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Bigger is Better......Or is it? 12" vs 15" vs 18" vs 21" (PA Drivers)

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  • Bigger is Better......Or is it? 12" vs 15" vs 18" vs 21" (PA Drivers)

    Strictly talking direct radiating, bass reflex cabinets, what are we looking at with the diameter of the speaker? I realize that other figures come into play here, but I have modeled countless 21" speakers and 18" speakers and usually achieved a higher custom amplitude and usually dig deeper than the 21" will. Some people would say that bigger speakers play "slower" than smaller, but I don't buy into any of that.

    When designing a PA sub-woofer, I look for a couple of key parameters: Qts, Vas, Sensitivity, Pe, Fs, and xmax. I'm still learning how the other numbers correlate to output and tuning. Ideally I go for the highest sensitivity, highest xmax, lowest Fs, and sub 0.40 Qts. I could be way off here, but that has typically yielded a sub-woofer that will play very low at high output levels in a relatively small ported cabinet.

    So back to the diameter or surface area of the driver, where does this come into play? for Example, companies such as Eminence, or B&C make a bunch of different drivers all in the same size. What makes them different from each other when most of the specifications seem the same across the models? I actually contacted eminence and asked which of their 18" drivers would play the lowest an loudest and the one they recommended was 180 degrees backwards from what I would have thought to choose.

    I know I'm all over the play here, but is a 21" speaker just a novelty or bragging rights? Any input from people with experience on these subjects would be greatly appreciated. I have successfully built PA sub-woofers that sound good, but have never gone past an 18" speaker. I'm still learning.
    "I don't know everything and do not claim to. I continue to learn and that is what makes me human."

  • #2
    Originally posted by STIchris722 View Post
    companies such as Eminence, or B&C make a bunch of different drivers all in the same size. What makes them different from each other when most of the specifications seem the same across the models?
    Mainly xmax. More than any other factor that's what determines their maximum SPL. Some have much higher Pe than others, but that's mostly for durability. 4kw Pe is of little practical value if the driver is displacement limited to 800w.

    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      When it comes to sub bass frequencies air displacement is king so larger drivers or horn loading is best. Given the materials we have to work with an 18" diaphram seems to be the sweet spot for conventional moving coil drivers at least, but throw some new technology at the problem and you get this...https://www.youtube.com/watch?v=yt8NLFsBWxw
      Paul O

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      • #4


        Mitsubishi Diatone D-160 Subwoofer
        PW-1600 honeycomb cone type woofer (160cm)


        Click image for larger version

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        "Not a Speaker Designer - Not even on the Internet"
        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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        • #5
          We used the B&C 21" woofer in a 17 cubic foot transmission line tuned to 20Hz. EQ was used to make up for the overall room response. But having that huge enclosure tuned that low made for an experience that is otherworldly. The owner actually had a pair of these installed, each powered by a 4000W amp.

          Each sub was capable of sustaining over 120dB from 20Hz on up without exceeding Xmax.
          R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

          Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


          95% of Climate Models Agree: The Observations Must be Wrong
          "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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          • #6
            How does one tune a speaker below its Fs with a Qts below 0.40? Again I'm a rookie so don't rag on me too much.
            "I don't know everything and do not claim to. I continue to learn and that is what makes me human."

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            • #7
              As you MIGHT have noticed, w/a Qts close to 0.40, you can typically get a driver (in a vented box) to get its F3 to = its Fs, in a box that's close to its Vas. My personal ROT is that w/every rise in Qts of 0.01, the F3 (given the same Fs) will drop by about 1Hz, while the Vb will increase by about 6%. (The inverse is also true.)

              F3 is determined by Qts and Fs. Vb is determined by Qts and Vas.

