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Multi-Way Conventional loudspeakers with crossovers

Relation between Power and Frequency
Relation between Power and Frequency
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Old 11th July 2018, 10:44 PM   #11
Lynn Olson is offline Lynn Olson  United States
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Here's a good way to think of conversion efficiency. The whole point of any loudspeaker is to convert electrical watts to acoustical watts, preferably with flat response and low distortion.

Turns out that's hard to do, since the requirement is for a 3-decade frequency response, which is extremely broad even for RF antennas, never mind an electroacoustic transducer. A very typical tradeoff is bandwidth vs conversion efficiency; a single ultrawideband driver will have very low conversion efficiency, and worse, very low headroom. So the traditional approach dating back to the 1930's is to split up the spectrum into bands, and design more efficient drivers for each frequency band. This buys more headroom as well as more efficiency, and better overall performance (flatter) in the desired band.

Here's a typical number for the conversion efficiency of a generic direct-radiator driver ... say, an 10-inch woofer to pick something out of the air. A fairly "efficient" woofer in this class would radiate 92 dB at 1 meter with 1 watt (or 2.83V rms) electrical input. However, in terms of conversion efficiency, this is only about 1% efficient. 99% of the amplifier watts do nothing more than heat up the small voice coil, which is not good, since voice coils don't radiate heat efficiently, and worse, there's a temperature coefficient with copper that decreases conductivity as temperature goes up. This results in a phenomenon called "thermal compression", which is a real concern in the pro-monitor market.

So amplifier watts can be traded around fairly freely, but too much electrical power does result in voice-coil heating, potential over-excursion, and other failure modes, as well as poor sound as the driver reaches its limits. Another thing to keep in mind with direct-radiator drivers is that excursion increases by 12 dB/octave as the frequency is decreased, so gigantic excursions are needed at 20 Hz if high levels are desired. Conversely, tweeter and midrange excursions are so small they are invisible ... the failure mode there is simply excessive heat in the voice coil.

Last edited by Lynn Olson; 11th July 2018 at 10:55 PM.
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Old 12th July 2018, 01:42 AM   #12
jolathe is offline jolathe  United States
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With active speakers you need lots of amplifier channels, looks like six in your case. My advice is to not get too hung up on what power per channel you want, just pick a robust, durable one, either 3 stereo ones all the same, or one multichannel. An 8 channel amp would be a good choice, say 60-100 W per channel. You'll find with active speakers if they are turned up more than a few watts per channel you'll get blasted out of the room; they get loud. The higher output range of the amplifier is primarily needed for the transients, so you have enough headroom to avoid clipping. The tweeter will need a capacitor between the amp and the speaker terminals to protect it, as the won't be a passive crossover present.
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Old 12th July 2018, 04:02 AM   #13
Tromperie is offline Tromperie  Australia
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I think you are overthinking this.

A multi-channel amp, such as the one in the link, will do the job (with an extra) cheaply. Remember, in this case, power is the amount of energy supplied to eack driver because of the energy of the musical content.
Sure Electronics' webstore 4 x 100 Watt 4 Ohm Class D Audio Amplifier Board - TK2050

I have three way actives and I use three LM3886 amps, all from the same voltage supply. The energy content of the music spectrum, derived from the crossover choices, is what determines the power supplied to each driver.

(You will, obviously, also need a power supply.)
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Last edited by Tromperie; 12th July 2018 at 04:04 AM. Reason: Spelling error.
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Old 12th July 2018, 05:16 AM   #14
Ghyduart is offline Ghyduart
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Youre missing the point. I understand very well the impact of the sensitivity and even the impedance. What I need to understand is the relationship with the ACTIVE crossover frequency. Thats why I said all parameters being equal. The only thing I want to change is the low frequency crossover. So if I need 100W for my mids when the crossover is at 80 Hz, how much power would I need if I set the crossover at 160 Hz? 50 W?
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Old 12th July 2018, 05:23 AM   #15
Ghyduart is offline Ghyduart
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Quote:
Originally Posted by Tromperie View Post
I think you are overthinking this.
It's part of the pleasure of DIY: I like to understand how it works.
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Old 12th July 2018, 06:34 AM   #16
RobWells is offline RobWells  United Kingdom
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Originally Posted by Ghyduart View Post
So if I need 100W for my mids when the crossover is at 80 Hz, how much power would I need if I set the crossover at 160 Hz? 50 W?
100w

This thread might help you.

