Bottom ports...how do they work, then?

[Muzz]

This just come up while I was looking at some cabs in another thread...anyone know how a bottom port works? When I used rear ported cabs, I was always careful not to back them up against anything to effectively block the port, but a bottom port has a gap the size of the feet (less than an inch?), does this not compromise port output? Or have I just been wasting my time all those years?

[Bill Fitzmaurice]

If the feet are too short it has the same effect as being too close to the wall with rear ports, although what happens probably isn't what you might think. When the distance is short enough the space between the speaker and the floor/wall becomes an extension of the port, lowering the tuning frequency of the cabinet. That's not a good thing. OTOH a well engineered bottom ported cab would take advantage of this and make the feet the right size so that the length of the port inside the cabinet could be shortened, allowing a reduction in the cabinet size. 

[scalpy]

So what am i listening for please?

[Bill Fitzmaurice]

A cab that's tuned too low is not going to be as loud in the lows as one that's correctly tuned.

[scalpy]

27 minutes ago, Bill Fitzmaurice said:

A cab that's tuned too low is not going to be as loud in the lows as one that's correctly tuned.

So can't really tell if i don't have a variety of cabs to AB. I will give it a go when the new cab arrives against my db112, but it wont be like for like obviously as the new cab is a miniscule 110. 

[Bill Fitzmaurice]

You'd only be able to tell with two otherwise identical cabs.

[scalpy]

53 minutes ago, Bill Fitzmaurice said:

You'd only be able to tell with two otherwise identical cabs.

That's what I suspected. Will be interesting anyway.

[yorks5stringer]

Ramsgate, Dover?

[Skybone]

Waiting for a fart joke...

[Muzz]

14 hours ago, Bill Fitzmaurice said:

If the feet are too short it has the same effect as being too close to the wall with rear ports, although what happens probably isn't what you might think. When the distance is short enough the space between the speaker and the floor/wall becomes an extension of the port, lowering the tuning frequency of the cabinet. That's not a good thing. OTOH a well engineered bottom ported cab would take advantage of this and make the feet the right size so that the length of the port inside the cabinet could be shortened, allowing a reduction in the cabinet size. 

Thanks Bill, authoritative as ever 😉

When you say 'make the feet the right size', the ones I've seen all have very short feet (like an inch or so), is that the sort of range you mean? Half an inch is too little but an inch would be too much, kind of thing?

[ezbass]

2 hours ago, Skybone said:

Waiting for a fart joke...

You are not alone. BC is slipping.

[Muzz]

I'd been braced for that (SWIDT?) since I posted the opening comment yesterday...

[Bill Fitzmaurice]

4 hours ago, Muzz said:

Thanks Bill, authoritative as ever 😉

When you say 'make the feet the right size', the ones I've seen all have very short feet (like an inch or so), is that the sort of range you mean? Half an inch is too little but an inch would be too much, kind of thing?

It depends on how skilled the cabinet designer was, and whether the prototype was tested to be sure of the result. I can see an inch being OK under those circumstances. It's easy enough to know if it's correct, you just run an impedance sweep to see what the in use tuning frequency is. I wouldn't blindly assume that was done, though. I've seen too many cab designs that never should have made it past the drawing board to do that.

[Muzz]

Ahhh, OK. I just hadn't imagined a flat surface an inch from a port NOT blocking it. Very interesting, thanks.

Edited November 26, 2019 by Muzz

[Bill Fitzmaurice]

Assume the port is 4 inches diameter, which gives an area of 12.5 square inches. If the floor/wall is an inch away the area between the port circumference and the flat surface is 12.5 square inches, so not only is the port not blocked, it's not even constricted.