Sub Frequency, Room Gain and Room size?

[efzauner]

hello

From my car audio days it is well known that a sub can take advantage of car cabin gain which adds 12db/octave below the frequency set by the interior dimensions.   Does the same hold for a HT room?  A room/wavelength of 20 feet corresponds to about 60Hz..

 

what dimension is the most important? say for a 15 x 17 x 8 foot room?  

 

The reason I ask is that most subs do not have flat response down to 20 Hz, but seem to roll off much higher, 30 or 40Hz. 

Do most sub installations use room gain to fill in the lower octaves?

 

I built a large sub using a 12 inch car sub, with a 20Hz vent resonance.  Huge it is, but works great.  But could I have made it smaller by tuning to 30Hz and gotten good in room performance?

 

thanks

 

[derrickdj1]

Subs do benefit from room gain.  Sealed will benefit more than vented subs.  C or the speed of sound is 1130 ft/sec2.  Your length of room for the longest dimension is 17 ft.

 

C/ 2*L or 1130/ 2* 17= 33

 

Room gain starts at 33 Hz and you will be up 12 db at 16.5 Hz.  A vented sub will use a 24 db slop for the hpf and will cut the output accordingly at tuning.

 

This is theoretical and things change in the real word and some frequencies will benefit more than others.

Edited March 27, 2016 by derrickdj1

[Chris A]

Does the same hold for a HT room? A room/wavelength of 20 feet corresponds to about 60Hz.. what dimension is the most important?

 

I think that you need a room that actually pressurizes down to the lowest frequencies that you mention, and you really don't have any open doors or large windows that can flex.  In fact, you'll need a room with pretty rigid walls (i.e., NOT drywall) to realize true cabin gain.  See http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/120190-does-anyone-understand-cabin-gain.html. Vented and TH subs apparently won't realize any additional gain in a room dominated by cabin gain.

 

Otherwise, what you're probably seeing is the effects of the "sparse room mode region" below its Schroeder frequency of your room: the peaks and the nulls due to the lengths of the room in three dimensions that are 1/2 wavelength of the frequencies that you're seeing gain, and anti-nodes at the halfway points at the center of the room in any dimension.

 

Typically, the most important dimension is the longest dimension, then the next longest, then the third dimension in regular-sized rooms.  For instance my room is about 40 feet long, so its lowest mode is 14 Hz.  That will determine where the room will switch over to pure cabin gain mode, if pressurizable.  But nothing in my room can be pressurized, and its drywall construction will eat all that "cabin gain" well above that frequency.

 

See https://community.klipsch.com/index.php?app=core&module=attach&section=attach&attach_id=79899

 

Chris

Edited March 27, 2016 by Chris A

[derrickdj1]

what is the size of the sub?  HxWxDepth,  Your room is not that big and should be easy to pressurize,

[mustang guy]

My favorite room-mode calculator. What I like about it is that it gives a bolt-area. If you can get in that region, your room is considered fine. Knowing the rest takes a bit of research, but the long and short of it is to experiment with dimensions starting with ceiling height, since it usually can't be changed.

 

here is that link: http://amroc.andymel.eu/?l=19&w=13&h=10&ft=true&r60=0.6

 

In your case, the room would be a bit better if it were 14' rather than 15'; however, being close to the bolt area is usually good enough.

[derrickdj1]

I built a large sub using a 12 inch car sub, with a 20Hz vent resonance. Huge it is, but works great. But could I have made it smaller by tuning to 30Hz and gotten good in room performance?

 

You need to run something like Win ISD or bass box on the two sub models and see how they compare.

Edited March 28, 2016 by derrickdj1

[Paducah Home Theater]

The reason I ask is that most subs do not have flat response down to 20 Hz, but seem to roll off much higher, 30 or 40Hz. 

Do most sub installations use room gain to fill in the lower octaves?

Yes but more importantly, most home theater subs leverage low tuning frequencies in ported boxes and/or DSP's to bump up the extreme bottom end. Just throwing sealed subs in a room with no EQ whatsoever would still sound bad regardless of any room gain.

[efzauner]

 

I built a large sub using a 12 inch car sub, with a 20Hz vent resonance. Huge it is, but works great. But could I have made it smaller by tuning to 30Hz and gotten good in room performance?

 

You need to run something like Win ISD or bass box on the two sub models and see how they compare.

 

I did use Win ISD Pro to design the sub......