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subwoofer loading: avoiding the band pass


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speakermakers 
Copper - Posts: 231
Copper spacespace
Joined: January 02, 2003
Location: United States
Posted: August 05, 2011 at 12:46 AM / IP Logged  
The last several subwoofer enclosures that I have built have all been down firing and loaded into a part of the vehicle that alters the response of the sub additionally beyond the cabin loading of the passenger compartment it's self. I have done this so many times that there is little guess work on my part as to just how the response will be affected.
Recently though on separate occasions a couple of my peers pointed out concerns that they had about my designs. This happened both times while in the construction phase. It dawned on me for the first time that my method might not be common knowledge. So I thought that sharing it here would be a good idea.
When you face a subwoofer into another chamber (trunk, foot well, storage space in a boat) you need to account for the acoustic interaction that the additional chamber, as well as the passage way from that chamber to the main listening area of the vehicle will have with your subwoofer. My method is to simply calculate the whole thing as a band pass in Bass Box Pro.
For example: I built a box for a boat recently that was located in the rear storage compartment. The subwoofer fired down at the floor. I built a frame around three sides of the bottom of the box (indicated by red) so that all sound was forced out one side of the box. This side of the box was partially obstructed by the fuel cell of the boat (green). The end result was a very small chamber beneath the box that the sub fired into (2" high X 14" X 11") and a long odd shaped path that led into the listening area. I purposely blocked off any acoustic path to the storage compartment.
subwoofer loading: avoiding the band pass - Last Post -- posted image.
I whipped up a band pass design in Bass Box that had a sealed chamber equivalent to the volume of my sub box and a front chamber equivalent to the volume of the space between the sub box and the floor. I then calculated the average cross sectional area of the path between the box and the listing area as well as the length, and plugged those numbers into my design as if they where port dimensions. Now Bass Box can graph the velocity and resonant characteristics of the entire path!
This design on the surface looked sketchy, but I knew that I would get the result I was looking for because I did my homework. This Box had an Fb of 41Hz and an F3 of 32Hz in the boat. I was able to confirm this after the fact by running an impedance sweep.
I should add that this type of simulation works for vented boxes as well. Also if you prefer that your sub box performs like a simple sealed or vented design and not be altered by the chamber that it fires into then simply keep the dimensions of your front chamber and port such that they are tuned out of band (above). The front chamber and path to listening area will then act as a conduit and will have no correlated affect on the end response.
let me know if anyone finds this helpful or interesting.
Haters need not apply.
impaladad64 
Member - Posts: 1
Member spacespace
Joined: September 15, 2011
Posted: September 15, 2011 at 7:28 PM / IP Logged  
Nice example! Enjoyed your post... I follow a similar design, but I like to build a bandpass, suspend it from the drivers side (starboard?!) dashboard and fire the port into the bow... I've been pleased with the results so far, but I have a 21' Sea Ray coming in next week and I may try to mount in the stern!

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