First up what is RMS and why is it so important?
This is the amount of continuous power, measured in watts. The higher the RMS figure, the louder your music can sound. Higher RMS numbers do not always mean cleaner sound though. The RMS output figure is what you should use when comparing different receivers, amplifiers and speakers.
Great your thinking all I have to do is match the RMS on my amp with the RMS on the speakers to get the best out of them. This is true except the marketing department didn’t come to the engineering meetings and decided they would fudge the figures to make them bigger so they could sell more stuff.
Even if we ignore the obviously wishful ILS ratings (If Lightning Strikes) written all over audio products different companies use different methods to get their RMS ratings.
Companies like Alp*** who spend a lot more on marketing than product development rate their gear using the standard put out by the Consumer Electronics Association (CEA) CEA-2006. This "voluntary" standard advocates a uniform method for determining an amplifier's RMS power and signal-to-noise ratio. Using 14.4 volts, RMS watts are measured into a 4-ohm impedance load at 1 percent Total Harmonic Distortion (THD) plus noise, at a frequency range (for general purpose amplifiers) of 20 Hz to 20,000 Hz. Signal-to-Noise ratio is measured in weighted absolute decibels (dBA) at a reference of 1 watt into 4 ohms. This applies to both external amplifiers and the amplifiers within in-dash receivers.
Only problem with this standard is its at 14.4V. Don’t know about you but if I get a volt meter and put it on my 12V car battery while the engine is running I don’t get 14.4V. As a consequence this standard overstates the RMS power because the equipment is not getting 14.4V but more like 13.3V with the engine on and 12 to 12.5V with it off.
I prefer it if everything was rated at this more realistic voltage and so do a lot of audio companies that are not controlled by the marketing department. You’ll find equipment from the real honest car audio companies is all rated this way so if their figures were converted to the unrealistic 14.4V it comes out a lot more powerful (some 20% to 30% higher).
The 2nd problem with this CEA standard is the 1% THD (distortion). For audio purists 1% is an unacceptably high level of distortion but again by using this level it boosts the RMS number.
Real honest car audio companies rate their equipment at lower THD levels. Some as low as 0.01%. If their RMS figures were recalculated at 1% their RMS figures would again be much higher.
This means you have to look not only at RMS but at what voltage and THD (distortion level) the RMS figure was taken at to be able to compare different manufacturers products and be able to match your amps and speakers correctly.
For example an Alp*** amp won’t cut the mustard powering a Crossfire sub even though the RMS appears to match up because the 14.4V 1% THD rated Alp*** amp doesn’t have the juice for the Crossfire gear all conservatively rated RMS at 12.5V 0.02%THD. A 700W RMS Alpine monoblock amp only does 500W RMS when run at 12V and even less when the THD is dropped from 1% to 0.02%.
Due to this matching difficultly a lot of people buy subs, speakers and amps from the same company because at least you can match the RMS correctly. Unfortunately not all car audio companies are good at making all car audio products. In fact most companies have their speciality. In the case of Alp*** its head units and in the case of Crossfire its subs. To get the best possible system then people want to be able to cherry pick the best products from a number of companies and mix and match.
Car Audio equipment Power Ratings and real RMS
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16 January 2007
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