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There are so many speakers rated at 50W RMS (and 250W PMPO), while absolute all head units are about 14-22W RMS per channel (50W max).

What am I missing? Could it be that those aftermarket speakers are all 4 ohms while head units are spec'd for 8 ohm speakers?

Edit: checked few head unit manuals and looks they are specified 20W RMS into 4 ohm channels. So my theory is wrong. Perhaps there is something with heat dissipation & default fuses?

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    Don't forget that speaker power ratings are sometimes exaggerated for marketing reasons. – barbecue Sep 7 '15 at 3:05
  • Typically they would market the max (or PMPO) performance and would hide RMS performance somewhere, so your comment is somewhat irrelevant. – Dzh Sep 7 '15 at 4:06
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The answer has to do with the available power supply. I'm sorry if the answer becomes too technical. A head unit uses the voltage supplied by the car, for the sake of argument lets say 12v. The head unit then turns that supply into a split supply (plus and minus 12V) because that is what speakers need to make sound. The maximum power to the speaker then is square(V)/R where R is the resistance of the speaker. What you get is 144/4=36. 36 watts is the maximum power delivered to the speaker but we are interested in the RMS power. To convert this to RMS divide it by 2 and that gives you 18 watts.

The not technical answer is that the head unit is powered by and only uses 12V from the car. This limits the power that the head unit can provide to the speakers. This is compared to an amplifier that has a DC to DC converter inside and can turn the 12V into a much higher voltage giving it the ability to deliver much more power. The reason they don't put these DC to DC converters into head units is cost and size. Even the smallest and wimpiest amplifiers are at least the size of a head unit.

  • Wait a second? Head units don't have their own dc-to-dc converter? So what if alternator is running and pushing way over 14V to the system? Wouldn't that sound about about 30% louder? – Dzh Sep 6 '15 at 21:53
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    No it would not. Even though head units do not have step up dc-to-dc converters they still regulate and stabilize the rails for their audio output. If they did not all kinds of electrical noise would be heard in the speakers. Also if you follow your hypothesis then if the system voltage dips because the AC came on or the rear defrost is turned on the then the volume would dip. – vini_i Sep 6 '15 at 22:01
  • Isn't RMS generally 70.7% of peak instead of 50% of peak? – TDHofstetter Sep 7 '15 at 1:39
  • For voltage and current it is correct that RMS is 70.7% of peak. But when calculating power (VI) both current and voltage have that 70.7% attached and since multiplication can be done in any order (VI*0.707*0.707) which boils down to (V*I*0.499). Another way to look at it is in actuality 70.7% is the square root of two. So if both voltage and current is a fraction with square root of two in the denominator then when multiplied together the square root of two becomes just a plane two. – vini_i Sep 7 '15 at 11:36
  • @vini 70.7% is the square root of 1/2 but yeah. – Jason C Sep 10 '16 at 14:24
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There are several factors that inform the choice for a 50W rating instead of 20W.

People who install aftermarket speakers may be likely to also install an amplifier. Those amplifiers can generally deliver much more power than the 18 W a headunit is limited to. If you were to design your aftermarket speaker to have an 18 W RMS limit, you'd miss out on that section of the market.

Another factor is distortion (clipping). 18 W is so little that people quite often drive the headunit at its maximum power, so the amplifier will start to distort. Distortion means the sound waves contain more energy: the sound wave looks like a square rather than a sine, so RMS is more than 70% of peak power. The distorted sound wave also contains extra energy at high frequencies. This energy is routed to the tweeter, which isn't designed to handle it.
That extra power has to be dissipated by the speaker. Some of it gets turned to sound, but most of it turns to heat. This heat will eventually melt the insulation on the moving coil that drives the speaker membrane. That will cause a short and/or jam the coil in its gap, making the coil and membrane unable to move. A 50 W speaker has thicker wiring in its coils than an 18 W speaker, so they'll take longer to overheat.

It is not necessary to match power ratings between the amplifier and speakers. In general, you'll want an amplifier with a higher rated power than the speakers, because then you can be sure the signal remains undistorted and clean until you get to the speaker's limit (instead of the amplifier's limit). At that point, the distortion becomes clearly audible and you can reduce the volume before damage is done.

You can ignore the peak and PMPO ratings, those are useless. The only rating worth comparing is the RMS rating.

  • One thing to add is that underpowered amplifier can break your speakers as squared off wave moves the membrane incorrectly (not sure how I can express myself). – Dzh Sep 10 '16 at 0:05
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    good point, added that. Although it's not the movement itself that's the problem, but the heat caused by the extra energy in the distorted waveform. – Hobbes Sep 10 '16 at 11:32

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