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In Reply to: RE: "Once you go dipole you never go back." posted by MikeCh on June 02, 2016 at 07:17:53
I personally feel that dipole bass radiators deals with room modes better because it does not excite them to begin with.
While I agree that dipoles load the room differently, room modes are still excited. How could it possibly be otherwise? If the loudspeaker is putting any energy into the room at an eigenfrequency, its mode will be excited.
The links you shared to Linkwitz's experiments include measurements which clearly show that the bass response is dominated by modal peaks and cancellation nulls in either configuration. The pattern is a bit different, but it's hard to say based on frequency response that one is better than the other.
What's more relevant to this thread is the low bass, since the OP is looking for a subwoofer to cover frequencies up to 40 or 50 Hz. I still don't see how a dipole subwoofer could be a viable option due to the two drivers cancelling each other.
Follow Ups:
SL's dipole woofers are flat to 20Hz, so they do a fine job with 40 to 50Hz.
As I've mentioned, SL's summary section #3 clearly states the significant improvement that dipole bass radiation has on "small" room acoustics.
Until you have first hand experience with what I'm saying, please take my comments at face value and FWIW. Once you do have experience, perhaps you too can help educate those who do not.
Also, unlike some who have commented in this thread, I have no financial motivation.
Good luck to the OP with whatever solution he implements.
SL's dipole woofers are flat to 20Hz, so they do a fine job with 40 to 50Hz.
They are not flat to 20 Hz that I have seen. The measurements you linked to show a rapid dropoff of > 12dB per octave below the lowest room mode, which was at 50 Hz in the small room and 35 Hz in the large room. This is exactly what you would expect from the design.
As I've mentioned, SL's summary section #3 clearly states the significant improvement that dipole bass radiation has on "small" room acoustics.
I can't find any such statement in section #3. The closest I could find is this:
"The degree of these differences is difficult to predict and will depend upon the specifics of a room and the placement of woofer and listener. However, the dipolar source can be expected to interact less strongly with the room and will, therefore, on average convey greater detail and resolution of complex low frequency material."
But then that seems to be contradicted by the following bullets:
"* It is nearly impossible to predict the room transfer function from a prior knowledge of room dimensions, woofer and listener placement, radiator type and the room's eigenfrequencies."
and
"* No meaningful conclusions could be drawn from the steady-state room transfer functions about performance differences between dipolar and monopolar radiation."
Further, when I look at the differences between the monopole and dipole frequency response measurements, I can't say qualitatively that one is better than the other.
I'm not here to argue with you and not sure what measurements you are referring to. The woofers system used in the LX design is flat to 20Hz in a listening room.
- Response -3 dB at 30 Hz (Q < 0.5) on ground plane, free-field
The difference in my rooms between a dipole woofer system and conventional sealed woofers has always been enormous - the difference between listening to music Vs mud - especially after getting used to not hearing the room's effects, which does not take very long. While results will vary to some degree from room to room, a dipole woofer system will always excite room modes less.
"By far the perceptually most uniform response in the range below 200 Hz is obtained with an open-baffle, dipole or figure-of-eight radiating source. Because of its directionality, the dipole excites far fewer room resonances than an omni-directional source. The measured room response is not necessarily any smoother than that for an omni-directional source. But the perceived difference in bass reproduction is startling at first, because we are so used to hearing the irregular and booming bass of the typical box speaker in acoustically small rooms. Quickly one learns to recognize the distortion of this combination and it becomes intolerable."
http://www.linkwitzlab.com/rooms.htm
Whether or not any of this matters to you, who knows? Regardless, try to keep an open mind until you have had an opportunity to hear a system like the LX521. It will likely render discussions like this one moot.
I have not heard the LX521. I have heard the Orion and it was very good.
I just don't believe the claim of flat to 20 Hz though. And it seems to contradict Linkwitz's own measurements that you linked to.
I do believe the claim of -3 dB @ 30 Hz in a ground plane measurement. But an anechoic or free field measurement of a loudspeaker is not representative of its performance in the "pressure region" of frequencies below the lowest room mode. For a monopole, the free field response will understate the in-room response in that frequency range. For a dipole, the opposite is true, the free field response will OVER-state the in-room response in that frequency range.
Here's why: In the pressure region, which is the frequency range below the modal region, the longest room dimension is less than one half of the shortest wavelength. Therefore, there is no wave front and no radiation pattern to speak of. The pressure in the whole room just rises and falls. With a sealed monopole subwoofer, the room pressure increases on the outward stroke and the box pressure falls, and the room pressure decreases on the inward stroke and the box pressure rises. With a dipole, with one side pushing and one side pulling there is no net pressurization of the room. If not for air resistance, you wouldn't get any sound at all. The reason why a free field measurement is different is that the volume is infinite so there is no pressure region. In free space, the dipole will radiate a dipolar pattern even down to the lowest frequencies. That doesn't happen in a room.
If you don't believe me, you can see it in Linkwitz's in-room measurements.
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