Calculating room gain - 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 19th January 2002, 07:53 PM   #1
diyAudio Member
 
Join Date: Jan 2002
Location: Wellington, New Zealand
Default Calculating room gain

Is there a way (other than by measurement - esp. if you don't have the equipment) - to predict / calculate room gain for a subwoofer?

Vented vs. a closed box design (for the same driver) - can yield upto 18Hz different in F3! (ie. F3 for a closed box Shiva MkII 12" is about 38Hz - vs vented (although larger box) = 21Hz).

I want to use a closed design - because it is supposed to give a better transient response (I mainly listen to music - and would rather have speed than bass extension) - however I feel an F3 of 38Hz is still too high for pop / rock music.

I know room gain will occur - it would be nice to be able to determine this - to see what the in room F3 would be of both boxes.

Thanks,
Dave.
  Reply With Quote
Old 19th January 2002, 08:12 PM   #2
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Here is a thread by Ignite where I gave several urls to show room gain:
Room Gain
  Reply With Quote
Old 19th January 2002, 08:41 PM   #3
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
The merits of closed box versus ported box for the type of subwoofer you are building were discussed and illustrated extensively in the following thread:
SPL good enough?

Just one thing to add. Tests were run and it was discovered that the lower the bass, the harder it is for the listener to detect such things as distortion and phase difference. And 18 Hz is deep bass indeed.
  Reply With Quote
Old 19th January 2002, 10:19 PM   #4
ucla88 is offline ucla88  Tahiti
diyAudio Member
 
Join Date: Dec 2001
Location: rocklin, CA
the short answer is-measure it if you really want to know. The problem is that room gain isn't really a uniformly rising function.
Art Ludwig's site is pretty informative.

you can take a wild and semi educated guess at where the first room mode is by 565/L where L is the longest dimension of your room. if the longest dimension of your room is 20ft, then the first axial mode will be around 565/20=28hz. there will be a peak here, or close. how big will this peak be? well, how much polyfill is in your overstuffed couch?

Art Ludwig's site

as far as that whole sealed vs. ported issue goes, well, there are objective differences. sub-jectively(couldn't help that), the differences are subtle. best to listen to a well built sealed and ported version of the driver you're going to use and make up your own mind.

there are extraordinary TL, IB, sealed, ported, and others.



Quote:
I know room gain will occur - it would be nice to be able to determine this - to see what the in room F3 would be of both boxes.
It's not entirely a fair comparison to simply look at the f3, as different designs have varying amounts of acoustic energy below f3. all things being equal, i'd take a TL at the same f3 than any other design, given it's shallow roll-off. (and if someone knows of a TL design with an f3 of 15hz, well, please let me know!)
  Reply With Quote
Old 20th January 2002, 02:23 AM   #5
diyAudio Member
 
Join Date: Jan 2002
Location: Wellington, New Zealand
ucla88 and kelticwizard - Thanks for the advice

I'll look through those URLs.

kelticwizard - one further question - you mentioned in the ported vs. sealed post that you should subtract the vent area (port * vent length) from the total internal volume. Is this with all programs?

Thanks,
Dave.
  Reply With Quote
Old 20th January 2002, 02:41 AM   #6
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Dave:

Not sure about other programs. I mostly use Boxplot and AVI, which is DOS based and really simple.

If your program calculates the vent dimensions, try going from a 2" port to a 8" port diameter, (or as large as the program allows). If that doesn't change the plot, then the program is not taking the port volume into account, and you should do so.

Actually, vent volume is virtually negligible in many speakers, but in subwoofers going really deep it becomes significant and should be accounted for.

For Boxplot, I subtract the vent volume plus 10 percent, it just seems to work best that way. One way to check your program is this: fill in a speaker with these parameters:

Qts = .38, (or Qes = .41, Qms = 7)
Vas = 2 cubic feet
Fs = 30 Hz

Then run it in a 2 cubic foot box (Vas). Make Fb, the tuning frequency, equal to 30 Hz, (Fs). If the F3 isn't equal to the Fs, in this case 30 Hz., then calculate your correction factor. In real life, that is usually how it works out.

Incidentally, this is known as the "classic" bass reflex. That is, put a speaker with a Qts = .38, (or .4), into a box equal to it's Vas, tune it to Fs, and the f3 should equal Fs. Other bass reflex setups are possible, of course. Some designers find them even more desirable than the "classic".
  Reply With Quote
Old 21st January 2002, 06:52 AM   #7
diyAudio Member
 
Join Date: Jan 2002
Location: Wellington, New Zealand
kelticwizard... thanks once again... but just to clarify

I'm using the ported.xls spreadsheet from diysubwoofers.com - simple but easy to use...

