Wattage ratings for active loudspeaker, tweet vs. Mid?

[maggiesnmacs]

I've read a little about what relative amplifier power ratings should be between different bands of an active speaker system, but wanted a little clarification from people with experience. I'm building a 4-way speaker with 500 WPC of class d power for the subs which are going to operate up to about 100hz, then most likely anywhere from 20-40WPC for the remaining bands which will be a PA mid bass operating from approx. 100hz - 300hz, "full range driver" operating from about 300 hz to 6khz and a tweeter from about 6khz on up. My dilemma comes from deciding power ratings for the amplifier. Im designing the amp myself and want to keep the necessary power down to keep amp cost down, but my full range will be about 96dB/w where as the tweeter is 88db/2.83v with a 6ohm rating so really about 86db/w. I know in average power the tweeter will never really need much power, but will I need way more peak power with this tweeter since my full range is so much more sensitive just for the tweeter to keep up or will the same amp power rating suffice for the tweeter?

 

[Ron E]

I've never done this, but this is how I would go about starting to figure something like this out. In music, the average power goes down as frequency goes up, but the peak power doesn't change nearly as much.

Pink noise has a bit more HF on average than music, at least above 5k or so. Pink noise has equal power per octave. This means that if you take 20-10k as 9 octaves (just ignore top octave for the calculation), if you need 9W RMS for a satisfactory average SPL from a full range speaker system with a given sensitivity, you essentially need 9W/9octaves, or 1W/octave. So for a woofer from 20-320 you need 4W, for a mid from 300-3k you need 3W, and for 3k-10k you need another 2W.

Then you tack on crest factor and just multiply power by 10 (or 15, or 20) and get 40, 30 and 20W amplifiers.

If a component of that system is less sensitive by 10dB, you need 10dB (or 10x) more power for that component... If more sensitive you need less power for that component.

One guy who went active and has some music spectra and other things, is Art Ludwig.
http://www.silcom.com/~aludwig/contents.htm

The specific section I remember reading (almost 20 years ago!) is this:
http://www.silcom.com/~aludwig/EARS.htm

 

[Ron E]

I did a check by processing roughly 30 second sections of some files with high and lowpass filters in Audacity
Rock1 - Rush
Rock2 - Toadies
Jazz1 - Flim and BB's

Process with 24dB/octave high and lowpass filters at 300 and 3000
Normalized files so peak levels were 0dB fullrange.

0-300Hz Lowpass
Rock1 -3.5dB or 44%
Rock2 -1.5dB or 71%
Jazz1 -5.2dB or 30%

300-3k Bandpass
Rock1 -2.6dB or 55%
Rock2 -1.5dB or 71%
Jazz1 -3.4dB or 46%

3k-22k highpass
Rock1 -2.9dB or 51%
Rock2 -5.5dB or 28%
Jazz1 -2.1dB or 62%

The numbers mean that a signal with the applied filter was down that many dB (peak, not RMS) from the fullrange signal. The filtered signal needed to be amplified by that much to reach 0dB.

The number in percent is the percent of full range signal peak power needed in that particular bandwidth. So if a single amp needed to be 100 watts peak to drive the full range signal, it would need to be 62Watts to run the 3k-22k band for the jazz track, but only 28W for the Rock2 track.

So the amount of peak power needed in each bandwidth is very program dependent....

 

[TMM]

Ron's calculations mirror what i've worked out in the past. Unless you like a very bass heavy sound, you can't get away with an amplifier that is much less powerful for mid-high because while the rms/mean power is much lower the peak power isn't.

The only exception to this is that if your tweeter amp hits the rail once in a blue moon you probably aren't going to hear it because many of the distortion products are ultrasonic, especially when you are crossing high. For that reason you can probably get away with a tweeter amp about 1/2 to 2/3rds the output power of that calculated using the method above. If you are designing your own amp then you might as well build it for the required peak power but skimp on things that are required for higher continuous output power - smaller heatsink, smaller powersupply filtering caps, etc.

 

[Ron E]

I expounded on this a bit more on a web page that I can edit...
https://sites.google.com/site/diyloudspeakerdesign/home/advanced/active-crossovers

 

[hahfran]

depends on distance. professional nearfield monitors have bass/mid 150 watts continuous and high range ( crossover frequency 2.4 kHz ) 60 to 80 watts cont.
For classical music the peak acoustic power is at 200 Hz there is no chance to reproduce
the sound pressure of a large chorus because the level is up to 130 dB in typical concert hall distance that is at least 10 meters. I think to browse through the ratings published by makers of active speakers is a good place to start because to account for acoustical properties of living rooms is a difficult job they should know better

 

[Charles Darwin]

Nearfield monitors are all over the place power-wise.

