Speaker RMS

Discussion in 'ICE, Sat Navs & Dash Cams Forum' started by turtlelips76, Jun 8, 2009.

  1. turtlelips76

    turtlelips76
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    Hi, im new to this site and car audio. I am looking to get a new system for my car. I am confused by RMS power and max peak power. I know what they mean, but do i have to match the speaker and subwoofer RMS to the amplifier RMS output? Any help would be greatly appriciated :)
     
  2. BlueWizard

    BlueWizard
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    RMS power refers to running an amp with a pure sine wave tone input, and measuring the output levels in terms of power. This, I think gives the truest indicator of an amps ability to deliver power under continuous demand conditions, and it is the first power rating I look at.

    Also, note that RMS power must be to 8 ohms. Rating power at lower ohms is a way of inflating the power. But the true limit of power, is not power, but available voltage. When you run out of available voltage, for whatever reason, then you run out of power. And regardless of the load impedance, the voltage is the same, or in most cases actually low at lower load impedances.

    You will also see RMS sometimes rated as FTC or RMS/FTC, that stands for the USA Federal Trade Commission which sets the standard for how RMS power is measured.

    Lastly, on the subject of 'RMS', while this is a common term used to refer to power, in technical reality, there is no such thing as RMS power. There is only RMS voltage and the power that results from the application of that voltage. So while purists can help pointing this out, it is none the less a common and accepted term when applied to power, even if it isn't technically correct.

    But, music isn't a single pure sine wave tone, it is complex blend of sounds at all levels and all frequencies. So using something approximating real music will frequently result in a higher power rating, and some feel that it is a much truer test of an amp.

    I personally feel 'music power' is valuable information, but to me it is nothing if it is not combined with RMS power.

    Also, power rating at 4 ohms are valuable, but only when used in conjunction with power ratings at 8 ohms. Technically, the 4 ohm power should be double the 8 ohm power, though that is rarely true, especially in consumer amps.

    And that is true for a variety of reasons. One reason could simply be that additional protection circuit could kick in when the load is 4 ohms, and that limits the power. But in a broad and general sense, the higher the 4 ohm power is relative to the 8 ohm power, the better the power supply is. Again, that's not an absolute. You need to weigh a full blend of information to made a final determination about an amp.

    Peak power is something of a farce. True, under certain testing conditions, it is possible to reach 'peak power' for a very brief period of time, but, while having some value, it is not a very realistic power rating.

    Now this is slightly different for Car Audio. Because cars only have a limit amount of voltage available, they make up for it by having much lower impedance speakers than home audio. You will frequently see 4 ohms car speaker, and sometime 2 ohms speakers. In this case, an amp with a power rating to 2 ohm implies that that amp is capable of functioning with a load that low. Something a home audio amps can't handle.

    In terms of matching, yes, you do need your speaker to be reasonably within the power of your amp. That doesn't mean only 100w speakers will work with only 100w amps. The speaker power rating is generally the highest power an speaker can sustain for a given period of time, though they never say exactly what that period of time is.

    So, you can use a 100 watt speaker, on 100 watt or SMALLER amps. If you don't get too crazy, you can use a 100w speaker on a 150w amp on the assumption that you are never going to use full power. But, you need to use an element of caution and common sense in a situation like this.

    So, the general rule for mating speakers and amps, is that the amp power must be equal to or LESS THAN the speakers rated power.

    Is your answer in there somewhere, or did I misunderstand the question?

    Steve/bluewizard
     
  3. turtlelips76

    turtlelips76
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    I am now wishing to add an active sub to my system, because my amp is only 2 channel and don't want to buy another. Can i hook the active sub up to my HU, or would i need to hook it to a new amp?
     
  4. koush

    koush
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    an active sub usually has the amp built in. It a quick way of getting a sub, but sound quality is not good as a separate amp and sub.

    Koush
     
  5. turtlelips76

    turtlelips76
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    i know that active subs have amps built in, but i want to know where to connect the input signal from. Not the power, remote or ground cables.
     
  6. koush

    koush
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    You probably have a high level and low level connections. The low level you just connect to the rca output from your hu, high level you tap into the speaker out and connect it that way. the low level connection give you better sound.

    Koush
     
  7. turtlelips76

    turtlelips76
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    OK cheers, is that off the HU speaker outs?
     
  8. koush

    koush
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    yes you can or tap it off near your rear speakers if the amp is in the boot, to save running wires to the back of the car

    Koush
     

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