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Old 17th September 2012, 07:45 PM   #11
jakubg is offline jakubg  Slovakia
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In active multi-way you can save power on midrange and tweeter due to their higher sensitivity.

Rod Elliot has a good example on his website:
"Assume we have a loudspeaker rated at 90dB/W/m (a softspeaker?) versus another rated at a much more respectable 100dB/W/m. With one Watt of electrical energy applied, one will have 10 times (10dB) the SPL of the other. While this is insignificant if we are happy with 90dB SPL, if we try to obtain 110dB SPL at one metre, the efficient driver will do this with only 10W, while the inefficient driver needs 100W. Another 10dB makes that 100W vs. 1000W - anyone want to guess which speaker will last longer before the voicecoil melts?"
Power Handling Vs. Efficiency
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Old 17th September 2012, 08:47 PM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by PrecisionAudio View Post
As I do know, very low frequencies require a lot of wattage. Very high ones, not so much at all.
I don't agree with either of your first two statements.
That probably puts be at odds with 95% of the Members.

I claim that all the drivers must be capable of similar maximum SPL and that the power of each of the frequency band amplifiers should be chosen to achieve that same maximum SPL in their respective frequency bands.

If you start with that as a hypothesis, you may end up with a balanced sounding active speaker system that does not grossly clip on any particular frequency band because you have selected a "too small" amplifier based on "average" power levels across the frequency bands.

But my suggested starting point will not appeal to many.
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Old 17th September 2012, 08:51 PM   #13
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This is a complex subject and not so good to generalize too much about, but some of your starting assumptions may be a little off.

First off, power requirements doesn't necessarily go up as frequency goes down. Within the range of a driver you can have flat response and not need any more power at, say 50 Hz as you need at 500, for the same output. If you want to go below the corner frequency of a woofer (below the system resonance) then sensitivity will roll off at a considerable rate and you would need lots more power to counteract the rolloff. If instead you wanted to extend the woofer response lower (to always be above the corner frequency) then you will have to drop efficiency considerably (such as heavy mass loading the woofer), but still, if you have flat response in the woofers range then power required will not change for frequencies in that range.

If you are talking about the power spectrum of music then search around and you will find a number of studies and curves. For the long term average the power requirements of music peaks in the midrange (around 200 Hz) and falls off either side. That means that you need to generate more acoustical power in the mids than at higher or lower frequencies. Classical music tends to have a more noticeable peak and jazz and pop less of a falloff away from the midrange, but they all have greater output requirements in the midrange. These are the long term averages, and the short term peaks (peak spectrum rather than long term average spectru) curves tend to be a little flatter, but still don't rise for the bass frequencies.

Now if you are talking natural driver sensitivity then midrange and tweeter sensitivities can be significantly higher than LF sensitivities. So, yes, you may need bigger amplifiers for the bass section of a triamped speaker than for midrange and treble. Again, this is a tradeoff between woofer sensitivity and extension. It is very easy to design a sensitive woofer but the bass extension will be inadequate. Achieving low bass extension requires low efficiency or large system volume, so you may see woofer sensitivities around 85 to 88 db (1m with 2.83 V input) while tweeters may more frequently be around 88 to 92.

Some of the above comments about true power (I x V(cos theta)) are correct but we tend to talk of power loosely as V squared over the nominal impedance. This may be referred to as nominal power or apparent power. If true power is considered efficiency will actually peak at resonance since the impedance curve goes high and current through the driver drops.

David S,
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Old 18th September 2012, 12:50 AM   #14
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
I don't agree with either of your first two statements.
That probably puts be at odds with 95% of the Members.

I claim that all the drivers must be capable of similar maximum SPL and
that the power of each of the frequency band amplifiers should be chosen
to achieve that same maximum SPL in their respective frequency bands.

If you start with that as a hypothesis, you may end up with a balanced sounding active speaker system that does not grossly clip on any particular frequency band because you have selected a "too small" amplifier based on "average" power levels across the frequency bands.

But my suggested starting point will not appeal to many.
Hi,

That is because you are wrong and always have been. The idea that
max SPL at a single frequency is the issue irrespective of the actual
bandwidth the driver is handling is just very, very wrong.

You suggest that a 3 way x/o at 3KHz and 10KHz needs the same
amplifiers as one crossed over at 300Hz and 3KHz, that is wrong.

You keep reiterating a standpoint / opinion in various threads you
cannot justify just because its a (wrong) assumption you once
made and used (or still use) to build something (or things).

We have been round the houses before with this, and you keep
refusing to accept the accepted theory (and measured practice)
about the subject, that is clearly right.

"I claim that all the drivers must be capable of similar maximum SPL and
that the power of each of the frequency band amplifiers should be chosen
to achieve that same maximum SPL in their respective frequency bands."

