The Care of Loudspeakers

Radio Record, Volume VIII, Issue 37, 22 March 1935, Page 46

The Care of Loudspeakers

HERE are no fewer than four varie- : ties, of loud-speakers which incor- ' porate permanent magnets. There is, first of all, the:one which is in reality -a glorified telephone receiver with. a circular’ diaphragm of stalloy or some. similar metal: ,Next there is the reed. type of loud-speaker, © Thirdly, there is thé balanced armature pattern. and lastly the permanent magnet moving. coil loud-speaker; For any of these instruments to give of their best ‘it ‘is essential that’ the magnets’ should. not have weakened. Loss of Magnetism. Not everybody realises that perman-' ent magnets. are permanent in name only. Should you doubt this) | pur-. chase a.small bar magnet and: ‘stuibject.

‘it toa minute or so of hard fiammert ing. . You will find that it will no longer sup‘port the, weight of iron that was once well’ Within its powers. "The qualities of a permanent magnet suffer not oiiky "under hard; blows but also under the effeets of vibration. Be careful not to let unnecessary "knocks:‘or jars come ‘the «way of your loudspeaker and _remember that whatever precautions you *'may’ take it is being: subjected to con- _ Stant vibration, . ‘ You » ean éasily: satisfy yourself ‘on ‘this. ‘point by placing ‘your hand on the case or.on the pot magnet of thé loud: speaker . ‘¢according, to, its type). ‘when . 4 it is reproducing speech or ‘music; at ‘good volume. The vibrations transfer"sired from the:diaphtagm or cone ‘can’ be : felt at*once, This being SO, even the. best of "permanent" magnets ein weaken in .time, The; symptoms "ane that the’ loud- » (Continued::on; page 49),- :

The Care of Loudspeakers (Continued from page 46), speaker is-no longer able to produce full volume without considerable distortion.: Often it will "zizz’ badly on certain sounds. ' , Dust and Damp. Dust is a great enemy of the movingcoil loud-speaker, whether of the permanent magnet or of the electro-mag-netic type. The clearance between the speech coils and the sides of the gap in which it moves is very small, and, if a _ thick layer of dust collects, as too often it does, the instrument may begin to distort badly. Every endeavour should be made to exclude dust from the working parts of moving coil loudspeakers. A bag made from dust-excluding material and fitted with a draw string is most useful for this purpose. The bag encloses the entire chassis.and cone and is held in position by the draw-string at the rear, The effects of damp are almost, if not quite, as serious as those of dust. They are particularly bad in loudspeakers of the "telephone receiver" and reed types but they can also have dire effects. in a moving-coil instrument if the gap becomes reduced in size by the formation of rust. Be careful therefore to keep the loudspeaker, of whatever type it may be, in a dry place. If it does become damp and deterioration sets in send it immediately for an expert overhaul. ‘In some moving-coil loudspeakers the cone is rather prone to lose its correct centring. Binding in the gap then occurs on loud passages and distressing distortion results. Centring methods vary somewhat, but an excellent outline of the general procedure is given in an article published. in the January "Radio Times." a