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Thread: size of subwoofer drivers vs. size of main speaker drivers

  1. #1
    Site Founder And Administrator Ron Resnick's Avatar
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    size of subwoofer drivers vs. size of main speaker drivers

    Some of us like to add one or more external subwoofer systems to even large, full-range speakers with 15" woofers (such as XLFs and Neoliths) for low frequency extension down to 20 Hz or below and for enhanced soundstage ambiance. Many of the subwoofers available today solve for small footprints (e.g., JL, Paradigm) with the result that the drivers in the subwoofers are smaller in diameter (10", 12", 13.5") than the woofer cones in the main speakers which are being supplemented.

    Other than manufacturers' desire to solve for a small footprint to avoid a large, tall tower subwoofer system, presumably to minimize cost and to maximize "wife acceptance factor," and other than the theoretical advantage that smaller drivers have less inertia than larger drivers, does it make any sense that our subwoofer drivers should be smaller than the woofer drivers in our large, main speakers?

    There just seems to be something theoretically inelegant about having 12" cones in the subwoofers supplementing 15" cones in the full-range speakers.

    1) If a main speaker has a 15" woofer, wouldn't we want, ideally, the subwoofer also to utilize 15" drivers?

    2) If the 15" woofer in a main speaker is in a ported enclosure as part of the main speaker cabinet, is it technically suboptimal or sonically problematic for the subwoofer to be a sealed box? (I know that Wilson and Martin-Logan use ported cabinets for their 15" cones but I have always been a bit skeptical about that, at least from a theoretical point of view.)
    Last edited by Ron Resnick; 07-10-2015 at 05:56 AM.
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  2. #2
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Disclaimer: I am not a speaker designer. Nor did I stay at a Holiday Inn last night, but I have piddled with this for a while and had a few grad courses long ago in a galax* -- oops, it was this galaxy.

    There are many factors that determine the performance of a speaker and size is just one. To use a fairly extreme example, a 15" driver with a small voice coil around a dinky little magnet that has half an inch excursion is likely going to be walloped by a 10" driver using a large voice coil around a huge magnet offering 2" of excursion. Designers have many parameters to vary in optimizing a design to fit a particular market. Small footprint can be addressed in several ways and the compromises (design trades) made yet provide much better performance than a larger driver in your main speakers.

    That said, most certainly there are main speakers out there with subwoofer-class performance. The majority of mainstream speakers are not, at least IME/IMO.

    One other reason for adding a sub is to smooth room response. That typically means placing the sub where it works best based on physical room dimensions, and that is rarely where the main L/R speakers live. A smaller sub is easier to place and, since it is often closer to the main listening position, may not require a large driver nor as much output.

    The phase and frequency response (roll-off) of sealed and ported systems is different. Sealed systems roll of more slowly below their -3 dB point where ported systems roll off fairly quickly below the port tuning frequency. But, the ported systems typically have higher output at and above the port frequency, all else equal. Around the port frequency those systems unoad the driver so the sound can become muddy. This can make integration more a challenge, but unless your room is huge the wavelengths are so long that it only really matters at and around the crossover point. You have to time/phase-align at the crossover anyway since they probably aren't at the same physical place in the room. IME experience the biggest factors in how well mains and sub integrate have more to do with other design choices beyond the sealed/vented debate. Several companies, including Rythmik (my current subs) use servo technology that largely obviates the sonic difference between sealed and ported, at least as far as distortion is concerned. Phase and frequency response differences still exist, natch.

    It's complicated...

    In the past I would only consider sealed, and preferred a servo design to control ringing and distortion. These days, while I own sealed, there are a lot of good designs sealed and ported, servo and not. It helps that now big amplifiers are common that can better control a sub.

    IME/IMO/usual disclaimers apply - Don
    Last edited by DonH50; 07-10-2015 at 11:39 AM.
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    Addicted to Best! LenWhite's Avatar
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    I believe active subwoofers are designed for one purpose - low frequency response. Using a subwoofer also relieves the amplifier(s) driving the stereo pair from the burden of reproducing deep low frequency response.
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    Adding to Don's good explanation and the same disclaimer of not being a speaker designer, the other variable is power. Subwoofers routinely have 1000 to 2000 watt class D amplifiers optimized for their loads. That power combined with the right driver, can put out incredible amount of bass, compensating for the smaller size of the enclosure and driver. I think it was Bob Carver who originally brought this thinking to the market with unusually small subs at the time. Something that is the norm today.

    Then there are tricks that are used in subs that can't be implemented easily in the main speaker. One is the so called "transmission line" subwoofers. I have one that has a tiny little 6 inch speaker in it. You would think it came out of a clock radio. Yet because of the special design of the enclosure, it puts out bass well above main speakers. Danley is one manufacturer of such subs and the bass it puts out is scary. At work, we custom build these kinds of subs and you have not lived until you try to talk over them and your voice gets modulated with the bass being played by them!

    Of course, having all of this and huge drivers helps even more . But unless you are going for outdoor sound, the techniques are sufficient to power our home listening spaces.

