Downsides to port noise

[curtis73]

Other than the noise itself, is there a downside to port noise? Does it "choke" anything or cause anomalies in the enclosure response? I have a small (about 0.6cf) PVC pipe enclosure that I want to put some isobaric 8" drivers into, but it looks like I start getting port noise if I go below 2.5" diameter for the port. Problem is, the port length (don't have all the math in front of me now) starts getting too long to be practical, and will be tough to package.

If I drop to a 2" port, it predicts audible port noise, but it will let me do a more "normal" length port of 10" inches.

Since this will be installed behind a plywood panel in a van, and the driver will be 10' from my ears, I'm not worried about actual audible port noise, but I don't want to sabotage the sound quality if port noise does other bad things. Make sense?

 

[GM]

The pipe resonances can comb filter out of phase with the driver's output, so might audibly distort all the way up into the mids BW since 10" = ~13543/2/10 = ~677 Hz fundamental minus some end corrections lowering it a bit.

 

[curtis73]

I suppose I could use a longer port and put it outside the enclosure. I have plenty of length in that cavity behind the plywood,

 

[MrKlinky]

The biggest downsides to small port area apart from noise are reduced output and increased distortion. I always start with a port area equal to Sd and compromise until everything fits! Remember to consider port rectification in the port design.

 

[stv]

If you want to dig deeper into ports, here is my thread about them:
https://www.diyaudio.com/community/...sonance-absorbers-and-port-geometries.388264/

Very short recap:

• flared port flanges help a lot
• straight hard edge ports start compressing at low levels but can produce output up to high levels (lots of port noises, however)
• central port diameter can be quite small
• necessary port flange cross section size mainly depends on tuning (deeper tuning needs bigger port termination surfaces)

Also keep in mind the cabin gain, maybe a closed chamber is enough?

 

[stv]

pipe resonances can comb filter

If used for subwoofer only and behind a plywood board i suppose that may not be a problem.

 

[curtis73]

Thank you all for the input. I think I'll maybe try a 2" first and test the response and see what my ears think. If it's too bad, I can always get out the saw and make it 2.5", but given the constraints of the enclosure, it would have to be at least partially external to the box.

 

[Turbowatch2]

You may underestimate the effect of trumpets at both sides of the port. You can get away with a fraction of the port area, if you fit them in the right dimension. Basically an exponential horn.
Another option for "too long" vents is the passive radiator. If you know basics of speaker development and can measure, this is a gret option.

 

[Booger weldz]

The biggest downsides to small port area apart from noise are reduced output and increased distortion. I always start with a port area equal to Sd and compromise until everything fits! Remember to consider port rectification in the port design.

Doesnt That make for a really big box volume if starting out with such a huge port/near TL dimensions ?

 

[cowanaudio]

This box has a port that is Sd/2. Driver is an 18LW1400. Port compression and noise were low enough. Usable box volume is 250L, the port added 100L or so to that, from memory.

An Sd/1 port would be crazy big, with little benefit, IMHO.

 

[MrKlinky]

Doesnt That make for a really big box volume if starting out with such a huge port/near TL dimensions ?

Usually, but not always, hence my qualifying comment: "...compromise until everything fits!". Large ports rarely experience problems as do small ones, and even though I hate the idea of building a deliberately highly resonant enclosure due to energy storage and long decay times, sometimes needs must when fidelity is of secondary, tertiary or even lower importance.