Nasty Port Noise?

[Triticum Agricolam]

1 hour ago, abdc said:

OK, so here is the new box I built per WinISD specs.  I have not put it together yet, but here is a quick mock up for you to see what it will look like.

External dimensions: 17.5 x 15.75 x 32.25

Gross Air Volume:  3.59 cubic feet

Net Air Volume: 1.75 cubic feet

Tuned to 32hz

Max Port Velocity 24m/s at 2200rms at 32hz with DC XL 12

 

Let me know what you think, almost looks like a T-Line at this point right?

Yeah that port looks a mile long, can you tell us how long it is?  

The problem you can run into is it can begin to become a high-tuned t-line if the port gets long enough.  Generally I don't like to see ports over 44" in length, but you can get away with a bit longer than that, it just depends on how low you are going to crossover from your subs to your mids at.  
 

Keeping the length of the port reasonable in situations such as yours where you have a small box, with low tuning, and high power can be difficult.  It definitely becomes a balancing act between reducing port noise/compression and not creating problems with an overly long port.  Often there no perfect solution and a compromise must be made. 

[abdc]

And for a 3 x 16 port slot vent, I had to make it 66 inches deep to reach tuning. So overly long perhaps lol.

[abdc]

 

Here are port dimensions. I went with these larger dimensions because reducing port velocity was the original goal, and unfortunately they called for a very long port.

[abdc]

Probably going to be a total mess.  Thanks for the feedback everyone, you guys are probably right about size and port length so this will be interesting.  WinISD sure seems confident though ha

 

I'll post results so everyone can say I told you so.

[Triticum Agricolam]

10 minutes ago, abdc said:

Probably going to be a total mess.  Thanks for the feedback everyone, you guys are probably right about size and port length so this will be interesting.  WinISD sure seems confident though ha

 

I'll post results so everyone can say I told you so.

Yeah, that's pretty long.  You have already cut the wood so might as well give it a try.  When you have it playing I'd suggest verifying the tuning.  WinISD doesn't do real well with slot ports and will usually tell you your port needs to be longer than it really needs to be to get the tuning you want.  When you get the box playing you can verify tuning acoustically by playing sine wave test tones at moderate volume.  Watch/feel the cone excursion while changing the tone frequency, right at tuning the cone excursion will be at a minimum.  As you raise or lower the frequency from that point the cone excursion will increase.  Just guessing I bet your tuning is going to come out somewhere between 28-30 Hz. 

As far as the pipe resonance issue goes, with a 66" long port your first resonance is going to be at about 106 Hz.  The general rule is you want the first resonance to be at least an octave above your crossover frequency, its not a hard rule though.  

 

[abdc]

Interesting, certainly more to consider here.

I wouldn't be comfortable bringing net air volume up past 2cu ft, but I guess I could bring it to 2, which would reduce my porting to 57 inches and keep my tuning relatively the same.  But the increased net volume would raise that 1st port resonance to something around 120 I believe (per WinISD)?  That would suit your last rule as my crossovers meet at 100hz.

Re-cutting the porting is not an issue.  Given your enclosure expertise, and knowing the DC XL likes smallish boxes, what would you do if it was you sitting here with this box (at this point assembled other than the porting)? Reduce porting/increase enclosure net volume slightly?

 

[abdc]

5 hours ago, Triticum Agricolam said:

The general rule is you want the first resonance to be at least an octave above your crossover frequency, its not a hard rule though.  

 

 

Oh, an octave. Yeah that is going to be a problem then lol.  No way I can get there on this build, I'm hardly breaking 100hz.  What kind of problem does this cause?