Experienced Opinions Requested : Sub Design Choices

pietschu · 16th January 2010, 09:43 PM

Hi All,

I'm looking for some opinions on a choice between 2 designs for my home theater sub. Due to spousal restrictions I'm confined to placing the 60" TV, the sub and the front speakers into an alcove 10 feet wide, 7 feet high and 2 feet deep. This should be plenty of room. The sub should be able to produce 106.5 dB in room and down to at least 18 Hz. Now I've selected and purchased 2 Creative Sound SDX-15 woofers and 2 Acoustic Elegance 18" 2500 gram passive radiators. My first thought was to go isobaric to reduce box size. This goes nice and flat to about 18 Hz (see below). I then thought about room gain and tried a normal passive radiator design and the output drops like a 0.5 Q sealed box but has a bit more output. This would allow for placing the woofers further apart and may help with room modes. The Right and Left speakers will be able to get down to maybe 28 Hz.

I will be using the soon to be released Outlaw Audio pre/pro that has room correction abilties so it can boost or cut the lows is required.

My request is that I get some opinions on what the pros and cons for the designs are. Any and all opinions appreciated, even if I don't agree!

davygrvy · 16th January 2010, 11:39 PM

Personally, I think isobaric is a waste of a driver as it doesn't improve low-end. It just drops the top-end. It's a nice idea when the driver aren't expensive, though. I like your choice of components.

What's the longest dimension of your room? Your space might have some room gain, so you might not need your speaker to go to 18Hz to get there. If these sims aren't taking into account the 1/4 space boundary effect I'm assuming you'll have also, you'll see a 6 dB jump in output compared to the sim's free space assumption (that I'm assuming it has).

If this was me, I'd choose the 330 liter alignment.

pietschu · 17th January 2010, 12:05 AM

A bit of an odd room. See drawing, largest dimenson is about 24 feet.

pietschu · 17th January 2010, 12:09 AM

Quote:

Originally Posted by davygrvy

If these sims aren't taking into account the 1/4 space boundary effect I'm assuming you'll have also, you'll see a 6 dB jump in output compared to the sim's free space assumption (that I'm assuming it has).

The sims are free space. I've seen articles stating either 3 dB or 6 dB per octave gain under about 80 Hz depending on the size and shape of the room and the location of the listener. Since I'm not going to do the calcs on room gain and find all the nodes etc, I'm leaning towards having 2 subs spaced about 8 feet apart in the alcove. More sources should result in a more chaotic distribution of those nodes. At least that is my guess.

davygrvy · 17th January 2010, 03:11 AM

Looks to me like you won't get much room/boundary gain at all. With the space opening wide to the right and far back, I don't think much pressure containment will happen. The 45 degree walls on the left (that I'll assume are curtained windows) might make some LF focusing, but probably not a strong antinode. When you close your eyes and listen, the room might have a left heavy tone in the LF region. The parallel back wall to front wall will make a resonance at around 100 Hz, but due to the opening on the right side, I doubt you'll have any nodes to be concerned about. If 100 Hz seems empty in the center of the room, you might consider some helmholtz resonators on the back wall tuned for 100. Sonotubes might work well as their round shape can also act as a specular diffuser if wall mounted.

pietschu · 17th January 2010, 03:21 AM

Quote:

Originally Posted by davygrvy

Looks to me like you won't get much room/boundary gain at all. With the space opening wide to the right and far back, I don't think much pressure containment will happen. The 45 degree walls on the left (that I'll assume are curtained windows) might make some LF focusing, but probably not a strong antinode. When you close your eyes and listen, the room might have a left heavy tone in the LF region. The parallel back wall to front wall will make a resonance at around 100 Hz, but due to the opening on the right side, I doubt you'll have any nodes to be concerned about. If 100 Hz seems empty in the center of the room, you might consider some helmholtz resonators on the back wall tuned for 100. Sonotubes might work well as their round shape can also act as a specular diffuser if wall mounted.

Thanks for the info. This is actually a basement and I have a thing for 45 degree corners. The right bottom opens into a small "office" pod. A 12 foot by 7 foot room with 2 more 45 degree corners and filled completely with a wrap around desk and upper cabinets. The opening on the upper right is a landing/foyer of sorts with doors for 2 bedrooms and the stairs giong to the main floor.

Does this all mean I should try for a flatter response from the sub?

davygrvy · 17th January 2010, 03:49 AM

Quote:

Originally Posted by pietschu

Does this all mean I should try for a flatter response from the sub?

