Wattage Required for Equal Output at All Frequencies - diyAudio
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Old 17th September 2012, 03:42 AM   #1
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Question Wattage Required for Equal Output at All Frequencies

As I do know, very low frequencies require a lot of wattage. Very high ones, not so much at all. But, what is the "wattage curve" if it exists and is there an equation?

My usage would be to determine, for example, how much amplifier wattage I would be saving if my active crossover design did not give a driver/speaker full range but rather just 200Hz and above. Or, any range in between certain frequencies for active bandpass crossovers. Or, instead of feeding 0Hz to xxHz, I cut that to 40Hz to xxHz sent to a subwoofer.

The big question and Why?: What is the wattage "saved" that can then be used to provide power to the frequencies that the driver/speaker can actually do very well?

I'm just curious to know if an equation exists. I am guessing that the power requirement increases exponentially as the frequency goes down? And, does this involve using the Fletcher-Munson curve also? Then, it becomes a subjective and not an objective calculation...arrrgggghhh.

One important example that comes to mind is that my mids/tweets in my vehicle are actively sent only 160Hz and above (-12dB/oct cutoff). The amplifier is 150Wrms/channel and each mid/tweet "speaker" is 125Wrms/channel. So, by not sending anything much under 160Hz to those "speakers," am I maybe overdriving them in the 160Hz to 20,000Hz range (grin)?

Thanks!
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Old 17th September 2012, 03:57 AM   #2
DUG is offline DUG  Canada
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It is not an simple answer.

This actually depends on what type of music you listed to.

Rock ? Classical ? Techno ? Other ?

Each type generally has a power vs frequency spectrum that is not the same.

I've seen Bi-amp / Tri-amp setup that call for 10W tweet, 20W Mid and 40W sub. (But that was a AF PWR IC mfg trying to sell their line)

What you save is not having to pay for "full power" at all frequencies when you may not need it.

Take a look at Rod Elliott's page:

ESP - Frequency, Amplitude and dB
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Old 17th September 2012, 04:00 AM   #3
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see the power distribution chart here: Why Do Tweeters Blow When Amplifiers Distort?

the catch with this idea is that in an ideal world, the HF amp need to be able put out the same peak power as the LF amp, in the real world you can get away with a smalle one
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 17th September 2012, 04:19 AM   #4
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Quote:
Originally Posted by PeteMcK View Post
see the power distribution chart here: Why Do Tweeters Blow When Amplifiers Distort?

the catch with this idea is that in an ideal world, the HF amp need to be able put out the same peak power as the LF amp, in the real world you can get away with a smalle one
Thanks DUG and Pete. I'll check out the links and learn all that I can.

As for music styles, mine runs the entire gamut.

I have found extremely low end music by a band named Front 242 where I can actually see the woofer cone moving in and out at around 3Hz! But, that MUST have been a mixing or mastering error. It is fun to watch the woofer cone move like that (and it isn't full excursion at loud volume either, just a slow fluctuation). The exact track is called "Soul Manager" from the 1991 disc Tyranny For You. Disc markings include EK 46998 and DIDP 073112 by Sony Music Entertainment. Pretty hard core music, I admit.

Then, after listening to something like that, I'll go to classical with a composition that includes cannon shots. Then, something that is acoustical guitar like Abba. Whatever comes to mind.

But, I do understand that it does depend on the type of music. I was hoping for an equation that took driver params into account and determined the wattage required to produce equal sound levels at each frequency. Oh well. Nothing in the art and engineering of sound systems works that simply.
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Old 17th September 2012, 04:56 AM   #5
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Slow cone movements in woofers can be due to ported enclosures 'breathing'.
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Old 17th September 2012, 05:32 AM   #6
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or infrasonics...i have quite a few DVDs that have silly levels of sub 10hz rubbish for 'effect', all it does is steal power from the amp, and unload the woofers in a similar way to turntable rumble. I think i have a couple of cds which do the same. Bad mastering or accidental recording of seismic events? Who knows? It shouldnt be there on a cd in my opinion. A lot of synth based music has the same problem, analogue noise generator synth sections used for 'snare' sounds being the worst. Ive seen more excursion from these synth snares than the kick drum sounds. Clearly the electronic music camp has yet to discover rumble filters.
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Old 17th September 2012, 06:21 AM   #7
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Quote:
I was hoping for an equation that took driver params into account and determined the wattage required to produce equal sound levels at each frequency.
Of course that formula would only be valid for that driver.
Several recent posts have concerned AC power determination in a reactive load. This stuff is not for the faint of heart! You really really have to be on your toes to get it correct. It ain't easy... no how, no way.
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Old 17th September 2012, 06:49 AM   #8
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reactive power, Q = VIsin(Phi) where Phi is the phase angle of the current with respect to voltage. Thats the simple part. Calculating the spectrum power, is a rather lborious extention of this i believe.
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Old 17th September 2012, 05:26 PM   #9
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Quote:
Originally Posted by PrecisionAudio View Post
But, I do understand that it does depend on the type of music. I was hoping for an equation that took driver params into account and determined the wattage required to produce equal sound levels at each frequency. Oh well. Nothing in the art and engineering of sound systems works that simply.
The power requirements are not only different for different music, but for different cabinet frequency response.
For instance, a sealed cabinet with a response 10 dB down at 30 Hz could be equalized flat using 10 times the power at 30 Hz than at 60 Hz, while another design that is flat to 30 and the same sensitivity at 60 Hz would use 1/10 the power at 30 Hz.

If you look at a musical spectrum that is 10 dB more bass heavy (say the difference between folk and hip-hop) and different cabinet designs, the difference could be 100 times, the folk music with an efficient cabinet would use 1 watt while the hip-hop with a sealed cabinet uses 100 watts.
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Old 17th September 2012, 06:31 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

Its a long running debate regarding average power to the frequency
ranges and the crest factor of each range. Certainly bass gets a lot
more average power than the mid, the mid more than the treble.

Your really stuck with rules of thumb. Its around 50:50 for a 3 way
with active bass to mid and passive mid to treble, active mid to treble
around 35:15 to 30:20, 2 ways around 80:20 though often less.

Active EQ and different driver efficiencies can really skew the numbers,
and of course they vary with the x/o points used, no simple answer.

Also note going active increases peak handling compared to average
handling, an active 3 way with a total of 100W (say 50+30+20)
can produce wideband peaks equivalent to a 270W amplifier.

rgds, sreten.
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