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Thread: How much power is needed for each driver in active 3-way setup with sub

  1. #1
    Member sebackman's Avatar
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    How much power is needed for each driver in active 3-way setup with sub

    Dear all,

    I have spent some time in trying to find info on how much amplifier power is needed for “sufficient head room” for each driver in an active three way setup. -Especially the mid/high and the high drivers.

    I do believe that I have sufficient power for the sub’s (UREI 6290 2x600 @4ohms) and the low/mids (Crown CTS 8200, 8x200W@4ohms). But how much do I need for the “mid/high” and “high” driver.

    The basics is that the need for power goes down with frequency. This of course subject to each driver, cabinets and the room it self. But assuming a perfect room and perfect drivers in their respective frequency window how would you suggest a normal power distribution would look like? What size of power amp would be needed for each driver?

    Let’s assume 3-way system with a subwoofer and all drivers have the same “linear” sensitivity of 90db and all are 8ohms. Assume filter slope of say 18db and that all drivers have power capacity in their respective band.

    Sub up to 80Hz
    Low/Mid between 80-1.000 Hz
    Mid/High 1000-10.000 Hz
    High from 10.000 Hz and up

    Given normal listening how much power would you use for each element? Is there any difference between music and movie’s? I understand that “the more the merrier” but that is beside the point. The relationship is the interesting point and from that it is easy to adjust for actual sensitivity for each driver.

    My gut feeling is that there is really little need for power above 1kHz and that one could do with as little as maybe 30 watts. But that is just a guess on my side.

    In reality it is a fully active 3-way 5.1 system with;
    Two SUB1500 subwoofers in closed cabinets, driven by a UREI 6290
    Each of the 5 other speakers use;
    Dual 8” low/mid drivers with about 93db sensitivity in parallel operation in closed boxes, driven by a Crown CTS8200
    Mid/high is 2451 driver on 2231 horn (JBL DMS-1) with 110db sensitivity, no amp yet
    Tweeter is Ti025K with I guess about 92 db sensitivity, no amp yet

    Thank you for input
    //RoB


    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  2. #2
    Administrator Robh3606's Avatar
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    It simple all you need to do is determine what you want as the max SPL level at your listening position. Scale the amps accordingly. I use 100 watts per driver and I can hit about 113db peaks clean for my stereo pair. More obviously if I am running in HT mode. That number is based on my woofer sensitivity of 98db from which all other driver levels are based. I can get more from my mids and compression drivers but simply don't need it. That is on the conservative side not allowing for any advantages due to using multiple amps, amp headroom. So what you need is base speaker sensitivity and listening distance plus what ever your max spl number is. Play with the calculator to see what numbers it comes up with. I listen at an average level of about 85db that gives me about 30db of clean headroom which is a good number.

    http://www.doctorproaudio.com/doctor...n.htm#calc_spl

    You can use as much power as you want but after a certain point it just turns into a watt contest with no real gain. The real advantage is to start with a high system sensitivity. I use all the same amplifiers for all drivers as I had enough SPL based on the woofer sensitivity and they were readily available.

    In your case the tweeter is the limiting factor as far as max spl is concerned. I have a 1.5" compression driver up top so I don't have any limiting up there.

    Rob
    "I could be arguing in my spare time"

  3. #3
    Member sebackman's Avatar
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    Hi Rob,
    Thank you for your input.
    I agree that this would be the appropriate mathematical approach given evenly distributed energy over the frequency range and that we are assuming average levels. Certain type of noise has this characteristic.

    I don’t think music and/or films do have evenly distributed energy over the frequency range. There is much more energy in the lower regions and I guess my question is what power requirements that translates into over the audible spectrum. Assuming normal mixed music and movie’s how much power headroom is really needed to handle the absolute level and the peaks.

    My sub’s are JBL SUB1500’s in closed cabinets and I feed them with 2x600W in 4 ohms RMS (UREI 6290). The calculator gives that such setup would give a max SPL of 106db at 4 meters assuming that the SUB1500’s are at 90db sensitivity.

    This means that since my 1,5” 2451 drivers on the 2332 horns are about 110 db they would need 7W to get to the same level and my tweeters (only used above10-12Khz) would need about 600W to give 106db assuming the same sensitivity as the SUB. I don’t think my tweeters would live very long…. J

    Starting from the sub’s, all the drivers above should need to produce less SPL with rising frequency regardless of sensitivity and still do accurate music reproduction. Hence I would need less power in the high regions compared to the low regions. But I don’t seem to be able to figure out how much less or how it varies over the frequency spectrum.

