Leeoh,
I suspect that you are not hearing much reflected sound with your drapes and such behind them. You could move them closer to the rear (less than 2 meters) and listen to how you like the sound. Remember you can work with either of two trains of thought. One would be to limit the rear wall reflections (as you have in your current situation) and the second would be place them close to the rear wall and have more reflections from it. For me in a underdamped room you blur the sound if you have too many reflections. But some people like the near surround sound effect. Hence, you can decide how lively or dead you want your sound.
Jim
I suspect that you are not hearing much reflected sound with your drapes and such behind them. You could move them closer to the rear (less than 2 meters) and listen to how you like the sound. Remember you can work with either of two trains of thought. One would be to limit the rear wall reflections (as you have in your current situation) and the second would be place them close to the rear wall and have more reflections from it. For me in a underdamped room you blur the sound if you have too many reflections. But some people like the near surround sound effect. Hence, you can decide how lively or dead you want your sound.
Jim
My wife says i snore so i must
dave
just on a different schedule than others - try phoning him at 7:00 AM PST
Hello Duke,
unfortunately the explanation about the wraparound energy and baffle step is not clear to me. First of all, the F3 baffle step of a 12inch wide baffle , believe is (380/1)=380 Hz as per the calculator here Baffle Step Compensation . If I am thinking on correct lines, the phenomenon of baffle step implies that at this and above frequencies, the sound waves are transitioning from 4pi to 2pi radiation and hence are not wrapping around the baffle. So the bipole dip can happen only at a lower frequency than this.
Now the the dip in frequency due to bipole, should happen at a frequency at half the wavelength of the distance between the bipole drivers. This will always happen at some frequency depending on the distance between the drivers. Increasing the driver offset or making the cabinet wider than deep only increases the interdriver distance, thus lowering the frequency at which the bipole-dip happens but would not reduce/eliminate it.
Widening the baffle will lower the baffle step but the bipole dip always will happen at a lower frequency than this frequency as the interdriver distance will always be greater than the baffle width in a cabinet with even the smallest depth. I am unable to understand how making the cabinet wider than deeper will eliminate or even reduce the bipole dip.
Perhaps the only way to reduce/eliminate the bipole dip is to cross over to a woofer for the LF at a frequency where the bipole dip starts.
Also BTW, has anybody noticed that if bipolar placed drivers are wired out of phase to get DIpolar radiation, the ambience effect is much more pronounced, albeit with reduced bass. Any idea why this happens ??
Thanks for your explanation.
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Greetings. Perhaps Duke will chime in, but here's my thought on wraparound and baffle step in a bipole.
They are two different phenomena. The wraparound dip is a (sort of narrow) cancellation notch, whereas baffle step is a shelf (i.e., a high point gradually transitions to a low point and stays there, or vice versa).
So by altering the cab's outer geometry, you can shift the baffle step point up or down, but it remains a shelf. Even if the F3 is 380Hz, that's the half-way point so it could very well start at 560Hz (and Duke is pointing out the starting point).
In terms of reducing, you can build a notch filter to remove the wraparound frequency from the rear-facing driver (as Duke points out elsewhere). Or you can ignore it in favor of dealing with larger in-room issues.
In terms of dipole's interesting ambient (but bass-free) sound: in bipole, the two drivers sum constructively (over some part of the bandwidth) but in dipole, the two drivers' output cancels pretty completely in the bass (and some midrange), and then for a while, alternately sums and cancels, like ripples in a pond (comb filtering). This can be useful for surround applications where you want a diffuse ambient soundfield.
Just one person's random thoughts on that!
P.S. You can simulate a bipole really, really well in MJK's worksheets. http://www.quarter-wave.com
They are two different phenomena. The wraparound dip is a (sort of narrow) cancellation notch, whereas baffle step is a shelf (i.e., a high point gradually transitions to a low point and stays there, or vice versa).
So by altering the cab's outer geometry, you can shift the baffle step point up or down, but it remains a shelf. Even if the F3 is 380Hz, that's the half-way point so it could very well start at 560Hz (and Duke is pointing out the starting point).
In terms of reducing, you can build a notch filter to remove the wraparound frequency from the rear-facing driver (as Duke points out elsewhere). Or you can ignore it in favor of dealing with larger in-room issues.
In terms of dipole's interesting ambient (but bass-free) sound: in bipole, the two drivers sum constructively (over some part of the bandwidth) but in dipole, the two drivers' output cancels pretty completely in the bass (and some midrange), and then for a while, alternately sums and cancels, like ripples in a pond (comb filtering). This can be useful for surround applications where you want a diffuse ambient soundfield.
Just one person's random thoughts on that!
P.S. You can simulate a bipole really, really well in MJK's worksheets. http://www.quarter-wave.com
Widening the baffle will lower the baffle step but the bipole dip always will happen at a lower frequency than this frequency as the interdriver distance will always be greater than the baffle width in a cabinet with even the smallest depth. I am unable to understand how making the cabinet wider than deeper will eliminate or even reduce the bipole dip.
The bipole dip does decrease as you increase the aspect ratio of the bipole. Attached is a simulation that Svante did (changing the depth of the box with a fixed width). The dip is the same as a monopole half as thick mounted on a wall.
The dip is on-axis, and also decreeases as you move off-axis. At 90 degrees off-axis it will disappear completely.
So dip can be minimized with bipoles of higher asprect ratio and toed such that you are listening off axis.
dave
Attachments
Thanks for the simulations Dave.
I still am unable compehend how a wider than deeper baffle reduces the bipole dip. Anyway, it seems to me the only foolproof and assured way to eliminate the bipole dip, is to just do a 2 way and cross over to a woofer at the baffle step as the bipole dip occurs at a lower frequency than the baffle step. In this case the only benefit of a bipole is the more ambient sound than a monopole, albeit only above the crossover. However crossing as low as 300Hz (only possible with a 15" wide baffle ) will have a omnidirectional radiation from the woofer as well !!
Have you tried bipoles in a FAST configuration ?!
I still am unable compehend how a wider than deeper baffle reduces the bipole dip. Anyway, it seems to me the only foolproof and assured way to eliminate the bipole dip, is to just do a 2 way and cross over to a woofer at the baffle step as the bipole dip occurs at a lower frequency than the baffle step. In this case the only benefit of a bipole is the more ambient sound than a monopole, albeit only above the crossover. However crossing as low as 300Hz (only possible with a 15" wide baffle ) will have a omnidirectional radiation from the woofer as well !!
Have you tried bipoles in a FAST configuration ?!
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