Confused on the dimensions of the subwoofer vented case

[superuserxam]

Been reading a lot the past few days about DIY subs.

I am not sure whether a 12" or 15" subwoofer to choose. The 12" will require a smaller case, less than 3 cubic feet sq (in volume). I have used WinISD and for a vented 15" case, it gave me a height of over 3' (feet)! Not sure if that is correct or not, but that is just scary!

I am planning to use this for HT 90% in a 25' x 10' room.

I am really confused on how does one come up with the size of the case (in my case, am looking at vented case, capable of 20 Hz or lower if possible).

I may be asking too much, and I know I am a newbie.

Been looking at this subwoofer ( http://www.parts-express.com/pe/show...number=295-468 ) and a 240W sub amp as well.

Would appreciate any pointers, or tips and hints on what to look and how to find the calculations. Too much information the last day, and my system is now overloaded with info.

Appreciate it!

[lhwidget]

Hi,
I haven't run any models on the driver you're looking at, but these guidelines are generally true:
1) Vented (bass-reflex) enclosures will almost always be larger than sealed (non-vented) enclosures.

2) A vented enclosure is tuned by two variables, a) the box volume, b) the vent's length.

The actual dimensions for a sub enclosure (its shape) aren't very important, as long as the internal volume is what your modeling software has specified.

For a monster like the 15" RSS sub you're looking at, use at least a 4" vent tube to prevent whooshing and chuffing noises. Two 4" vents would be better.

The down side is that as the vent's cross section is increased, the length will increase, and two 4" vents at a low tuning point (about 20 Hz) will be very long.

The vent doesn't have to be inside the enclosure. The first thing most people imagine, is a box with long tubes sticking out. While this will work, most of us don't like the look. Instead, wrap a rectangular vent around the outside of the enclosure. It's cross-sectional area should be equal to the two 4" vents, and its length should be equal to the length of the two 4" vents. Also, avoid wide skinny vent shapes, go for closer to square.

Lastly, you might want to look for a sub amp with a low frequency cut-off filter. A vented enclosure does nothing to control the driver's cone excursion below its tuned frequency. If rumble from LPs, or low frequency effects excite the sub below this frequency at a loud listening level, you'll probably drive the voice coil into the back plate (you'll hear the clack).

[john trials]

Here's a cool site with some info (and photos of external ports).

http://www.subwoofer-builder.com/index.htm

[lhwidget]

Hey, those are fun, and actually usable.

I was thinking more along straight pipes or a bundle of snakes approch, like The Blue Man group uses on their wilder instruments

[superuserxam]

The vent doesn't have to be inside the enclosure. The first thing most people imagine, is a box with long tubes sticking out. While this will work, most of us don't like the look. Instead, wrap a rectangular vent around the outside of the enclosure. It's cross-sectional area should be equal to the two 4" vents, and its length should be equal to the length of the two 4" vents. Also, avoid wide skinny vent shapes, go for closer to square.

Lastly, you might want to look for a sub amp with a low frequency cut-off filter. A vented enclosure does nothing to control the driver's cone excursion below its tuned frequency. If rumble from LPs, or low frequency effects excite the sub below this frequency at a loud listening level, you'll probably drive the voice coil into the back plate (you'll hear the clack).

LOL - half of what you said is, well, rocket science. Trying to figure out the first paragraph. The science of the second paragraph will be analyzed!

Going for a 12" sub, would that reduce the size of the volume drastically?

I am thinking the 15" for that extra oomph.

[fbov]

LOL - half of what you said is, well, rocket science. ...

Actually, it only seems that way because you're new and on the steep part of the learning curve. For me, this was some of the more easily understandable stuff, but it makes a big difference what we assume about your skill level when replying.

Subwoofers are popular projects, and great places to start DIYing speakers. Here's the basics you need to know to get started.