    What is the energy spectrum of music?

    Regards
    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  4. #4
    Senior Member jcrobso's Avatar
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    Also consider the Fletcher-Munson curve

    http://en.wikipedia.org/wiki/Equal-loudness_contour

    Note how the bass frequencies require far more SPL for the same apparent loudness level.
    This is why Bass speakers have to be BIG and need a lot of power!

  5. #5
    Administrator Robh3606's Avatar
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    Starting from the sub’s, all the drivers above should need to produce less SPL with rising frequency regardless of sensitivity and still do accurate music reproduction. Hence I would need less power in the high regions compared to the low regions. But I don’t seem to be able to figure out how much less or how it varies over the frequency spectrum.
    Average power may be lower but peaks may not be.

    http://sound.westhost.com/bi-amp.htm

    Rob
    "I could be arguing in my spare time"

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    Senior Member timc's Avatar
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    We are hitting 40w (McIntosh meter) on a tweeter with 100db/1w/1m. Flat phase, and flat impedance. Crossover at 850Hz.

    Thats LOUD. But man, it pays off to have good headroom. No strain whatsover.
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

  7. #7
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by jcrobso View Post
    http://en.wikipedia.org/wiki/Equal-loudness_contour

    Note how the bass frequencies require far more SPL for the same apparent loudness level.
    This is why Bass speakers have to be BIG and need a lot of power!
    Sorta kinda... bass drivers need to be big due to the physics of sound. It is absolutely possible to create a woofer that will produce 20Hz and only be 2" in diameter... but it will play very softly.

    In any event, a zillion years ago there was a chart someone published that I used for contemporary music that broke up the power requirements for various bands of the spectrum. It was something like 50% of the power is below 250Hz, 25%-35% between 250 Hz and 1KHz and 15%-25% above 1KHz... this will obviously vary based on musical type but it is more or less accurate for most music.


    Widget

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    Member sebackman's Avatar
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    It seems no sense in having a 500W amp feeding the tweeter or the driver so my point is, was does make sense?

    If the reasoning under the link provided above is correct, the ”equal power point” is around 250 Hz.

    Wanting extensive bass as with the .1 (subwoofer) channel in films probably moves the point lower.

    The below table is from the link above.

    XO Frequency in Hz / Power to Bass (%) / Power to Mid+High (%)
    250Hz / 40% / 60%
    350Hz / 50% / 50%
    500Hz / 60% / 40%
    1.200Hz / 65% / 35%
    3.000Hz / 85% / 15%
    5.000Hz / 90% / 10%
    12.000Hz / 95% ? / 5% ?

    Source of table is here.

    Given this table I get to something like this.

    My JBL SUB1500 subwoofers should be fed as much as I can afford or they can stand !

    For the 5.1 system I can use Rob’s suggestion above and decide how loud I want to play and dimension power amp's after that. If I feed my dual 8” woffer/mids with 125W each (250W power amp / 8”s in parallel). The calculator shows about 109 db SPL max.


    Source of calculator is here.

    My XO introduces the 1,5” 2451 drivers at 1.000Hz (like in DMS-1) and according to the table above they would need about half of the power above 1.200Hz. Given that the sensitivity is 110db the calculator gives me 12W for 109db for one driver/horn. And as their need is only half of the power above 1.200Hz it comes to 6W.

    The tweeter needs less than 10% of the power if cut-off above 5.000Hz in table above and probably much less for me using XO point at 12kHz. The tweeter is 92db sensitivity and the calculator gives me 800W to get to 109db SPL. As it only needs 10% of the power above 5kHz, or less, it comes to 80W or less for the appropriate SPL level. ….Assuming it can handle it….

    So from the above conclusions I would get

    * Sub’s = as much as possible and not really included in the reasoning… (OT, new 2x900W amp replacing UREI 6290 tonight)

    * Dual 8” woofers/mid’s @ 94db and 80-1kHz = 250W for 109db SPL (125W per driver)

    * 2451/2332 mid driver @ 110db and 1kHz to 10Khz = 6W for 109db SPL (given that the Bi-Radial link is compensated electronically and does not effect sensitivity as with passive filters)

    * Ti025K dome tweeter @ 92db and 12Khz and up = <80w for 109 db SPL. Probably 40W is enough from a simple calculation.


