# Relative Power in Audio Spectrum



## Kral (Mar 6, 2011)

I’m interested in speaker design and construction. One of the design decisions that must be made is determining the power rating of each driver in a multi-driver system. Anybody know if there is an available study that shows the typical relative power contained in each segment (1/2 octave, 1/3 octave etc. of the audio spectrum for various types of program material (classical, heavy metal, TV, etc)?
Kral


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## WooferHound (Dec 8, 2010)

For sound in general, each full octave that you go up has half the watts in it. In Physics you would say that, everytime you double the frequency you have twice the energy and if doubling the frequency is twice the energy then you need half the watts. 

You can look at this in the other direction as, everytime you go down 1 octave you need twice the power in watts. this means that there is very little power in the highs and can be a huge amount of power in the lows.

Lets look at the octave centers and the amount of power you would need to have available if we started with 1 watt in the uppermost octave...

Freq - Watts
16k = 1
8k = 2
4k = 4
2k = 8
1k = 16
500 = 32
250 = 64
125 = 128
60 = 256
30 = 512


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## fusseli (May 1, 2007)

WooferHound said:


> For sound in general, each full octave that you go up has half the watts in it. In Physics you would say that, everytime you double the frequency you have twice the energy and if doubling the frequency is twice the energy then you need half the watts.
> 
> You can look at this in the other direction as, everytime you go down 1 octave you need twice the power in watts. this means that there is very little power in the highs and can be a huge amount of power in the lows.
> 
> ...



Exactly why I try to explain to people with big mains that they are saving amp power on their AVR by using an HPF when using a sub.


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## pepar (May 30, 2006)

Could you please link me to something that I can use to make this power per octave point on another forum? I can't find a reference. I'd use you as the reference, but you know how people on forums are. 

Thanks,
Jeff




WooferHound said:


> For sound in general, each full octave that you go up has half the watts in it. In Physics you would say that, everytime you double the frequency you have twice the energy and if doubling the frequency is twice the energy then you need half the watts.
> 
> You can look at this in the other direction as, everytime you go down 1 octave you need twice the power in watts. this means that there is very little power in the highs and can be a huge amount of power in the lows.
> 
> ...


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## GranteedEV (Aug 8, 2010)

I think you want equal power accross the spectrum AVAILABLE (although 10db more for Movie LFE)

But in music content IE Classical, the majority of the amplifier energie is from 100hz to 400hz or so. That's where the fundamental frequencies for many subjective "dynamics" just happen to be located. What makes it especially worse is that it's also where baffle step compensation for monopole speakers normally comes into place. With passive 2/3 way speakers we may need to apply resistive attenuation to get flat response because much energy is lost to the backwards portion of a large wavelength. With 2.5/3.5 way speakers we may have a dipping impedance profile because multiple woofers are used to compensate for the backwards dropoff.


So I'd say in the region from 100hz to 400hz, you often want a good 105-110db capability. Below that, it doesn't hurt to have the same amount, although it might not get called upon. Above that, you're often limited by your tweeter (and perhaps midrange's) power handling (both thermal and mechanical) and this should be taken into account but i think the same amount of power is a good choice.


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