Port/Vent

[kingfisher]

Hello Every, I have a question about one large vent in comparison to two or more vents. All things being equal is there a different sound for a single large vent as aposed to a dual or multi vent in the same box Volume? I followed Joe of JLI and saw that he is using two vents rather than a single vent! Any thoughts?

[Brett]

If the CSA for a pair of ports is the same for a single larger one, the tune in the enclosure will be the same as will the port air speed for each port. I doubt there will be any sonic difference between the two, everything else being equal.

The reason for choosing one over another could simply be availability of tube in a given size from another use, such as PVC pipe for plumbing. I have a design currently that will use a pair of 8.6cm ID tubes because they fit internally better than a single larger unit and they are a standard stormwater pipe size here, so very cheap.

[GM]

Note that the dual vents of the same CSA as one will need to be slightly longer for the same Fb due to friction losses, so is slightly better damped. Not audible, though as the number of vents increases it eventually becomes audible and at some point it becomes ~aperiodic which is very audible.

Going from one to two of the same CSA will tend to sound better damped and if the single is already large enough to not need to be any bigger, then port gain can be great enough to cause some peaking at Fb which may/may not be audible due to the room dominating down low.

GM

[Jay]

I just can speak from experience that I like smaller vent better. I'm hard pressed to explain why, but let me just try.

It is probably true that in HIGH SPL a small vent MAY produce more noise, but in low SPL we seem to lost more sonic with bigger vent?

It seems that we should create a vent that can change itself based on SPL

But look at another way how to solve the problem. Move the port location "away" from the woofer. Most practice is to put the vent just behind the tweeter. If you put the vent closer to the woofer you will lose more (sonic/detail in low SPL). Troel made his rectangle vent just on the top of the enclosure. I like such solution, just not practical when I have to experiment with the exact length of the vent.

May be there is a sweet spot for the vent location, based on enclosure dimension and everything else?

[Brett]

Originally Posted by Jay

It is probably true that in HIGH SPL a small vent MAY produce more noise,

They will, depending upon the design, and smaller ports will also compress earlier. The Devantier, Button and Salvatti paper on the Harman site goes into this in detail.

Originally Posted by Jay

but in low SPL we seem to lost more sonic with bigger vent?

Makes no sense to me.

Originally Posted by Jay

Most practice is to put the vent just behind the tweeter.

I would never put the port there.

Originally Posted by Jay

If you put the vent closer to the woofer you will lose more (sonic/detail in low SPL).

Can you give some practical idea of why you think this is so? Doesn't make a lot of sense to me otherwise.

Originally Posted by Jay

May be there is a sweet spot for the vent location, based on enclosure dimension and everything else?

I typically put them on the bottom; that's certainly where they're going on the current builds.

[Jay]

Hmmm... you must first understand that I mostly write based on empirical observation. May be there's some misunderstanding because of my English, but from your response it seems to me like you don't experiment a lot, but only rely on your software/equipment.

Me: "But in low SPL we seem to lost more sonic with bigger vent?"
You: "Make no sense to me"

The difference is like vented versus sealed (where in sealed you have the optimum transient/sonic/detail or whatever you call it). The smaller the port the closer it is to sealed characteristic.

[Jay]

Me: "Most practice is to put the vent just behind the tweeter."
You: "I would never put the port there."

What I mean is something like the one in the picture. I forget that I was talking about bookshelf and you were thinking about floorstander? But the point is not to put it just behind the midwoofer (again this is about bookshelf) because the midrange information is there.

[Jay]

Me: "It is probably true that in HIGH SPL a small vent MAY produce more noise,"
You: "They will, depending upon the design, and smaller ports will also compress earlier. The Devantier, Button and Salvatti paper on the Harman site goes into this in detail."

Actually I "know" it is true. Well, theoretically. In practice I have never had problem such as "Oh, my vent is too small/noisy, I need to make it bigger!". Probably because the thing that theoretically is considered a bad thing, in practice turns to be favourable. Just like 2nd order distortion

[Xoc1]

I recently discussed the subject of port size with a PA speaker manufacturer.
They told me to avoid making the ports too large.
They were of the opinion that a large port allowed too much mid range out of phase output from the rear of the speaker.
They tended towards 4 tubular ports.
The port velocity they chose was fairly high, but adequate to avoid compression.
This can have other advantages with high power speakers. The ports can be distributed at the top and bottom of the cabinet to encourage convection cooling, and the higher port velocity also helps to cycle the air through the cabinet.

[speakerdoctor]

A rear-facing port in a bookshelf application may not be a good idea because of the possible close proximity of the port to a wall, thus chocking off the air flow and attenuating bass response.

Originally Posted by Jay

Me: "Most practice is to put the vent just behind the tweeter."
You: "I would never put the port there."

What I mean is something like the one in the picture. I forget that I was talking about bookshelf and you were thinking about floorstander? But the point is not to put it just behind the midwoofer (again this is about bookshelf) because the midrange information is there.