BWC (Big Woofer Club): Why Big Woofers Matter

Discussion in 'Speakers' started by nishan99, Aug 5, 2020.

  1. purr1n

    purr1n Desire for betterer is endless.

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    The no passive xover on the woofer argument is huge. Low DCR coils are expensive and with woofer coils, DCR will not be anywhere near zero. On woofers than can touch 4-ohms, that will be a lot of wasted power. Active woofer means separate amp. Sure I could throw an extra $5k on a boutique SET or PP tube amp on it, but as bass frequencies don't seem to benefit as much from high quality amplification, better to throw cheap class AB or D with a ton of power on it.

    Depends how much of purist / complexity.
     
  2. Priidik

    Priidik MOT: Estelon

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    Some fine post by good folks, I will only add a few:
    - biggg woofer --> high moving mass & strong motor = high back emf (low powered amps can be destroyed by this)
    - 98dB/m/W might be the datasheet sensitivity at 200..800 Hz, but what is the sensitivity in actual use through the xo for hi-fi use? Probably 93..95.

    On the other hand as have been hinted already, a lightweight 15'' (say, real bass sensitivity of 94dB) can sound much better with low powered amp than a low sensitivity 10'' bass (example 89dB) with the same amp. The 15'' will use a fraction of the power to get similar result, significantly easier for the amp to put out same SPL.

    What @purr1n and @Hrodulf said are worthy of repeating and condensing: beefy amps and active sound 9/10 times better than the same bass with weak amp. Regardless of driver size.

    A caveat is timber integration and, while hard to believe, bass from 90..200 Hz actually benefit from TOTL sensibilities (plankton, microdynamics, color shades) the likes of uber tube amps can give.
     
  3. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    For me any notion of bass purism goes out the door once I'm indoors. Positioning kills dips, EQ kills peaks - what's left is the purest bass the room will allow for. And I haven't really heard about people craving dat toob bāss. Audiophiles generally are afraid of bass and will happily opt for no bass as it will sound "faster".
     
  4. Poleepkwa

    Poleepkwa Friend

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    From my experience the "purist" form of bass is outside at a live gig with loads of horn drivers and plenty of power. However I doubt it will go flat till 20hz, ( most likely start to roll off at about 30hz). My pro sub elements have a sensitivity rating of 98db, but if I remember correctly they are a lowish 80db@20 Hertz in my smallish sealed box. These would never work outside. Put them inside a room though and the room gain will raise the sensitivity and flatten the frequency response flat till 10hz. These are pretty light weight drivers compared to those monster HT subwoofer elements with their monster rubber surrounds.
     
    Last edited: Jan 4, 2021
  5. Cspirou

    Cspirou They call me Sparky

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    Kinda of forgot that sensitivity is at a particular frequency and not across the bandwidth.

    Looking at the datasheet for an Eminence Beta it is rated at 98dB but that is at 1khz, where no one really uses it. 100Hz is at 94dB and 45Hz is at 83dB. Which is going too need a lot of watts if you want to reach down low.
     
  6. dmckean44

    dmckean44 In a Sherwood S6040CP relationship

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    Most of those 2-way 15" PA speakers cross over at 1.2khz so I wouldn't really say that's where no one uses it. Those trapezoidal speakers usually use an undersized box with a low port turning so that bass can be safely boosted, which again requires even more power.
     
  7. 9suns

    9suns [insert unearned title here]

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    https://www.pearl-hifi.com/06_Lit_A...wers_Wilkins/B&W_Nautilus_801_Development.pdf

    Here we can read the B&W Nautilus 801 development paper, which happily explains why multiple small woofers are for losers and that the newer models (with two 10' woofers) are designed by their marketing department.

    Page 17:

    "One remarkable fact was consistently noticed during the development of the new 380mm bass unit for the Nautilus™801, and that was that a single large and stiff bass cone always sounded better than a number of smaller cones, even though they may well have had the same aggregated properties. One possible explanation for this is the concept of the production of a “coherent wave front”. This will be produced by a single large very stiff cone, which can couple with the air in a uniform manner over the whole of its surface area unaffected by differences in loading over that area. This behaviour is to be compared with that of several cones which, even though they may be closely spaced, will still leave gaps of “uncoupled air” between them. The very stiff cone material of the large single driver, which is a thick sandwich of Kevlar® reinforced paper fibres with a very stiff skin, makes it less responsive to local changes of acoustic impedance or unbalanced modal pressures either behind or in front of the cone.

    A “coherent wavefront” simply means there is either a constant or a smoothly changing phase relationship between neighbouring parts of the wavefront. So even if two drivers are relatively close together compared to a wavelength. Also, even if their contributions are equivalent to a single large driver, their different acoustic environments will mean that their outputs are slightly different, in terms of both amplitude and phase.