Speakers are described mathematically by "T/S" or Theil/Small parameters. T/S parameters are driver specs like you find listed in the catalog:
Le: 1.00 mH * Impedance: 4 ohms * Re: 3.3 ohms * Frequency range: 18 - 800 Hz * Fs: 18 Hz * Magnet weight: 150 oz. * SPL: 90 dB 2.83 V/1m, 87 dB 1W/1m * Vas: 9.95 cu. ft. * Qms: 3.10 * Qes: .49 * Qts: .42 * Xmax: 14mm

This data is used by simulation software to predict low frequency response curves for a driver in a specific size box. This allows you to select various drivers and change the box to see what affect that has on bass performance, without actually buying drivers or cutting wood. Hopefully, you see the benefit of this approach: model many but build once.

Two popular box simulators are Unibox (Excel-based) and WinISD. I use the former, found here:

http://audio.claub.net/software/kougaard/ubmodel.html

You will need to enter the target driver T/S specs into the Dbase tab, if it's not already listed, and then select the driver in the design sheet. You spec the box volume, and tuning if required, and the simulator does the rest. Give it a try and ask questions is probably the best way to get you started here. Note that Unibox won't ask for all the T/S parameter as some are derived from others, and it will ask for data you don't have. I copy values from another, similar driver.

Note that this only gives you a volume. You get to choose dimensions to fit your application, as long as the internal volume is close to the target and the driver still fits. It also gives you a port length, based on a port diameter you specify. Then take a look at the worksheets for your design to see what FR, driver excursion, port air speed, impulse response, etc. look like. you should feel lost at this point, so ask questions.

One final thing: room gain. My room is ~22x15x8, and I get to 10Hz fairly flat with 1 sealed 15" sub. The sub simulation is down 23dB at 10Hz, the difference is room gain. Energy at higher frequencies can bounce all over the room because the room is large compared with their wavelength. Not so at low bass frequencies. First, you get "modes;" standing waves. Eventually, the lowest mode (dimension = 1/2 wavelength) no longer fits so energy in that dimension begins to build up, first in the short dimension (ceiling) and eventually in the longest (~30Hz in my room). Room gain is why I get response to 10Hz.

Lots here. Read and think, research and ask. If fully frustrated, I bet there's a dozen regular posters who've already modeled the RSS395HF, but the real value is in doing it yourself...

HAve fun,
Frank

[lhwidget]

LOL - half of what you said is, well, rocket science. Trying to figure out the first paragraph. The science of the second paragraph will be analyzed!

Going for a 12" sub, would that reduce the size of the volume drastically?

I am thinking the 15" for that extra oomph.

Sorry, I wasn't very clear. If you have a box, say 24" x 18" x 18" (inside dimensions) for a net volume of 4 1/2 cubic feet, and you find that a 4" vent would be 30" long, how do you install a 30" long pipe inside your box? Easy, you use 90° elbows and turn the vent tube every time it runs into an inside surface (don't forget to increase your enclosure volume to compensate for the 1/4 cubic foot you lost to the vent tube's volume).

If you don't want to fiddle with that, you could actually build the box with the vent tube mounted externally. You could install a 4" tube 30" long sticking straight out of the top, it would still function properly, just look bizzare.

What I meant to explain is that the best way of making an external vent is to make a rectangular passage (usually following the inside of the skinny walls of the enclosure) that is the proper cross-sectional area and the right length. This passage connects the inside of the box to the room, and serves as the vent. A lot of people would call this a slot port because of its shape.

What most folks never consider, is that the vent tube is now external to the resonanting air volume inside the enclosure now.

Wasn't trying to be confusing, just didn't explain myself very well.



Switching to the RSS 12" driver will require a smaller enclosure (I'm guessing 2.5 to 4 cu ft, based on its Vas spec of 3 cu ft). The RSS 15's Vas = 9.9 cu ft, it will be looking for a an enclosure volume of 5 to 10 cu ft, depending on how you tune it.

I think you would be pleased with either one, but the 15 will give more oomph.

[lhwidget]

Late thought...

Frank's right, get the modeling software and figure out what you're going to need to build to suit your needs & wants, or get the guys here to help you out by modeling the driver.

The numbers I've used above are random, I didn't model the driver.