So, what are the speaker specs I should pay attention to?

That's a good question. For the purposes of the average shopper, there are three specifications (aside from size and price) that you should pay attention to when shopping for car speakers — sensitivity, frequency response, and minimum power handling.

Speaker sensitivity
A speaker's sensitivity denotes how loud a speaker plays for a given voltage level from an amplifier.

Huh?

Stay with me now.

Think of a speaker's sensitivity like this — it tells you how effectively the speaker converts power into sound. The higher a speaker's sensitivity rating, the louder it will play on the input power it receives. A speaker rated 3 dB higher than another, for example, requires much less power to produce the same output, or volume.

Sensitivity or efficiency?
Now, here's the tricky part — due to the lack of an accepted standard for measuring speaker sensitivity, different manufacturers have different ways of determining a speaker's sensitivity.

1. 1 watt/1 meter: The manufacturer measures the sound pressure level (SPL) that a speaker produces with one watt of power at a distance of one meter. But this measurement is actually a measure of a speaker's efficiency, and not its sensitivity. That's because what determines the value of 1 watt of power depends on the frequency of the signal and the speaker's impedance. A speaker requires much more power to produce low frequencies (200 Hz and lower) than high frequencies (1 kHz and up), so an efficiency rating based on the SPL at 1 watt/1 meter depends on the frequency used in the test.
   
   "Efficiency is strictly defined as how much acoustic power the loudspeaker puts out for how much electrical power it is being driven with. If you feed a loudspeaker with 100 electrical watts, how many acoustic watts of sound does it produce? The answer is 'not many,' a typical moving-coil loudspeaker being about 1% efficient." ³



2. 2.83 volts: The manufacturer measures the SPL a speaker produces at 1 meter on 2.83V of input power. Today's solid-state amps do a pretty good job of maintaining their output voltage in comparison to older, tube-style amps. So the measurement of a speaker's voltage sensitivity is considered a more accurate measurement.
   
   Now, a voltage of 2.83V will produce 1 watt from an 8-ohm speaker (the impedance of a typical home speaker), but car speakers are almost universally 4-ohm designs. So using 2.83V to measure a car speaker actually gives you the speaker sensitivity rating of 2 watts input power measured at 1 meter — that's an effective 3 dB gain in sensitivity from an actual 1 watt/1 meter SPL measurement!!

For a truly accurate measurement of a 4-ohm car speaker's voltage sensitivity, you'd need to measure the SPL of the speaker at 1 meter, on 2 volts.

Tom Breithaupt, Amp and Speaker Engineer and Product Manager for Blaupunkt, explains:

Computing power (watts) from an amplifier (the voltage device) to a speaker (the resistor) is simple math — P=(V x V)/R. Thus, running 1 volt into 4 ohm speaker, this computes to (1 x 1)/4 = 0.25 watts. Now, for 2.83volts/4 ohms, this equates to (2.83 x 2.83)/4 = 2 watts. As for 1 W/1 m, this requires 2V of drive voltage, which is (2 x 2)/4 ohms. Now for an 8 ohm speaker, for 2.83volts/8ohm, THIS NOW is 1 watt!! What is the difference? It is simply a 3 dB higher value when you reference 2.83V over 2V for a 4 ohm speaker as 99% of car audio is.

³ "Measuring Loudspeakers, Part I," Stereophile, November 1998