question on port air velocity... - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-05-12, 09:49 PM Thread Starter
Shackster

Join Date: Nov 2012
Location: Wheeling, IL
Posts: 45
question on port air velocity...

i know a lot of research has been done on this matter already...and i feel like i have read almost too much of it...i have also used the flare-it program...but i still have a question...

for my current design...

given the max input signal is 300W which equates to an output of 109dB the port air velocity maxes out @ 32 m/s. The port length = 20" and port diameter = 4" for this design.

given that the 32 m/s is at the max input signal...how much of an issue will this be for me in regards to port chuffing ???...i have heard two numbers thrown around as limits...35 m/s...and then a more current 17 m/s...i do plan on using a port with double flared ends...

From what I have read, the port length should not be greater than 30" due to port resonance or something to that effect...and if I were to go for a larger diameter port...say 6"...the length would have to be over 40"...

I have also tried to play around with the size of the box and played a little with the tuned frequency (did not want to budge too much on this though)...but i have not had much luck in really bringing down the max port air velocity below 30 m/s with a 4" port...
cnp112 is offline

Old 12-05-12, 10:40 PM
HTS Senior Moderator

Russ

Join Date: May 2007
Location: The Inland NW, USA
Posts: 1,788
My System
Re: question on port air velocity...

You are right that 30m/s at full power is less likely to happen on a regular basis, there's a chance you could be safe. The truth is that "maximum" tolerable port velocity is a little bit up for interpretation, and it also varies with the area of the port. Bigger ports can allow higher velocity without chuffing, the 17m/s figure is for ports closer to the 3-4" diameter range.

I personally look at the 32m/s figure with disdain, I once built a sub with a flared slot port with modelled velocity of just 22m/s assuming I'd be safe. It chuffed at high SPL down low.

Here's an insightful set of tests that someone completed. From the sounds of your build, if it were me I'd make a bigger port possible. Otherwise you are taking a clear risk at audible port noise.

 My Setup SI HT18, Sealed, NU3000dsp "HT 2.0" Monitors (704D, XT25) Flatpack Computer Speakers (ND91-8) The In-Walls
fusseli is offline
Old 12-06-12, 07:02 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: question on port air velocity...

I've been modeling subs with a 4" flared port to a maximum port speed of 26 m/s peak (not rms). In real world applications of HT there has not been one complaint yet of port noise. Slot ports are a different animal.
Mike P. is offline
Old 12-06-12, 10:24 AM Thread Starter
Shackster

Join Date: Nov 2012
Location: Wheeling, IL
Posts: 45
Re: question on port air velocity...

thanks guys...extremely helpful...it seems i need to bring down the air port velocity as the input signal is the RMS value...and also because it seems i should model for closer to ~25 m/s give or take a few m/s...

could one of you guys take a look at my design and let me know if there is something i should do to bring down the port noise while still maintaining fairly flat low frequency extension...the FR plot in the design seems to be ideal for me because it maintains a flat response down to the low 20's...-3db @~21...and falls relatively steeply beyond that...thanks...
cnp112 is offline
Old 12-06-12, 11:30 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: question on port air velocity...

Small box low tuning is hard to do. Your options are add another 4" port, go with Passive Radiators, or reduce the power input.
Mike P. is offline

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