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  • #31
    Originally posted by bradley.s View Post

    I don't know why anyone would specifically want dipole bass in a small room.
    Dipole bass in a normal listening room really isn't dipole. It just multiple sources at slightly different locations with some having inverted phase. This configuration alters the way room mods are excited compared to multiple source with the same phase. The other issue is no room pressurization below the room fundamental with "dipoles".

    John k.... Music and Design NaO dsp Dipole Loudspeakers.

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    • #32
      Originally posted by bradley.s View Post
      I don't know why anyone would specifically want dipole bass in a small room. You have long wavelengths that wrap around the enclosure anyway; 250Hz is a 4.5 foot wavelength. Also, why would you want 2-4 15"s per channel? You can't hear low bass in stereo and you'd make it impossible to control the low frequency small room modes. You wind up introducing problems rather than solving them.
      As long as you can have them positioned far enough from the walls, no reason why dipole can't be good in a small room. I've found dipole bass to be better than closed or ported systems in any situation. This is equalized dipole, not passive large, open baffle systems, that is.

      WRT multiple woofers per channel, outside of the driver numeric "overkill", multiple drivers per side could have an ameliorating effect by having sources from multiple locations in the room, especially floor/celiing reflections and possibly a small amount of room mode change as well, probably more in the lateral room mode range rather than longitudinal. Not quite the multiple subwoofer approach promulgated by Geddes, but may still have some benefit. Call it a possible smoothing effect. I suspect the smaller the room, the more effective that might be.

      dlr
      WinPCD - Windows .NET Passive Crossover Designer

      Dave's Speaker Pages

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      • #33
        Woofers play mid range frequencies (>250Hz.) In that case they are above the frequency threshold zone. Humans can hear stereo in the mid ranges. Although, I don't know which mid range frequencies we're best able to identify in stereo. I don't even want to say what I think I remember because I might cause someone else to mis-remember. I need a source to reference so I can remind myself. Thus, placing multiple woofers per channel to play the mids is fine. Bass, however, is a different story. Two to four 15" woofers used to play bass in stereo doesn't make sense to me.

        Here's a Harman paper on multiple subs with sound field management. Multiple subs placed in different room locations is the first step. The next level up is bass sound field management with measurements + DSP. JBL offers this service but they come to your home with mics and proprietary algorithms.

        https://pdfs.semanticscholar.org/00d...e976a1a72e.pdf

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        • #34
          Originally posted by johnk... View Post

          Dipole bass in a normal listening room really isn't dipole. It just multiple sources at slightly different locations with some having inverted phase. This configuration alters the way room mods are excited compared to multiple source with the same phase. The other issue is no room pressurization below the room fundamental with "dipoles".
          Why would someone want that? Other than experimentation. I understand why someone would want to experiment. Harman/JBL already worked out bass optimization in small rooms. Is there science or engineering available for open baffle bass sound field management? If not, Harman/JBL bass sound field management is the way to go.

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          • #35
            Originally posted by bradley.s View Post
            Woofers play mid range frequencies (>250Hz.) In that case they are above the frequency threshold zone. Humans can hear stereo in the mid ranges. Although, I don't know which mid range frequencies we're best able to identify in stereo.
            By and large frequencies below 150Hz cannot be directionally located. Based on that one might think that crossing over from subs to mains at 150Hz would suffice, but there would still be a lot of above 150Hz content from the subs due to the harmonics created by the movement of the cones even at low levels, let alone at levels high enough to create high THD. For that reason 80Hz is the de facto preferred crossover frequency, as it minimizes the ability to directionally locate the subs.

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

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            • #36
              Originally posted by billfitzmaurice View Post
              By and large frequencies below 150Hz cannot be directionally located. Based on that one might think that crossing over from subs to mains at 150Hz would suffice, but there would still be a lot of above 150Hz content from the subs due to the harmonics created by the movement of the cones even at low levels, let alone at levels high enough to create high THD. For that reason 80Hz is the de facto preferred crossover frequency, as it minimizes the ability to directionally locate the subs.
              I believe there is a mid range where we lose our ability to interpret stereo. I read it in a research paper but didn't save it. It was odd because there was a gap in the middle of the mid ranges where our ability to hear in stereo degrades. We are good between upper bass and low mids, then our ability degrades in the mid mid ranges, then we're good at upper mid ranges. I can't remember if our ability to hear in stereo continued above upper mids.

              I can't remember how I found that paper in the first place so I've had a hard time finding it again. Wish I could because it might influence where we should locate our frequencies in the room. And it would be possible to do something like that today with DSP.

