Re: Power Ratings for Speakers

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Posted by JackAttack on January 27, 2001 at 16:08:03:

In Reply to: Power Ratings for Speakers posted by Moyle on January 27, 2001 at 13:56:57:

: What is the difference between RMS handling power and Program handling power. The Ampeg speaker enclosure PR-410HLF has a rating of 600watts RMS and 1200watts Program power handling. What does this mean?

First of all, there is by definition no such thing as an "RMS watt"; marketing flacks decided it sounded cool about thirty years ago, while engineers cringed. You can have RMS volts, or RMS amps, but not "RMS watts."

The correct term for "RMS watt" in this application is 'average sine power', only people who don't know any better say "RMS watt".

Anyway, you have entered into a relatively grey area.

The "600 watts RMS" means that "8 ohm cabinet" (again misleading--that is the nominal value of the impedance minimum above driver fundamental resonance and typically varies widely with frequency because uncompensated moving coil drivers are extremely reactive loads) should be able to withstand by some (again arbitrary) standard of required/acceptable acoustic output at a given distortion criterion around 70VRMS long term.

Even that is misleading because virtually all drivers exhibit some degree of power compression (the motor can't dissipate heat fast enough, the voice coil windings get hot, the resistance goes up and the output drops. All of a sudden it could take twice the voltage (four times the power) to get the same output--that would be a pretty severe 6dB power compression, most are 2dB-3dB, about 40% more voltage from the amp). In good designs compression is fairly transient and the cabinet will not audibly "quiet down."

The deal with "1200W program power" is also bogus. "Program power" is somewhat analogous to "peak power" or "instantaneous power" as opposed to average sine power.

Which in turn IMPLIES that the cabinet can withstand a half sine wave peak of (sqrt2)*70VRMS or 98V peak which in turn works out to 1200W peak.

That's where the 1200 W peak comes from--using the peak voltage in the power calculation instead of the RMS voltage.

It's kind of telling you that you have 3dB of headroom in your amp (double the power for some short period of time, usually in the 10 millisecond range).

The cab will theoretically take transient voltage spikes of around 100V peak, so you can pop/slap/tap with less audible distortion.

Some manufacturers that are really full of it will use peak-to-peak voltages in the calculation, yielding a super-inflated power rating of ruor times the average sine power--in your case 2400W--and they would call it something like "music power" or "peak music power".

How do manufacturers get away with it? Becuase they know the typical duty cycle for a music signal, even for bass rigs, is low. I'll have to double check the exact texbook definition for duty cycle, but it's basically an indicator of what portion of the peak voltage or current must be available for a given period.

Hope this helps.


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