Basreflex tube cut with 45 degree angle?

[heinz1964]

Hi

What happens when you cut the basreflex tube at an 45 degree angle on the inside of the box?

The tube will have different lengths, how does that effect the behavior of the tube.

What about different cutting angles, how does that influence the output of the tube.

best regards

uwe

 

[JMFahey]

No big deal.

Truth is, *length* by itself is not *the* main parameter, ,it´s not an organ pipe, but airmass inside it, and no matter how you cut the end, you will still have one definite mass of air inside.

Of course, when drawing cabinet plans or even calculating tuning, it´s far more practical for the builder to specify "use a 2" diameter by 4" long tube" than "use a tube which holds 2 grams of air" or whatever but in fact tube geometry already implies that.

 

[Patrick Bateman]

I've been doing this for a while.

One neat side effect of cutting the port tube at an angle is that it works similar to a flared port. (Because the area is greater when you cut at an angle)

 

[Brian Steele]

A tube cut at a 45 degree angle should produce an Fb that's slightly less than the same tube of the same average length cut at 90 degrees.

 

[steveMc]

I have a 100mm port that will end up just 50mm from the back panel of the cabinet. Would making the port a bit longer and cutting the end at a 45-degree angle help minimise the undesirable effects of having the port so close?

 

[Coltpix]

Port length

Simple answer. The port length is measured from center of angle (tube ctr.) The short side and long side are equal so they are offsetting penalties.

 

[steveMc]

To clarify, you normally need to maintain a clearance equal to the diameter of the port between the rear of the port and the back of the cabinet. In this case, I don't have sufficient clearance - so, in theory, the pathway into the port will be constricted and impair the effectiveness of the port.
The idea is to angle the end of the port so that it no longer faces the back of the cabinet but is angled away from it. The theory seems reasonable but I'd be interested to know if anyone has done this and whether it actually works out in practice.

 

[Booger weldz]

Taper the port slightly. And model it in horn response. You probably have a plastic tube, but if cardboard its quite easy? You can shorten it if its already got enough length to build on with a taper to make up for shorter, if careful not to get an air velocity issue

 

[weltersys]

I have a 100mm port that will end up just 50mm from the back panel of the cabinet. Would making the port a bit longer and cutting the end at a 45-degree angle help minimise the undesirable effects of having the port so close?

Steve,

Choking off a port's cross sectional area causes turbulence, which can be heard as "chuffing" or "wind noise".
If the port's inner termination is more than 25% of it's diameter from a wall, it's cross sectional area is not reduced. Your example of a 100mm port terminated 50mm from the back panel of the cabinet would still have double the cross-sectional area of the port available at it's entrance.

The "undesirable effect" of a port terminating closer than a clearance equal to the diameter of the port between the rear of the port and the back of the cabinet is the port effectively becomes a bit longer than it would be in "open air", lowering the Fb compared to what would be predicted.

Your 100mm diameter port 50mm from the back panel could likely be slightly shortened and tune as predicted with no adverse effects.

 

[steveMc]

My rule of thumb has always been to keep the port one diameter away from the back wall. Where I got this idea from is lost in the mists of time. I was under the impression that turbulence would result from having the port too close to the wall.
If 25% is acceptable, weltersys, that will make my life a lot easier. Thanks for that.