Subwoofer Tests Explained - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

 
Thread Tools
post #31 of 45 Old 08-19-08, 10:03 PM
Senior Shackster
 
DrWho's Avatar
Mike Bentz
 
Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Subwoofer Tests Explained

Quote:
SteveCallas wrote: View Post
The sealed 118db sweep most closely matches the PR'd 115db sweep in terms of output.

But there is something causing a difference in the distortion profile - I'd still have to say the PRs. If we look at the SVS Ultra 20hz, 15hz, or 10hz tunes vs the sealed version, we see less distortion at higher output levels everywhere above tuning with the ported versions. In my mind this is because the air in a port is more linear than a PR.
There is no way air in a port is more linear - the non-linearity of air in a port begins because you can't have absolute pressures going below a perfect vacuum. It's totally a physical limitation and the only way around it is to move to a PR.

The distortion I think you're referring to is very likely because the PR's have too narrow of a Q. You don't get to adjust the Q of a port, but you can choose the Q of a PR. When you go too narrow, the cone excursion of your active will go up around the tuning frequency...thus the distortion is higher there too. Take some mass off your PR's and let the tuning frequency go higher and that behavior goes away...the rise in response at the tuning frequency at higher driver levels starts to go away too.

-Mike Bentz
~It's all about compromise~


"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
DrWho is offline  
Sponsored Links
Advertisement
 
post #32 of 45 Old 08-19-08, 10:19 PM
Shackster
andru
 
Join Date: Aug 2008
Location: Kewarra Beach
Posts: 14
Re: Subwoofer Tests Explained

Hi all,
Just wondering... if you are testing your sub in a free field environment... that is out doors.
Would you be better to raise them away from the ground by 20' or 30"..? On a pole or a extending speaker stand or something....
Is it possible?... otherwise you do have a boundary effect to consider, and not really a totally free field. From a scientific point of view.

Andru
andru is offline  
post #33 of 45 Old 08-20-08, 03:16 PM
Elite Shackster
 
Ricci's Avatar
Josh
 
Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: Subwoofer Tests Explained

20 or 30" is not going to have much of an effect at the frequencies being tested here. Also some of the subs weigh upwards of 100lbs, so that becomes a factor.
Ricci is offline  
 
post #34 of 45 Old 08-20-08, 03:22 PM
Elite Shackster
 
Ricci's Avatar
Josh
 
Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: Subwoofer Tests Explained

Quote:
DrWho wrote: View Post
There is no way air in a port is more linear - the non-linearity of air in a port begins because you can't have absolute pressures going below a perfect vacuum. It's totally a physical limitation and the only way around it is to move to a PR.

The distortion I think you're referring to is very likely because the PR's have too narrow of a Q. You don't get to adjust the Q of a port, but you can choose the Q of a PR. When you go too narrow, the cone excursion of your active will go up around the tuning frequency...thus the distortion is higher there too. Take some mass off your PR's and let the tuning frequency go higher and that behavior goes away...the rise in response at the tuning frequency at higher driver levels starts to go away too.
What about all the suspension non linearities, and distortions caused by the surround and spider/s at large displacements like in these tests? I'm pretty sure that there will be more noise/ non linearity from a PR or 2 that is close to it's limits than from a port that's close to it's. Also the PR's will have a hard mechanical limit.
Ricci is offline  
post #35 of 45 Old 08-20-08, 07:43 PM
Senior Shackster
 
DrWho's Avatar
Mike Bentz
 
Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Subwoofer Tests Explained

The SPL of a port near its limits is nowhere near the SPL of a PR near its limits...

The suspension non-linearities of a passive aren't going to be any worse than your active. In fact, increasing the surface area of the passives (ie, doubling up) is going to reduce whatever distortion you're worried about. The thing about passives is that you're free to use as many as you want...

Ports will show measurable power compression at only 10m/s air flow - and yet most designs are barely able to get under 50m/s at full output. As you make the port larger to reduce air flow, the cabinet gets bigger, and your natural resonances (of both the port and the cabinet) start moving into the passband of the driver. And even if some of these resonances are just above the passband, the harmonic distortions from the driver can trigger them too. The point being that it's not a free lunch to add more port...

-Mike Bentz
~It's all about compromise~


"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
DrWho is offline  
post #36 of 45 Old 08-20-08, 10:09 PM
Elite Shackster
 
Ricci's Avatar
Josh
 
Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: Subwoofer Tests Explained

Yes the linearity problems with your driver will also be present but these are added to by the PR or multiple PR's also most PR systems don't have enough displacement to handle the drivers full output at the tuning freq. So they are under PR'd just like most systems are underported. Where are you getting this port compression at 10ms statistic? I don't think this is accurate for a 6" or much larger heavily flared port. True the PR will trap the noises that sometimes escape from a port inside the box. There is no free lunch with PR's either. Don't get me wrong I like PR's but I"ve never seen anything to prove that a PR will always be cleaner than a port. I think it depends on the application.
Ricci is offline  
post #37 of 45 Old 08-21-08, 12:28 AM
Elite Shackster
 
SteveCallas's Avatar
Steve
 
Join Date: Apr 2006
Posts: 2,398
Re: Subwoofer Tests Explained

Quote:
DrWho wrote:
The SPL of a port near its limits is nowhere near the SPL of a PR near its limits...
That's a pretty generic, all encompassing statement. Are you suggesting a single 15" PR can provide more clean output than a heavily flared 10" port, as an example?


