Toccata Grande, Part 1
By Lars Mytting
20 Jan 1998
Rattle and hum: Resonances
Now, how on Earth do we make a cabinet that suits our monster? The cabinet will be quite big, 125x46x32 cm in total. With this size, wall resonances becomes a factor we must deal seriously with. All materials that vibrate produces sound waves. It is very fair to consider the cabinet walls as a second speaker cone. Since we have dealt a lot with decibels and cone areas the last minutes, let us investigate the cabinet walls with the same methods.
The final Toccata cabinet has two walls of 125x46 cm and a rear wall of 125x32 cm. This gives the astonishing area of 15 500 cm2 -- sixteen times larger than the drivers; equal to twenty 15" drivers! If this area (15 500 cm2) moves 0,0625 mm p-p, they will generate the same sound level as when the speaker cones moves 1 mm! At 270 Hz (a frequency where MDF and chipboard usually have strongest resonances) a movement of only 0,01 mm p-p will generate a sound level of 90 dB!
That is theory. But tests made by Colloms(6) agree, and state that many cabinets at certain frequencies can give higher output than the drivers. And the sound the cabinet produces is distortion and distortion only. Some speaker manufacturers claim they tune their cabinets like a guitar, to the A note of 440 Hz, to the Italian violin maker tradition of the 16th century, or to whatever that appears "musical" in the advertisements.
Let me humbly point out that what they are "tuning" is a board of wood, perhaps 20 mm thick, bolted and glued to a box and coupled to a stand or to the floor. It is interesting that these manufacturers claim to have benefit of modern cone materials like carbon paper, magnesium or polypropylene; since they can make 20 mm chipboard sound so good.
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