Vent machs(easy question!)

[xstephanx]

Well, ive read enough to know that there are SO many opinions out there on what is an acceptable vent air speed. I just need to know what people use, and how much chuffing they experience at what frequencies(easy question for most of you).

I am using an atlas 15" in a 6.5 foot^3 vented enclosure, with two UNFLARED 4" ports(tuning the box to 20hz), the airspeed works out to have a peak at 68fps at 14 hz. Is this acceptable? keep in mind this is with all 240 watts going to it, which probably wont happen often, i dont often listen to 20hz test tones at 110 db+...Yet

Sorry for the repeat question, but hopefully ill get to the bottom of all this port speed junk for once and for all!

 

[Rademakers]

I use 90 ft per second as the upper limit for PA, with more ft per second I found it to be eaily hearable even with 126 dB of kickbass behind it Ussually for Hifi only the half is allowed.

If you're sure that you will only use at say halfpower you should be doing just fine (the ventmach will most likely not get halved, but will probably be reduced to below or around 45 ft per second.

Most programs calculated with one end of the ports flared while the other end is just straight. Rounding the corners will help and in this case perhaps just about enough.

But wouldn't it be far easier just to try it tho? It's not harming the speaker as far as I know so there isn't much risk. You could even enlighten us with the results of you fieldknowledge

Cheers

 

[xstephanx]

no harm in trying....unless it doesnt work...then im stuck with a box that sounds like a train . WinISD is set so that it calcs for both ends straight, would I achieve a significant improvement if I did a 3/4 round with a router on the end of the port?(routing over the outer layer of 3/4 mdf and leaving the port hanging on the inner layer)

 

[simon5]

Most people told me to stay under 0.1 mach, so less than 34 meters per second or less than 112 feet per second.

 

[Rademakers]

I am using an atlas 15" in a 6.5 foot^3 vented enclosure

Here you state that you are using the enclosure but as I now understand you've actually not build it yet??

As simon5 states, 34 m/sec is the upper limit, as I was told too. With the exception that 34 m/sec is for PA-use and 17 m/sec is for hifi-use. Which sound pretty logic to me, since PA will get much higher SPL-levels and thus portnoise will be less noticeable. My personal experience took me to the conclusion that 30 m/sec as upper limit for PA-use is a better and safer limit. Thus that would mean 15 m/sec for hifi-useage as upper limit.
Which would be done by taking more or bigger ports, also resulting in longer ports.

Once I designed two cabinets with ports as small as possible to get a maximum compact design (2,4 cubic ft outside, for 15" PA). It was designed only to take 500 Wrms. Later I found out that even with 700 Wrms the cab was practically asking for more power. To bad this was limited by the very present portnoise.
The redesigned cab takes 1000 Wrms now with a slightly larger port that actually doesn't inflict with the size of the cab.
Conclusion: I Spend 200 euro on material and hours of building and testing, while a little adjustment could have saved me a lot of time and effort.

Flaring the ports just makes a slight difference (as does a round port over a square port). But since rounding the corners (any corner on the box really) gives the cab a much nicer and more profesional look it's welcome anyway.

By the way: which version of WinISD are you using?

Mvg Johan

 

[xstephanx]

UH OH! grammar police

Correct the box isnt built yet(or should i say tube?). But the longest thing thats gonna fit in there, is two 4" ports with a 90* elbow(interestingly enough, i dont need any length after the 90* elbow!) Even if i crammed a 6" port in there, is it really worth the 2 sq inches over the dual 4s?

I am using winISD pro [alpha]

The ports are facing DOWN, into the ground, will i experience significantly less noise becuase of this? This is another reason why i wasnt going to rout over the holes in the first place.

I need a solution quick .

 

[simon5]

64 ft per second will probably be acceptable since you said it won't be used to 240 watts often.

You could buy two flared port ends to go with your ABS 4" tube...

Anyway you aren't gonna run 14 Hz sinewaves into it...

 

[xstephanx]

...unless i had a stressing day at school and need a massage

I thought that 64fps oughta be ok too, besised the elbows actually flair out a bit.

and I think I better put in a good word for chad after the great customer service Ive recieved at AA, I should be ordering the thing today.

 

[bwbass]

WinISD's documentation says 5% of the speed of sound is a good rule of thumb. That's about 56ft/sec.

I just built a small sonotube sub with the Adire Koda 8", and based on the 56ft/sec vent speed limit, I wound up using a 3" port. Even at full blast, I can barely tell there's air moving in the port with this design, so I'd say this is a pretty conservative number. Higher is probably fine, as stated above.

 

[Rademakers]

If you use 240 W or less it should do just fine. Only the day you would like to use the sub to it's full potential you would hit it's shortcoming.

Mvg Johan

 

[xstephanx]

Do you have a picture of the kodatube? Even some measurements? Im curious as to the extension of a "midbass" driver.

 

[weikertball]

hmm, what about in car? My 2 Dayton Quatro 15's are only getting about 33 square inches of port and at with 200 watts to each, it peaks at about 85 feet per second at 16 hz and at the tuning freq. it's around 75 feet(21) hz. I figured that since it would be in a trunk, the backseat would help filter it out some?

 

[xstephanx]

Youre never going to have great SQ in a car, most of the time. Port chuffing shouldn't be too bad, but i find if the ports is significantly too small, it gives the bass this strainging sound. Id worry more about rattles before you consider rebuilding a box.