    Furthermore, the air between the drive units is not being driven at all and this will translate into a change in phase across the resulting wavefront as the air tries to “fill in” the lost contribution. One can postulate that, at low frequencies, air can “spill off the edges” of the individual cones more easily in an array of small cones, which obviously have more edges for it to spill off, than from a single large one. For instance, two 12 inch drivers have a combined circumference of 1630mm whereas one 15 inch cone has a circumference of only 1037mm. It is also interesting to note in this context that the radiation resistance and reactance at low frequencies of one 15 inch cone, is actually larger than that of two 12 inch cones, even though the area of the 15 inch cone is 0.02m2 smaller than two 12 inch cones. This is because the change from a steadily rising radiation resistance characteristic at low frequencies, to a horizontal one at high frequencies, occurs at a lower frequency with one large driver than with two smaller ones (Fig 15). Mutual radiation impedance effects will redress this imbalance to some extent, provided that the two individual drivers are close enough together for one driver to acoustically load the other, though it will only be totally redressed if the array of small drivers produces a totally contiguous surface in all directions"

    That's just a part of it, the whole document is a very interesting read.
     
  8. Tachikoma

    Tachikoma Almost "Made"

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    So, I've been on the lookout for sensitive speakers, and I just realized that PA speakers also come in the passive variety with 90+ dB sensitivity levels. Since we've tried PA amps with audiophile speakers, has anyone tried PA speakers with audiophile amps?
     
  9. Armaegis

    Armaegis Friend

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    Yes. It's really no different, and plenty of the horn type stuff from JBL isn't that far off the mark from their hifi offerings in the first place. The part to be wary of though is that those PA cabinets are typically built to throw sound far away. You will have zero cohesion between the tweeters and woofers at close range for a 2 or 3-way vertical cab that isn't coaxial in design. My mains are Yorkville U15's that I run bi-amped, stacked on top of 200 lb subs. Great for filling a gym or community centre, but lousy in a living room unless you're sitting a minimum of 12' away and it poses a danger to any china cabinets nearby.
     
  10. Tachikoma

    Tachikoma Almost "Made"

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    The JBL SRX8** series looks especially interesting, given that they are actually available in my locality, unlike most other BWC speakers.
     
  11. Armaegis

    Armaegis Friend

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    Looking at the specs here: https://jblpro.com/en/product_families/srx800-passive-series

    Dispersion on the 812 and 815 looks better. They have a wider coverage pattern and smoother FR (beamwidth and directivity) than the 835, which as a very general rule of thumb means they will play a little nicer in a home setting instead of shouting your face off. Stack it on top of an 818 sub and you probably won't even need a stand (or you can trim a wooden dowel to bring it up to height and allow you to rotate it).
     
  12. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Listen before you get any PA speakers. Those compressions horns aren't always ear friendly.
     
  13. Tachikoma

    Tachikoma Almost "Made"

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    The crossover point in the 812/815 is rather high though, at 1.9 kHz. The 835 pulls that back down to a much more reasonable 330hz/2.4khz with the same LF and HF drivers.

    Anyway, I'll try to arrange for an audition before I do anything rash.
     
    Last edited: Feb 21, 2021
  14. Grattle

    Grattle Friend

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    I have an Infinity 12” sub in a sealed enclosure. Powered by a Behringer KM750 in bridged mode. Should the woofer feel warm to the touch after heavy usage? The cap feels very warm after a 30 min loud session. Normal? Reason to worry?

    edit: it’s rated for 1200 watts. It’s an Infinity Kappa and it’s set to 4 Ohm. I bet I’m overpowering it. I think I’ll add a 2nd one and run in stereo mode so that I’m not pushing it so hard. Bridged mode is probably sending it too much current. I can see the room lights dim on heavy sustained notes.
     
    Last edited: Mar 19, 2021
  15. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Interesting. Subs rarely get much heat stress in normal use because bass is usually very transient in nature, so the coil is not conducting much most of the time (and it's usually huge af). Maybe measure DC on the amp outputs?
     
  16. wormcycle

    wormcycle Friend

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    Your sub looks like 2 Ohm to bridged Behringer KM750. That usually leads to some problems.
     
  17. Grattle

    Grattle Friend

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    I didn’t realize that. Thank you. I will take it easy on it until I get a 2nd sub and can run them one per channel.
     
  18. ergopower

    ergopower Friend

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    I don't understand how a load impedance changes with respect to source, could you explain further?
     
  19. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Low impedance load will cook the amp, not the driver. But yeah, for a bridged amp each half sees half of the load impedance.
     
    Last edited: Mar 20, 2021
  20. wormcycle

    wormcycle Friend

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    Here is the explanation, example from Wiki:
    Example. Two amplifiers, each rated 100 watts maximum for an impedance of 4 Ω (four Ohms); in bridge mode they will appear as a mono amp, rated 200 watts into 8 Ohms. This is the most commonly misunderstood mode of operation and it requires additional circuitry to implement if the pair of amplifiers does not have the facility built in. The image shows two identical amplifiers A1 and A2 connected in bridge mode. The signals presented to each amplifier of the pair are caused to be in anti-phase. In other words, as the signal in one amplifier is swinging positively, the signal in the other is swinging negatively. If, for example the maximum output voltage swing of each amplifier is between a peak of + and – 10 volts, when the output of one amplifier is at + 10 volts the output of the other will be at –10 volts, which means that the load (a loudspeaker) now sees a 20 volt peak difference between the “hot” (normally red ) output terminals. Driving the load between two signals of opposite electrical polarity makes each amplifier see only half the load's electrical impedance.[3]
    [​IMG]
     

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