So I have a 3W amp that I want to use, and I've been told that that means each speaker channel outputs 1.5W. If that's correct, I have my mind set on two 1.5W speakers, but I don't know if I need 4 ohm or 8 ohm speakers. I've been told that the ohms shouldn't matter, but I've been told by another that it is what determines volume range. The datasheet of the amp itself says 4 ohms, so should I match the speaker ohm rating with that? Obviously I want to have the widest volume range of the volume knob possible.
Electronic – What do ohms of a speaker do
amplifierspeakers
Related Solutions
If the source has enough power, then it will work. If it doesn't, then it will need an amplifier.
Simplest is to have an amplifier anyway, then you don't care about the source.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Series connection of resistor.
Since you are increasing resistance in a series circuit the amplifier won't know or care where in the circuit the resistance is. Convention would be to put it in the red or '+' wire.
Figure 2. Simplified equivalent circuit.
The problem is that your speaker isn't just a resistor. It is largely an inductive load and since it has mechanical resonance (favourite frequencies to vibrate at) the situation becomes more complex. A good amplifier will have a strong drive to force the speaker to the correct position at any instant and overcome the inductance. Low resistance speaker cables are part of this equation. Adding your 2 Ω series resistance will affect how well the amplifier is able to control the speaker and some loss of fidelity will result.
Try it and see if you notice. The resistors will need to be rated to handle the power. Start with ones rated for at least 1/4 of the nominal amplifier output power rating.
Best Answer
Almost all audio amplifiers today are approximately voltage sources -- they have relatively low output impedance. If you change the speaker impedance, to first order, you change the power delivered, which goes as $$P = V^2/R$$.
This applies both to the power delivered for a given input signal and volume setting, and also to the maximum possible power output.
You can't get infinite power out by lowering the speaker impedance. There is a minimum possible impedance that the amplifier can effectively drive. This will be specified in the data sheet, and will generally also allow the maximum power output.
Beyond that, there isn't much difference -- a similarly designed 8 ohm vs. 4 ohm speaker will operate mostly the same, as long as you drive it with the right type of amplifier.
There are often multiple ways to connect a speaker to an amplifier. For instance, many multi-channel amplifiers can be connected in "bridge tied load" mode, where two ampliifer channels drive opposite signals to the speaker + and - (rather than having - at ground). Other amplifiers always work this way, and still others allow multiple outputs to be connected in parallel to drive more output power. Each of these modes may have a different minimum impedance and power vs. impedance curve, so make sure to look at the data sheet for the exact topology you use.