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              • #8
                Originally posted by STIchris722 View Post
                How does one tune a speaker below its Fs with a Qts below 0.40? Again I'm a rookie so don't rag on me too much.
                ​There's nothing really stopping you from selecting an Fb that's above or below the driver's Fs, if the overall response curve matches what you are aiming for.
                Brian Steele
                www.diysubwoofers.org

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                • #9
                  Originally posted by STIchris722 View Post
                  How does one tune a speaker below its Fs with a Qts below 0.40? Again I'm a rookie so don't rag on me too much.
                  You just build the enclosure to the proper tuning. The enclosure has its own tuning independent of the driver installed. Here's what the 21SW115 looks like in 17 cubes tuned to 20Hz. You just EQ to fit the room and the 20Hz tuning lets the woofer relax at the most demanding effects frequencies. This particular installation had the subs limited to about 50Hz.

                  Click image for larger version

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                  R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

                  Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


                  95% of Climate Models Agree: The Observations Must be Wrong
                  "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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                  • #10
                    Originally posted by Paul O View Post
                    When it comes to sub bass frequencies air displacement is king
                    +1, but it doesn't take a 21" driver to get it. The B&C 21DS115 has 2520 cc Vd. For the same price a pair of B&C 18TBW100 have 2900 cc Vd.

                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

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                    • #11
                      Originally posted by billfitzmaurice View Post
                      +1, but it doesn't take a 21" driver to get it. The B&C 21DS115 has 2520 cc Vd. For the same price a pair of B&C 18TBW100 have 2900 cc Vd.
                      But a 21" woofer looks more awesomer than a wimpy 18"
                      R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

                      Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


                      95% of Climate Models Agree: The Observations Must be Wrong
                      "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

                      Comment


                      • #12
                        Originally posted by Brian Steele View Post

                        ​There's nothing really stopping you from selecting an Fb that's above or below the driver's Fs, if the overall response curve matches what you are aiming for.
                        So what happens at the resonant frequency of the driver if it is placed in a box that is tuned below it? I guess my understanding was that at the Fs, the driver freely resonated causing a mess. Does that change once it is placed into a box with a different tuning frequency? If so what happens as it approaches the Fs?

                        Again for arguments sake, let's assume we are talking about the B&C 18TBW100 as this is one that I have modeled several different ways.
                        "I don't know everything and do not claim to. I continue to learn and that is what makes me human."

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                        • #13
                          Originally posted by STIchris722 View Post

                          So what happens at the resonant frequency of the driver if it is placed in a box that is tuned below it?
                          ​Nothing. The only way to modify the resonance frequency of a driver is to modify the driver. You don't often see a box that's tuned below the driver's resonance frequency because usually the result is less efficient between Fb (the resonance frequency of the box) and Fs (the resonance frequency of the driver. But in some cases Fb might be more important than the loss in efficiency - see Pete's design for an example.


                          Brian Steele
                          www.diysubwoofers.org

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                          • #14
                            Originally posted by STIchris722 View Post
                            So what happens at the resonant frequency of the driver if it is placed in a box that is tuned below it?
                            You don't get flat response, as seen above. It's called an extended bass shelf alignment, EBS, often quantified by how many dB the shelf is below the transfer function sensitivity of the driver, like EBS -3dB or EBS -6dB. The lower the shelf extends the more dB the shelf lies below the transfer function 0dB line on the chart. IMO if you want 20Hz response start with a driver that's intended to give 20Hz response. By and large pro-sound drivers aren't designed to give 20Hz response, because pro-sound doesn't need 20Hz response, and to get it you have to sacrifice what pro-sound drivers are designed to give, high sensitivity.

                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

                            Comment


                            • #15
                              Originally posted by STIchris722 View Post
                              ...So back to the diameter or surface area of the driver, where does this come into play? ... I actually contacted eminence and asked which of their 18" drivers would play the lowest an loudest and the one they recommended was 180 degrees backwards from what I would have thought to choose....
                              There are some fundamental relationships that come into the performance envelope - Ka and Acoustic Impedance, and the displacement demands as frequency drops.
                              "Not a Speaker Designer - Not even on the Internet"
                              "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

                              Comment

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