Thoughts on Ports... - diyAudio
Go Back   Home > Forums > Design & Build > Construction Tips

Construction Tips Construction techniques and tips

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 Search this Thread
Old 21st July 2011, 08:40 PM   #1
diyAudio Member
 
Join Date: Apr 2011
Location: London
Default Thoughts on Ports...

I was just thinking about ports for a vented enclosure.

I'm curious as to what would happen if you were to use two ports per box, but to have them tuned to different frequencies, for example, one tuned to 30hz and one tuned to 45hz (per box)?

Anyone have any idea/experience in this?

James
  Reply With Quote
Old 21st July 2011, 08:55 PM   #2
diyAudio Member
 
revboden's Avatar
 
Join Date: Feb 2011
essentially they average, although it dosent really work because one (45hz) blows out before the other (30hz) really gets going.
  Reply With Quote
Old 21st July 2011, 09:20 PM   #3
diyAudio Member
 
vacuphile's Avatar
 
Join Date: Apr 2011
Location: Seaside
You will lower the Q of the enclosure untill it will stop working at all as a BR.

Another thing, if it can be avoided, work with one port only. Two ports generate more resistance and huffing and puffing than a single one with the same surface area as the two combined.
  Reply With Quote
Old 21st July 2011, 10:21 PM   #4
diyAudio Member
 
DBMandrake's Avatar
 
Join Date: Nov 2010
Location: Glasgow, UK
Quote:
Originally Posted by jmechie1 View Post
I was just thinking about ports for a vented enclosure.

I'm curious as to what would happen if you were to use two ports per box, but to have them tuned to different frequencies, for example, one tuned to 30hz and one tuned to 45hz (per box)?

Anyone have any idea/experience in this?

James
Quote:
Originally Posted by revboden View Post
essentially they average, although it dosent really work because one (45hz) blows out before the other (30hz) really gets going.
You can't tune two ports to two different frequencies in the same enclosure by using two different lengths - there is only ever one box resonance frequency.

Note that I said box resonance not port resonance, as a port by itself doesn't form a resonator. (At least at bass frequencies... there is an undesirable tube resonance in the midrange of most cylinder shaped ports typically between 500-1000Hz depending on tube length)

It's the combination of the compliance of the air in the box acting as a spring, and the mass of air within the port(s) acting as an attached mass that together form a mechanical resonant device, very much like a mass hung from the end of a coil spring and bounced up and down.

If you were to add two unequal mass weights to the end of the same spring there will still only be one resonant frequency, which will be determined by the sum of the two masses. (And the spring stiffness, which is equivalent to box volume)

Likewise using two unequal length ports will still resonate at only one frequency, and it will be higher in frequency than if both ports were the same length as the longer port, but lower in frequency than if both ports were the same length as the shorter port. (A similar effect applies if you used two ports of different diameters but the same length)

As far as the bass tuning goes, if you use multiple ports you want them to all be the same diameter and length, so that their air velocities are balanced. If you make one port smaller in diameter than the other, the smaller one will have a higher air velocity than the larger one, increasing turbulence.

Likewise, if you use two ports of the same diameter but different lengths, the air flow will favour the "easier" path of the shorter port, resulting in higher air velocity and turbulence in the shorter port, and more net turbulence than if both ports were the same length for the same tuned frequency.

The only reason you might ever want to make the ports slightly different lengths, is to stagger the high Q "tube" resonances in the midrange apart so they don't both occur at exactly the same frequency - by doing so you could reduce the peak in the midrange by up to 6dB, but I wouldn't make them more than 20% different in length, and a far better solution would be to just put the ports on the back of the cabinet facing away from the listener, which will essentially eliminate the port induced midrange resonance/leakage at the listening position.
__________________
- Simon

Last edited by DBMandrake; 21st July 2011 at 10:31 PM.
  Reply With Quote
Old 21st July 2011, 10:41 PM   #5
diyAudio Member
 
chlorofille's Avatar
 
Join Date: Jul 2008
Location: Malaysia
Awesome post!!
  Reply With Quote
Old 21st July 2011, 11:48 PM   #6
diyAudio Member
 
vacuphile's Avatar
 
Join Date: Apr 2011
Location: Seaside
Quote:
Originally Posted by DBMandrake View Post
.

If you were to add two unequal mass weights to the end of the same spring there will still only be one resonant frequency, which will be determined by the sum of the two masses. (And the spring stiffness, which is equivalent to box volume)

Likewise using two unequal length ports will still resonate at only one frequency, and it will be higher in frequency than if both ports were the same length as the longer port, but lower in frequency than if both ports were the same length as the shorter port. (A similar effect applies if you used two ports of different diameters but the same length)
Hi DBmandrake,

I agree with your post, but I have my doubts about the part I quoted. I do not think that the analogy with two different weights on top of one spring is correct. The reason is that two weights on top of one spring would always have to move in unison, whereas the air mass in two tubes of different lengths would not.

It is therefore a much more complicated situation, and I would expect a system like that to go through motions that are impossible to model with the sort of mathematics you use to calculate a Helmholz resonator. It is much like the sort of executive desk toy you might know, a sort of cardanically suspended inbalance. A system like that displays erratic behaviour, which can be modelled with a mathematical equivalent, but that cannot be calculated in the classical sense.

Vac

Last edited by vacuphile; 21st July 2011 at 11:49 PM. Reason: Grammmar and speling
  Reply With Quote
Old 24th July 2011, 04:45 PM   #7
diyAudio Member
 
DBMandrake's Avatar
 
Join Date: Nov 2010
Location: Glasgow, UK
Quote:
Originally Posted by vacuphile View Post
I agree with your post, but I have my doubts about the part I quoted. I do not think that the analogy with two different weights on top of one spring is correct. The reason is that two weights on top of one spring would always have to move in unison, whereas the air mass in two tubes of different lengths would not.

It is therefore a much more complicated situation, and I would expect a system like that to go through motions that are impossible to model with the sort of mathematics you use to calculate a Helmholz resonator. It is much like the sort of executive desk toy you might know, a sort of cardanically suspended inbalance. A system like that displays erratic behaviour, which can be modelled with a mathematical equivalent, but that cannot be calculated in the classical sense.
I'm not sure that I see where the "erratic behaviour" is coming from - yes the air velocity will be different when two ports are not the same dimensions, but that doesn't constitute erratic behaviour. It's the total momentum of the air in the ports that controls the resonance.

It's easy enough to test experimentally. Do a nearfield measurement of the output of each port on a cabinet when the two ports are significantly different lengths.

You should find the resonance peak occurs at exactly the same frequency for both, but the amplitude will measure higher on the shorter port due to a higher velocity.

I'm not advocating unequal size ports by the way, anything other than equally sized ports is non optimal in a number of ways, but it does still work as a single Helmholtz resonator...
__________________
- Simon
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
five ports is better than one? birdyfoot Construction Tips 6 13th June 2011 12:18 PM
help on ports sidd_raj2002 Subwoofers 3 17th January 2005 10:48 AM
Need some help regarding these ports Xristo Multi-Way 2 5th November 2004 09:24 PM
Ports Ryan_U Multi-Way 6 2nd November 2004 04:16 PM
curved ports vs. multiple ports Craig Multi-Way 1 22nd September 2002 07:30 AM


New To Site? Need Help?

All times are GMT. The time now is 03:59 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2