Port Noise, Wind Speed................. - Page 2 - diyAudio
Go Back   Home > Forums > Loudspeakers > Multi-Way

Multi-Way Conventional loudspeakers with crossovers

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 11th April 2003, 02:26 AM   #11
OMNIFEX is offline OMNIFEX  Jamaica
diyAudio Member
 
OMNIFEX's Avatar
 
Join Date: Mar 2003
Location: Locked Up In The Amp Rack
Quote:
Originally posted by bostarob
Are you sure? Then why is it that when you have a ported speaker with the vent hanging outside of the box, (for testing purposes) Fb is barely effected?

If you have a speaker in a lets say 6 cubic foot box,
and, your port takes up 3 cubic feet out of the box,
in which, the speaker needs 6 cubic feet, to achieve
40 Hz. You will sacrafice some lower octaves to
achieve less port noise.

Scale a 16 cubic foot box.

Use any 18 inch driver you like.

Make the vent 24 H by 22 W by 12 Depth.

Now, try to get 40 Hz (- 3dB) out of the box.

You can't, for the port has taken up too much space.

However, make the port smaller, you can achieve
40 Hz. But, will have more air passing through the
port than the larger port.
__________________
OMNIFEX
  Reply With Quote
Old 11th April 2003, 06:04 AM   #12
diyAudio Member
 
Join Date: Apr 2002
Location: Baltimore
I'm not convinced that the volume of air inside port is to be subtracted from total enclure volume, and I haven't found any solid arguments either way.
  Reply With Quote
Old 11th April 2003, 09:32 AM   #13
diyAudio Member
 
Join Date: Jan 2003
Location: Zurich
Quote:
Originally posted by bostarob
I'm not convinced that the volume of air inside port is to be subtracted from total enclure volume, and I haven't found any solid arguments either way.

Think of it this way...

The box volume forms an acoustic capacitance. This means that a volume velocity ((vibrating driver) acting into the box will tend to compress and decompress the air in the box. It is the compliance of the air that forms the acoustic capacitance. The port, on the other hand, is an acoustic inductor. This means that a volume velocity acting into the port will not compress the air in the port and the air in the port moves like a solid column of air. It is the mass of this solid column of air that forms the inductance.

Now if you put a port inside a box, you are dividing the total volumn of the box into two regions. In one region, the acoustic capacitance, there is compression and decompression, but no partical (air) velocity. In the other region (the port), there is no compression and decompression, but there is partical velocity. Now if you make the port bigger and keeping the box the same size, you are decreasing the arrea of the box that acts as an acoustic capacitor. The only way around this is to use a passive radiator. A passive radiator works exactly like a wort. The only difference is that a port relies on the mass of the air inside the port to form the acoustic inductance, while the passive radiator relies on the mass of the radiator to form and acoustic inductance.

Hope that helps, John
  Reply With Quote
Old 11th April 2003, 10:33 AM   #14
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
I think the port sizes of most P.A. speakers are a compromise anyway.
While there is general demand for high accuracy sound even at high SPLs there are also econcomy requirements: The boxes have to be reasonably sized.

I think most of the vented P.A. subs generate quite a bit of port noise when driven to their peak SPL capabilities.
But because a P.A. should have some reasonable headroom they are only driven at these levels during short peaks.
OTOH there is music with almost no dynamics at all and P.A. systems might be fed with this stuff at almost full power endlessly.
But then it is not the vents, that generate most of the noise anymore

Regards

Charles
  Reply With Quote
Old 11th April 2003, 11:08 AM   #15
diyAudio Member
 
Join Date: Jan 2003
Location: Zurich
Default editing posts

I just reread my last post and there are a ton of typos. Does anybody know how to edit posts? I notice at the bottom of the page it says editing of posts is allowed, but how?

Thanks, John
  Reply With Quote
Old 11th April 2003, 02:51 PM   #16
diyAudio Member
 
kingdaddy's Avatar
 
Join Date: Mar 2002
Location: Fort Worth, Texas
I believe Hancock is right; 17m/s is probably very close to max, I know this form some experiments Im doing right now. Im building a pair of Maelstrom Subs and have been using WinISD Pro to model the enclosure in (355L Vented @ 20Hz) and Ive made a jig that allows me to change out ports on the fly so to speak. My first model used a 6 diameter port and they all had port noise ranging from severe to medium, the longer the port the less severe the port noise. Then I tried an 8 diameter port and to my surprise, I still got some audible port noise although its probably not audible in a room while playing music. So I went back and checked my WinISD modeling and found that Im right at 19m/s with the 8 port and the 6 ports had anywhere from 35m/s (shortest port 4long) to 29m/s for the longest port (30).

