Why oval ports?

[Gibsonman63]

I've been doing some thinking about speaker boxes lately. The semi-open backs have been proving the most practicle for gigging lately because I can turn my speaker cabinet around and still crank my amp a fair amount when I need to hold down the volume.

The amps and speaker cabinets I have with open backs have a rectangular space. Is there some benefit (and science) to the large oval ports vs. my big rectangles? I have to admit, the ovals are cool to look at.

[DonMoose]

The oval ports help you get those round tones.

/ sorry

[NickC]

The oval ports help you get those round tones.

/ sorry

Will pentagram shaped ports make one a star?

[Structo]

I think the oval port like on Dumble speaker cabs adds a bit of back pressure to add focus and a bit of low end to the speaker cab.

Not so sure a rectangular opening of the same size wouldn't sound similar but I think Dumble was after a unique design.

I wonder if he borrowed the idea from somewhere?

[Lonely Raven]

I think the oval port like on Dumble speaker cabs adds a bit of back pressure to add focus and a bit of low end to the speaker cab.

Not so sure a rectangular opening of the same size wouldn't sound similar but I think Dumble was after a unique design.

I wonder if he borrowed the idea from somewhere?

Backpressure would be the result of the size of the opening compared to the size of the cabinet combined with the square inches of speaker moving the air. Shape would be irrelevant.

IMHO, round openings are for looks only.

One could probably hypothesize that the shape of the port might effect different resonant frequencies in the wood of the cabinet itself, but considering how little mathematics is really done in guitar cabinet design, I'd say that's pretty far fetched.

For example, if you have a cabinet that's 24" X 24" X 24", then there is a good chance the cabinet will have a resonant hump at a specific frequency who's waveform happens to be 24".

[surfsup]

I believe the area of the port opening, combined with the depth form a volume which can be used to tune the frequency of the box. I believe this is largely ignored by Guitar Speaker Cabinet builders. I'm not sure if that's because it is felt the cab should not be optimized for a certain set of frequencies, or what....?

Most of the equations to do this are based on a round port but the area of any shape can be converted to the area of a circle with a set radius.

http://www.clubknowledge.com/Car_Audio_FAQ/?t16

[Lonely Raven]

I believe the area of the port opening, combined with the depth form a volume which can be used to tune the frequency of the box. I believe this is largely ignored by Guitar Speaker Cabinet builders. I'm not sure if that's because it is felt the cab should not be optimized for a certain set of frequencies, or what....?

Most of the equations to do this are based on a round port but the area of any shape can be converted to the area of a circle with a set radius.

http://www.clubknowledge.com/Car_Audio_FAQ/?t16

Since very few guitar cabs have a port with any depth (tube), I would agree with this. Bass cabs on the other hand (and some very few guitar cabs) were designed with Thelie ports (I think I spelled that right).

It's subwoofer 101.

[NickC]

I have a Bag End 2 x 12" guitar cab. Each speaker has it's own compartment, and each compartment has a 3" round port on the back.

[Lonely Raven]

I have a Bag End 2 x 12" guitar cab. Each speaker has it's own compartment, and each compartment has a 3" round port on the back.

That would be one of the few examples of a tuned cabinet.

The problem is, they were probably designed around that speaker. You change speakers, and the sound could go to hell with the cabinets tuning.

Mesa Boogie also has some tuned port guitar cabinets.

[Aurora]

Will pentagram shaped ports make one a star?

Diabolic performance .. ??

A port in a 3/4" panel, irrelevant of the form, round, square etc, is too short to follow modern bass reflex loading theory. A variance of 1/32" will shift the tuning way out of designed parameters.....
Loading may still occur, but the result is more of an empirical ( read cut and try ) nature..... that is not to say that some of these cab's still sounds good for their task...

[briane]

My belief, is its really a cheap karlson curve. Look that one up.

What it does, through frequency roll-off, is acentuate certain frequencies, in general those on the low end.

Of course the karlson concept/rolloff is much more complex than that.

[Ulysses]

Hi there from a forum newbie! What a great source of information you guys have put up there. Thaks for sharing your experience.

Now my guess about the oval ports is, that there`s a simpler reason:
It`s much nicer and easier to wrap that Tolex around the edges of largely diametered curves than to cut and glue it into sharp negative corners.
How `bout that?

[Lonely Raven]

Hi there from a forum newbie! What a great source of information you guys have put up there. Thaks for sharing your experience.

Now my guess about the oval ports is, that there`s a simpler reason:
It`s much nicer and easier to wrap that Tolex around the edges of largely diametered curves than to cut and glue it into sharp negative corners.
How `bout that?

Damn your green blooded logic; there's no place for it here!


Good idea actually, when combined with my thought of aesthetics.

And welcome to our madness!

[billyz]

If you only factor in the volume of the port then the shape is irrelevant. But I believe the shape and the position affect the phasing of the wave as well. A more complex shape ,like the oval will result in a more complex wave. It will smooth out the phase response.

here is an interesting link on acoustic physics that might explain my thoughts with a little more clarity.

http://www.kettering.edu/physics/drussell/demos.html

You could argue that since it is only a guitar speaker you don't need to get all fancy, but I think a guitar speaker cabinet merits just as much attention to design as any quality instrument.

[jjman]

The "partially" open back cab pics I've seen have had openings that seem to large to function as a true port does. Not that there's anything wrong with them. I think the idea is for the speakers to behave as with an open back but curtail the "extra" sound that comes out the back. I think an "infinite baffle" result is the goal?

I'm not even clear on the functioning of a true port that's in the back. Does the math change for that as apposed to a front tuned port?