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Thread: Woofer size and power usage.

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    Woofer size and power usage.

    Sup guys , I was told a while ago that the smaller the woofer the harder it is to drive for eg. 6 inch vs 8 inch, i was told the 6 inch would be harder to drive (need more watts) because it has to move more air so i thought id see what you guys thought.


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    Only true for the same volume (loudness) the frequency is set say 50Hz that is the speed the cone is moving in and out now think of the area of a 6" cone and that of a 10" cone that is the amount of air being moved (loudness) now think of the amplitude of the movement say 6mm start multiplying these together for an idea of the difference in audible output as frequency drops. Adding more power is only good up to the limit of the speaker extrusion (the max the cone will go in and out)

    If you actually know such things as impedance this calculation can be done properly (I'd have to look up the equation)

    Want to learn about tis stuff? try to find a copy of the loudspeaker design cookbook by Vance Dickerson there are many editions but even an early one would serve its purpose here.

    To give an idea on power I once had an active system with 30w amps on the treble and 300w amps on the bass (both 8ohm loads)


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    largely dependent on room size and box design (sealed, ported, t-line, horn, x-order, etc etc etc..), motor rest position (overhung/underhung), motor strength (dependent on coil length, material, magnet material, magnet design, and how close the magnet is to the coil), excursion (not extrusion), and even the frequency bandwidth your driver is capable of using.

    Of importance as well is that the weight of the diaphram between a smaller driver and a larger one will be different. So even though a 6" needs to move further to create the same decibel level the power usage will be similar overall, until you run into the point you start causing power compression.
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    The general biggest factor is room size and aesthetics. You might be surprised as I was to try a 300 watt sub in a small room, think it sounds just great until you bring it to a room that is double in size. Suddenly you are low on power and have lost the room shacking prowess. These guys summed up this stuff pretty nicely.
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    Quote Originally Posted by Nanometer View Post
    The general biggest factor is room size and aesthetics. You might be surprised as I was to try a 300 watt sub in a small room, think it sounds just great until you bring it to a room that is double in size. Suddenly you are low on power and have lost the room shacking prowess. These guys summed up this stuff pretty nicely.
    After reading this, it took a few days but I tried mine at friend's with a den that 20 x 40 feet, with hardwood floors. I don't know, but I call that big. My dinky cheap Pioneer with only a 240W Dayton Plate AMP like that room better than my Den that's something like 11 by 24 rectangle.

    In the corner and a large enough area for the Bass to expand was awesome! I'm no big fan of SPL as STEVil can tell you but my bud's son is now copying my smaller Sub.

    Nothing special went into this sub and at least 3 people have recreated it and all are happy with the results. I was so impressed I ordered another 250w when I was about to get the 500W version for my next Sub.

    As one of my friends would say, "You can't hold good Bass Down".
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    Bigger speaker = more mass to move in and out. You'll always require larger amounts of power to properly drive larger speakers, it's just common sense.

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    Quote Originally Posted by SabreWulf69 View Post
    Bigger speaker = more mass to move in and out. You'll always require larger amounts of power to properly drive larger speakers, it's just common sense.
    No.

    Larger speakers usually have larger voice coils which provide them with higher or equal motor force to diaphram weight ratios compared to smaller drivers. If you buy "large" speakers with tiny voice coils they probably suck anyways.
    Last edited by STEvil; 01-29-2011 at 02:03 PM.
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    I swear this is the case. Science of whatever aside, common sense and experience says, bigger driver, bigger power requirements. I dare you to run an MTX Jackhammer from a 30W tweeter amplifier and have any sort of decent result, and vice versa with out the tweeter exploding in a fraction of a second. Is simple, if you have 2 drivers made out of the EXACT same stuff, in the EXACT same way, but one is larger in every component and material that is used compared to the smaller driver, the bigger one will use more power. More practical tests, hook it up to measuring equipment, the bigger one will use more power. This isn't the science of schrodinger's cat, nor the complex semantics of theology or philosophy, it's just plain Jane common sense.
    Last edited by SabreWulf69; 01-29-2011 at 11:25 PM.

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    Ever bothered to look at the efficiency or sensativity ratings of the driver? Most tend to be in the range of 80-95db @ 1w. Some can be as high as 110db @ 1w.

    Comparing a jackhammer (you mean the 22/24, right?) and a tweeter just isnt a valid argument at all given that they are meant for completely different frequency ranges.

    Again, given a proper build method, you do not "need" more power for a larger driver.
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    Efficiency and sensitivity are only one part of the picture, then there's damping control, what you want to get out of the speaker to begin with and a whole other range of things to consider.

