Go Back   Home > Forums > >

Tune Box:  Which is it? Port length or port vol
Tune Box:  Which is it? Port length or port vol
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools
Old 23rd April 2019, 12:04 AM   #1
scott68 is offline scott68  United States
diyAudio Member
 
Join Date: Nov 2016
Default Tune Box: Which is it? Port length or port vol

Question: Which is it, port length or port volume (air mass) that tunes a box?

I'm interested in making an insertable disk circular port (not a tube) that sides into a non tall cylinder to get a longer length port where a tube would not fit. See figures 1 to 3. I can also slide in various "Discs" with different volumes easily.


I can vary the air mass by changing the center circle dia and/or the depth of the disc (sandwich plates seal the channel.)



I can vary the port length by changing the entrance and the exit locations.
Couple of questions:



1. In a bass reflex (not a T-line), if you have two ports with the same volume, but one is longer than the other (thereby smaller cross sectional area) , would the performance be the same assuming there is no chuffing in either?
2. In other words: is the weight of the air mass inside the port that determines the box tuning or the length of the port? ie, 2" dia times 6" long port has the same vol as a 3" x 2.67" long port about 18.85 cu in.


Would both ports perform the same?


Separate note: see Fig 4. From what I read, two different length ports do not work but then there is the neat cornu spiral horn that seems to work well. So do two different length ports work or not?


Thanks
Attached Images
File Type: jpg Circular non tube port.jpg (41.5 KB, 130 views)
  Reply With Quote
Old 23rd April 2019, 04:25 AM   #2
RHosch is offline RHosch  United States
diyAudio Member
 
Join Date: Sep 2003
Location: SouthEast
More or less a ratio of length to diameter. Doesn't follow mass, in that a larger diameter port needs to be longer, not shorter to keep the same tuning.

Or stated a different way, as you add air column mass by increasing port length for a fixed diameter, tuning frequency drops. AND, as you decrease air column mass by decreasing port diameter for a fixed length, tuning frequency ALSO drops.
  Reply With Quote
Old 23rd April 2019, 05:22 AM   #3
scott68 is offline scott68  United States
diyAudio Member
 
Join Date: Nov 2016
Good, what I have been reading about calculating the air mass inside the port and then using that as the weight to add to a passive radiator is wrong. That is what brought on the question. Anyway, the 2nd part of the post based on fig 4 in the picture: Is it also wrong that you shouldn't have two different length ports? And yet, the popular Cornu has them???
Thanks
  Reply With Quote
Old 23rd April 2019, 06:09 AM   #4
picowallspeaker is offline picowallspeaker  Italy
diyAudio Member
 
picowallspeaker's Avatar
 
Join Date: Apr 2007
The original question should be: do I tune the box or the speaker ?
The original answer is: you do both; someone would say " It depends", as it usually happens in speakers...
In deep but also simply you would use what the best is available for making that speaker "sing".
Generally, the maker of the speaker knows how it behaves in closed boxes, Helmoltz boxes, infinite baffle with more volume that Vas, horn expansions in front.
The cornu is a backloaded horn, isn't it ? so it has a compression chamber and two/three/four lines, the expansion ratio, the length etc. it may resemble a BVR ( big vent reflex, with the exponential expanding duct ) but it is not.
Btw there are already big problems in defining aerodynamics & other phisics associated to the presence of ONE duct in a chamber, the chamber itself offers soooo many opportunities to all the particular problems to arise, and they do
Like viscosity, turbulence, and all the likes. Have you ever heard of a speaker filled with a gas other than air ?
__________________
there’s no staying at home when you do not have one
  Reply With Quote
Old 23rd April 2019, 07:45 PM   #5
chrisb is offline chrisb  Canada
diyAudio Member
 
Join Date: Aug 2002
Location: victoria BC
Well, if memory serves me correctly, the Dayton Wright XG8 were a sealed ELS filled with an inert gas that enabled them to operate at higher bias voltages and achieve over 20dB theoretical gain in max SPL over standard “air loaded” designs.
Of course that bears no relevance to the case at hand, but does answer the specific question
__________________
It’s been a slice kids
  Reply With Quote
Old 25th April 2019, 02:32 PM   #6
Dave R is online now Dave R  United States
diyAudio Member
 
Dave R's Avatar
 
Join Date: Aug 2006
Quote:
Originally Posted by scott68 View Post
Question: Which is it, port length or port volume (air mass) that tunes a box?

1. In a bass reflex (not a T-line), if you have two ports with the same volume, but one is longer than the other (thereby smaller cross sectional area) , would the performance be the same assuming there is no chuffing in either?
2. In other words: is the weight of the air mass inside the port that determines the box tuning or the length of the port? ie, 2" dia times 6" long port has the same vol as a 3" x 2.67" long port about 18.85 cu in.

Would both ports perform the same?
Attached is a formula for determining the length of a (round) vent for a "normal" vented box. It involves:

c = speed of sound in air
R = radius of vent
Vb = box volume
fb = tuning frequency.

Without going into too much detail . . .
The modeling behind this starts with a vent containing a mass of air that vibrates back and forth, and the volume of air behind it acts as a spring. The "1.463R" term is an end correction factor (determined from empirical data), assuming one end free and one end flush mounted.

The tuning frequency is found from a ratio of the spring "constant" of the air in the box, to the mass of air in the port. The mass density term cancels out, and does not appear in the final result.

Think of the air in the box as a compression chamber, with the mass of air in the vent being a piston moving against it. The cross section area of the vent determines the "stiffness" of the air in the box. A larger diameter vent sees a larger force from the air being compressed than a smaller diameter vent would see.

The formula shows you that a larger radius vent requires more length to achieve the same tuning frequency as a smaller radius vent, given the same box volume.
Attached Images
File Type: png ventL.png (3.9 KB, 53 views)
__________________
- Dave R
the 200% Norske
  Reply With Quote
Old 25th April 2019, 02:46 PM   #7
mbrennwa is offline mbrennwa  Switzerland
diyAudio Member
 
mbrennwa's Avatar
 
Join Date: Mar 2005
Location: Zurich, Switzerland
Tune Box:  Which is it? Port length or port vol
Quote:
Originally Posted by scott68 View Post
Question: Which is it, port length or port volume (air mass) that tunes a box?
It's the ratio of the air mass in the port and the area that couples that mass to the internal volume of the box (neglecting the details of internal losses / turbulence in the port, and end correction stuff).
  Reply With Quote
Old 25th April 2019, 04:45 PM   #8
scott68 is offline scott68  United States
diyAudio Member
 
Join Date: Nov 2016
Thanks guys, because of the way to figure the weight to add to a passive radiator, it caused me to question what I thought I knew. Your explanations was spot on.
  Reply to this post

Reply


Tune Box:  Which is it? Port length or port volHide this!Advertise here!
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Port length nskii Multi-Way 10 13th July 2017 04:43 PM
Help with a port length, A7.3 lorienblack Full Range 0 20th November 2014 04:01 PM
Slotted port length and aero port length. deebeez92 Subwoofers 3 26th March 2013 04:18 AM
How to tune the Port Length? endrek Subwoofers 9 21st April 2011 06:13 PM
port length jwhit67 Multi-Way 1 6th March 2008 12:29 PM


New To Site? Need Help?

All times are GMT. The time now is 03:24 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 13.64%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki