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Multiple subs vs. bass traps
Multiple subs vs. bass traps
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Old 13th February 2018, 06:56 PM   #181
scottjoplin is offline scottjoplin  Wales
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According to Linkwitz dipole subs can be better than monopoles because they excite fewer room modes, I'm wondering also whether because they radiate primarily in opposite directions if that is also an advantage and if they almost act as two in one, or is it just wishful thinking on my part?
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Old 13th February 2018, 07:38 PM   #182
bentoronto is offline bentoronto  Canada
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Multiple subs vs. bass traps
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Originally Posted by scottjoplin View Post
According to Linkwitz dipole subs can be better than monopoles because they excite fewer room modes, I'm wondering also whether because they radiate primarily in opposite directions if that is also an advantage and if they almost act as two in one, or is it just wishful thinking on my part?
Attention DIYers: the whole issue of what to do about that out of phase rear wave has been hijacked by commercial interests who, of necessity, are best off if they sell boxes. Likewise for the sub forum, it is as if there are no good solutions except boxes fulfilling the model Thiele advanced.

Think statistically. Better to say the eigentones (and box resonances) are always lurking, just a matter of how much the speaker activates them. As long as the rear wave does not uniformly conflict with the front wave in a wide band, it is not an inimical presence.

An irregular-shaped large baffle (or long Vogt tube, Transmission-line-like path, or real big sequestration) meets that stricture and you've got great sound.

But for a smallish dipole baffle, you need Linkwitz' naive-model math, vast bass boost, and luck.

B.
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Last edited by bentoronto; 13th February 2018 at 07:45 PM.
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Old 14th February 2018, 02:16 PM   #183
Zwiller is offline Zwiller  United States
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Originally Posted by bentoronto View Post
An irregular-shaped large baffle (or long Vogt tube, Transmission-line-like path, or real big sequestration) meets that stricture and you've got great sound.
B.
Can you please expound? I think I understand (rear wave does not uniformly conflict with the front wave) but would like to confirm. Contemplating a TL HT sub. Planning on running it near field on back of sofa.
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Old 14th February 2018, 03:14 PM   #184
bentoronto is offline bentoronto  Canada
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Multiple subs vs. bass traps
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Can you please expound? I think I understand (rear wave does not uniformly conflict with the front wave) but would like to confirm. Contemplating a TL HT sub. Planning on running it near field on back of sofa.
To avoid irritating those who ardently believe in the Theory of TL as Taught Us by MJK, I'll only say that when the exit door of the tube is remote from the driver, you have a welcome benefit of having the exiting wave more randomly interacting with the driver front-wave.

Seems like an important consideration in building a TL, eh. Like Nelson Pass's beautiful article, I've been thinking about a folded Vogt tube about 15 feet long. The folds have to be an even-number so that the exit is remote from the driver. Call it Bentoronto's TL Theorem.

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Old 14th February 2018, 08:20 PM   #185
scottjoplin is offline scottjoplin  Wales
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Can you please expound? I think I understand (rear wave does not uniformly conflict with the front wave) but would like to confirm.
Do you mean OB/dipole? There is some cancellation at lower frequencies depending on baffle width and shape. Less at higher frequencies as they become more directional and with reflections the output can be greater than a monopole
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Old 14th February 2018, 08:56 PM   #186
AllenB is offline AllenB  Australia
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whether this method also means the room is not pressurised?
John does say this with regard to the DC mode.. I can't see the issue. When a wavelength fits in the room, different parts of the room will be at opposite pressures so what does room pressurisation mean.. that the room is elevated in pressure in a spatially consistent manner? At lower frequencies this just means the bass is being produced. If the room keeps it in, this is just a simple resonance. Its level can be adjusted like any other resonance.
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Old 14th February 2018, 09:21 PM   #187
bentoronto is offline bentoronto  Canada
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Multiple subs vs. bass traps
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Do you mean OB/dipole? There is some cancellation at lower frequencies depending on baffle width and shape. Less at higher frequencies as they become more directional and with reflections the output can be greater than a monopole
I used the words "conflict" and "band". While phase isn't usually relevant to hearing, it is definitely relevant to acoustics. If the baffle is uniform (textbooks might use circular baffles) and you are close to Darth Vader phase, you get some degree of annihilation. Otherwise, the annihilation is conditional, esp if the rear wave has bounced around some.

That same logic applies to all boxes where the rear wave comes loose to attack the front wave. In a BR box, the rear wave coming out of the port annihilates the front wave below the box tuning*. That's why a BR box whimpers away at low freq at 18db/8ave.

I think some folks on this forum who have never used a dipole sub (but who remember how long a sound wave is) think every note below 500 Hz is totally annihilated.

B.
*the port output is copacetic with the front wave above the tuned freq
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Last edited by bentoronto; 14th February 2018 at 09:26 PM.
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Old 14th February 2018, 10:30 PM   #188
scottjoplin is offline scottjoplin  Wales
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John does say this with regard to the DC mode.. I can't see the issue. When a wavelength fits in the room, different parts of the room will be at opposite pressures so what does room pressurisation mean.. that the room is elevated in pressure in a spatially consistent manner? At lower frequencies this just means the bass is being produced. If the room keeps it in, this is just a simple resonance. Its level can be adjusted like any other resonance.
Linkwitz refers to a dipole as a velocity source and a monopole as a pressure source. Here is a quote from a response comparison Dipole-monople response

" Since a dipole source consists really of two spaced monopole sources of opposite polarity, it generates a large particle velocity between them and a local acoustic field impedance (p/v) which varies with observation point, and which is generally lower than that for a single monopole. Is our hearing in some way sensitive to sound field impedance? Does it affect coupling to the ear canal? Certainly a dipole in a room, at the same location as a monopole, couples to a different degree to the same room mode."
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Old 14th February 2018, 11:08 PM   #189
scottjoplin is offline scottjoplin  Wales
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Old 15th February 2018, 01:54 AM   #190
AllenB is offline AllenB  Australia
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Room modes have regions of pressure and regions of velocity. I don't believe that's it. Probably the more interesting thing about dipoles is they have directivity.
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