BAFFLES FOR CONES

Sun (Auckland), Volume III, Issue 675, 29 May 1929, Page 16

BAFFLES FOR CONES

ELIMINATING DISTORTION : IMPROVEMENT OF TONE Owners of speakers of the various cone types will often find it possible substantially to improve the quality of music yielded by the speaker by ! equipping it with a baffle board. | The baffle was first used with the new dynamic cone speaker, to the successful operation of which it is essential, but it is now being applied to the ordinary types of cone with excellent results. A baffle board consists of a suitably treated wooden board, cut square, and about 6in longer on each side than the diameter of the cone in the speaker. In the centre of this board is cut a circular hole the same size as the cone in the speaker, and the speaker is attached to the baffle board in such a way that the edges of the hole in the board make an airtight joint with the rim supporting the cone of the speaker, but at the same time allowing the cone perfect freedom of movement. The purpose of the baffle board is to prevent the loss of sound energy from the cone. Sound is produced by the speaker by the vibration of the cone surface setting up alternatively areas of high pressure and areas of slight vacuum in the air in contact with its surface. When the cone surface is forced forward, for instance, it will press on the air against it, and create an area of high pressure. Alternatively, when it recedes, there will be a low pressure area created immediately adjacent to its surface. It is the formation of ruse alternate high and low pressure areas which creates the sound waves. It will be clear, however, in considering the movement of a cone diaphragm that when the front of the cone moves forward against the air to form a high pressure area the back surface of the cone will be moving away from the air against it, causing a vacuum. There is, as a result, a considerable pressure difference in the air in front of and behind the speaker. As a result of this difference there is a tendency for air to leak round the edges of an ordinary cone, flowing from the high-pressure area which always occurs on one side of the cone to the low-pressure area on the other side.

This leakage results in a considerable loss of power, as the outer edges of the cone cease to create sound, and the sound producing medium then becomes the central portion of the cone from which the air is unable to escape. Not only does this air leakage result in loss of volume, but it also causes distortion. This distortion arises from a tendency for the lower notes pro-

duced on the speaker to be suppressed. When a low note is being produced the speaker diaphragm is not making so many motions to and fro a second as when a high note is produced.

As a result the actual time taken for each complete vibration is longer, and when air leakage is occurring round the edges of the cone it follows that a greater leakage will take place on the lower notes than on the higher ones, because the low notes allow more time for leakage. Thus, the sound loss on lower notes will be more pronounced than on high ones, and the musical perspective will suffer.

When the baffle board Is fitted to the outer edges of the cone, the distance over which air leakage must take place to produce the effects which occur when no baffle is used is very much increased, and, in addition, leakage can only take place against the considerable inertia of the body of air round the baffle board, a distance of some inches. Thus, if the baffle board is carefully fitted to the supporting frame of the cone, it can prevent the effect.

It should be noted that the same source of trouble does not exist in a speaker of the horn type, as the moving diaphragm and air column in the horn are completely enclosed and cannot suffer from air leakage.