That's the direction I would go. Wait for others to respond as well.

jason_watkins · 17th January 2010, 09:56 AM

The good news is with that very irregular space you should have pretty randomized modes other than the floor to celling axial modes. So limited placement options may matter less. As for room gain, little way to know.

pietschu · 17th January 2010, 01:44 PM

Quote:

Originally Posted by jason_watkins

The good news is with that very irregular space you should have pretty randomized modes other than the floor to celling axial modes. So limited placement options may matter less. As for room gain, little way to know.

Thanks for the feedback. The ceiling is just short of 8 feet. The joist space is 14" which has been filled with a combination of fiberglass, a floating layer of drywall and rock wool insulation. The surface consists of drywall with square and rectangular cutouts (28"x28" and 23"x28") that will be covered by cloth covered picture frames. The floor is a laminate on a OSB subfloor which is spaced up about 3/4" over concrete. Those are my uneducated attempts at reducing sound transmission and reflection.

head_unit · 25th January 2010, 06:46 AM

That's a pretty huge space, so I don't see why you would want to go isobarik at all. Much better to get the two sub drivers next to each other along the viewing axis. That creates a kind of virtual acoustic pressure wall between the two.

I'm not sure what you have in mind by a "flatter" response, but something with a gentler rolloff is probably good. Don't worry about just f3, look at f6 and f10 etc.

As for "More sources should result in a more chaotic distribution of those nodes."-nah. I don't think so. The nodes are just from geometry at the LISTENING position. The sub positioning just excites some modes or not. Multiple sources may randomly not-excite resonances at the listening position and happen to sound better, but that's luck not acoustics. Better to load the drivers next to each other, adjust the listening position, and apply time-domain EQ (is that what Outlaw is bringing?)

16th January 2010, 11:39 PM	#2
davygrvy diyAudio Member Join Date: Mar 2007 Location: Richmond, CA Blog Entries: 1	Personally, I think isobaric is a waste of a driver as it doesn't improve low-end. It just drops the top-end. It's a nice idea when the driver aren't expensive, though. I like your choice of components. What's the longest dimension of your room? Your space might have some room gain, so you might not need your speaker to go to 18Hz to get there. If these sims aren't taking into account the 1/4 space boundary effect I'm assuming you'll have also, you'll see a 6 dB jump in output compared to the sim's free space assumption (that I'm assuming it has). If this was me, I'd choose the 330 liter alignment. __________________ Think out of the box

17th January 2010, 03:11 AM	#5
davygrvy diyAudio Member Join Date: Mar 2007 Location: Richmond, CA Blog Entries: 1	Looks to me like you won't get much room/boundary gain at all. With the space opening wide to the right and far back, I don't think much pressure containment will happen. The 45 degree walls on the left (that I'll assume are curtained windows) might make some LF focusing, but probably not a strong antinode. When you close your eyes and listen, the room might have a left heavy tone in the LF region. The parallel back wall to front wall will make a resonance at around 100 Hz, but due to the opening on the right side, I doubt you'll have any nodes to be concerned about. If 100 Hz seems empty in the center of the room, you might consider some helmholtz resonators on the back wall tuned for 100. Sonotubes might work well as their round shape can also act as a specular diffuser if wall mounted. __________________ Think out of the box

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17th January 2010, 09:56 AM	#8
jason_watkins diyAudio Member Join Date: Jul 2005 Location: none	The good news is with that very irregular space you should have pretty randomized modes other than the floor to celling axial modes. So limited placement options may matter less. As for room gain, little way to know.

25th January 2010, 06:46 AM	#10
head_unit diyAudio Member Join Date: Feb 2004 Location: Los Angeles	That's a pretty huge space, so I don't see why you would want to go isobarik at all. Much better to get the two sub drivers next to each other along the viewing axis. That creates a kind of virtual acoustic pressure wall between the two. I'm not sure what you have in mind by a "flatter" response, but something with a gentler rolloff is probably good. Don't worry about just f3, look at f6 and f10 etc. As for "More sources should result in a more chaotic distribution of those nodes."-nah. I don't think so. The nodes are just from geometry at the LISTENING position. The sub positioning just excites some modes or not. Multiple sources may randomly not-excite resonances at the listening position and happen to sound better, but that's luck not acoustics. Better to load the drivers next to each other, adjust the listening position, and apply time-domain EQ (is that what Outlaw is bringing?)