[genjamon]

A bit of a physics question here.  In several threads dealing with bass systems, especially open baffle bass drivers and also front/rear combination subwoofers, people talk about the beneficial effects of front/back waves meeting and cancelling.  I guess I'm confused.  I know what deconstructive interference in waves is, and that when out of phase waves meet they deconstructively interfere (cancel).  But I thought the waves still continued on their original paths.  That is, the cancel at that one spot where they meet, but they continue on and exist beyond that point.  The waves don't cancel and then cease to exist.  Am I missing something?  I guess I don't understand the advantage of an open baffle bass system.  Just because the front and back waves cancel at 90 degrees (the plane of the speaker), it doesn't mean they cancel elsewhere, right?

Can anyone point me to some resources describing the acoustical physics here?  I took engineering physics in college and did really well in it, but that was over a decade ago and I'm not in that kind of technical profession to keep up the knowledge.

[*Scotty*]

http://vbn.aau.dk/files/12831869/AC-phd.pdf
This doctoral dissertation does a good job of explaining how and why it works in  addition to
 why it is a very desirable way to achieve flat bass response.
Scotty

[rjbond3rd]

Just because the front and back waves cancel at 90 degrees (the plane of the speaker), it doesn't mean they cancel elsewhere, right?

My understanding is that the front and back are 180 degrees out of phase, and totally cancel at the sides (figure 8 radiation pattern, the sides having nulls and the front and back having something like an 18/dB rolloff):

http://www.quarter-wave.com/OBs/OB_Theory.html

I would say the benefit is the lack of a box's drawbacks (i.e., the cancellation is merely a consequence, not a benefit per se).

[genjamon]

Thanks, guys.  It will take me a bit to digest these.

[JohnR]

I know what deconstructive interference in waves is, and that when out of phase waves meet they deconstructively interfere (cancel).  But I thought the waves still continued on their original paths.  That is, the cancel at that one spot where they meet, but they continue on and exist beyond that point.  The waves don't cancel and then cease to exist.  Am I missing something?  I guess I don't understand the advantage of an open baffle bass system.  Just because the front and back waves cancel at 90 degrees (the plane of the speaker), it doesn't mean they cancel elsewhere, right?

Yes, that's correct.

The issue that is being assumed in all of those discussions is excitation of room modes and the time domain behavior of bass in the room. By changing the way that bass is generated in the room, the theory is that these things can be improved. The part that is still missing though is measurements in real rooms to see how well the different approaches work in practice.

[*Scotty*]

Quoting from the Bass Place Sticky http://www.audiocircle.com/index.php?topic=106838.msg1095060#msg1095060

[/"The CABS 2.2 has been simulated and implemented in two standard listening rooms. The system utilizes two loudspeakers in the front wall of the room to create a traveling plane wave and two extra low frequency loudspeakers in the back wall delayed and in opposite phase to remove the reflection of that wall giving a uniform sound field. After measurements of the implemented system in the two rooms one can conclude that the system works effectively in small and middle size rectangular rooms.http://www.audiocircle.com/index.php?topic=106838.msg1095060#msg1095060

"The system can achieve good responses not only in a single listening position but also in the whole room from 20 Hz to 100 Hz having spatial deviations in a large listening area of only ± 1.3 dB in the ITU Room and ± 1.6 dB in the IEC Room, contrary to the advanced room correction systems that typically optimize to a single listening position.


"Informal listening with music signals integrating CABS with full range stereo loudspeakers has shown evident improvement by removing the booming sound which is always present in small or middle size rooms. It presents a clear front sound image to the listeners and the back loudspeakers were not heard at all. Since the system works in the time domain it works effectively with transient sounds as well as with long durations tones."/]


Scotty

[genjamon]

Yes, that's correct.

The issue that is being assumed in all of those discussions is excitation of room modes and the time domain behavior of bass in the room. By changing the way that bass is generated in the room, the theory is that these things can be improved. The part that is still missing though is measurements in real rooms to see how well the different approaches work in practice.

I see, so it's more a matter of the front and back wave of an OB taking different paths means they will cross with different path lengths, hopefully evening out frequency response by disrupting nulls and peaks through constructive/deconstructive interference.  Kind of like automatically multiplying the number of drivers in different room locations - a la the swarm system concept.  Makes some intuitive sense.