  5. #5
    Member Sponsor [VIP/Donor] GaryProtein's Avatar
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    Quote Originally Posted by DonH50 View Post
    . . . There are many factors that determine the performance of a speaker and size is just one. To use a fairly extreme example, a 15" driver with a small voice coil around a dinky little magnet that has half an inch excursion is likely going to be walloped by a 10" driver using a large voice coil around a huge magnet offering 2" of excursion. Designers have many parameters to vary in optimizing a design to fit a particular market. Small footprint can be addressed in several ways and the compromises (design trades) made yet provide much better performance than a larger driver in your main speakers.

    That said, most certainly there are main speakers out there with subwoofer-class performance. The majority of mainstream speakers are not, at least IME/IMO.

    One other reason for adding a sub is to smooth room response. That typically means placing the sub where it works best based on physical room dimensions, and that is rarely where the main L/R speakers live. A smaller sub is easier to place and, since it is often closer to the main listening position, may not require a large driver nor as much output.
    That's what I was going to say.
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  6. #6
    Addicted to Best! Rodney Gold's Avatar
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    You need to know what levels of bass you are looking for .. ie what level are you listening at
    Do you want thump you in the chest and suck the breath out f you whilst pressurising your ears or are you listening at levels where you can converse..
    Pointless spending money on a pair of SVS sb13 ultras if you in the latter category.

    A swarm approach with 4 smaller subs vs 2 x big ones is something you can try..apart from delivering the bass and spl , they will also just kill all room bass issue to a huge degree.

    http://www.enjoythemusic.com/diy/081...buted_bass.htm
    and
    http://www.theabsolutesound.com/arti...woofer-system/

    The secret with subs is integration ..
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    VIP/Donor [VIP/Donor] microstrip's Avatar
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    Although I do not remember the details any more, I have a read that an important aspect of subwoofers is the total area of the cones because of the way they couple to the air in the room. The impedance matching between the subwoofer acoustical parmaters and the effective mass of the moving gas was said to be critical.

    The article (in french ..., I think it was in Revue du Son ) addressed why the authors felt that a 500 square centimeters cone with 1 cm max excursion could be better than a 250 square centimeters with 2 cm max excursion, even with equal measured distortion (not these numbers, surely).
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    Site Founder And Administrator Ron Resnick's Avatar
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    Don, thank you for the background. I am familiar with transmission line designs, Amir. And I agree with Amir and Don on all counts!

    Rodney, I am looking only for linear extension to 16 Hz for two channel music. The more than two subwoofer ("swarm") approach appeals to me. If I get the Neoliths I would probably get four Martin-Logan BalancedForce 212 subwoofers (eight 12" drivers in total). Thank you for the links!
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  9. #9
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Quote Originally Posted by microstrip View Post
    Although I do not remember the details any more, I have a read that an important aspect of subwoofers is the total area of the cones because of the way they couple to the air in the room. The impedance matching between the subwoofer acoustical parmaters and the effective mass of the moving gas was said to be critical.

    The article (in french ..., I think it was in Revue du Son ) addressed why the authors felt that a 500 square centimeters cone with 1 cm max excursion could be better than a 250 square centimeters with 2 cm max excursion, even with equal measured distortion (not these numbers, surely).
    microstrip (glad you did not choose "stripline", not sure I could live with that image! ),

    I have read pro and con articles about that over the years and it was the topic of a chapter or two in my acoustics class back in college. The main counters I recall are that the wavelengths are so large that acoustic coupling differences were not the main issue once you got enough area to do the job (note 500 cm2 is about 12.6 cm radius, or about a 10" diameter cone, if I did the math right). The sub is a point source long-wavelength emitter, a sort of specialized thing. Lower excursion generally leads to lower distortion due to both staying better within the linear range of the voice coil and further away from modulating (flexing) the cone itself.

    In any event this is not my area of expertise so I do not know who is right. I'd like to think the weight of evidence I have is that a good 12" driver is fine for most of us, but that is probably because it is all I could afford and would fit in my space. I am sure if I had space (and funds) for a pair of 18" subs in place of my 12" pair I would find a way to argue the other side...

    RonR -- Glad I could help, but keep in mind I am no speaker expert. FYI, my pair of little Rythmik F12's (12" sealed subs) is down about 3 dB around 14 Hz and 6 dB around 10 Hz in my small'ish room.
    Don Herman
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    [Industry Expert] Addicted to Best! DaveC's Avatar
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    Quote Originally Posted by microstrip View Post
    Although I do not remember the details any more, I have a read that an important aspect of subwoofers is the total area of the cones because of the way they couple to the air in the room. The impedance matching between the subwoofer acoustical parmaters and the effective mass of the moving gas was said to be critical.

    The article (in french ..., I think it was in Revue du Son ) addressed why the authors felt that a 500 square centimeters cone with 1 cm max excursion could be better than a 250 square centimeters with 2 cm max excursion, even with equal measured distortion (not these numbers, surely).
    Right, this is why compression drivers are effective when used with a horn, the horn acts as an impedance transformer between the diaphragm and the air. So a driver cone with larger surface area does this more effectively vs a smaller one but ime other factors are more important with subs and it's possible to get good sub performance out of smaller subwoofers. Like most things, implementation is the most important aspect of the design. For woofers, I am a believer that bigger is better because it allows better macrodynamics, for example the impact of a kick-drum is more realistic with a larger woofer vs a smaller one. OTOH, there are examples of speaker with exceptional dynamics that use smaller woofers like the TADs, but they are not inexpensive.
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