200RMS. Once again you cant find or afford (scenario again) an amplifier that gives out this power at 1 ohm, but you can get two amps that deliver 600RMS each at 2 ohms, I think you get the idea.
I hope that clears things up for you 
Originally submitted by AustinPXX
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