There are many guidelines some more conservative than others.

I'd say get it as low as you can go within reason for your build. Remember that port noise is a result of port compression which causes distortion in the sound. The reason we have DIY Subwoofers is we want to have loud clean bass. Generally the most cost effective solution is to use a folded square port. The easiest port to do is probably a Passive Radiator build. Kevin has insane deals on them and you can use a mass kit to tune the PR for your needs.

I only suggest round ports in Sonotubes or Speakers, not subs. But I'm very conservative when it comes to port air velocity because I can afford to be a snob on it.

In general I shoot for no more than 14 m/s with a subsonic filter applied. If you are a couple above that you should be fine. I'd never build anything to be greater than 20m/s unless constraints required it.

If you must use a small port you can use a high pass filter to bring down port noise/compression.

Low port noise is not damaging at all.

 

The two 11 cu ft 13.5hz slot ported subs I will be building next week will have a port velocity of 38 m/s without a hpf at max power of 1200 watts and 30 m/s at the tuning frequency of 11.5hz. My ports are 20.5" wide by 2" high by 49" long. With the hpf engaged it will drop to 20 m/s at full power and stay under max excursion which will be fine. With running at 1000 watts rms without a hpf it drops down almost 2 m/s and only a couple of db. But to give you an idea when compared to a 15hz tune sub in the same size enclosure and a hpf engaged the 13.5hz tune will have 5db more at 10hz and the same output from 15 to 50hz as the 15hz tune.

Hence why I am going with the lower tune.

At the end of the day you are advised to get a hpf anyway to be on the safe side as you wouldn't want to damage your drivers.

cheers

Graham

 

2" is much too narrow you want your square ports to be as square as possible.

 

Thats strange as I have been advised by Mikep that 20 m/s velocity is fine. Sub junkies slot ported sub has a velocity of 34 m/s with no hpf and he only gets a whisper of noise at the tuning frequency of 14.5hz.

Could you elaberate please.

cheers

Graham

 

20m/s usually is, but I'm a more conservative builder. Still your port design isn't advisable because it's not square enough. 2 x 20" isn't as good as 4" x 10" Plus the 4" x 10" will make the box stronger.

I realize there may be reasons you want to go with the 2" x 20" and you'd probably be fine with it. But why make the job harder. I do suggest using a dado joint for the port and box if you can. It is far more resistant to air leaks and you can use quarter round in the turns to reduce turbulence using this method. On the exterior it allows you to hide the seams avoiding the hassle of veneering.

You can cut these with a straight router bit or a table saw.

 

My design goals usually are to keep port velocity to less than 5% the speed of sound (17 m/s) in the audible range (>20Hz) and to less than 8% the speed of sound (26 m/s) below 20Hz - although I don't really care what happens to the air speed below 20Hz. The only reason to care about it is to minimize the possibility of hearing the port noise.

 

Dyonn,

On my lower tuned sub if I increase the hpf to 13hz instead of 12hz I get a max port velocity of 17 m/s at 12hz so all should be fine. I wouldn't need to change the 2" port height. Above 20hz it is 10 m/s and lower.

cheers

Graham

 

Technically speaking Graham, you would never see the 30m/s port air speed when your not running a HPF. You probably wouldn't push the subs past 60mm excursion (due to massive distortion and possibly damaging the subs), so you air speed will stay down according to how you have your levels set if your not running the HPF.
I'm not sure if your models in WinISD have changed any lately, but that's just what I remember from the last time that I modeled them.

Steve, the larger you make the box, the less port length that you will have to have to maintain the same tuning frequency. So, if you have a small box for a ported 18" sub (say 3-4 cu. ft.), the port length will have to be massive to keep the air speed to an acceptable speed, and to keep the port resonances at the right hz.

 

Still port resonance isn't a major design issue and is far less problematic than compression. This is why we can build a speaker with a port resonance in the middle of it's operating range.

The problem is once you get above 60" it's very hard to do a proper port in a reasonable box(something wives would allow in house). For the newb I suggest you do a sealed build as it's much simpler and far less problematic. A sealed Shiva-X would smash nearly every commercial sub under 1000 bucks when used with a parametric eq.

 

Lsiberian,

Woods all cut up now mate. Am from FI has advised today that I will be ok especially with a hpf engaged. He was telling me that they built a massive god knows how many watts sub and that showed up that it should have port noise but there wasn't. Even without a hpf at full power at max excursion I am getting 30 m/s but that is at 1200 watts but I don't often watch movies at full power unless the neighbours were out but I aim to get the reckhorn asap so that port issues won't be a problem.

A 10" x 4" port gives the same port velocity in winisd as my 20.5" by 2" by 49". There is no difference. You can make it bigger say, 20" x 4" but it then ups the tune to 18hz which I don't want.

cheers

Graham

 

WinISD doesn't factor in the skin effect, but you've already made the cuts. :dontknow:

Just build it and give no more thought to it.

 

Will do mate. Really excited now. I will starting it first thing tomorrow. Just need to sort out the stuff to line it with. I will read your response on the other thread.

cheers

Graham