Box type explanation

[phuddleston]

I have been looking at enclosure design and am still not sure what I want to do. I am familiar with a typical bass reflex enclosure design. I have noticed the JL Audio HO enclosure, though, and wonder how that slight tweak affects things.

Does it change the way the box is tuned by porting across the cone of the sub, or is it smoke and mirrors? Would a box like this perform just the same as one with exact internal net volume, and port specs, just with the sub mounted on a flat face?

[Triticum Agricolam]

...Would a box like this perform just the same as one with exact internal net volume, and port specs, just with the sub mounted on a flat face?

Yup.

[philrab]

My guess is they angled the sub to make the box dimensionally shorter.

[NathanW]

Actually the geometry of the enclosure does have an effect on the output of the driver(s) in it. You can build two different enclosures with the exact same internal volume and tuning frequency. One rectangular and one trapezoidal, and they will sound different due to several factors including: internal standing waves and panel resonances, as well as air flow, to name a few.

There's a lot more to a box than volume and tuning frequency.

[philrab]

Actually the geometry of the enclosure does have an effect on the output of the driver(s) in it. You can build two different enclosures with the exact same internal volume and tuning frequency. One rectangular and one trapezoidal, and they will sound different due to several factors including: internal standing waves and panel resonances, as well as air flow, to name a few.

There's a lot more to a box than volume and tuning frequency.

Care to explain?

[Triticum Agricolam]

Actually the geometry of the enclosure does have an effect on the output of the driver(s) in it. You can build two different enclosures with the exact same internal volume and tuning frequency. One rectangular and one trapezoidal, and they will sound different due to several factors including: internal standing waves and panel resonances, as well as air flow, to name a few.

There's a lot more to a box than volume and tuning frequency.

I know that you are technically correct, but in this case there is no way that there would be an audible difference between the enclosure as pictured and one with a flat face. I'm highly doubtful there would even be any kind of measurable difference.

Car audio subwoofers are generally too small and crossed over too low for standing waves to become as issue. A properly braced box shouldn't have any panel resonances in the subwoofer frequency range regardless of external shape.

[NathanW]

Standing waves are created by parallel surfaces reflecting sound waves back and forth, they create peaks in the output. Also at certain frequencies you'll have dips due to phase shifts. A rectangular (or cubical) box has six parallel surfaces (three pairs). A trapezoidal box on the other hand doesn't have any parallel surfaces, sound waves are instead reflected at different angles from each internal surface.

As for tuning frequencies not always giving the same result, that has a lot to do with air flow (and pressure) through the vent(s). You can have two round ports for example, even using the same size tube, if one is flared and the other is not the flared port will be able to handle higher air velocities without audible port noise. And with rectangular ports adding 45 deg. angles to the insides of the port will improve air flow within the port giving you more output vs. a port of the same dimensions without 45's.

Also internal bracing will reduce panel vibrations resulting in cleaner sound by preventing the panels from making any audible noise.

[Triticum Agricolam]

Standing waves are created by parallel surfaces reflecting sound waves back and forth, they create peaks in the output. Also at certain frequencies you'll have dips due to phase shifts. A rectangular (or cubical) box has six parallel surfaces (three pairs). A trapezoidal box on the other hand doesn't have any parallel surfaces, sound waves are instead reflected at different angles from each internal surface.

Yes, standing waves are bad and they are caused by parallel surfaces but that's only half the story. For a standing wave to form you have to be playing a frequency with a half-wavelength less then the distance between the parallel surfaces. If you have a box with 48" between surfaces (which is on the upper end of what we see in car audio) you aren't going to get any standing waves until you play content above 140 Hz through that box. Pretty much nobody plays content that high through their sub box. For a box with 24" between surfaces it twice as high at 280 Hz.

I challenge you to find me a picture of a sub box with no parallel surfaces. It would be an absolute bitch to construct with compound angles everywhere.

[Ritch40]

Standing waves are created by parallel surfaces reflecting sound waves back and forth, they create peaks in the output. Also at certain frequencies you'll have dips due to phase shifts. A rectangular (or cubical) box has six parallel surfaces (three pairs). A trapezoidal box on the other hand doesn't have any parallel surfaces, sound waves are instead reflected at different angles from each internal surface.

Yes, standing waves are bad and they are caused by parallel surfaces but that's only half the story. For a standing wave to form you have to be playing a frequency with a half-wavelength less then the distance between the parallel surfaces. If you have a box with 48" between surfaces (which is on the upper end of what we see in car audio) you aren't going to get any standing waves until you play content above 140 Hz through that box. Pretty much nobody plays content that high through their sub box. For a box with 24" between surfaces it twice as high at 280 Hz.

I challenge you to find me a picture of a sub box with no parallel surfaces. It would be an absolute bitch to construct with compound angles everywhere.

thats no joke! i recently had to help someone build a box that was that way for his boat. and bitch doesnt even half begin to describe what a pain that was. needless to say just to make dang sure it was good and air tight we ended up doing some fiberglass work...lol

[NathanW]

I was asked to explain, so I did. I never said it was easy or that you had to use a trapezoidal design. I used that as an example of one of the best designs out there. And the JL HO box that was in the first post is a semi-trapezoid design with fewer parallel surfaces than a standard box. It is a fact though that there are many factors that play a role in just how a sub sounds in a given box volume.