3 Responses

1. Jatznic

   12|Jan|2010 1

   You want your speakers to handle your system’s output power, so you’ll need at least 150W speakers.
   The 91 dB sound pressure is an indication of the efficiency of the speakers. Higher is more efficient, lower is less efficient. Remember that dB is logarithmic, so a 3 dB change is a factor of 2 (88 dB uses twice the power to sound as loud as your 91 dB speakers).
   A key spec of the frequency range was left out, and that is the variation from one frequency to another — i.e. +/- 3 dB. Ideally, you’d like +/- 0 dB so all frequencies would sound the same with the same power input, but that’s not going to happen, so most spec how close they come to the ideal. A speaker with +/- 0.5 dB across the frequency range will sound far more accurate that one that is +/- 3 dB (remember that 3 dB is a factor of 2).
   The wider and flatter (+/- 0dB) the frequency range, the better.
   Impedance by itself generally doesn’t relate to sound quality; you just need to match your amplifier’s preference for best results.
   But remember that the specs are the manufacturer’s specs, not independently certified specs. You’d do well to consult audio review magazines for their test results of the speakers you’re thinking of instead of believing the manufacturer’s specs. They aren’t perfect, either, but they will probably be more objective than the OEM.

2. sd_ducks

   12|Jan|2010 2

   The above answers are correct except that it is not always necessary to match the impedance of the speaker to the amplifier any more. This used to be a requirement (and still is for a few high-end amplifiers) for tube amplifiers using output transformers. Nearly all modern consumer-level amps use transistor output stages that are inherently low impedance (voltage sources). They are capable of driving a wide range of speaker impedances; the acceptable range is usually indicated in the amp’s specs. The range is limited on both ends, however. Too low a speaker impedance will draw too much current at high power levels, overheating the output stages and possibly damaging the speaker. Too high an impedance limits the maximum power output of the amp (since the output is a voltage source, the power output is E^2/R, so the power output increases with lower impedance). The amp will usually be rated at a nominal impedance (often 8 ohms) but will indicate maximum power for other impedances as well. Remember, this applies to maximum power and only to those conditions that may drive the amp to near maximum power. Those conditions occur only for very low frequencies at high volume levels.

3. gp4rts

   12|Jan|2010 3

   sd_ducksoup is right, impedance doesn’t really affect sound quality and all you have to do is match the speaker’s impedance with the amplifier.
   To your amplifier, the speakers appear as impedance. From Ohm’s law, we know that Voltage/Impedance = Current. So, if you plug in a speaker with a lower impedance than what the amplifier wants (e.g. a 4 Ohms instead of more current will go through the speaker (in that case twice as much). Since Power (measured in Watts) increases with the square of the current, you risk feeding your speakers too much power and breaking them.