I couple months ago I built a small powered 10″ sub for my Dad’s family-room theater. It was designed to fit in the small space at the bottom of one of his bookcases, one of the reasons for going with a sealed design, to keep the box nice and small. Even though the model of this enclosure predicts an f3 of only 39Hz, I expected the in-room response to be somewhat better, since after all the model doesn’t take that into account. It shouldn’t after all, since the response of the box is what you’re designing to. What it does in the room is just a bonus. Ever tried to get a lot of deep bass outside in the backyard? It’s a little tougher. Anyway, today I found my Radio Shake SPL meter (that thing had been lost for the last year) so I thought I’d do some quick measurements and see what it looks like on paper.
We’ve watched a bunch of movies with this little sub and I’ve been extremely impressed. There is no active bass boost, so the input response is as flat as it can be (without worrying about non-linearities in the amp or sound card). The beginning of The Polar Express when the train is pulling up to the first little boy’s house just rattles and rumbles the whole house. I couldn’t believe a small 10″ 22L sealed box could have so much deep bass, and the sub isn’t even corner-loaded either, there is only the one wall behind it.
So I downloaded TrueRTA, which is available from www.trueadio.com onto my HTPC, then I set up my SPL meter on a tripod across the room on the couch at about the listening position, and ran the sine wave tone generator in 2Hz increments from 10Hz up to 80Hz and recorded and plotted the results. I kept the volume kinda low, one can only tolerate pure sine wave tones blaring away in the house for so long. I couldn’t believe that the in-room response of this sub actually has some decent bass down as low as 26Hz before it finally starts to rolls off. I compared the modeled performance to the measured performance and was happy to see that for the most part it looks like it should, however the low-pass crossover on the amp appears to be kicking in a little early, I may need to dial the knob up a little bit to pick up some more bass above 50Hz. There is most likely a standing wave at or around 15-20Hz which might explain the dip below the projected response in that region. I’m getting a bit of constructive interference around 28Hz and 42Hz where there’s some bumps in the response there. Overall however the measured in-room response is within +/- 4dB of the modeled anechoic response, so all in all, that’s not bad.
I also measured the near-field response by locating the meter about 1″ away from the dust cap on the driver and then repeated the measurements. The graph is shown below. This type of measurement mitigates the effects of the room and measures more accurately the response of the box/driver. When compared to the modeled performance, it’s just about right on target. If you took out the 80Hz low-pass filter, the rolloff appears to begin dropping right at about 40Hz and then follows the modeled response all the way down to 10Hz. I should take out the low-pass filter, I bet the two graphs would line right up. That just proves that you can achieve predicted results through careful desing, modeling and building. T/S parameters really work, once again. Of course the response of a sealed box is easy to predict, it’s the TL boxes and EBS 4th order boxes that get more difficult.
In the end it really comes down to how does the sub sound when kicking back and watching movies, and this little sub really can perform. It’s not going to be breaking any SPL competitions, and I’m not going to do a max SPL in-room measurement either (I’ve cooked too many subs by just grilling them with 20Hz sine waves and watching the cones go in and out, ooh, neat, pop! dang!). I’m anxious to see what kind of in-room response I can get from my dB-500 12″ sub in a box that actually is suppose to be flat all the way down to 20Hz. That should be downright awesome! So I can’t wait…