For 8" 2ways they range from 56W+56W for the JBL LSR308 to 320W+200W for the Quested VS2108 and everything in between. Mind you the JBLs retail for less than a tenth of the Questeds.

I think RonE's calculations get you closer to an answer than comparing monitor specs. ;-)

 

[hahfran]

I think RonE's calculations get you closer to an answer than comparing monitor specs. ;-)

not agreed music/program specific speakers who wants that? It is listening distance that matters for power of driving amps and that power must cover all types of program. And listening distance is a raw approximation as any reflection on "acoustically hard" areas of the living room is unaccounted .

What a difficult discipline of physics acoustics is can be seen how acoustic engineers - and physicists- design car motor sounds. Far more demanding than amp diy! And how does beer sound which the ad promotes as light and refreshing? Another sound designers.
No acoustics is far above amp and speaker diy. So i keep it find answers by comparing monitor specs and in general multi way active speaker specs

 

[AndrewT]

The required drive voltage depends almost totally on the sensitivity of the speaker drivers.

Choose the drivers having the same impedance and the same sensitivity and you will find that they will reproduce at similar SPL levels and sound balanced when driven with similar peak output amplifiers. They don't need more, nor less powerful amplifiers.
Make the peak SPLs match.

Here's an example using different sensitivity drivers, all of the same impedance.
bass 89dB/Wm, 100W gives 109dB @ 1m
lower mid 91dB/Wm, 63W gives 109dB @ 1m
upper mid 93dB/Wm, 40W gives 109dB @ 1m
treble 101dB/Wm, 6.3W gives 109dB @ 1m

 

[billshurv]

certainly accoustic music is expected to have a pink noise 'like' spectrum. If you look at section 1.3 of this link BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1 you can see the table Fane use and I suspect is copied a lot in the pro industry.

Using that suggests that you would want
50% bass
40% midrange
10% tweeter.

Compare that with ATC SCM100A which has 200W, 100 and 50 for not dissimilar ranges, which doesn't match the Fane recommendations.

Suspect there is no easy answer to this, esp if you listen to electronic music that can and does break every rule of expected power densities. But certainly interesting to know if there is a believable rule of thumb.

 

[MAAC0]

Magiesnmacs:

I'm no expert but take o look at Rod Elliot's page about blowing tweeters. Perhaps the sound distribution graph can help

Why Do Tweeters Blow When Amplifiers Distort?

You should also first match the driver's sensitivities. There is also some info on designing crossovers.

Passive Crossover Network Design

Usually you want to pad down the mid-range and tweeters, since they have more sensitivity

 

[hahfran]

One has to account for power handling capacity and sensitivity the latter is defined as mounting on infinite baffle and half-sphere sound pressure field. Power handling : most commercial active speakers come with frequency dependent limiters to make use of the full
xmax headroom. Who wants a deeper understanding of "active" : read R Stahl's US patent. That is active

 

[ICG]

My dilemma comes from deciding power ratings for the amplifier. Im designing the amp myself and want to keep the necessary power down to keep amp cost down, but my full range will be about 96dB/w where as the tweeter is 88db/2.83v with a 6ohm rating so really about 86db/w. I know in average power the tweeter will never really need much power, but will I need way more peak power with this tweeter since my full range is so much more sensitive just for the tweeter to keep up or will the same amp power rating suffice for the tweeter?

Well, it's not that complicated. You need only to determine two things:

1. At what sound level will you listen? If you know that, you can easily calculate how much power you'll need. You have to make sure your tweeter can handle the needed power and have to have enough dynamic reserves and especally low distortion at the needed spl.
2. The tweeter doen't need much power but a clean amplitude. Your amp has to be able to deliver the signal clip free, a high enough rail voltage is therefore important, a huge transformator isn't needed though, the high impulses are usually very short.

To be honest, I personally think it's not a good idea to pair up a high efficiency fullrange (+subwoofer!) with a low spl tweeter. There are very few tweeters which can keep up dynamically, maybe an AMT or a big ribbon tweeter. Maybe the power isn't the issue but the sound most probably is.
Besides that, you should keep an eye on the dispersion, a full range with 96dB sensitivity usually has probably ~10", but even 8" ones narrow down a lot at 5kHz but 'normal' tweeters are still wide open at that frequency. What kind of tweeter are you going to use?

 

[Globulator]

Besides that, you should keep an eye on the dispersion, a full range with 96dB sensitivity usually has probably ~10", but even 8" ones narrow down a lot at 5kHz but 'normal' tweeters are still wide open at that frequency. What kind of tweeter are you going to use?

Very true. If you are lucky you even get dispersion graphs like this:
FRS 8 M - 8 Ohm

It turns a rising response into a falling response just but moving a few degrees off axis.

Checkout their tweeters at the top, amazing dispersion.

Shame so many drivers don't even come with frequency plots though, let alone dispersion data.