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              • #37
                (From the OP): Thanks to everyone for their wisdom. After reading all this, and thinking about it, I have come to the preliminary conclusion that there are at least two main factors in the sound of open baffle or dipole bass:

                1. Because of the LF rolloff, the effective damping is typically higher than a boxed woofer, or equivalently, the effective Q is lower, resulting in a more Bessel type response shape than ported cabinets. As Bessel filters are known for having the best transient response, I think this helps account for some of the 'tight' or 'fast' descriptions I have heard about. Because of my experiments with listening to induced phase shifts via digital allpass filters through headphones, I believe Dr. Toole (JBL) when he says we can't hear non-linear phase as long as the group delay variation doesn't exceed something like 2 ms. At least in the mids and highs. But I think you can hear group delay variations in the LF region, since group delay typically varies widely "down there", especially with ported cabinets, and the periods below 100 Hz become long enough that the brain can begin to perceive such group delay variations as altering the waveform/transient response.

                2. The figure-8 pattern of dipole speakers interact with the room differently than the near omnidirectional pattern of a boxed woofer at low enough frequencies. And at low frequencies, these room interactions play a significant role in the perceived frequency response, as I'm sure most of you are well aware!
                "She don't love my speakers anymore..."

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                • #38
                  Originally posted by Unbiasedsound View Post
                  Plus the type of direct servo (Rythmik?) amps used by GR Research can enhance control over the driver for better quality bass.
                  https://www.youtube.com/watch?v=1Q3QQPO7y04&t=27s

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                  • #39
                    Originally posted by billfitzmaurice View Post
                    , but there would still be a lot of above 150Hz content from the subs due to the harmonics created by the movement of the cones even at low levels,
                    ??? Harmonics created by the movement of the cone? That would be distortion, at any level.
                    John k.... Music and Design NaO dsp Dipole Loudspeakers.

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                    • #40
                      Robbing Peter to pay Paul maybe a bit?

                      Truly can not say I have enough experience in this subject to have a well founded opinion, but people that know more than me seem to....

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                      • #41
                        I can see getting some Doppler distortion (really phase intermodulation) because of the extreme woofer cone motion. Can't say I sat down and calculated the level of distortion, though.

                        http://sound-au.com/doppler.htm
                        Francis

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                        • #42
                          Originally posted by johnk... View Post

                          ??? Harmonics created by the movement of the cone? That would be distortion, at any level.
                          True. It's unavoidable, and can be substantial.
                          http://www.readresearch.co.uk/loudsp..._article_1.pdf

                          Since the level of distortion goes up with excursion it's another reason why using multiple subs with lower excursion is better than using one with higher excursion.
                          www.billfitzmaurice.com
                          www.billfitzmaurice.info/forum

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                          • #43
                            To the OP who was wondering what OB bass sounds like: If you are really curious, request to audition the Linkwitz LX521 speakers in the LX521 support forum. They have a "Seeking Auditions" thread where you might be able to hook-up with someone near to you for a listen.

                            Descriptions are useful, but there's no substitute for being there and listening for yourself.

                            Disclaimer: I built LX521 speakers, so I have a confirmation bias.
                            Bill Schneider
                            -+-+-+-+-
                            www.afterness.com/audio

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                            • #44
                              Originally posted by billfitzmaurice View Post
                              True. It's unavoidable, and can be substantial.
                              http://www.readresearch.co.uk/loudsp..._article_1.pdf

                              Since the level of distortion goes up with excursion it's another reason why using multiple subs with lower excursion is better than using one with higher excursion.
                              You aren't going to get much modulation distortion in the midrange with a woofer crossover at 150 Hz. There just isn't the bandwidth. For example, a woofer doing 30 and 100 HZ will produce 130 and 70 Hz IM distortion. HD is another issue, but if you are talking about 30 Hz, then the distortion is at 60, 90, 120, 150...At 150 you are already up to 5th order distortion which won't be significant. All these analyses are great, but they don't always reflect what is heard. And Elliot's paper on IM being phase distortion is BS. I showed 15 years ago, while it's a convincing argument, it's the same dam thing.

                              http://web.archive.org/web/200908092.../Doppler1.html

                              This is why I don't post much anymore. To much time spent paying attention to minutia and not enough spent actually on what matters.
                              John k.... Music and Design NaO dsp Dipole Loudspeakers.

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                              • #45
                                What's the point of auditioning bass speakers if you don't bring the room home with you. Attached image is from chapter 6.1 of Floyd Toole's book Sound Reproduction Third Edition. Don't mind crime yourself into thinking an audition will help you with low frequencies.

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