    Does this make sense???

    //RoB
    Last edited by sebackman; 10-26-2010 at 12:46 PM. Reason: Tables garbled
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  9. #9
    Senior Member 4343's Avatar
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    Cool

    Quote Originally Posted by Mr. Widget View Post
    ...
    In any event, a zillion years ago there was a chart someone published that I used for contemporary music that broke up the power requirements for various bands of the spectrum. It was something like 50% of the power is below 250Hz, 25%-35% between 250 Hz and 1KHz and 15%-25% above 1KHz... this will obviously vary based on musical type but it is more or less accurate for most music.

    Widget
    This falls in line with what I've always used as a rule of thumb. I remember a Cerwin-Vega publication that espoused the line that half the power in music was below 200Hz, half above...
    Mike Scott in SJ, CA
    Drive 'em to the Xmax!

  10. #10
    Senior Member pos's Avatar
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    Quote Originally Posted by sebackman View Post
    * 2451/2332 mid driver @ 110db and 1kHz to 10Khz = 6W for 109db SPL (given that the Bi-Radial link is compensated electronically and does not effect sensitivity as with passive filters)
    Hi

    Even if the conpensation is done before the amp, you still need to take it into account. If you compensate, say, 10dB at 10khz then you have to consider the sensitivity to go from 110dB at 1khz to 100dB at 10khz in your calculation

  11. #11
    Member sebackman's Avatar
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    Hi pos,

    That depends if you do bi-radial compensation by amplifying above 2 kHz or if you attenuate below 15 kHz, as with passive bi-radial circuits, to flatten the curve.

    I'm amplifying in the filters using the same inputs as JBL states for their big monitor DMS-1, which use the same horn and driver combination as I do. They are acctually doing both, they amplify by high shelving 12db/oct +10db @ 8,28kHz and attenuating Q0,25 -1,5db @ 3,24Khz for the 2450SL on the same horn as I'm using, the 2332. I use 2451 with SL membranes so that should be the same or very close I guess.

    I'm using digital filtering and can't really do large amounts of attenuation as the only way of doing that in the digital domain is by dropping "bits" and that means reducing resolution. -Especially on low listening levels. As long as there’s head room in the digital domain amplification is normally fine. In my simple "non sientific" tests the loss of resolution is clearly audible. -And I'm feeding full digital signal to my filters so it cannot be a A/D issue.

    But does the above play with numbers make sense?

    //RoB
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  12. #12
    Super Moderator yggdrasil's Avatar
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    IMHO you are creating a problem for yourself with those low sensitivity tweeters.

    The numbers seem correct, they will need a huge amp to keep up, but will they stay alive for long?

    Pos is also right about the compensation - it doesn't matter where it is done, you will need the extra power.

    Regarding the frequency spectrum and power needs - that is entirely dependent on the music. The fact is that, given driver(s) with 100dB/1m sensitivity, it will take 10W to produce 110dB/1m regardless of frequency.

    The complexity of the signal will add to the power needs, as Rod Elliot showed in your link.
    Johnny Haugen Sørgård

  13. #13
    Senior Member timc's Avatar
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    I have lots of music, where the voice is the most power demanding off all the instruments. It depends on the mix/music.
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

  14. #14
    Member sebackman's Avatar
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    Agreed, I made a misstake in the calculation above.

    As the driver/horn dropps from 2kHz I need to compensate with more power. Say that the dropp is 10db, then I would need about 10 times the above stated power according the calculator. So I would need about 60W to the driver to compensate for the sensitivity drop.
    So;

    * Dual 8” woofers/mid’s @ 94db and 80-1kHz = 250W for 109db SPL (125W per driver)

    * 2451/2332 mid driver @ 100db and 1kHz to 12Khz = 60W for 109db SPL (it is probably less because as the bi-radial compensation needs more boost in higher frequencies the need for power is reduces as per above, so the real need is probably lower...)

    * Ti025K dome tweeter @ 92db and 12Khz and up = <80w for 109 db SPL. Probably 40W is enough from a simple calculation due to the fall off in program information in higher frequencies.


    Does this make sense?


    //RoB
    Last edited by sebackman; 10-27-2010 at 02:18 AM. Reason: spelling
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  15. #15
    Senior Member timc's Avatar
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    My guess is that 40W for the tweeter wil cause compression. I think other tweeters might be better suited for you.
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

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