vent turbulence and high excursion drivers

[paulspencer]

I'm designing a vented box for the Stryke/AE speakers AV12 which is a high excursion 12" driver with 23mm one way xmax. I'm trying to determine acceptable vent velocity. I've heard figures ranging from 5-10% of the speed of sound or 17 - 34 m/s, however, what is not clear is what is acceptable when the vents are flared. And there are flares and flares! A 10mm flare radius is not the same as 60mm!!

* what are your experiences with vents for high excursion drivers?

* has anyone done a vented box for this driver (AV12)?

* what vent velocity do you find acceptable for your ports and driver?

It would be good to have points of reference. For example:

"I used Tempest in a 200L vented box tuned to 22 Hz with a 5" vent with a 2" radius flare and simulations showed 33m/s max velocity in the vent but it wasn't a problem even for LFE use."

[rabbitz]

There's some info under Peerless engineering notes that it should be below 1/10 the velocity of sound, approx 35m/sec.

http://www.d-s-t.com/main/tech/appxls.htm

The main info I've found is that the flare opening at the edge of the box should be at least 1.5x greater than the port diameter and assymmetrical flares work better with a smaller flare on the inside than outside but extreme flared ports can be undesireable (B&W Flowport excluded of course).

[Eric]

Hey Paul,

My sub used an Audiomobile MASS 12 driver (xmax of 20mm one way) in a 135L vented enclosure. The vent is 6" diameter PVC that is 36" long, providing an overall tuning of about 19Hz. The end of the vent is not flared and I have never detected any "chuff" or other obvious sign of vent turbulance at levels up to about 110dB in a room that measures 24x15x8 feet.

WinISD models the vent mach in the sub at about .07 Mach. Here is the web page write up the sub - the WinISD model is in the middle of the page.

Hope this helps.
Eric

[mike.e]

Paul-
I use a jbl2226 on 175litres internal, and 33hz tuning with ~360cm^2 port area. With 240watts rms input, vent airspeed is ,<12.5m/s.




Keep it sealed until your sure that you wont get chuffing. Ofcourse you could tune it low like 20hz, and remember that music will not normally have that so no problem!(could even EQ it slightly down at tuning to prevent it) But then EQ ,you shouldnt need to.

EDIT: I did see somewhere that flare ports of twice the radius have can have 'double the airspeed' for the same chuffing level. Now I hope that is double radius,not diameter,etc. I forget where! Perhaps Isaac from subsim said it.

[BassAwdyO]

Eric, I was looking at the webpage with your sonosub. Do you think those fishes like sitting on top of the subwoofer enclosure when its pounding away? I bet they go nuts when it hits a low note. Back to the thread topic, flared ports can definately handle much higher velocity than non flared ports. The chuffing is a whistling caused by the air moving past a edge which creates small eddys. If the edges are more rounded, the eddys are not created as easily. Back in my newbie days (though i'm still a newbie compared to some) I made a vented box that had a airspeed of ~120m/s as calculated by winISD. The air velocity peaked there at 16hz and was plainly audible to say the least. I believe vent noise is not just a factor of airspeed in the vent however. Frequency also plays a large role. If you have an enclosure tuned at an easily audible frequency such as 30hz it will take much more air velocity to become audible chuffing than say 15hz where you cant actually hear the tone.

[paulspencer]

Thanks Rabbitz, I had a look at the Peerless note and it confirms 10% or mach 0.1. It seems this is what I should aim for and then add flares. Perhaps power compression is also an issue.

However, with this driver, a vented design means the driver won't go to its xmax. In fact excursion will be more like 16mm, short of its 23mm xmax. This could mean power compression is less of an issue. In which case I wonder if it is possible to design a smaller than optimum (according to the 10% rule) vent with good performance.

What is undesirable about extreme ports? (What do you call extreme?) In my case I'm thinking of 52 - 72mm radius flares for 100 - 120mm vents. This is 3 or 4 layers of 3/4" MDF.

What is it about asymetrical ports? I have heard just the opposite, that they should be symmetrical! I would think that it is the same as trying to achieve a symetrical magnetic field for driver motors. However, I have considered that the outside flare may be more critical as internal turbulence is most likely masked.

Eric,

Your driver is very similar to mine in output, slightly less xmax, although I may in fact be using less than you (impossible to get to xmax above tuning with a sane amount of power). I would think that a 6" vent could be replaced with a 5" flared vent.

[paulspencer]

Keep it sealed until your sure that you wont get chuffing. Ofcourse you could tune it low like 20hz, and remember that music will not normally have that so no problem!(could even EQ it slightly down at tuning to prevent it) But then EQ ,you shouldnt need to.

Mike, for music it would not be worthwhile, however I'm designing this for those bursts of LFE where there might not be anything to mask the sound. Also I plan to have them close to the listening position, so vent noise is critical.

EDIT: I did see somewhere that flare ports of twice the radius have can have 'double the airspeed' for the same chuffing level. Now I hope that is double radius,not diameter,etc. I forget where! Perhaps Isaac from subsim said it.

Don't you mean a flared port of half the area is acceptable? Hmmm, seems a bit unclear, will have to look into this ...

WinISD indicates that lower tuning lowers vent velocity. However, this means a longer vent. A larger diameter vent must be tuned higher to fit in the box.

If I go with a 120mm flared vent, I can get it to 880mm in my 60L box, hence tuned to 24 Hz. Peak velocity is 29m/s. If I use a 100mm vent of the same length it will be tuned to 20 Hz. This peaks at 19 Hz with 33m/s. Hence the bigger vent tuned higher is better from a turbulence point of view.

[mike.e]

A passive radiator,would save minutes of your life due to not debating the different port options that arent optimal

[paulspencer]

Looking at SV subwoofers, it seems their top of the range sub B12plus uses 4 drivers very similar to AV12 (made by TC sounds, the cone, surround and magnet look identical). They use what appears to be one 4" precision port per driver. If I model this box with 2500 watts total input (from the recommended Crown K2), the vent velocity is quite high, unless a rumble filter is used. 20 Hz 6th order HPF gets vent velocity down below 29 m/s, but without this it goes above 40m/s.

While previously I had preferred lower rumble filters (Fc @ 16 Hz) due to their lesser impact on resonse around 20 Hz, it seems a higher fc helps a lot with vent noise.

[paulspencer]

Originally posted by mike.e
A passive radiator,would save minutes of your life due to not debating the different port options that arent optimal

This is not a big ugly box sub! I'm working on an elegant curved design where the size and look are critical. Passive radiators are IMO a waste of money, force the use of a rectangular box and save only a small amount of volume (5-10L) while at the same time further compromising the stiffness of the box (due to the holes), hence requiring thicker walls and losing all of their benefits! They offer no improvement in performance!

Yet it would save time but if I were trying to save time, then I wouldn't be doing DIY!