    ...OK then, power a Jackhammer with a headphone amplifier, and run a low-pass filter to it, or is it again, 'meant for different things'?. Proper build. I give up, your a genius engineer. Who needs big power for big speakers?, I mean clubs do, and concerts, and big subs, and generally every other large speaker from anyone of significance that can build 'proper' speakers like KEF, Bowers & Wilkins, Wilson Audio just for example. But yeah, your somehow right, and I'm somehow arrogant and dumb. No psuedo sciences for me, just plain 'it makes sense'.

    Give a real life example of this miracle massive speaker that can for example be powered by a 1-watt amp and play extremely loudly and clear and then I'll be listening. You sure are re-writing the engineering books on this one and throwing out all the required power rating systems they have on I think every speaker in existence.

    I have briefly read the Loudspeaker Design Cookbook by Vance Dickinson plus many more including Audio Engineering - Know it all, Master Handbook of Acoustics, Sound Reproduction - Loudspeakers And Rooms by Floyd Toole, and High Performance Loudspeakers - Fourth Edition by Martin Colloms. In which and where are you referring to your theory, if you can point it out without mincing their words, then I'll listen, until then, small speakers = small power, big speakers = big power. Even with tube amps and electrostatics your theory doesn't work in reality, even though 'normal' drivers is what the OP is talking about.

    Trust me I'm willing to listen, if you can convince me otherwise. I'd love to power my dual 12" towers at home without a power thirsty amplifier, I really would. I'm talking about if *everything* is larger on the larger driver.

    [Edit]
    Technically your saying, maybe 2 *different* drivers of different sizes, and having the magnet and voicecoil on the larger one extremely small and having only the diaphragm, frame and surrounding significantly larger than the small one then it would be in this situation that you would need significantly less power than the smaller one?, or the other way around?, I don't know, I prefer my common sense. I look at this as mincing meanings, when I hear bigger driver, I mean bigger driver overall, I say what I mean as a whole and mention if it is only in part.
    Last edited by SabreWulf69; 01-30-2011 at 03:05 AM.

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    I think you've forgotten to look at the basic laws of physics in play or you were misreading something.. anyways.

    Driver diaphragm area (note: general numbers, not taking into account surround width).
    8" = 50.27 in^2
    10" = 78.54 in^2
    12" = 113.1 in^2
    15" = 176.71 in^2
    18" = 254.47 in^2

    Voice coil area (note: assuming a 2" winding depth for all coils).
    2" = 12.56637 in^2
    2.5" = 15.70796 in^2
    3" = 18.84956 in^2
    3.5" = 21.99115 in^2
    4" = 25.13274 in^2
    6" = 37.69911 in^2

    Assuming motor strength (magnet power) is the same between all drivers and discounting gauss saturation the moving mass (diaphragm, coil, suspension compliance) is pretty much the only factor at play. Most loud speakers generate between 1 and 5 teslas of force. 1 tesla is the force of 1 newton being exerted on 1 wire carrying 1 amp of current. After the math that comes out to a force that accelerates 1kg of mass at 1 meter per second squared.

    So, if we have a tesla of "1" for simplicities sake and compare a 10" driver with 2" voice coil to a 15" driver with 3" voice coil we can see that although the 10" driver has a better diaphragm area to voice coil area ratio (6.25 vs 9.37) the 3" coil is also providing 1.5 times more effective area for the motor force to of the magnet to act on. Also the 10" driver must move 2.2 times further than the 15" driver to attain the same amount of air excitation. In short, you will need more power for the smaller driver to attain the same performance as the larger driver given that they are constructed with reasonable similarity aside from voice coil size. Of course voice coil size will also increase moving mass (a 3" coil can be 2-3 times the weight of a 2" coil depending on construction), but if you are using a larger driver you should be considering your application anyways. Asking a 15" driver to perform the same characteristics of an 8" or 10" in the vocal ranges is generally not going to happen while also handling lower frequency output.

    Of importance is that you seem to think I am indicating that reduced power is required. I am not suggesting this. What I am saying is that believing you need large power for a large diaphragm is a misnomer. Yes, more power can be utilized. Yes, a larger speaker is more likely to be found in a larger auditorium environment where it can make use of more power. However 150w for a 10" or 150w for a 15" is probably going to give you basically the same effective loudness in a confined area such as a home. In a large area the drivers chosen for the venue will likely be constructed with the ability to handle more power as well, and thus will require more power be default as well.

