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In the Forum: Audio Discussions
In the Thread: Exceptional loudspeakers drivers

Post Subject: Baffle DimensionsPosted by NBC on: 4/7/2007

Thanks Romy,

Yes, I received your advice in the past privately regarding proper size for 1808 enclosure, and I thank you for pointing me in right directions. Yes, sealed box is simple calculation using e.g., WinISD. I am experimenting with thick walled (5/16") 24" diameter Sonotube and fiberglass stuffing, yielding aproximately 14cft.

(Structural reinforcement could include placing a larger 30" cylinder around the 24" cylinder, and filling the gap with sand, which would allow me to break down, move or transport the sub if necessary.)

However, in the above post regarding "dimension of the baffle", I was referring to actual ***baffle*** dimensions, as opposed to cabinet volume. Although my cabinet would logically have a round baffle, it would be a simple matter to create a rectilinear one.

What baffle dimensions did you settle on?

Michael Green of Green Mountain Audio, for example, wrote the following about baffle geometry for low bass systems. (However, at the end of the article he states there is some (unknown to me) type of math involved in calculating proper baffle size, in relation to the room boundaries:

Considerations of shape

A cabinet face can be too large or too small. Too large, and it delays some of the energy in the low bass from reaching the wall behind the subwoofer as soon as possible. In the middle bass, it will also throw an 'acoustic shadow' onto that wall. Yet, this wall reflection is needed as soon as possible, because it will reinforce the low bass that is still being pushed out from the subwoofer driver. The result is less uniform bass in the room and a subwoofer that is more difficult to 'place.'

If the cabinet face is too small, the woofer cone's air pressure in the middle-bass range slips around that cabinet face before the cone can complete its pressure cycle. The result is an uneven tone balance and lack of impact in that middle-bass range.

The reason for this loss of energy is interesting, because it affects so much of what we hear: As a woofer cone begins to move, it initially pushes against the nearest air molecules. Those molecules say "Thank you!" and immediately speed away. As those molecules speed forward in all directions at a speed 10 times faster than a car on a highway, they collide with other air molecules. Through those collisions they transfer their momentum.

Thus, the energy of that cone's initial push continually slips away, which is called a 'loss of radiation resistance.' The lower down the scale towards the bass we go, the greater that loss -- because the cone is required to spend an even longer time pushing out and then 'pulling' back.

That longer time gives the air molecule collisions even more time to flow away from the cone. For a 30Hz, very low bass note, the first collisions have spread out to a radius of nearly 3 meters distant by the time the cone finishes its first outwards push. Those distant collisions are blissfully unaware that the woofer is still pushing forward. Behind those first collisions lies a trail of more collisions, extending all the way back to the cone's surface. Each is unaware that the woofer is still pushing, because the woofer is only moving forwards at 0.5 meter per second while those collisions take that push and distribute it at 340 meters per second.

Thus, the energy of the woofer's initial push and subsequent 'pushing' continually slips away, in all directions, and does so more and more as the scale is descended. Its radiation resistance is decreasing with decreasing frequency.

In this regard, however, the floor is our friend. It holds some of those collisions near the subwoofer's cone and helps increase the radiation resistance, particularly in the middle and lower bass. The wall behind the subwoofer and your ceiling also closely hold some of those collisions -- the ones from the lowest bass -- to increase the radiation resistance in that range.

The goal is to have a uniform radiation resistance. When the effects of the wall behind and the floor and ceiling are calculated, there is math which shows the size of the subwoofer's cabinet face that will smooth the transition between all those surfaces.

http://www.greenmountainaudio.com/Speakers/Hammer%20Brothers/DesignConcept2.htm

Regards,
Neil

PS: This may be getting off track from your original thread topic about the best drivers. Feel free to move and rename the thread to something like "Implementing sealed lower bass solutions" if you like.

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