15. More about the dustcap story.


15. You want to know more about the dustcap story.
Normally, it is not necessary to remove the dust cap. In exceptional cases, for instance, in case of units from the PA that have been used heavily for an extended period of time, or units with a heavy cone and a supple centre ring, it may be necessary to centre the cone in another way.
You can simply check this by keeping the unit first down with the magnet pointing downwards and then turn it upside with the magnet pointing upwards. If the cone then ‘falls’, there is a rough clearance maximum. For an 8 inch speaker, the clearance should be about 2 mm, for a 10 inch one it would be 2.5 mm and for a 12 inch one it would be 3 mm. The centre ring may not be completely flat, but still retain its sturdiness.
In such an exceptional situation (about 0.5% of the cases), the cap can be removed. In my opinion, it is better to use the original cap for such a revision. This is important, especially for smaller speakers which produce more middle and high tones. It is certainly not the point to cut the caps off, to centre the cone and to install another cap afterwards. There are several reasons for this:
  1. A part of the old cap remains in place, which renders the cone heavier and slower. It also reduces the performance.

  2. The cap plays an important role, especially in case of smaller units, for the production of higher frequencies. Installing a larger cap (which has to be larger in order to cover the remainders of the old cap) affects the production of these higher frequencies. The following is a direct quote from the Dutch edition of the book ‘‘The Loudspeaker Design Cookbook’ by Vance Dickason. This is not just any book, but a Bible among books used by loudspeaker designers:

    --- A dustcap has also influence on the of the high frequencies. ---

  3. The new cap is not an original one and will, in most cases, be made of a standard material, from which the whole product line of dust caps has been produced. This may also affect sound production. (see above).
You can probably imagine why, both from a visual and a sound-technical point of view, it is better to leave the dust cap in place and to use our way of refoaming. There are various types of dust caps:
  • plastic
  • paper
  • felt
  • fine meshed fabric
  • aluminium
  • air permeable fabric

For the last type, the air-permeable fabric, construction plays an important role. If left uncorrected, there may be an accumulation of compressed air behind the cap. This results in a distortion of the sound. Different manufacturers have found different solutions to this problem. For instance, by making ventilation holes in the voice coil holder, in the pole core, or simply by making the dust cap air-permeable. The installation of just any cap ‘that fits’ can cause unexpected problems. In a case like this, Audiofriends tries to deliver a dust cap that is most similar to the original one.