cheers

Dan

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cheers

Dan

Dan:

To be safe always go with the RMS rating of a speaker. This is the actual amount of work the speaker can do safely. I suspect the total music power makes the little boppers feel all warm inside and allows them to show off but it is terribly misleading to someone who doesn't know any better. Manufacturers know better than this but it improves sales. Total music power is a useless spec.

normally I would tend to agree with you but the manufacturer I am referring to is Fostex a japanese manufacturer of among other things wide range drivers. I don't think their market is teeny boopers or DB drag racers. This is what puzzles me about the use of such a figure.

Dan

Hope this helps,

ding said:

cheers

Dan

All these terms are truely confusing! Their definitions are obscure to most people at best. For example, the term "RMS power" is actually a bit of a misnomer in itself. It really means average power dissipated. RMS or root-mean-square current and voltage (not power) is handy for trying to calculate the equivalent dc V or dc I in a mains electrical circuit that would give the same heating power into a resistive load as the ac V or ac I. So the average power dissipated by a resistor R is the (Vrms)^/R or (Irms)^2xR. This only works with a resistive load. A speaker is far from being a resistive load. Also, RMS V and I are 0.707 of their peak value in the case of a perfect sinewave, and this leads to the rule of thumb of the average power being half the peak power. But this isn't true of a music waveform - you would have to work out the rms by integrating the VxI across the load. I think music power is an attempt to better quantify the power, but I don't know how it is calculated either!

Traderbam:

The way I had it explained to me in school is all I know. You are correct "root mean square" and ".707". If you look at a sine wave of a signal .707 or 70% of the way up that signal starts to peter out as it prepares for the ride back down. This peak is unable to contribute any usable power to the signal and through the use of RMS we ignore it if we don't need to see it. The reverse of .707 is 1.414 which we all know will give us the DC voltage after the rectifier, which happens to be the peak of the original AC voltage. Most people would be suprised to know the actual peak of a 120v residential circuit. You are right in the respect that other factors influence the conclusion. Inductance and capacitance can cause current to lag or lead the voltage. This is whats known as the difference between true power or apparent power unfortunately we can't measure this with a volt,ohm meter. I generally try to ignore all but the rms spec only because it is the least of the three and probably the safest.

I don't know if there is a rule to define it, but I've already seen many different ways to measure it. One good tip is to search in good speaker manufacturers sites the definitions for these measurements. One site I've seen says it uses normal musical and voice music programs allowing 5% maximum distortion. The call it "musical power", like Fostex in its International site.

regards

one consideration is the thermal limit. that is, how much power can go into a driver before it blows the voice coil. this is, obviously, key, because this is sort of an event horizon-once you exceed this, there's no turning back.

the other consideration is the displacement limit. now this gets fuzzy. this is where alot of manufacturers fudge. all speakers are at least weakly non-linear, and, when driven to excess, become even more non-linear. what that means is that distortion tends to increase with increasing power. you can exceed a displacement limit and not exceed the thermal limit and what do you get--alot of distortion.

but it's not like the graph of a drivers distortion is a step function. distortion is relatively low through the lower portion of the driver's power range, then at some rate, the driver becomes more non-linear and more and more distortion occurs.

different manufacturers quote different displacement limits because they pick different acceptable distortion levels-5%, 10%-often they don't even say. so, the same driver could be rated at different power levels by different manufacturers.

the moral of the story is that published specs are a good starting point, but not really all that helpful. usually, most drivers distort prior to melting so you can get an idea of the driver's ability this way. (I'm NOT saying you should drive your units to audible distortion-unless you want to kiss your tweeter goodbye.)

PassFan said:This is whats known as the difference between true power or apparent power unfortunately we can't measure this with a volt,ohm meter.

*There are two parts of apparent power -- imaginary and real.

*The imaginary part, Q, in volt-amps-reactive or VAR, is due to the reactive component of the load impedance caused by L or C.

*The real part, P, in watts, is due to the resistive component, R.

Apparent power, S, is described volt-amps or VA. The equation relating the three is as follows and is related to a triangle using pythagreon's theorem. P and Q form the sides, and the diagonal is S.

S = sqrt(p^2 + q^2) or abs(p + j*q)

j is the same as imaginary "i" in EE world.

Your multimeter will only arrive at apparent power S since it only sees the magnitude of the current or voltage.

0.707, or sqrt(2), is arrived at by integrating the absolute value of a 1V peak sine wave over one period - this is the average value of the signal. Another way of looking at it, is to add up all the area under the sine wave for one period. This area is the same as the area under a DC signal of the same length, with the RMS voltage instead.

Back to the subject... I have found a definition in Randy Slone's amp book:

Originally from R. Slone

Music Power - An ilegitimate term used to describe the output power of an audio amplifier. In reality, it is a schrewd way of squaring peak output values instead of RMS values when calculating power, and amounts to nothing more than a marketing gimmick to promote sales.

D

I guess the only reason you would need this with your speaker specs is when your amp is rated in "music power" (I hope your amp isn't rated in music power!!)

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