Similar maximum SPL in each band at multiple frequencies requires
amplifier power to be proportional to each bands bandwidth, QED.

rgds, sreten.
proportional to each bands bandwith
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Old 18th September 2012, 01:04 AM   #15
Pano is offline Pano  United States
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Yes, exactly what David said in post #13. The midrange often receives a great share of the musical power, but would take no more to reproduce than the bass, given the same speaker sensitivity (and BW, as sreten points out).

The problem being that bass drivers and cabinets are often less efficient than midranges and tweeters.
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Old 18th September 2012, 02:23 AM   #16
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Quote:
Originally Posted by sreten View Post

"I claim that all the drivers must be capable of similar maximum SPL and
that the power of each of the frequency band amplifiers should be chosen
to achieve that same maximum SPL in their respective frequency bands."

Similar maximum SPL in each band at multiple frequencies requires
amplifier power to be proportional to each bands bandwidth, QED.
Its just not that simple.

Lets say I have a 2 way sytem with a passive crossover and a 100 Watt amplifier. I take long term measurements and find that the average power distribution is about 80% to the woofer and 20% to the tweeter. If I biamp with an 80 Watt woofer amp and a 20 Watt tweeter amp have I achieved equivalence (lets assume similar driver sensitivity and little attenuation in the previous passive crosover)?

Unfortunately not. With my passive system there might have been instances where full peaks fell in the tweeters range. The full 100 Watts would be available for these peaks.

When going from passive to active we can calculate the average power distribution and also the peak power distribution. They will give different answers. The only way the active system is guaranteed to surpass the passive system, for all input signals, is when each section has the full power the original passive system had.

David S
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Old 18th September 2012, 03:11 AM   #17
Pano is offline Pano  United States
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Yes, very true. But the typical cheat is less power in the top end because
  1. It's usually more efficient
  2. Music tends to have less power up there
  3. We can cheat for a smaller, lighter, cheap amp because of 2

It kinda works, in a practical sense. But can result in blown tweeters, for sure.
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Old 18th September 2012, 09:04 AM   #18
AndrewT is offline AndrewT  Scotland
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Sreten,
My reading of posts16 & 17 is that they are part of the 5% of the Membership that agrees that peak SPL can occur anywhere in the audio bandwidth.

It is clear from post14, that you are of the Majority.
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Last edited by AndrewT; 18th September 2012 at 09:11 AM.
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Old 18th September 2012, 11:40 AM   #19
DrDyna is online now DrDyna  United States
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I've often thought about this too, but in looking around online and the answers generally presented seem to dance around the topic without ever really being solid.

Let's say you generate a noise signal that's equal in level across all frequencies from 20 cycles to 20,000 cycles. Let's also assume all the drivers have perfectly even impedances and sensitivities. You look at your RTA and you've got a perfectly straight line going from 20-20,000. You put a voltmeter on your subwoofer driver and increase the level until it's getting 1 volt.

Now, move the meter to the tweeter driver. Does it also read 1 volt?

That seems like what the question really is once you strip away the variables. If the answer is yes, then there is no chart.
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Old 18th September 2012, 12:17 PM   #20
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by speaker dave View Post
Its just not that simple.

The only way the active system is guaranteed to surpass
the passive system, for all input signals, is when each
section has the full power the original passive system had.
David S
Hi,

I never said its that simple, but that the simple principle cannot be ignored.

The above is overengineering and ignores a basic principle of active speakers,
they can handle wideband transients far better than passive speakers, at
the cost of lower narrowband peak levels, for the same total amplifier power.

e.g. A 50W+50W FAST two way crossed over at around 300Hz will on
most quality material equal the dynamics of a 200W amplifier, though
clearly in each band it can't beat a 100W amplifier.

For an active 3 way with properly chosen amplifiers, say each band
is equivalent to a 100W amplifier, wideband transients go up to an
equivalent of a 900W amplifier, but narrowband it is still 100W.

One could argue you'd be better off with a 300W amplifier and passive x/o.

Its a self defeating argument regarding 300W passive versus 3x100W active.

The statistical nature of music signals indicates the active system would
be superior most of the time and nearly all the time across an LP side.

Intelligent choice of x/o points and amplifier power is needed. That is
why I often advocated a stereo amplifier with active bass/mid around
300Hz, active EQ and a simple passive mid/treble x/o for active.

If you have a say 35W per channel valve amplifier for mid/treble the
argument does not go you need a 30W per channel amplifier for the
bass, it does go 70W to 100W for the bass will be a better choice.
(Might as well have the extra bass handling for bass heavy stuff).

Its not simple regarding the most cost effective way of doing things.
For a typical active 3 way 170W/90W/40W will better 3x100W.

rgds, sreten.
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