To view links or images in signatures your post count must be 10 or greater. You currently have 0 signatures.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 signatures.
SteveCallas is offline  
post #38 of 45 Old 08-21-08, 02:58 AM Thread Starter
Elite Shackster
 
Ilkka's Avatar
Ilkka
 
Join Date: Jun 2006
Location: Finland
Posts: 1,468
Re: Subwoofer Tests Explained

Quote:
darrellh44 wrote: View Post
It guess its not so much a question of restricting the higher frequency excursion, but adding distortion to it. A case in point can be seen in some of Ilkka's Round 5 results if you compare the % THD plots between the sealed LMS-5400 and the 2xPR LMS-5400. For every SPL level 105db and higher, there is significantly more THD between 35 Hz and 100 Hz for the passive radiator version (even though it has twice the volume).

2xPR LMS-5400:
http://personal.inet.fi/private/zipm...200l%20thd.png

Sealed LMS-5400:
http://personal.inet.fi/private/zipm...100l%20thd.png

-Darrell
Hi guys,

I too noticed the difference in upper bass THD right after plotting the results. When looking at the individual HD components more closely, I noticed the difference was mainly caused by 2nd order harmonic. Now, there could be a few reasons for that.

1) The drivers weren't identical. A small difference in BL linearity could cause that kind of difference in 2nd harmonic.

2) The enclosures weren't identical. The upper bass performance of a small sealed enclosure is different from larger sealed or ported/PR enclosure.

3) I'm almost 100% sure it isn't because of passive radiators. The radiators don't contribute at those frequencies, so all THD is produced by the active driver. The passive radiators are too "stiff" to move at those frequencies. There's no non-linearity problems because the radiators aren't simply moving at all (or at most very little).

When wanting a proper comparison (same driver, same enclosure) between ported and sealed, look at the SVS PB13-Ultra (as you already did).
Ilkka is offline  
post #39 of 45 Old 08-21-08, 03:04 AM Thread Starter
Elite Shackster
 
Ilkka's Avatar
Ilkka
 
Join Date: Jun 2006
Location: Finland
Posts: 1,468
Re: Subwoofer Tests Explained

Quote:
andru wrote: View Post
Hi all,
Just wondering... if you are testing your sub in a free field environment... that is out doors.
Would you be better to raise them away from the ground by 20' or 30"..? On a pole or a extending speaker stand or something....
Is it possible?... otherwise you do have a boundary effect to consider, and not really a totally free field. From a scientific point of view.

Andru
You are being correct, the testing environment isn't free field. But actually it doesn't have to be because the ground acts like a perfect mirror for frequencies that low (below ~200 Hz). Therefore we simply get 6 dB more SPL compared to free space (usually compensated by measuring the subwoofers at 2 meter distance). The measuring technique is called ground-plane. I suggest looking into the following paper.

Gander, Mark: "Ground-Plane Acoustic Measurement of Loudspeaker Systems," Journal of the Audio Engineering Society, vol. 30, no. 10, October 1982.
Ilkka is offline  
post #40 of 45 Old 08-21-08, 11:52 AM
Elite Shackster
 
Ricci's Avatar
Josh
 
Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: Subwoofer Tests Explained

There were 2 different LMS drivers? All this time I thought that you tested the same one and just switched alignments with the one driver. I never noticed the slight increase in distortion above 50hz for the PR'd system before.
Ricci is offline  
 

Bookmarks

Tags
explained , subwoofer , tests

Quick Reply
Message:
Options

Register Now




PLEASE COMPLETE ALL REQUIRED FIELDS BELOW... THANKS!

REQUIRED FIELDS ON THIS PAGE
YOU MUST COMPLETE ALL OF THESE

Username
Password
Confirm Password
Email Address
Confirm Email Address
Random Question
Random Question #2




User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
PLEASE READ BELOW PRIOR TO ENTERING AN EMAIL ADDRESS!

ATTENTION!

YOU MUST ACTIVATE YOUR ACCOUNT!

Activation requires you reply to an email we will send you after you register... if you do not reply to this email, you will not be able to view certain areas of the forum or certain images... nor will you be able download software.

AN INVALID EMAIL ADDRESS WILL CAUSE YOUR ACCOUNT TO BE DELETED!

See our banned email list here: Banned Email List

We DO NOT respond to spamcop, boxtrapper and spamblocker emails... please add @hometheatershack DOT com to your whitelist prior to registering or you will get nowhere on your registration.


Email Address:
OR

Log-in









Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



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 is not allowed!
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 


For the best viewing experience please update your browser to Google Chrome