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I'm shopping on Sony's site for all the parts I need for a car audio system, and they have me confused on their power statistics. I understand what peak power is, and I know that I don't want my amp putting out anywhere near that much. But some items also have listed rated power, and RMS (CEA 2013) power listed. For example, one subwoofer has 1300w peak (380w rated), and 500 watts CEA 2031 RMS power. I researched RMS, and everything is telling me that is the rating I should be using for my amp output, but I don't want to be wrong and buy an amp that is going to eventually burn up my speakers. Thanks very much for any assistance.

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Because waveforms are only at the peak for a very short period of time, the most useful figure for amplifiers is RMS - which actually gives a realistic number for the power requirement.

That said, you want a speaker to cope with peaks, as these are what will kill it. Generally subwoofers do give both figures, as they are fed the greatest power in your audio system.

If your amp gives 350W RMS output on the sub channel, for example, then you will be safe with a 500W RMS sub with 1300w peak.

Simple view: Just make sure your speaker numbers are higher than your amp numbers :-)

In my car, I have a 650W amp - although some of my speakers are only rated to 500W - but I feel safe, as my volume controls on the amp are only at around 2 (out of 10) because any louder means I can't have a conversation with anyone in the car even if I shout.

  • Thanks for the answer. Since asking this question I've gotten away from Sony's site and I'm looking at Crutchfield's site, which has everything listed in RMS. Now I just need to figure out how much I need. Maybe you could help with that here in the comments since it's more of an opinion based question. I want my music to sound good first, I want to feel the bass, and it should be loud when I want it to be. I don't need it so loud that the whole block can hear it, though. Is 60 watts each to two 5 1/4s and 2 6 1/2s and 300 watts to a 12 in the trunk going to be more than I need? – Dave Jan 27 '13 at 10:02
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If your sub is 500rms, get an amp that can do that and more. 800rms and up to 1200rms. The more power the more you tune it down, then you don't stress the amp --- 350 rms to 500 rms sub = hot amp clipping = sub damage.

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Peak power is the instantaneous power at any given moment. RMS (Root Mean Square) power is the equivalent power in DC in a resistive load.

In the case of 60 Hz fixed frequency power line current the calculation is relatively easy. However with complex waveforms such as audio the calculations get far more complicated as it involves the summing of the all the frequencies amplitudes in the new complex waveform. Pretty complicated eh?

It gets even more complicated with speakers. Speaker power rating are rated at specific power levels and specific frequencies. This is because the speaker coil is a varying inductance whose impedance changes in relation to its position in voice coil assembly. The average 8 Ohm speaker varies from 2 Ohms to 32 Ohms depending on the speaker design.

Here is an interesting tidbit of information if you want to increase the power rating of a tweeter. Raise the crossover frequency from 1600 Hz to 4000 Hz. Your 10 watt tweeter can handle 50 watts.

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I'm surprised only one answer mentioned impedance and that didn't include the importance of impedance-"matching" (yep, I know this isn't exactly the correct term) the speakers to the amplifier. Shortly said, if the amplifier says e.g. 8-16 Ohm (unlikely in the case of car audio), and your speakers are 4 Ohm, you cannot use the full power level of the amplifier. Or if the amp says 4 Ohm and your speakers are 2 Ohm, you have the same situation. So, be sure to check the impedance of your speakers and select an amplifier that supports this impedance.

If your speakers are capable of more power than your amplifier, it means you don't get everything out of the speakers. If, on the other hand, your amplifier is capable of more power than your speakers, you cannot use the full power level of your amplifier or else you risk ruining the speakers. So, either way (less power than speakers, more power than speakers) can be chosen. Just be careful when turning the volume to the highest possible levels. If the sound isn't clear, stop increasing the power level! But in a space as small as a car, I would guess your ears will start to hurt before that happens.

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