    Heck, i've got a set of towers with dual 10's in them that are very much more sensitive than my smaller towers with dual 6.5's in them. Can barely turn the amp up past 1/8th power before they are obnoxiously loud for average listening. This is just on a tiny old phillips 2x50w amp.

    As for a real-life example of a "massive" speaker playing loudly on a small amount of power, I have a 21" Pyle Pro. Its rated at 105db with 1 watt. That rating is a load of dung, but i'll agree that its loud and efficient as it can keep up with the dual 10 tower in a free-air environment quite easily on the same power. The dual 10 towers are SPL Monitor 4000's. They were cheap and sound decent The Pyle works well up to about 170hz.

    edit

    I was going to add a link to some "extreme" testing but I cant find it anymore. Had to do with "Team Riprock" and their quest for 180+ db.
    Last edited by STEvil; 01-30-2011 at 05:41 AM.
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    So as long as the voice coil increases in size proportionately along with the size of everything else in the larger driver not just in 'steps' of inches compared to the smaller driver i.e. just larger for the sake of more power handling when you give the smaller and the larger driver the same amount of power they will be equal to the same loudness, correct? I can see some downfalls of this as mentioned with power compression from over-excursion if all competing drivers are being driven to their limits.

    Also, with SPL, (just an example) would a 500W 8" sub put out the exact same amount of volume as a 500W 12" sub without over-excursion when driven with identical loads? Realistically would you really expect to see a 8" driver having higher or equal operating mechanical and electrical limits compared to a similarly priced 12" driver? Also with damping factor, aren't larger loads (specifically the relationship with impedance changing with frequency) more demanding from larger speakers than smaller? I am also guessing that with frequency changes speakers can become (depending on size) more or less sensitive and efficient.

    I guess the aim to designing a good speaker is getting as close to a hypothetical perfect speaker which has perfect infinite pistonic motion across the entire frequency spectrum with infinite stiffness and zero mass as much as possible.
    Last edited by SabreWulf69; 01-30-2011 at 07:58 AM.

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    Quote Originally Posted by SabreWulf69 View Post
    So as long as the voice coil increases in size proportionately along with the size of everything else in the larger driver not just in 'steps' of inches compared to the smaller driver i.e. just larger for the sake of more power handling when you give the smaller and the larger driver the same amount of power they will be equal to the same loudness, correct? I can see some downfalls of this as mentioned with power compression from over-excursion if all competing drivers are being driven to their limits.
    Until you hit the points of compression and saturation, pretty much yes.

    Also, with SPL, (just an example) would a 500W 8" sub put out the exact same amount of volume as a 500W 12" sub without over-excursion when driven with identical loads?
    As long as the load is within their mechanical ability this could be possible. The 12" will have more mechanical ability, however.

    Realistically would you really expect to see a 8" driver having higher or equal operating mechanical and electrical limits compared to a similarly priced 12" driver?
    Examples of this do exist in the car-audio world..

    Also with damping factor, aren't larger loads (specifically the relationship with impedance changing with frequency) more demanding from larger speakers than smaller? I am also guessing that with frequency changes speakers can become (depending on size) more or less sensitive and efficient.
    Impedence will change less on a driver experiencing less excursion than one experiencing more excursion. With less movement of the coil there will be less change of dampening factor over frequency range and even phase-plugs and pole vents for horn enclosures become less of an issue. If you stay at lower excursion levels.

    I guess the aim to designing a good speaker is getting as close to a hypothetical perfect speaker which has perfect infinite pistonic motion across the entire frequency spectrum with infinite stiffness and zero mass as much as possible.
    Of course
    Last edited by STEvil; 01-30-2011 at 12:31 PM.
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    That pretty much clears all of that up about what I was assuming, cheers.

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    Yea, interesting read there. Far more than I ever considered in the speaker world.
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    Man some of the stuff you guys know lol its like a frizzy gorilla doing accounting haha, good read good debate .

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    A good reference on this / these topics, besides a load of books, can be found on these pages -

    Overpowering, Underpowering, Distortion, Clipping, and Everything in Between

    Heavy Load: How Loudspeakers Torture Amplifiers


    and as a little bonus this is just really awesome :-) v

    Bowers & Wilkins - A Sound Experience Video Series (How they make their 800D series speakers)


    [Edit]
    I believe you can overpower something, just don't overdrive it, if you want to hear the maximum fidelity and loudness from a speaker, make sure it is amplified properly, have your gains set correctly among other things, then you can't go wrong. I have always said music no matter what it is can't kill a system, people, incorrect configurations, and poorly made components kill systems. The general science is --> 3 things kill speakers 1) Exceding thermal limits. 2) Exceding mechanical limits. 3) Material degradation. "Generally, amplifiers use the formula V1=G*V2. To power the amplifying device, you usually need two reference points, +Vcc and -Vcc. For symetrical reasons, they usually have the same absolute value. If these values are not high enough, the amplified peak value may exceed +-Vcc, causing clipping". Clipping can = harsh and distorted sounding speakers.
    Last edited by SabreWulf69; 02-02-2011 at 05:41 AM.

