DIY subwoofer question... [Archive] - Audio & Video Forums


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Keith from Canada
01-21-2004, 01:30 PM
I currently have an Adire Shiva 12" sub mounted inside of an 88.5 liter sealed enclosure (Q=0.6) that was originally used for an equal amount of music and movies. This design worked very well for both however, it did lack a little punch when it came to movies. After some remodelling, my current set-up will now be used almost entirely for HT and I am hoping to add some of the punch back to the sub by building another box.

My question is, would I be adding more "punch" to my HT bass if I went down to a 54 liter sealed enclosure with Q=.707?

Many thanks!

This Guy
01-21-2004, 02:17 PM
but making the box smaller will have you lose some efficiency. The smaller box would probably be more beneficial to music reproduction which you won't need. I personally would build a big ported box that Adire or others reccomend. If it's a proven design it will sound punchy and still have that slam for HT. My dad built my half sister a sub using a 12" driver that preferred a 2 cubic foot sealed box, so we tested it in the one cubic footer we already had and it sounded HORRIBLE. Every note it played was distorted sounding and wasn't even close to xmax or clipping the amp. As soon as we put it in the 2 cubic footer it was great, loud and clean bass. So thats what happened when I used too small of an enclosure, noty sure if it will happen to you. I'm guessing your using the 250 watt in 4 ohms plate amp right?


01-21-2004, 06:38 PM
Have you tried downloading the Shiva white papers from Adire's website? They pretty much suggest that a higher Qts will result in more slam than the tighter sound that a low Q will give you. I know that the Rava is rated at Q=.67, and the Q=.707 that you're gunning for is as theoretically close to a flat frequency response as you can get. As TG suggested, you could always go with a ported design as well.

Or you could go back to the drawing board and build something around one of Adire's 15" Tumult drivers!

Richard Greene
01-23-2004, 01:00 PM
I could not hear a Qtc difference of 0.1 (0.6 to 0.7)

If you want more punch try a Qtc of 0.9 to 1.0

You can easily reduce the volume of an enclosure by removing
the driver and gluing a block of styrofoam inside the enclosure
whose volume equals the enclosure volume reduction you want.

While Qtc above or below 0.707 does not have a flat
frequency response, the peak for a 0.9 Qtc is only 0.69dB
and the peak for a 1.0 Qtc is only 1.25dB -- the peaks will
tend to be in the middle of the octave from 50 to 100Hz. where
most of the bass music is located (kick drums, bass guitar notes).

These frequency response deviations are minor compared
with in room bass frequency response (including standing waves)
at the listening position that's typically +/-10dB or more !

Keith from Canada
02-09-2004, 07:48 AM
Many thanks Richard.

The option recommended by the techs at Adire would be to add a vent to the current sealed enclosure that I am currently using. Does adding a vent lead to potential problems (e.g. tuning) that I should be concerned about? FYI -- I am not overly educated in terms of the science of subs...I just built the box recommended by Adire and that's about it (in short, I'm a laymans terms kinda guy).

Thanks again!

Richard Greene
02-09-2004, 10:51 AM
Porting a 12" Shiva driver requires a specific-sized enclosure and port -- the port tuning frequency is selected based on measurements of your driver (you should not rely on printed specs.). This is a complex job. The result would be extra output under 30Hz.
That could increase the impact of explosions in action movies. It would have little effect on four-string bass guitar and kick drums.

If you want more impact from bass instruments such as kick drums and bass guitars, then you must increase output above 40Hz. A high Qtc sealed subwoofer will do that.
I built a sealed Shiva 12" sub in 1999 using a 1.7 cubic foot enclosure (internal size) stuffed with three lbs. of polyester pillow stuffing (three bags). It provided high impact bass guitar reproduction. However output under 30Hz. was weak. That was not a problem for playing music where there's little output under 30Hz. ... and that's exactly what the subwoofer was designed for.

Keith from Canada
02-09-2004, 11:10 AM
The result would be extra output under 30Hz.
That could increase the impact of explosions in action movies. It would have little effect on four-string bass guitar and kick drums.

This seems to be exactly what I'm looking for. As I mentioned previously, my original design was based on a 50/50 HT-music split that has now turned into close to 100% HT. The sub is phenomenally tight and controlled for music and blends seemlessly with the rest of my system however, it lacks the punch that is needed for good sounding HT reproduction. The goal of my redesign is to a) take the existing box and find a way to improve the HT 'kick' or b) rebuild a box using the same amp and driver to one better suited for HT applications.

One of my options is to take the existing box (an 88.5L sealed Shiva design) and simply add a vent to it in a similar fashion to the 85L vented design that's part of the Vented Shiva blueprints (however, mine is front firing, the vented design indicated is down firing). Or, I could simply build another box that is more suited to HT applications, be it a vented or sealed design.

I guess my questions are these: 1) If I simply add a vent to my existing box, will I get better 'kick' for HT (or could this botch things up and make things sound horrible)? and 2) Should I rebuild the box completely, what design would yeild the best results for an almost exclusively HT system (keeping in mind that I just plan on building it to spec...I am not one who knows enough to mess around with tuning or doing mathematics to determine the amount of fill required etc)?

Again, many thanks for any help...

02-09-2004, 11:50 AM
Hi Keith.

Something else I would check into do you have your sub hooked up? Thing is...we can only read what you write you can't always get the entire picture of what's going on. Mr Greene certainly won't steer you wrong (as you already know).

But...something I would check into is your sub hooked up? Is the cross-over function being performed by a plate amp or an HT receiver? What frequency is it set at?

What I'm getting at (without knowing your system) is...if the sub is crossed to may never get the effects you want out of it. Because your main speakers are handling this area. Far too often people think low effects happen lower than they do...and high effects happen higher than they do.

Along with everything else you have planned to try...try raising the crossover point up see if you get the desired effect. then..(if you achieve some of what you want) slowly lower the crossover point until you find the lowest optimal setting for you and your desires. It doesn't sound like your looking to get a true flat audio set up just go for the effect you want for HT...and if the "kick" for HT you crave is higher than the corssover is set on the sub. Well...>>>>>>

I'm just not sure the sub...or the enclosure is to blame. It may be...but I would look at other things too. If the "kick" you want is at 80-120Hz...and your sub is set for 40,60 or 80...and you main sepakers aren't capable of moving some's likely that you won't achieve what you want by changing the enclosure.

just another angle to try.

This Guy
02-09-2004, 12:44 PM
Download a subwoofer program to help design a new ported box. I like "speaker box design software" others like "WinISD". these two programs will help you design the best ported box for your application and driver. It's a very useful software and is quite accurate.