What response should I be in search of for home theater sub?

[CFC]

Hey guys,
What kind of f3 extension would be ideal for a home theater sub? Should I strive for as low as possible? or do I need to design to account for room modes?
If so, where can I get some info on designing a HT sub around room modes?

Thanks,
CFC

[David Gatti]

IMO a sub worth having should be approaching -3dB at 20Hz. Otherwise it's really just a woofer.
Obviously the lower it goes, theoretically the better it is, but you have to deal with the real-world constraints of size and cost.

[simon5]

You can build your subwoofer to take into account room gain. You can't build to take into account room modes. Some people use a parametric equalizer to correct room modes a bit, but it's far from a perfect solution.

[CFC]

Originally posted by simon5
You can build your subwoofer to take into account room gain. You can't build to take into account room modes. Some people use a parametric equalizer to correct room modes a bit, but it's far from a perfect solution.

Ouch... "room gain" is what I meant actually ... not room modes...
so I'll repost my question:
where can I get some info on designing a HT sub around room gain?

Thanks,
CFC

[simon5]

Well everyone have something to say on that subject, but I got great results in the past with a -2 dB per octave curve.

If your reference is at 80 Hz, design it to be 0 dB at 80 Hz, -2 dB at 40 Hz, -4 dB at 20 Hz, -6 dB at 10 Hz.

I guess -3 dB per octave is good too.

Some people here will advise you to use -6 dB per octave instead.

It depends on your room construction, if it's plaster walls or concrete, etc.

The best thing to do would be to do your room gain curve in the room you want to have your subwoofer, but usually you want to build a subwoofer because you don't have one in the first place hehe!

[CFC]

Originally posted by simon5
Well everyone have something to say on that subject, but I got great results in the past with a -2 dB per octave curve.

If your reference is at 80 Hz, design it to be 0 dB at 80 Hz, -2 dB at 40 Hz, -4 dB at 20 Hz, -6 dB at 10 Hz.

I guess -3 dB per octave is good too.

Some people here will advise you to use -6 dB per octave instead.

It depends on your room construction, if it's plaster walls or concrete, etc.

The best thing to do would be to do your room gain curve in the room you want to have your subwoofer, but usually you want to build a subwoofer because you don't have one in the first place hehe!

Actually, I do have a small sub in the room already, but obviously I'm looking into replacing it. Could I estimate the room gain curve through that? I'm not sure how to get that measurement.

CFC

[chops]

I don't think all of the measuring is necessary for theater use, unless you're also using the same system for music listening, which then you WOULD want the flattest response possible.

My subwoofer project is just going to involve building two somewhat small-ish 7 cu.ft. enclosures (I'm going to be running dual subs) tuned to 16Hz for my Dayton 15DVC subs. That way, they will be fairly tight through most of the listening range, and then get a little boost from about 20Hz on down to about 12Hz.

At 12Hz, I'll be at around 95dB for just one sub, and that's not even including room gain. I'm guessing that with both subs up and running in the room, I'll be somewhere around 105dB @ 12Hz, give or take a dBs. (probably something like 120dB @ 20Hz)

But anyway, to get back to my point... Tuning that low (16Hz) the enclosure will pretty much act like a sealed enclosure until around the port tuning frequency, then the cone movement will become very well controlled at the tuning freq where it will need it the most.

[simon5]

CFC, you can easily measure your room gain curve if you have a SPL meter like the one from RatShack.

Bring the subwoofer outdoors far away from all walls, measure the frequency response at low volume from 15 Hz to 100 Hz.

Put it back in your room, measure the frequency response at low volume from 15 Hz to 100 Hz.

Substract the two curves, you now get your room gain curve.

An easier way of doing it is if you already know what driver is in your small subwoofer and what's the internal net volume of your small subwoofer. Enter the data in WinISD will give you the outdoors curve. Substract with the indoors curve you did and you get your room gain.