Horn throat velocity (looking at compression ratio from a different perspective) - diyAudio
Go Back   Home > Forums > Loudspeakers > Subwoofers

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 14th November 2011, 05:55 PM   #1
diyAudio Member
 
Join Date: May 2006
Default Horn throat velocity (looking at compression ratio from a different perspective)

I learned how to use Akabak about a year ago. Not sure why I didn't think to check this before (Akabak makes it so quick and easy) but it looks like horn throat velocity can get pretty high.

First I checked the Labhorn at full power. Wiith it's ~2:1 compression ratio, throat velocity was up around 13m/s. (Obviously the Lab is an offset driver horn, so velocity at the throat is actually 0, max velocity happens a bit further down the horn). I only checked a couple of points near the throat so it's possible that it gets higher than 13m/s, I only spent a couple minutes on the Labhorn sim.

Next, I checked my last personal horn project. With it's ~5.5:1 compression ratio it's throat velocity is up over 25m/s at full power. (It's actually an offset driver as well, but for this I modelled it as end loaded to make it easy to see velocity right at the throat without having to search for the spot of highest velocity).

Usually we look at compression ratio only as far as force (potential driver destruction) but throat velocity looks like it might be a big issue too.

To make matters worse, usually horns with big compression ratios are built with very high aspect ratio throats (elongated rectangle as opposed to square), which is not necessarily the best shape to funnel high velocities through.

Anyone have any idea how much throat velocity might be safe before distortion goes way up? There's rules of thumb for ported boxes, is there one for horn throat velocity?

Last edited by just a guy; 14th November 2011 at 06:04 PM.
  Reply With Quote
Old 14th November 2011, 08:45 PM   #2
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by just a guy View Post
Usually we look at compression ratio only as far as force (potential driver destruction) but throat velocity looks like it might be a big issue too.

To make matters worse, usually horns with big compression ratios are built with very high aspect ratio throats (elongated rectangle as opposed to square), which is not necessarily the best shape to funnel high velocities through.

Anyone have any idea how much throat velocity might be safe before distortion goes way up? There's rules of thumb for ported boxes, is there one for horn throat velocity?
My observation from distortion testing of various drivers in various horns with differing compression ratios leads me to believe the speaker construction and cone stiffness is more the determining factor for high distortion below Xmax than throat velocity.

Using my Keystone tapped horn using two Eminence Lab 12, or one Eminence 4015LF, or a BC18SW115 as an example, the 15, with the least compression ratio, and hence less throat velocity, had by far more distortion than the other two driver choices. Itís cone was not stiff enough for high power and low distortion.

I donít think high throat velocity per se causes distortion, whether the speaker cone can take the pressure without deforming will determine distortion levels below Xmax.
  Reply With Quote
Old 14th November 2011, 09:06 PM   #3
diyAudio Member
 
Join Date: May 2006
Thanks, that's what I was hoping to hear.

BTW, what's the throat size of Keystone?

And do you think there's a line that just shouldn't be crossed, like maybe 30 m/s? 40? 50? I bet it's pretty easy to get up pretty high with a very high xmax driver and a horn with a very very small throat.

Last edited by just a guy; 14th November 2011 at 09:09 PM.
  Reply With Quote
Old 14th November 2011, 09:24 PM   #4
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by just a guy View Post
Thanks, that's what I was hoping to hear.

BTW, what's the throat size of Keystone?

And do you think there's a line that just shouldn't be crossed, like maybe 30 m/s? 40? 50? I bet it's pretty easy to get up pretty high with a very high xmax driver and a horn with a very very small throat.
In ported designs, too high a speed blows the air out of the port, chuff chuff, flap flap.
I have never really given m/s much thought in horn throat design.

The BC18SW115-4 loaded Keystone had more distortion at the same drive level as the BC18 in a ported cabinet, but also had 6 dB more output.
I'd suspect that reducing the drive level by 6 dB would result in less distortion from the Keystone than the ported cabinet.

