Right now I'm designing an eclosure for two(2) 15" American Bass XO-1544 subwoofers (Vas=4.629ft^3 Fs=27Hz Qts=0.698). I've read that anything with a Qts that high should be put in a sealed enclosure, but the owner really wants a ported enclosure.
I went ahead and did calculations for a ported enclosure and this is what I got.... with a BoxVolume=4.51ft^3 it will reach -3db at about 29hz. The box frequency comes out to 15.086Hz and with a 4inch Port the length would have to be 30inches SAD .
These number are much different from the other enclosures I have made in the past. Is a ported enclsure of this design even worth the hassle, or should I just recommend a sealed enclosure?
I've built several enclosures in the past (a couple of sealed and one ported) but I still consider myself a novice by all means. My main source as far has been "The Car Stereo Cook Book" by Mark Rumreich and all of the above imformation was obtained from this book and several varoius online sources. Any comments/help is much appreciated.
Replies (7)
MrBrownstone on 09/12/2006 05:07:15
Designed for sealed only, huh? Well, box selection is primarily based upon the owner's use. Let's not read too much into specifications. You can make a vented enclosure work well with this woofer, but likely, it's not as well as selecting another woofer.
Although I usually don't recommend a vented enclosure, if a customer of mine want's 'the ultimate' woofer, I most certainly recommend that he/she purchase a very beefy woofer that boasts a vented application. That way, we don't try to make Walt Disney do Stephen King movies.
In your case, When selecting a vented enclosure design, I'd consider the application. If your customer is using the woofer for a daily driver, venting around 32-35Hz yields the minimum in frequency bump while still preserving relatively tight bass in the upper ranges.
Of course, if he's a DB Dragger, that's an entirely different application.
In your case, try inputing the port FREQUENCY and solve for the length from the software.
PS Here's a dirty little secret....usually the recommended vented box size is about 1.4 to 1.5X the recommended sealed enclosure. In this case, that's about 2 cubes per woofer. The tuning frequency dictates the Port length and diameter...the woofer doesn't make any difference so long as displacement is relatively the same size.
lessismorespl on 09/12/2006 08:39:08
Mr. B is correct when it comes to specs. Try going here:
http://www.bcae1.com
Look up the enclosure building calculators there. Also go to the American Bass website and speak w/ a tech. about suggestions for tuning. Techs are always will to think outside the box a little. There specs are set for your basic consumer which would most likely buy a prefab enclosure. They can get you on the right track for building the ultimate enclsoure.
http://www.americanbassusa.com/main.html
Good luck.
less
cdmckee on 09/12/2006 09:43:20
Sorry, I forgot to mention that this is for a daily driver and I'm pretty much stuck with the speaker choice because he has them.
So I can set the port frequency to whatever I want (prefferably between 32-35hz). I was lead to beleive that I had to use a port frequency from this equation, Frequency= 0.39*(Fs/Qts), which with the woofer gave me about 15hz. I went back and used a frequency of 33hz and got a much better length.
Also, is the square port calculations of www.bcae1.com pretty spot on? Thanks again for the repsonses.
b_bass59 on 09/12/2006 14:17:46
A quick question, when we tune a port to a frequency. Is the frequency we tuned it to is it the high point if the wave or when the wave comes back to zero, one more thing, does the port size, have an effect to the enclosure,( except port noise reduction or tunning the port Frequency)
swez on 09/12/2006 16:20:56
A PTF of 35 Hz., is plenty good. The higher the PTF, the shorter the port length will be, when using round or slotted ports. If you want to cross-check the figures fro Bcae, I use this site aas a double check. Try this one: (It does all the basic parameters for you)
http://www.the12volt.com/caraudio/boxcalcs.asp
Bass59,
Yes, ports have a displacement value inside a given enclosure and we have to consider this when calculating the internal box space required for a given woofer design. We also consider the sub displacement as well. We look for Vb as the final value.
Vb = Gross internal box airspace - sub disp. - port disp. = Usable airspace the sub will actually be working in.
The high and low points of a wave form are considered "Amplitude" or power in the audio signal. This parameter has little to do with tuning frequency cxalculations. However, it helps us determine the port size mins, so we don't have port noise, chuffing and whistle effects.
The number of complete wave cycles that pass through in 1 second, is considered frequency. (Expressed in Hertz) Think of a port as a length of rigid pipe. Let's say we have 2 lengths of 1" diameter pipe. One is 12" long and the other is 6" long.
If we stop up one end and blow across the open end, we get a tone. As length of pipe changes, the tone we get, will also change. (Basically, a one note flute) The longer the pipe, the lower the tone we can expect.
Ports work on a similar principle, but we don't stop up one end of the port.
Got all that?
Swez
MrBrownstone on 09/12/2006 20:54:40
In a nutshell, the purpose of a port is to increase (boost) output near the port frequency. ex: a port at 35Hz will yield more output from 35Hz to about 50Hz. Above 50Hz (about 1/2 octave), there'll be little benefit and below 35Hz, the driver will be basically unloaded---essentially acting as though there is no box present as well.
Vented enclosures are good for getting a little more efficiency out of a woofer at the bottom of it's dynamic range, however, sacrificing transient response (tightness) is the trade off. If your customer likes a loud sounding system (rap music, dance music, etc) but doesn't care about the quality of sound, then vented is definitely a good choice. If he/she isn't all that aware of quality of sound, he may not notice the difference other than 'one seems louder' etc.
Ash on 09/12/2006 21:07:44
Man! With the high Qts on those subs and large Vas, there is no way I would try a ported. But I guess ya gotta do what ya gotta do.
I'm with "Mr. B", however I do like ported as some of the material that I listen to takes advantage of it. Yet, only with small subs with a way lower Qts (.35 and under). These mate better with them in compact boxes and have far better transients than higher Q subs.
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