Dynamic Speaker Faults

Radio Record, Volume IV, Issue 27, 16 January 1931, Page 16

Dynamic Speaker Faults

~Some Common Sources

N many. ways the dynamic cone speaker is an extremely rugged mechanism. The field magnetism, since it'is produced by the flow of an electric current, never weakens. so long as the current flow is maintained. The dynamic speaker has definitely improved tone quality. But its wide frequency range of tone coupled with the method of construction sometimes accentuate faults that would be unimportant in a less efficient unit. On the very low notes, for instance, it may’ actually have a motion of as much as nt quarter of an inch-many times the possible motion of even the best magnetic speaker. This imposes a severe

strain on the diaphragm and on the mechanism holding it in place. The accompanying illustration shows where trouble can occur and where to look if ever the speaker begins to produce queer, rattling noises or scraping sounds, or the volume falls off appreciably. When the dynamic speaker is not in use the diaphragm is under no strain.

The front edge is maintained in a central position by a thin, soft leather ring, the outer edge of which is clamped or otherwise fastened to the metal frame, the inner edge being cemented to the paper cone. The cement is not infallible. When subjected to a severe strain, such as a erash of static, it may break loose. at ‘some point. If this occurs on the edge of the paper diaphragm the loose edge of the paper will vibrate against the leather and produce a_ rattling

sound that. usually ‘will be most pronounced on one particular tone frequency. The leather itself may become loose at its outer edge and produce a similar but less evident noise. Many dynamic cones are made from a single piece: of paper with one cemented seam 'running from the apex to the edge of the cone. Occasionally this seam gives trouble. The cement gives way at some point and the loose ‘edges’ of the paper rattle together, Looseness also may develop in the cone. support arms, Muffled and distorted music and speech, if it is not due to some defect in the set itself, often is caused

by the ring at the apex of the cone rubbing on the pole piece, The clearance at this point is very small, in some cases not over five thousandths of an inch. ‘Testing for this trouble is easy. Take a piece of writing paper and see if it can be slipped in the crack between the pole piece and the centre ring of the cone. If so, move it all the way

around the circle without binding at any point. If it can be so moved the cone is not in trouble at this point. Extremely weak reproduction on A.C. type dynamic units may be due to an exhausted rectifier unit. A burned-out voice coil or field winding weuld put the speaker out of commission at once, but such troubles are so rare‘that they need not be considered.