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Contrary to popular opinion, beautiful hardwood is not a good acoustic material since it tends to resonate, or vibrate, creating its own sound patterns. Made from wood ships, particleboard does not suffer from this problem-- and it is cheap. However, it has numerous undesirable characteristics such as moisture absorption and brittleness that makes it almost impossible to fasten two pieces together securely.
On the other hand, fiberboard is processed by "rubbing" apart wood into fiber bundles of approximately the same size that are combined with a resin binder to form a homogeneous mat of random fiber orientation. The result is the ideal construction material for speaker enclosures. Its high density resists moisture. It's strength allows the thickness to be reduced with attendant weight savings. Its uniform composition permits parts to be securely fastened. And, the absence of long wood grains affords minimal resonance.
 The infinite baffle - When a speaker cone moves forward, compression of air takes place infront and rarefaction takes place behind it. When a loudspeaker is mounted on a relatively small baffle board the compressed air spills around the edge of the baffle into the zone of rarefaction still present at the rear, thereby inhibiting the excursion of the cone. This acoustic short
circuit as it is called worsens towards low frequencies. This can lead to acoustic cancellations. Hence it is important to seal the speakers in enclosures. !
So larger the baffle board the lower will be the acoustic cancellation frequency becomes. Theoretically, the minimum length of the side of the baffle board to prevent acoustic cancellation to prevent a short circuit is half the wavelength of the tone. So for 50Hz frequency, the baffle should have an area of 3.4X3.4 = 11.56 m2..! This is impractical and hence the folded baffle or sealed enclosure concept is what is practiced.
 Base-reflex enclosures - At low frequencies, the radiated power from the rear of the cone represents half the total radiated power for obvious reasons. The bass reflex loundspeaker systems make use of this radiation. To do so, involves the reversing the sense of the air particle motion at the rear of the cone before adding it to the vibration infront so that no phase reversal happens while adding so as to cancel the power. Reversal of the direction of particle motion is achieved by the resonance effect associated with the vented cabinet. |
