Speaker Design, Speaker Building: Loudspeaker Design & Construction

Acoustic Dampening in Loudspeakers

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Acoustic Damping for the manufacture of speaker enclosures

Placing acoustically absorbent material at the antinode positions (where the vibration motion is the greatest) can dampen dimensional resonances.  It has virtually no effect at the nodes.  This is done in most small enclosures using thick rolled up layers occupying most of the space.  Larger enclosures present a problem in that the absorbent material tends to be compressed under its own weight at the bottom.  One solution to this is to suspend nylon nets within the enclosure to support layers of absorbent:

It should be noted that besides preventing standing waves within an enclosure, ‘stuffing’ a box has effects on the following parameters:

  1. Compliance Increase.  Compliance is increased within the enclosure.  This is equivalent to increasing the box size by as much as 15-25%.
  2. Efficiency Increase.  Proper selection of the amount, type and location of the material can cause an increase in efficiency by as much as 15%, as cancellation no longer occurs.
  3. Mass Changes.  Air flow behind the driver is reduced.
  4. Damping losses.  Some energy may be lost through friction within the absorbent material, and between it and the sizes of the enclosure.

Popular materials for damping include:


Fibreglass

Readily available material, used usually in either 2 or 4 lb/ft3 densities, but acoustic properties are not as good as some others and can be a health hazard to work with.

Long Fibre Wool

Considered very acoustically sound, but very prone to crushing under it’s own weight, so some retention method should be used such as netting in fig 2.2.  Also needs to be moth proof if the enclosure is to be open (eg in the case of bass-reflex or transmission line enclosures).

Dacron®

Purpose made dampener available from loudspeaker parts suppliers.

Acousta-stuf®

Purpose made dampener available from loudspeaker parts suppliers.  Like Dacron, but with crimped fibres reportedly giving the product the same sonic qualities as long fibre wool.

Polyurethane foam

Convenient as it can be obtained in various sizes, blocks and sheets.

Bonded Cellulose Acetate Fibre (BAF) wadding

Most commonly used due to its convenience and acoustic properties.

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