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HASIB'S ARTICLES AND AUDIO PROJECT REVIEWS
10” SEALED SUBWOOFER (PART 2) Introduction For those who follow me before, I have six subwoofers in my HT system. Yeah, that’s quiet a lot. Right now I want to downgrade my subwoofer system to just three. Maybe you want to know why I want to downgrade my six subwoofers to three – six subwoofers in HT setup make the bass unreal and I’m kinda too old for that kind of “big” bass. I have several complains from my family including myself that the subwoofer boxes make my listening room full and ugly too also the others are really not suited with the subwoofer controller I used (box resonance frequency is too high). I want to make new boxes because the old one has small internal volume (I want to make the box a little larger) and the height is too tall and looks a little tedious to my eyes. Rectangular shape box is my target because it looks cool and easy to build too. The old subwoofers have sold to a friend of mine. Driver Specifications I use 10” car subwoofer model. This driver is really a subwoofer driver. A pair of them cost RM450 – quiet expensive but the performance is quiet good. This is the same subwoofer driver I used previously used on my old subwoofer that sold to a friend. Here are the specifications I measured myself:
For nominal impedance, sensitivity rating, maximum power handling and structure magnet, they’re from manufacturer specification. Design goal I want to design sealed subwoofer because the subwoofer controller was designed for sealed subwoofer. Have a look at my DIY 3-way active crossover project for full description. Sealed subwoofer system also has other pluses – very good transient response, offers the easiest most tolerant design and construction.
Driver’s math I’m using WinISD version 0.44 software for box simulation. This is freeware software and you can download from www.linearteam.org.
Well, close to my design goals. Maybe one will wonder that –3dB frequency (F3) of 37Hz is too high for subwoofer application. To decrease it I must make larger box and out of my option. Here is the plotted response from the simulation:
Frequency/gain response Based on plotted gain response, there is a slight hump around 50Hz to 200Hz. The only solution to make the response flatter is making the box bigger. The Box The box is made from 11-mm plywood that doubled together to give 22-mm thickness. Excuse me – no MDF available in my area.Externally the box measured 50cm x 45cm x 47cm (WxHxD). After subtracting the volume displaced by the driver, braces, terminal cup etc clean internal volume is 71 liters. There are two internal braces made from 22-mm plywood install inside the box to make it stiff. For attaching the panels/braces I use woodscrews, nails and lots of wood glue – the joints must be very secure to make it strong and to avoid air leak. I use jigsaw to cut holes for driver and speaker terminal cup cutout. After the glue was secured, time for wood filling and serious sanding process. I don’t have electric sander and I done this manually. This is very tough job, not too hard but need patient and internal strength. I use medium grit sandpaper and rub the panels till I feel my hand want to fell off! For finishes I use Matex paint (water based wall paint). I applied three coats that every coat sanded with medium grit sandpaper before another coat applies. This will make the finishes have very smooth surface. For final assembly I use black shine wood paint and applied two coats of them. Painting the box manually using paintbrush is not really good compared by using air pressure gun painter. I just used very traditional method since I can’t afford such luxury. The finishes really good, it has very smooth surface when touched by hand but looks a little rough by bare eyes (blame the paintbrush). Mounting screws for driver and speaker terminal cup drilled. Before mounting them, I use double foam tape attached behind driver/speaker terminal mounting flange to keep them airtight. Remember just peel off the double tape one side only, if not after you mount them you will never remove the driver/speaker terminal again. I use thick rubber sheet under the subwoofer secured using double foam tape to keep the box from rattle/vibrate on floor. Before break in period I have tested the box airtight or not. Using my own technique I ran sine wave signal about 40Hz and drive the subwoofers from its dedicated power amplifiers. I use my hands/ears to detect air leak between joints, speaker terminal and the driver – yes it passed. Also I have another technique. This can be checked by pressing in the cone and then watching how fast the cone moves back up to its rest position. My cone sure enough took about three seconds to return so I was definite I have good sealed subwoofer boxes. The Sound I have measured box resonance frequency (Fb) before/after installing fiberglass. Fb is 50Hz without fiberglass filing and 47Hz with fiberglass. I use 1” of fiberglass installed internally on every panel excluding front panel. When comparing the Fb, the driver really fooled after fiberglass installed because it thinks it mounted in larger box (about 100 litres). Looks like I have gained 29% of “free” space and other plusses mentioned below. How about the sound? The sound really exceeds my expectation. It really shows up after two weeks (after full break in). It sound deep and tight. I don’t hear any unpleasant bass (whoompa whoompa or thunka thunka bass). It just sound louder and louder if I push them hard. Does the box rattle? Oh yes, that the disadvantage using plywood on loudspeaker construction. No matter how strong you build the box, it will rattle. Using three or four (!) sheets of 12mm plywood didn’t help. I really can’t get rid it off even I sat on it. Luckily when fiberglass installed the rattle did reduced but still there. I’m very lucky the rattling box didn’t “smear” the sound output. It just heard when I put something on it ie Cd, remote control etc. I did not perform real life frequency response because I don’t have SPL meter. Performance between the two subwoofers is quiet same except one of them need a little more fiberglass filing to keep the Fb stay at 47Hz. It doesn’t frighten me because even the two drivers are from same manufacturer/model the TS parameters will vary a little. Cost for this project is about RM600 and I can say it is a real bargain. That’s all folks. I hope I have given you some ideas from this project. Completed Project Pictures
Fig1 - 10" Sealed Subwoofer (Front View)
Fig2 - 10" Sealed Subwoofer (Rear View) Muhammad Hasib bin Md. Nor June 2002 Any Comment And Question Email to : hasib_tangkak@yahoo.com
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