At the narrow end, the Keystone throat is about 69 square inches, the compression ratio in the center of the expansion is 2.5/1 for an 18" IIRC.

Art Welter
Attached Images
File Type: jpg Keystone.jpg (83.4 KB, 167 views)
  Reply With Quote
Old 14th November 2011, 09:58 PM   #5
diyAudio Member
 
Join Date: May 2006
Thanks for your comments.
  Reply With Quote
Old 14th November 2011, 10:16 PM   #6
diyAudio Member
 
revboden's Avatar
 
Join Date: Feb 2011
Quote:
Originally Posted by just a guy View Post
Thanks, that's what I was hoping to hear.

BTW, what's the throat size of Keystone?

And do you think there's a line that just shouldn't be crossed, like maybe 30 m/s? 40? 50? I bet it's pretty easy to get up pretty high with a very high xmax driver and a horn with a very very small throat.
Based off 69 square inches with squared edges, the boundary layer thickening per velocity would start to detune the throat at ~30m/s and be completely turbulent at ~38m/s. (that's calculated with a very smooth surface, sheet PVC, if the wood is rough that number would head south a bit)
  Reply With Quote
Old 14th November 2011, 10:34 PM   #7
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by revboden View Post
Based off 69 square inches with squared edges, the boundary layer thickening per velocity would start to detune the throat at ~30m/s and be completely turbulent at ~38m/s. (that's calculated with a very smooth surface, sheet PVC, if the wood is rough that number would head south a bit)
revboden,

My figure of 69 square inches at the narrow end of the throat was incorrect, it is actually 71.875 square inches there, 78.125 mid cone, and 84.375 at the bottom of the 18" speaker.
Could you please explain what you mean by "detune the throat", and what results you would expect from a "completely turbulent" throat.

Art
  Reply With Quote
Old 15th November 2011, 12:43 AM   #8
diyAudio Member
 
revboden's Avatar
 
Join Date: Feb 2011
Hi Art,

It's basically about displacement thickness. As air moves over a surface the boundary layer gets thicker with velocity so the effective size of a port or horn shrinks in cross-section while the length stays the same. For a port this will tune the port lower because it has less area vs length. As the horn gets larger it would have a higher volume to surface ratio so it would be less and less effected as the sound moved down the length.

In a completely turbulent state the particle flow is chaotic and the effects would be very hard to predict. my guess: It would sound muddled, like sound trying to get through randomly changing densities of fill.

rev.
  Reply With Quote
Old 15th November 2011, 01:01 AM   #9
diyAudio Member
 
Join Date: May 2006
How do you calculate this?
  Reply With Quote
Old 15th November 2011, 01:30 AM   #10
diyAudio Member
 
revboden's Avatar
 
Join Date: Feb 2011
I suck at explaining things this complex... I'll try to find a good description. This may take a while. I'll also try to come up with some simplified equations.

these have some of the basics.

Stokes boundary layer - Wikipedia, the free encyclopedia
Boundary layer - Wikipedia, the free encyclopedia
Boundary-layer thickness - Wikipedia, the free encyclopedia
Reynolds number - Wikipedia, the free encyclopedia
http://www.engineeringtoolbox.com/re...ber-d_237.html
Attached Images
File Type: jpg 800px-Moody_diagram.jpg (106.7 KB, 155 views)

Last edited by revboden; 15th November 2011 at 01:51 AM.
  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
Horn experts - Please help to determine optimal throat size for low-mid horn pk Multi-Way 9 19th August 2012 06:29 PM
Compression ratio/new build Dag Johansen Multi-Way 28 24th July 2012 01:13 AM
Compression ratio/cone failure weltersys Subwoofers 24 27th February 2011 02:39 PM
Compression ratio and diaphragm size in compression drivers inkasound Multi-Way 50 8th January 2009 03:38 PM
horn with low compression ratio patriz Multi-Way 6 17th May 2006 10:45 PM


New To Site? Need Help?

All times are GMT. The time now is 09:10 PM.


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