I entered your sample driver / enclosure - and got F3 = Fb = 30Hz - however when I bumped up the port diameter (therefore increasing the vent length) - the frequency response plot did *not* change....

does this mean I still need to subtract the vent volume from the predicted Vb? and in subtracting - does that mean I need to build the box smaller than calculated by the spreadsheet? - I would have thought since it does not take into account the vent volume - you need to *add* the vent volume to the enclosure volume.

eg. If you are meant to subtract.....
say the Vb = 57 litres (approx 2 ft3)- should I then make the Vb in construction = 57 - vent volume * 0.9 (being the additional 10% subtraction you state).

Thanks,
Dave.
  Reply With Quote
Old 21st January 2002, 03:35 PM   #8
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Dave:

I should have made myself clearer with this "add/subtract" thing.

The vent is not to be considered part of the enclosure volume. You can either build the box larger to compensate for the vent, or lessen the box volume in your calcualtions.

Therefore, in construction, the box should be Vb PLUS volume of vent.

Under many circumstances the vent volume is negligible. In our above example, a 4" diameter port would be only 6% of the box volume. I wouldn't even bother.

However, if we make the port 8" diameter, our port volume is greater than the box volume. Even a 6" diameter port will use almost half the volume of the enclosure. If your plot does not change from a 4" diameter port to an 8", then clearly your program is not taking this into account.

If your program is showing an F3 of 30 Hz in our test box, then you probably don't have to allow an extra 10% assuming:

A) your vent volume is negligible-4" diameter is very generous for any speaker likely to go into our box, (3" diameter is more usual);

B) you are not putting an extraordinarlily large speaker in there-the speaker itself has a certain amount of volume as well. That has to be subtracted, but in most cases it is negligible.

I have built the test example and in most cases it comes out to an F3 of 30 Hz. So it works in most real-world instances. If you are going to use an extraordinary amount of bracing-well, then the bracing volume has to be compensated for.

Even if all these factors throw the calculation off by 10%, so what? You might end up with being 4 dB down at 30 Hz instead of 3 dB down? Not exactly something to keep anybody up nights worrying about, LOL.
  Reply With Quote
Old 21st January 2002, 04:04 PM   #9
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
PS:

We had a project discussed here where a Blueprint 1503 was contemplated being put into a 4 cu. ft box, tuned to 18 Hz. The Blueprint 1503 required either a 6" diameter vent with flares, or an 8" diameter vent without them.

The 6" diameter vent would take up about 21% of the box volume. The 8" diameter vent would take up about 70%.

As you can see, it is with subwoofers tuned really low that the vent dimensions really come into play. Subwoofers, and really compact speakers.

Incidentally, the fact that the vent cannot be counted as part of enclosure volume is the reason the Passive Radiator, (or "Drone Cone") was invented. It is a substitute for a vent. It takes some trouble and expense to make, so it is used in places where the size of the vent has gotten out of hand.
  Reply With Quote
Old 22nd January 2002, 08:08 AM   #10
diyAudio Member
 
Join Date: Jan 2002
Location: Wellington, New Zealand
kelticwizard - ahhh.. I see - thanks.

I don't want to build the box larger than necessary.... (I'm quickly reaching Xmax on the SAF / WAF parameter - but I may as well test the limits.... :-)

So I'll tune the box, then at build time add the extra space for the vent.

One last question (where have you heard that before? :-)....

I read your reply to someones posting about "ideal" port diameter - you said you had lost the formula. David Weem's book suggests no less than 1/7th the area of your driver. Therefore for a 12" driver - no less than 5" (not flared I presume) would suffice (I know about the length of the pipe increasing with diameter - so will check it fits lengthwise for the Fb selected - and if not I'll "elbow" it).....

Anyway - my question - have you found the formula? Should I base the port diameter on some function of driver Vd? I'm probably being a bit pedantic about things - but know what can result when one "near enough" judgement is based on another "near enough" judgement .....

Thanks,
Dave.
  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
calculating gain overdrajv Tubes / Valves 9 5th January 2009 08:22 AM
Room gain Vikash Multi-Way 31 13th March 2004 10:10 AM
Room gain Raka Multi-Way 6 16th September 2003 08:13 AM
Room Gain Ignite Multi-Way 5 12th March 2003 01:10 PM


New To Site? Need Help?

All times are GMT. The time now is 07:25 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