Try searching the forums there's plenty of discussion on this topic.
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Old 12th July 2018, 07:05 AM   #17
mdpaudio is offline mdpaudio
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Ive always heard that if you put white noise into a speaker at 100 watts the point at which 50 watts will be below a frequency and 50 watts above is at 200hz.
In other words, 200hz is the 50/50 point in power needed in a system.
That is why if you use an active xp the high frequency amp now has double the usable power, not twice as much power, it can now use the power that would have been used below the xo above that frequency. The amp will appear much larger than it is. The same goes the other way for below the xo frequency amp.
Its the added benefit of active xo, dont turn the power into heat and waste it but move it where you can use it.
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Old 12th July 2018, 07:08 AM   #18
RobWells is offline RobWells  United Kingdom
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If you look at the graphs Pano posted in the thread I linked to above you will see that most (except hip hop) music seems to have a fairly flat response between 100hz and 600Hz. So basically you'll need to design for your mid / midbass speaker to have enough output required for maximum levels in this frequency band. Below 100Hz use subs that complement your mids but also to suit your musical tastes. (If you listen to lots of hip hop and electronica or watch films on the system then bigger/more subs). Above 600Hz the output requirements roll off a little.

Basically big amps for subs / midbass, smaller amps above 600Hz. The actual amp power outputs will be determined by your personal room size and maximum spl requirements. Of course if your midbass speaker only needs 40w to reach xmax then there's not much point putting a 300w amplifier on it.

Last edited by RobWells; 12th July 2018 at 07:17 AM.
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Old 12th July 2018, 12:20 PM   #19
TMM is offline TMM  Australia
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Quote:
Originally Posted by Ghyduart View Post
You’re missing the point. I understand very well the impact of the sensitivity and even the impedance. What I need to understand is the relationship with the ACTIVE crossover frequency. That’s why I said “all parameters being equal”. The only thing I want to change is the low frequency crossover. So if I need 100W for my mids when the crossover is at 80 Hz, how much power would I need if I set the crossover at 160 Hz? 50 W?
Depends if by 'how much power' you are asking:
A) Can a lower power amplifier be used if the highpass is moved from 80Hz to 160Hz?
or
B) If I play a certain song and my mids are driven with 100W average power when the crossover is set to 80Hz, how much power will they see when I change the crossover to 160Hz?

In the case of A, no not really because all things the same a lower powered amp will have a lower voltage headroom and this may be needed to satisfy transients that still exist above 160Hz.

In the case of B, it highly depends on the spectral content of the song. If it is a violin solo and there is no content below 160Hz, then obviously the power delivered to the mid won't change after you alter the crossover. On the contrary, if there was a loud bass guitar playing around 110Hz then the power will be reduced dramatically when the crossover is changed from 80 to 160Hz as now the bass driver is going to handle that loud tone at 110Hz instead of the mid.

Last edited by TMM; 12th July 2018 at 12:25 PM.
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Old 12th July 2018, 02:44 PM   #20
DougL is online now DougL  United States
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Quote:
Originally Posted by Ghyduart View Post
You’re missing the point. I understand very well the impact of the sensitivity and even the impedance. What I need to understand is the relationship with the ACTIVE crossover frequency. That’s why I said “all parameters being equal”. The only thing I want to change is the low frequency crossover. So if I need 100W for my mids when the crossover is at 80 Hz, how much power would I need if I set the crossover at 160 Hz? 50 W?
In an active speaker, power is independent of frequency.
Power is per driver. Assuming flat frequency response and impedance, any note the driver plays will have the same voltage and power required to play at the same db.

Here is an example:
I want a 3 way active speaker to reach 115 db at any frequency.
My hypothetical tweeter is 115 db.
My Hypothetical midrange is 94 db
My hypothetical Woofer is 100 db

I need 1 watt for my tweeter amplifier. this is independent of frequency, as long as the tweeter maintains the 115 db sensitivity.
I need 128 watts to drive the midrange to 115 db
Finally I need a 32 watts to drive the woofer to 115 db.

What the paper I linked basically said is:
If we assume we are playing acoustic music, as a designer I may decide I only need a 103 db target if I decide to cross the tweeter at 4 kHz, rather than 1 KHz.
(The paper stated that acoustic music power requirement fell at 6 dB per octave starting at 1 kHz. Therefore 4 kHz being 2 octaves higher may require 12 dB less power)
If on the other hand, I decide I need to have the speakers play electronic music, I may want the 115 db, and require an amplifier that can produce the required power.

I understand this is counter-intuitive. Just keep asking until you understand.
By the way, I have an actively crossed 3 way Open Baffle driven by a 6 channel, 30 watt power amp. It can ring my ears if I'm not careful.

HTH
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Last edited by DougL; 12th July 2018 at 02:51 PM. Reason: Eddited for clarity
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