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Old 17th September 2003, 06:43 AM   #1
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Location: NZ
Default 50m/s port >> 20m/s port- more SPL ?/

hi this is an important and interesting topic to me-port study in depth-what REALLY is going on

hi i have a mate whos box models at 50m/s (modeled in winisdPRO)port air velocity with rms power he uses-what gains can i expect reducing it to 20m/s ? other than chuffing and noise.

from '' high performance loudspeakers by martin colloms 5th edition'

Gander[37] investigated both vent and thermal compression effects in some depth.
As regards temperature,a 380mm bass mid driver fitted with a 63mm motor coil was tested,a unit intended for musical instrument applications.The response was compared for a 1w and 100w input (100db and 120db SPL)No compression was evident at the sealed box system resonance of 50hz; elsewhere a significant 2.5db of compression was seen at higher power,almost halving the effective rated power input.

Vent compression was illustrated in a 125L enclosure tuned to 42hz by one of two choices of vent
the first was a 110mm aperture in 19MM thick front panel the second a pair of ducted 110mm ports with 125mm tubes,173cm^2 twice the area.At 96db(1w) the vent efficiencies are the same.

However at 116db(100w) the smaller vent lost a surprising 4db in its working range while the loss from the larger held to just 1db.

btw i think the text means- one port of SAME 125mm tube length, and one port of TWO tubes-or maybe not! ..it isnt worded well

does AES have much on this sort of topic?
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Old 17th September 2003, 07:02 AM   #2
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Join Date: Jan 2002
Location: Canberra, Australia
Umm, the way I figure it, the port with lower m/s should chuff and compress less, not more. you're trying to squeeze the same volume of air out of a larger hole. Of course to tune lower with a larger hole you need a longer pipe that encloses a larger mass of air so...

In most cases, the bigger you can make the vent for the same tuning frequency, the better.

If you're using WinISD, keep an eye on the group delay curves when designing your boxes too. the sammer the delay you can achieve, even at the expense of low freq extension the better (IMO). Some folks want BOOM and don't care how slow and bloated it is but not for me.

Hope I've helped a bit

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Old 17th September 2003, 08:33 AM   #3
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Join Date: Nov 2002
Location: NZ
yep i agree.

yeh GD is important-but not for SPL boxes,,however for my jBL i did look at it..and for my jBL box i made a low airspeed port 45x9x 46cm long ,175L

the port with lower m/s should chuff and compress less, not more
yes ,did i say otherwise?oops.
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