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Power Ratings

Discussion in 'Amps and Cabs [BG]' started by Jeff Corallo, Dec 26, 2000.


  1. Jeff Corallo

    Jeff Corallo

    May 30, 2000
    I know that this has been discussed before, but I cannot get the concept straight. Power ratings on cabinets are generally listed using the acronym RMS which is a measure of the amount of power on a continuous basis the cabinet an handle. Then the term "program handling" is used which means the cabinet can take a momentary spike of up to a given wattage (usually twice as much as the RMS wattage). I purchased Fender PA speakers cabinets this weekend. On the cabinets, both terms are used but a new term is mentioned being "maximum watts". As a result, the cabs say the following: 100 watts, 200 watts progam handling, and 400 watts maximum watts. No explanaition is in the owners manual received with the cabinets and the Fender web site makes no mention of what this means. Running the cabinets in parallel (4 ohm) thru a Crest 900 amplifier producing about 300 watts at 4 ohms, will the cabinets be able to handle survive this load?
     
  2. MikeyD

    MikeyD

    Sep 9, 2000
    It's as clear as mud, isn't it? Here's my two cents' worth:

    1. RMS (root mean square) is a way of finding the average quantity (e.g., voltage, current, or power) of an alternating current waveform. In the case of sine waves (the usual context we deal with in audio), the RMS *power* will be 1/2 the peak power of the wave.

    2. RMS doesn't necessarily mean *continuous*. It could be momentary (transient) power over at least one complete cycle. However, in most audio contexts, what is meant as "RMS continuous power" is loosely thrown about as simply "RMS". It is a little misleading, but that's my take on it.

    3. Program handling, I believe, is a way of recognizing that typical music program signals vary a bit. In your case, with the "program handling" being twice the RMS level, the manufacturer is essentially claiming that the average level of the signal is 1/2 of the program peak level - or 3 dB down. So if you have an amplifier that can deliver 200 watts RMS continuously, without clipping, the manufacturer says you can feed a 100-watt speaker under normal "music program" conditions, because such a program will see occasional 200-watt (RMS) peaks, yet its average level (the level that effectively heats the coils) is only 100 watts.

    Another view is that "program" refers to the actual peak power level of the sine wave, in which case 200 watts peak corresponds to only 100 watts RMS (average).

    4. By "maximum watts", I believe they mean the momentary absolute peak power *spike* that can be handled for a very short period (say one whole wave cycle) without damage. I think this rating is useless for most non-technical people.

    I can't tell if the cabinets will handle your amplifier, because you said they were in parallel, but didn't give the quantity or impedance per cabinet. If you meant that you connected two 8-ohm cabinets in parallel to get 4 ohms, then the power will be split evenly between the two: each will see 150 watts RMS. If this is the case, there's a good chance that the cabinets can handle the power *if* the signal almost never clips, if you don't feed them a lot of square-wave (overdriven or fuzz) waveforms, and if you don't have long, sustained notes at full power. In other words, typical clean music signals will probably be okay. If any of the above is not the case, you might want to limit the power to the speakers somehow.

    I welcome any rebuttals from those more familiar with the customary power rating methods.

    - Mike
     
  3. Jeff Corallo

    Jeff Corallo

    May 30, 2000
    The Crest 900 has two channels and can power each channel with 200 watts at 8 ohms, approx 325 watts at 4 ohms (I originally said 300, but I'm going by memory) and deliver 450 watts at 2 ohms. Both cabinets (in parallel) will run off of one channel and 1- 4 ohm subwoofer will be powered off of the second. Both 8 ohm cabinets will be daisy-chained making 4 ohms and will pull 325 watts out of the one channel (assuming that the Crest is asked to deliver this amount which probably will not happen as we don't usually run the amp full open). By what your telling me, I should have no problems with the cabinets. But your right-the terminology is as clear as mud. Thanks for your response.

    [Edited by Jeff Corallo on 12-26-2000 at 06:13 PM]
     
  4. Hey Jeff,
    Here's the scoop: the 100 watt rating is the continuous power rating. This means it can take a steady signal of 100 watts for a (theoretically) indefinate period of time. This type of signal would be like a test tone or pink noise(all frequencies at the same level, sounds like the static on a radio when it's not tuned to anything). You might as well ignore that, because no music is constant like that. The 200 watt rating = program power = RMS power = average power (more or less). This rating is higher than the continuous power because it takes into account the dynamics in music. The speaker can take more program power because levels rise and fall constantly in music. The 400 watt rating is the peak power rating, which as the name suggests, is the maximum power the speaker can take for short periods of time (i.e. microseconds - milliseconds). Your cabinets should easily take the amount of power you stated because if they are in parallel they will only be recieving 150 watts each. If the amp is clipping (peaking) all the time, this will damage your speakers even if they are not recieving their max power. In most cases overpowering your speakers is preferable to underpowering them. Distortion is the thing that ends up killing most speakers not excessive power. Hope this helps, feel free to ask any questions, there are a lot of very knowledgeable people on this board.
     
  5. WOW, both MikeyD and you responded during the time it took me to write mine!! There weren't any responses when I started.... I must be slow:D
     
  6. MikeyD

    MikeyD

    Sep 9, 2000
    Hi, Spacegoat! I guess I gotta argue a little here. In my view, the pink noise is a random, time-varying signal to approximate the power spectrum of real music. On the other hand, "RMS" implies a steady sine wave tone (your "test tone"), without any music-like peaks and valleys to its envelope. You say program power = RMS power, and I think this is confusing. My previous post suggested that the program power rating represents the peak transient power that a typical program signal (music or pink noise) might produce during its transient peaks, whereas "RMS continuous" would mean the average, steady, sine wave power. Your follow-up suggests you were writing yours before having read mine. Anyway, do you disagree? If so, maybe you can point out something I'm not aware of. I think it's important for the original poster for us to come up with a unified explanation. Thanks!

    - Mike