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    Quote Originally Posted by STEvil View Post
    Ever bothered to look at the efficiency or sensitivity ratings of the driver? Most tend to be in the range of 80-95db @ 1w. Some can be as high as 110db @ 1w.

    Comparing a jackhammer (you mean the 22/24, right?) and a tweeter just isnt a valid argument at all given that they are meant for completely different frequency ranges.

    Again, given a proper build method, you do not "need" more power for a larger driver.
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    Yeah, clipping can kill a voice coil very easily. I've seen tests done on extreme car audio subwoofers where they were fed 7,000w RMS (two separate coils for 1 driver so 3,500RMS per coil) for hours on end of clean power and didnt die. Feeding that same power below the tuning frequency or using clipped signals/power would result in thermal destruction in just a few minutes. Even 1000w of clipped power can cause damage to the same drivers.
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    Quote Originally Posted by Donnie27 View Post
    QFT! It should have simply ended here!
    I made my points, which I thought were valid.

    I still don't see how a smaller speaker would directly load an amplifier *more* than an equivalently made larger speaker. I get told one thing from those articles I posted and read certain things about it in various books, and get told differently, I don't know what to think now. Trolls aside, could you once more explain to me in terms that other people use from articles like the ones I have posted, in how this is possible with proportionately larger drivers? Could you please, please quote me or link me to some reference material?

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    I'm not sure what exactly I could point you at to clear it up any further. Maybe you need to think of it in terms of decibels versus distance (or room size, discounting construction materials/strength) from the diaphram/speaker. For example if you had an 8" driver and a 12" driver both rated at 88 decibels efficiency at 1 watt at 1 meter, the 8" driver may only reach 75 decibels at 5 meters, whereas the 12" may still be near to 80-85 decibels.
    Last edited by STEvil; 02-09-2011 at 05:39 PM.
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    Quote Originally Posted by STEvil View Post
    I'm not sure what exactly I could point you at to clear it up any further. Maybe you need to think of it in terms of decibels versus distance (or room size, discounting construction materials/strength) from the diaphram/speaker. For example if you had an 8" driver and a 12" driver both rated at 88 decibels efficiency at 1 watt at 1 meter, the 8" driver may only reach 75 decibels at 5 meters, whereas the 12" may still be near to 80-85 decibels.
    So therefore the smaller driver is quieter than the larger one, well that is indeed simple and relates to everything that I was trying to say from the beginning.
    Last edited by STEvil; 02-09-2011 at 05:39 PM.

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    I cleaned out donnie's.. posts..

    Anyways, yeah. The smaller speaker is "quieter" for a given amount of power, but that doesnt mean a larger speaker requires more power. Most of the efficiency has to do with the voice coil being inside the magnetic gap. The more it leaves the gap the less efficient the coil is.
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    The woofer size's differentiate in SPL and in the performance of how well it can handle a given frequency, what makes sense to me is that maybe at frequency's where the cone does not have to move very much ( 1khz ) that they may not have a very different power usage level.
    But If given a frequency to the drivers that forces it to move at its hardest you would see a big difference id say, generally the bigger the woofer size the more air it can move and the more power is needed to move that driver at its physical peak.

    Now obviously a smaller driver can try to produce what a bigger driver can do much more easily and fail because it does not have the physical properties to naturally reproduce this frequency, so the smaller driver would not pull much power trying to produce what the bigger driver is built to do.

    I have not looked much into SPL but it seems to me as the driver size increases the easier it is to drive ( less watts ) but then as you increase the size of the woofer the more capability the driver has to produce lower frequency's and then the physical limit is increased and more power is needed to drive to its peak.

    My guess is if SPL is your goal at a frequency that's the easiest for the given driver size then generally the bigger the more easy it is to drive, but the way i would do it is the physical limit pushing its max and measuring those results because speakers are made to reproduce at its best the entire frequency range.

    Be gentle with me i could be wrong as most of this is best guess at how i think it all works, but i do believe that the bigger the driver the more power is required to drive at its peak frequency ( impedance dip ).

  25. #25
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    peak frequency is not related to this. Impedance change (rise and dip) are due to excursion. More excursion means more power required.
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