So based on this I believe that the 17m/s figure is a good max level, also I was playing a pure 20Hz test tone at very loud levels to get this port noise with the 8 port at 19m/s velocity, so this probably wouldnt be a problem with any music material.

The lower the tuning freq the longer the port, the longer the port the slower the velocity, also the bigger the box the higher the air velocity of the port. So the box size and the port length and diameter all play a role here.
Attached Images
File Type: jpg maelstrom test.jpg (49.6 KB, 116 views)
  Reply With Quote
Old 11th April 2003, 03:58 PM   #17
OMNIFEX is offline OMNIFEX  Jamaica
diyAudio Member
 
OMNIFEX's Avatar
 
Join Date: Mar 2003
Location: Locked Up In The Amp Rack
Quote:
Originally posted by bostarob
I'm not convinced that the volume of air inside port is to be subtracted from total enclure volume, and I haven't found any solid arguments either way.

That's why I gave you an example, for you to try
in your program.



Quote:
Originally posted by OMNIFEX


Scale a 16 cubic foot box.

Use any 18 inch driver you like.

Make the vent 24 H by 22 W by 12 Depth.

Now, try to get 40 Hz (- 3dB) out of the box.

Box Dimensions: 48 H x 24 W x 24 D

This should convince you.

Quote:
Originally Posted By hancock
I just reread my last post and there are a ton of typos. Does anybody know how to edit posts? I notice at the bottom of the page it says editing of posts is allowed, but how?
If your new, its going to take some time to edit your
post. It has happened to all of us

If your a regular poster, just click on the "Edit" next
to the blank box that says Quote.
__________________
OMNIFEX
  Reply With Quote
Old 11th April 2003, 04:46 PM   #18
diyAudio Member
 
Join Date: Apr 2002
Location: Baltimore
Just press edit, it's easy. A passive radiator acts like a wort. Ill keep that in mind. Hancock, thanks, that's a solid argument, I'm convinced. Thanks all for putting up with me (haha)
-andy
  Reply With Quote
Old 11th April 2003, 05:10 PM   #19
diyAudio Member
 
Join Date: Jan 2003
Location: Zurich
Quote:
Originally posted by OMNIFEX



If your new, its going to take some time to edit your
post. It has happened to all of us

If your a regular poster, just click on the "Edit" next
to the blank box that says Quote.
Ahh, so eventually I'll see an "edit option" at the end of my posts. I guess I'll have to proofread before submitting. Don't want to make any more posts about worts...

John
  Reply With Quote
Old 12th April 2003, 03:37 AM   #20
CarMan is offline CarMan  United States
diyAudio Member
 
Join Date: Mar 2003
Location: Reno
Default subtracting volume

I did some measurements in my car with a 4 inch port 15 inches long. If the port sticks out of the box the resonance drops. It pretty well matched what I expected in winISD, after I add cabin gain for the car. This tells me that the port volume is subtracted from the volume of the box.

If the volume of an external port were added to Vb I would have observed an 18hz resonance and -2db at 18hz. However, resonance was at 20hz just as expected with external, and ~21.5hz with it tucked in. Just as reported by winISD with subtraction of the port volume I get a flat response to 23hz, -3db at 20hz.

Just my tests, trying to get the car sounding good. Try working with a van someday... Lots of room but the shape/resonances suck. It is much easier to get flat 20hz in a sport coupe! At much louder volumes too!
  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
Belkin Hi-Speed USB 2.0 5-Port PCI Card for sale. supernet Swap Meet 1 13th March 2007 01:27 PM
Port noise CARTRulz Subwoofers 6 13th June 2006 01:27 PM
Pls help a poor DIYER make a low wind speed! kornmong Subwoofers 10 2nd September 2005 08:48 AM
Port air speed versus excursion Vikash Multi-Way 1 16th September 2003 07:04 PM
Port Noise Redeye Multi-Way 19 11th February 2003 10:47 PM


New To Site? Need Help?

All times are GMT. The time now is 08:10 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