Does port area affect sound?

[Avi3ator]

Hi!

I'm trying to design my first vented box. I was wondering if port area affects anything? Or is it just the length, width and height that matters?

I'm using winISD and the air velocity with the "correct" port size is at 33m/s, so to get that down to around 20 I made the port wider, and winISD automatically makes it shorter. But this also affects the total port area, the port area increased with around 400cm2. And this also increased the 1st port resonance to 148hz.

 

So the question is does it matter? If so, how much? 

Thanks :)

[Joe X]

Low port area results in port compression and port noise, you want to stay below 30 m/s at all times but lower airspeed is better.

 

Also bear in mind that WinISD will not work for slot ports and the frequency response plots are invalid for car audio.

[Avi3ator]

16 minutes ago, Joe X said:

Low port area results in port compression and port noise, you want to stay below 30 m/s at all times but lower airspeed is better.

 

Also bear in mind that WinISD will not work for slot ports and the frequency response plots are invalid for car audio.

 

So me increasing the port area wont really affect anything except lower air speed?

[Joe X]

Higher airspeed kills port output so you your box will be less loud around tuning and you may get port noise which is caused by air moving too fast through the port.

[SnowDrifter]

There's a goldilocks zone.

Too little port area will result in audible chuffing, loss in output, can cause unloading due to turbulent airflow and loss of backpressure.

Too much can complicate box design and also result in unloading - particularly below the tuning frequency.

There's also port shape, which will have an effect on how much port area you need - an 8" round port will NOT be the same as a 2x10" slot port with the same area. How much port velocity you can have without chuffing and turbulence is a function of how wide the port is along the smallest axis - boundary layer behavior. I can't seem to find it again, but a while ago, I read some info discussing this subject. Will post back if it pops up.


The very loose rule of thumb is 12-16" of port area per cuft of box  displacement. More realistically, it's a function of tuning frequency and power to the driver. Just.... Don't read too far into that.

The port resonances in winISD given are based on the speed of sound (think like a transmission line box). Unless you're playing close to ~150hz with your sub, don't worry about that one. The other bit with that program - it calculates on an open area. Like, box in the middle of a field, not in your car - so whatever response profile it gives needs to be taken with a grain of salt.

 

[Avi3ator]

4 minutes ago, SnowDrifter said:

Too much can complicate box design and also result in unloading - particularly below the tuning frequency.

Alright! Thanks for all the info! Hopefully the box will turn out great!