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A
workshop is not a real workshop without a sound system! And with a
disgust of local radio stations in mind something better was needed.
The answer was an elaborate 5 speaker sound system that directly hooked
into a networked P.C that was able to play streamed on line radio
stations. The network and the computer are still in the works but in
the meantime a Creative Zen MP3 player or a netbook supplies the input to the
amplifier.
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Main speaker design and construction: No off the shelf speakers even came close to what was wanted so something custom was planned (not surprise there!). Using the program Blau-box and with a Jaycar catalog in hand the components where chosen: 6.5" woofer: Initially the 5" model was chosen however this speaker has a resonant frequency (Fs) of 49 Hz vs 67 Hz, had 1 Db better sensitivity and didn't need a much bigger enclosure to be tuned correctly. Ribbon tweeter: Unlike the more traditional dome style tweeters the ribbon tweeter design uses a moving ribbon membrane, the result supposedly allowing for very sharp and crisp sound. Jaycar had a stock clearance so I managed to buy these very cheaply. Adjustable port: An adjustable length 50mm diameter port was chosen because the calculated port length was longer than the maximum available fixed length. A ported design was chosen because it allowed for a lower frequency response than a sealed enclosure. Cross-over: Initially a 2 way 3500 Hz cross-over was used, this was later changed, more details under 'testing'. Speaker cloth: A covering cloth was also bought however the cover and the material will be attached later on as a low priority. Other components: Amplifier: A cheap 5.1 channel amp was sourced from oo, this may be upgraded later on but in no rush. Cable: Initially a cheap 2 core cable was used however this was later changed, more details under 'testing'. Mid range speaker: Because of the irregular workshop shape the sound would of been uneven at the smaller assembly bench. To stop the sound appearing to come from one direction a single orphaned driver that was found on ebay. This would use the amps center channel and be mounted above the assembly table facing down. Because of the quirks of human hearing a partial supplemented sound in one direction will give the impression the complete sound source has moved, so in this case a more even and full sound. A small wooden enclosure was made for this driver. Speaker brackets: To allow for full adjustablilty a design was made that bolted/screwed to the walls and allowed the speakers to pivot in both directions. A simple design using 13 x 25 thin RHS steel with a top pivot using an M5 bolt was used. Testing: Speaker testing: Although the system was and still is far from complete the pre-mentioned Mp3 player was supplemented for the P.C output and the amp temporarily placed on a tub for trial runs. During very early testing the sound quality of the first newly assembled speaker was poor. For the lack of a better description the mid range was sorely lacking in that is was flat and a 'muddy' sound. Going off a hunch and some advice from some Jaycar staff a new cross-over was tried that switched at 3500 Hz instead of the previous 5000 Hz. This meant the woofer would do much less of the midrange and the tweeter would being doing much more work. The result was excellent and the new setup sounded quite rich and sharp. Covering and uncovering the speaker port lowered the mid range and some of the low end response so it was exactly as calculated. So all in all each speaker enclosure sounded rich and complete in the sound whist still being quite cost effective (under $80 AU each) Amplifier: During testing and trial fitments of speaker cable it because increasingly hard to get one of the channels to function properly in the amplifier. The channel would cut in and out when the terminals where manipulated. This suggested a dry soldered joint, so with the warranty and caution thrown into the wind it was taken apart. The soldered join looked great but for the sake of it the two joins where re-soldered. Upon re-assembly and testing the problem was completely fixed. Cable: When the first speaker was mounted in the far corner it was significantly quieter. The first suspect was the long cheap cable. When fitting the cable one of the two conductors didn't even accept solder no matter what. This brought up serious questions about the material used, such as if it was even copper. So the choice was made to put together something custom (again!). With a large surplus of four meter long single core cable that had a 1.5mm cross sectional area these where joined end to end and wrapped as a pair. With the new cable in place the sound quality was indistinguishable from the speaker sitting directly on top of the amp, so a total success. |
