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Old 20th February 2008, 01:05 PM   #1
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Default speaker wattages

hello all,
I am new to the world of audio design so I was just wondering how wattages of speakers are divided up in a system.
For example: if i have a 100 watt woofer and a 50 watt midrange in one system, and i feed this system 60 watts; will each speaker take 60 watts, distorting the mid, or will the woofer take the bulk leaving them both in safe limits?

This has been bugging me for some time since so many systems i see have woofers that can take 200-300 watts while the rest only 50 or so.

Thanks in advance for any answers.
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Old 20th February 2008, 02:18 PM   #2
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I think you are looking for power distribution curves, though I don't have direct reference to them, I do know they are out there. Wikipedia might be a good start.

To the best of my memory, the bulk of orchestral power is centered around 500hz. In general, the bulk of the acoustical power of all sound is between 1khz and 5khz.

I think electrically, it takes more power to generate low notes. Woofers are big and heavy and take a substantial force to move them. Mid and tweeters are smaller, lighter, and have to move less distance, so they take less power to move them.

Consequently, the power of the Mid is typically less than the bass, and the power of the tweeter is less than the Mid; all adding up to one power rating for the overall speaker system. Usually, the power rating of the bass speaker.

So, likely in your case, with a 100watt woofer and a 50 watt midrange, and likely an even lower watt tweeter, you probably have a safely rated 100watt system. Though to allow an additional safety margin, 80 watts would be better.

Again, I do know this audio power distribution has already been worked out, and I vaguely remember seeing it recently, but I don't remember where.

Steve/bluewizard
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Old 21st February 2008, 08:28 AM   #3
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thanks for that reply, just what I wanted to know
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Old 21st February 2008, 08:51 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
don't worry about power distribution. That is an averaging procedure that has little relevance to designing a driver grouping.

My philosophy relies on accepting that each driver will have a similar peak transient SPL for the worst case signal in any frequency band.
It allows for the use of the crest factor in normal music. Here we have an average level of music (or speech/sound effect) and peaks that rise between 10 and 30db above the average. The LF peaks can and will be of very long duration (lots of heating effect) the mid F peaks will be of significantly shorter duration. The treble peaks will be very transient in nature and correspondingly have little heating energy in them.
It is this transient nature of the peak that prevents the voice coil burning out.
Equally, you cannot test a combination of drivers using a high power swept sinewave. The mid and treble drivers are at risk.

I generally try to obtain sufficient overhead in the music system for 20db crest to average. Listening at 1W average is loud and the system can take voltage peaks 10times this with minimal distortion/clipping.
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Old 21st February 2008, 04:19 PM   #5
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Excellent information Andrew, but I think what the original poster is getting at is that there must be some general rule for determining how much rated power a Mid or Tweeter needs to be the functional equal of the woofer.

We know less rated power is needed in the mid and high range, but how much less? Would a ratio of 100%woofer:50%Mid:25%High be preferred or would some other ratio?

What I mean is, as a general rule, if you have a 100watt woofer would a 50watt mid maintain the overall speakers 100watt rating, and would a 25watt tweeter maintain this 100watt system rating. How about 100:30:10? How about 100:20:5?

People make this judgment call everyday, they must be using some general guideline.

I am aware other consideration come into play. If you are pushing your tweeter near the low end of it's rated response, you probably need to de-rate the power, for example.

But still, people combine woofer, mids, and tweeters everyday into complete systems, they must at least have a vague idea of what power ratings are workable.

Just a thought.

Steve/bluewizard
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Old 21st February 2008, 04:33 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

The equal power point for the bass range compared to the rest is
~ 350 Hz, may be higher with classical and lower with pop / rock.

Note that the above is clipping level requirements, not average power.
Dynamic range in the bass is often compressed and consequently can
run far higher unclipped average levels than the midrange / treble.

/sreten.
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Old 21st February 2008, 04:38 PM   #7
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Quote:
Originally posted by BlueWizard
some general rule for determining how much rated power a Mid or Tweeter needs to be the functional equal of the woofer.
I think any ratio would be highly dependent on the XO point and chosen slope so it would vary tremendously.
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Old 21st February 2008, 05:14 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I believe it's down to maximum SPL of each driver and that is inextricably linked to sensitivity and power handling.

Take two drivers both of 90db/W/M and of equal impedance.
Match them up through a crossover and they will play at roughly the correct level relative to each other and will have a similar peak SPL if chosen by that method.

Now take an 84db bass and 90db mid and 96db treble drivers.
The bass amp will need four times as much power cf the mid and sixteen times the power driving the treble just to get matching SPLs when approaching maximum loudness.

It's this last scenario that leads to recommendations for enormous power handling in wideband bass drivers, which are very different from high sensitivity narrow band bass drivers used in PA.
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Old 21st February 2008, 08:45 PM   #9
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Cal Weldon said:

"I think any ratio would be highly dependent on the XO point and chosen slope so it would vary tremendously.

True, and I acknowledge that in my post, but this is a decision that people making countless times a day. They must be basing that decision on some guideline. They must have some very basic criteria to make that decision on.

For the record, attached is a Orchestral Power Distribution. Note that the peak is at 10 Acoustical Watts. That should make it easy to scale.

Steve/bluewizard
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File Type: jpg powerdist-a.jpg (42.9 KB, 147 views)
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Old 22nd February 2008, 07:16 AM   #10
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Thanks for all the help guys!

So according to Blue Wizard's graph, a the woofer to mid crossover should be around 500 hertz to allow the woofer to take that huge wattage load that occurs from 250 to 500 hertz?
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