SB acoustics SB12MNRX25-4 "full range" Build thread - Page 6 - diyAudio
Go Back   Home > Forums > Loudspeakers > Full Range

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 24th January 2013, 07:54 AM   #51
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
OK thanks Dave, That's different to what I'm seeing so there might be something wrong with my nearfield measurements still or there may be something else going on! Anyway regardless it sounds pretty good so I probably should stop obsessing

Blue is the unibox sim, red is the nearfield response with merged port response. The difference isn't huge but it is enough to bug me.

Upping the series resistance in the sim (second pic) to 2 ohms gets it a bit closer to the actual measured result, so maybe it is my dodgy alligator clips...

Anyway I won't get a chance to do anything more till the weekend most likely. I will report back if I work out what is going on!

Tony.
Attached Images
File Type: png sim_comp.png (18.7 KB, 389 views)
File Type: png sim_comp2.png (18.1 KB, 387 views)
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 24th January 2013, 08:03 AM   #52
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Isn't that bump expected from the measurment technique?

You see this all the time:

Quote:
The rise in the upper bass is mainly an artifact of the nearfield measurement technique used below 300Hz
Click the image to open in full size.

dave
__________________
community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  Reply With Quote
Old 24th January 2013, 08:35 AM   #53
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Could be (time to dig out my d'appolito testing loudspeakers)!

Strangely my recollection is that my MTM sealed nearfield matched the sim pretty much exactly as did my 70L vented.

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 24th January 2013, 11:44 AM   #54
diyAudio Member
 
Bob Brines's Avatar
 
Join Date: Jan 2003
Location: Hot Spring Village AR
Or that bump could be a box too big tuned too low.

Bob
  Reply With Quote
Old 24th January 2013, 07:19 PM   #55
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Hi Bob, unlikely in this case I think. I deliberately made the volume smaller with higher tuning to get a more gentle rolloff with better transient response. Or at leas that was how it simmed

I suppose my T/S params might be an issue.

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 24th January 2013, 08:01 PM   #56
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Bob, actually thinking about that don't you mean too small and too high? bigger and lower usually results in a droopy response... Leaving the volume the same (at 2.5L) and increasing the tuning to 110Hz in the sim gives me that rising effect.

I might try blocking off some of the port to lower the tuning frequency and see what happens. putting something down the middle would serve as a brace as well...

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 24th January 2013, 09:01 PM   #57
diyAudio Member
 
5th element's Avatar
 
Join Date: Oct 2002
Location: England
Any bump towards the low end tends to get thrown into the

Quote:
The rise in the upper bass is mainly an artifact of the nearfield measurement technique used below 300Hz
Statement from stereophile. It's simply a diplomatic way not to have to really discuss baffle step losses, the chosen amount of compensation used and any wonky cabinet alignments.

In absolute terms, the true stereophile bump occurs because you're measuring nearfield, on a loudspeaker, that has baffle step compensation applied. As you're nearfield the loss doesn't occur and you see a hump as a result.

In terms of the small bump that you're getting Tony, it is most likely caused by the box alignment. The two things that I would mention here is that adding in the port can be tricky. As you measure everything nearfield, a small difference in measurement distance can grossly affect the measured SPL between the port and the driver, this can screw up how accurate the combined response is. The other thing to watch out for are any tone controls in the mic preamp, as even when set to zero they may not be.

The first is easy to fiddle with as you can simply alter the level of the port and see if you can get things to match up better. I tend to ignore measuring the port and rely on an impedance measurement to make sure that the port is tuned to the correct frequency.

The second problem you can only sort out by bypassing the controls internally via a bit of handywork. The behringer preamp mixers are popular but some don't have defeatable tone controls.

If you are concerned about the alignment then I'd go back to the simulator, keep all the box parameters the same, but remove the port (ie use a closed box). Check what the simulator predicts, then measure your box with the port plugged and compare the two.
__________________
What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz!
  Reply With Quote
Old 25th January 2013, 06:15 AM   #58
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Thanks 5th there is no BSC as yet. I like the suggestion of modelling sealed and measuring with the port blocked off

It's not a huge deal as they sound fine, but I like to get to a point where my sims correlate with what I do , and if they don't I like to get to the bottom of it! (read that as I'm overly obseesive )

When I was measuring my 70L BR boxes it wasn't matching and I spent a lot of time trying to work out why. In the end I'd dropped the mic at some point and damaged it, the LF response was affected. I tried a new mic and the sim matched the measured result

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 26th January 2013, 10:52 AM   #59
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
I had a bit more of a play today. I haven't tried sealing up the port yet. but I did try blocking roughly half of it off (in the middle) to get two separate ports. This dropped the box tuning to around 55hz (both in the sim and in actual measurements).

The FR flattened out a bit, though the difference is not that great. Interestingly it has a minimal effect on the transient response as well. It does however affect the curve up higher, though obviously with a NF measurement the higher frequencies can't really be trusted.

Apart from the fact that my port area halved (and there would be potentially more problems with port noise) This doesn't seem like a bad option.

I'm still rather stumped at how different the results are to the modeled response, but one thing is nice, the design is not too sensitive to box tuning!! I'm wondering if I really pushed the limits a bit far with how small a box I could build, and the sim gets inaccurate when pushing the boundaries this far.

Blue trace is with the lower 55Hz tuning, red is the original 70Hz. I Iike the rolloff of the lower tuned box better.


Tony.
Attached Images
File Type: png nearfield_compare.png (64.8 KB, 338 views)
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 31st January 2013, 08:32 AM   #60
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
OK I think I have finally worked out what is going on re the sim not matching the reality. I will do some further tests to verify but maybe not till the weekend.

It suddenly dawned on me that I should try the sim with nothing other than a higher driver fs and see what happened. I chose 70hz instead of the 61Hz that I had measured. what do you know but the sim matches the measured response almost exactly!

The woofer I have been measuring with is the one that I did not do 12 hours of breakin on. I only did about 10 minutes of 45Hz on it before measuring the T/S params (which seemed to be enough and was the recommended method on the SB website). I should do the same before doing a nearfield measurement and see what happens. I suspect that the woofer returns to it's un-broken in behaviour when it has only had such a short breakin period!

Tony.
Attached Images
File Type: png sim_vs_actual_2.png (35.7 KB, 308 views)
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  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
Hemp Acoustics 8" Full Range Drivers $130 (Pair) Dilmonkey Swap Meet 7 5th May 2012 04:04 AM
First time Full Range Build tshaq Full Range 5 10th March 2012 06:00 PM
FS: Hemp Acoustics 8" Full Range Drivers $130 (Pair) Dilmonkey Swap Meet 1 29th February 2012 03:02 AM
Yet another 'help me choose a full range project' thread poynterama Full Range 21 27th October 2011 05:49 PM
Hi-Vi/Zaph/SB 3-way MTM build thread dmitriy167 Multi-Way 6 28th August 2009 02:17 PM


New To Site? Need Help?

All times are GMT. The time now is 12:14 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