What type of sub for HT system?

[sjalloq]

Hello all,

I'm new in the world of DIY'ing and have decided that I'd like to build my own sub for HT and possibly some 2ch listening. My fronts are Monitor Audio RS6's so I'm not let down in the bass department for 2ch stuff too much. If I can build something I can use for both then great, if not, then I'm happy with building something to take care of the LFE channel in a 5.1 setup.

My goals are fast, dynamic bass and I'm trying to decide on what type of subwoofer to build. I've done a lot of reading and have decided I'd like to build either a sealed, ported or TL design. I'm drawn towards the TL type because it's a little more exotic and I'd like to learn more as well as give myself a challenge. I should say that I'll probably be buying a plate subwoofer amp rather than building my own. I can handle some woodworking, but building an amp is not something I want to do on a first project.

I'm going to be running my HT system with the recommended THX cutoff frequency of 80Hz. As such, I'd like to build a sub that extends from 80Hz down to as close as it can get to 20Hz. One of the questions I'm unsure about is what sort of SPL I'm looking for. I think I'd prefer to build something that was controlled and doesn't distort rather than go for all out SPL.

Can anyone offer any words of wisdom before I spend the next month reading all the posts in the forum.

Thanks for any help.

[badman]

Yes, spend a month reading all the posts in the forum There's a lot to know in speaker design, fortunately, subs are easier to do, build a super quality cabinet for a tried and true design, add a plate amp, and enjoy the boom. Read through http://www.diysubwoofers.org/ to get some ideas. I tend to prefer sealed enclosures, I'm working on a sealed sub right now, 350W into a 15" in about 5 ft^3 enclosure. Not the most extended bass the driver can do, but with room gain, should hit quite low. With the monitor audios, I'd definitely recommend one of two solutions- a low-q sealed alignment (between .5 and .707) or a variable q sealed alignment ( resistively damped operation ). The RDO sacrifices efficiency to be able to adjust the Qts to suit your taste.

[simon5]

I tend to prefer ported or a passive radiator design because you can easily hit 20 Hz or below, without any EQ. It's also louder and less THD down low because of lower excursion.

[badman]

Yeah, you can get more 'extension' with a port... but beneath tuning the rolloff is faster, so you'll get less REALLY deep stuff (if you like that sort of thing), less group delay with a sealed box, and the distortion, well, beneath tuning the driver starts to flop like a fish in a ported box. Of course, throwing high pass filters of amps into this mix changes things a bit, one must consider any boosts or whatnot the plateamps include, which can greatly increase group delay, reduce or increase response depending upon frequency, and other crap. I intend to run my sealed box with a 2nd order highpass at 12Hz.

[sjalloq]

One question you'll be able to answer concerns WinISD. If I'm going to have a play with the software and look at the SPL plot, at what point does the sound become too quiet? I.E. on the rolloff under 25Hz, is it the -3dB or -6dB point that I should look at when comparing how low a particular design drives?

Also, how much gain do you have to factor in for room gain. I understand every room is different, but is there a general ballpark figure?

Thanks.

[simon5]

For room gain, I use a curve in my head starting at 0 dB at 80 Hz. Now the curve climb by 3 dB per octave.

0 at 80
+3 at 40
+6 at 20
+9 at 10

What's quiet? You choose.
-1 dB barely audible difference
-3 dB audible difference
-6 dB big difference
-10 dB two times less loud for our ears

I use -3 dB because it seems to be a generally accepted "standard".

BTW, use WinISD Pro Alpha.

[tade]

I couldnt be happier with my single dayton 12 hifi in a sealed box of two cubic feet. In a week i will have two. Perhaps i will be happier!

[sjalloq]

Thanks for those figures, very helpful.

With respect to WinISD Pro, I had a go earlier and tried entering the T/S parameters for the Volt R3853. It just barfed saying that the relationship between Qts, Qms and Qes wasn't in line even though I got the specs from the web and WinISD itself. Seems a bit flaky. Any suggestions?

[odysseybmx414]

use the help file in the pro version to enter specs in the right order that happend to me when i used pro first too.

[simon5]

Just remove a parameter, remove Qts for example, it will autocalculate the right number.

Following the help file will correct your problem as odysseybmx414 said.