Back in the 1990s I bought a set of home stereo speakers called Magnat Project II which I used with joy til a few years ago when during a party the bass speakers blew up. These speaker boxes are dual speakers type, meaning each box has a tweeter for high tones and a woofer (or ‘low speaker’) for low tones. Other speakers boxes might have triple speakers, thus for respectively low-mid-high. Some have even more speakers covering the sound range.
My current tweeters are still fine and so are the electronics inside the speakerbox. This bit of electronics is in both speakerboxes and is called a high/low-pass filter or filter for short. This filters the incoming audio signal so only high tones go to the tweeter and low tones go to the woofer. I believe some filters from other brands or speaker models may just pass anything to the low-speaker and just act on the high tones, that I am not sure of.
A cross-over frequency is the frequency at which the audio is either filtered for woofer or tweeter. For instance, my filter has a cross over frequency of 3000 Herz, I believe . An optimal speaker setup will have woofer, tweeter and filter all matching, provided a margin. A woofer, in general, could have a frequency range of 30hz-1000hz, or perhaps 20-3000Hz, a tweeter might have a range of 1500hz-20,000Hz or perhaps 2500hz-30,000Hz, depending on class of quality and setup.
When you decide to re-use bass speakers into an existing speaker box, try to find out if the combined frequency ranges between new bass speaker, the existing tweeter and the filter don't overlap too much, or worse, leave large gaps in the frequency range. If you have no technical specification of your setup, no worries, it is worthwhile to have a try at this DIY project. Fun it is! And anything better than speaker boxes you don’t use at all, right? Oh, and make sure the new speakers have no rips and tears in the fabric, it’ll sound awful. Use undamaged goods.
The speakers of a previous car or mine (a Saab 9000, 1996) had a noticably good sound, also having tweeter and low-speaker combination on each channel (that is, left and right). The filter was mounted on each of the low-speakers, altough I eventually decided not to use them since the quality of my Magnat filter is better. Yet I wasn't sure if it had an approximate same cross-over frequency. I tried it anyway and it worked out very nicely. Here is how I did the whole thing.
- electric cutter to cut out the contours for the new speaker
- a 1.5 volt battery to confirm the polarity of your new speaker, in case you are in doubt. I used a DIY power supply with 3.3V output
- some wires and flat connectors as often used for DIY in cars
- a small chisel, the obvious screwdrivers
- regular electric drill
- screws, tape, some paint, pliers, hammer.
- electric Hole Saw
- measuring gauge
- A4 (US: ‘Legal’ ) sized blanco paper, lead pencil, scissors
Step 1: Imagine, Measure and Mark, Pre-fit
Take away any front decoration of the speaker box and remove the existing low speaker. Note to carefully try to release the speaker until you see it moving – don’t yank it out or start prying with a screwdriver. Do it delicately and mind the wires attached to the speakers. Make a photo of the wiring for reference. If the speaker had flat connectors, unplug it, other wise just cut them close to the speaker but leave a few millimeter of the wire attached so you can see what color was attached. Or take that photo. Then remove the speaker.
Take the new speaker and see how the whole looks like when you put the speaker loosely in the existing hole. See if the dimensions fit well between the edges of the speaker box frontboard and estimate if the speaker does not touch the backside of your speaker box or other components or whether it unwantingly covers screwholes or so. In my case I had to remove the Magnat logo decoration and reattached it a little lower.
Step 2: Draw Contours
Take the new speaker and put it on the A4 (US: ‘legal’) sized Paper with the front facing down. Take a pencil and draw the contours on paper, including the screwholes. Then, take a measure and see how wide the flange (or ‘rim’) of the speaker is and draw this on the contour on paper. This takes a little handyness in drawing. Then, take scissors and cut roughly HALF of the internal curve and the outer rim curve, depending on the shape of your speakers. Leave it, don’t remove it.
Step 3: Overlay and Pre-cut
Take the paper and put it in the most economic and effective way over the existing hole of the speaker box, with the part you just cut in paper to fit over what is still untouched board of the speaker box front. Fix the paper to the speakerbox front using tape. Take a leadpencil and draw the inner rim curve and outer contour on the speakerbox front by following the slits in the paper you cut earlier. When done, remove the paper and keep it stored wrinkle free for your other speaker box.
Step 4: Cutting the Board
Before you start cutting remove padding like sound dampening whool or sometimes insulation material is used, and you may want to insert a damp cloth inside the speakerbox so it catches wood chipping and dust while working.
Then, preferrably using an electric cutter, cut over the line you drew on the frontboard, which is the INNER contour of the new speaker. DON’t cut the outer line, because then your speaker falls in the newly cut hole.
Step 5: Chisel the Rim
This is a little tricky and takes some craftmanship. The rim (or ‘flange’) of the new speaker might need to be ‘sunk’ in the wooden front board, as was in my case. So you may need to chisel from the-outer- contours inwards and downwards until you have half-thick-board-space for the rim of the new speaker. Some other speakers have raised mounting holes on the outer edges and so may not need chiseling. Yer lucky then.
Cut, trim, chisel in small chunks at a time (have patience!) till the new speaker fits in at rest. Make sure to store away the speaker, don’t put it next to the toolbox because you’ll drop a screwdriver through it. When finished you can decide to paint any exposed wood in the original color, as I did in black.
Step 6: Test Polarity and Connect
Test to reconfirm polarity of existing wires of the new speaker. With a multimeter measure which wire on the connector plugs on the back side of the speaker BOX (typically red and black) and write down which wires correspond to the wires you cut earlier. Typically, inside the speakerbox a black wire means PLUS and a white means MINUS, which might contra-intuitive. Just measure it, let’s not play philosopher or Chief Electronics Engineer.
So, take the new speaker and take a 1.5 volt battery or any other low power source up to 3 Volts and put both wires very very shortly to the speaker and see what direction the inside (conus+fabric) the speaker moves. If it moves outwards, you have got the right polarity. Write it down or remember it. Attach matching connectors, disconnect any alternative filter. Then, use and apply standard connectors that fit the new speakers.
Also, In my case I decided to leave the filter on the new speakers so I needed to disconnect the wiring from it, since I was using the filter in my Magnat speaker box.
Step 7: Re-assemble and Optional Relocate the Bass Reflex Hole
After putting any insulation or other sound dampening material back in the speaker box as it was, mount the speaker. You may want to pry a little hole for the screws to prevent wood bursting or splitting. Don’t overtight or you’ll ruin the screwholes after all.
In my case I decided to move the bass-reflex hole at the back side to the front side. I used a metal plate to close to the hole in the back side and used the Hole Saw for a new perfectly circular hole on the front side. The little plastic pipe for protection and decoration fitted right in after a little padding with black tape.
Step 8: Connect and Test
Test your speakers with frequency sweeps from an audio test CD or using your favorite High Quality song. Try left and right channel independently and listen if both low and high speakers work in conjunction.
Let’s hear it !
Qwertypat, february 2nd, 2016