Dynamic Cone Speakers Explained

Radio Record, Volume II, Issue 53, 19 July 1929, Page 38

Dynamic Cone Speakers Explained

Lecture by’ Mr.

L. H.

Wright

to Radio Society

PFORE the Wellington Ama-. teur Radio Society on Tuesi day last Mr, L. H. Wright, of ithe technical staff. of the, #\ Radio Record," delivered an address on the operation of %)| dynamic cone speakers. .The following is a resume of that’ y lecture :-- oy RerERENce toa ‘dictionary tells 1 us * that sound is "the sensation produced by stimulation of the auditary ~ nerve. by aerial vibrations." This does not convey much idea of the great vari- . ation of soufid. But these variations of sound ‘simply fellow variations, in the nature of the vibrations, of to" be* more precise in the rapidity with which." they follow. one another. Thus pitch, or the distinction between a low note

and a highite gine As determined. ‘purely ‘py the rapidity. of. ‘the- alternate compression and rarifaction’. resulting from this vibration, It ‘is.well known that a high note is produced by: rapid Vibrations: and -a- low, ‘note’ by ‘slower vibrations. It is. not; every vibration that affects the auditary system,» but providing’ these vibrations are within certain: limits, varying with different people, the sensation of sound is produced. Roughly, the -useful limits of audibility are. between 30. and ‘7000 vibrations per second. For the sake of demonstration the middle'C on piano. has a frequency of 256:.Octaves above and below are double or. half- ‘this number as the case may be,- , ‘No ‘doubt the ‘frequency tests- from 2YA. will .be. remembered .by .. many.

‘Having a south: idea: of what: ‘sound is, itvis as well to study’one or tw0 of its. ‘sources,: In -the. case: of..a stringed instrument the ‘vibrations ‘ of: ‘the ‘whole. instrument set the surrounding ’air in motion, In. a wind instrument the shape is such that by: blowing into it, ‘the air is‘ set. in rapid: vibration.: In the case’ of aloudspeaker -the:.object is: tobe’ able to.reproduce all frequencies in the audible’ seale with no undue: _prominence. : Kinds .of Speakers. HIS. brings us to a study of the different kinds :of speakers. The merit of the speaker can be judged’ by the -extent.of.the variation in the intensity of sound at different points of the frequency scale.’ Thus’ a good speaker will--have sin even- response curve, and Sa poor: one ‘will suppress some frequencies. "-or ‘make . some frequencies : predominate. ‘ A horn type of speaker has the greatest Tésponse.at,the high frequencies. A cone (nagnetic) type or reed driven has a high-response at a middle. frequency .depending upon. -tle construction and design of the speaker... | The: M.C. speaker has ‘a- frequency response that can be adjusted by ‘a. process‘of varying: the. number of turns ‘on the. coil,.alterations in the shape and size‘ of baffle,. and the introduction of: filters for waprensing., any frequency, ° PHD: principle of opetatien was first demonstrated in 1898 by Sir Oliver Lodge. It-is not a new idea, but simply | the revision of ‘an undeveloped theory. Briefly, it can be said that when a wire, carrying’ alternating or fluctuating current is.suspénded in a° powerful magnetic field, the wire tends‘to move in. sympathy With the variations of cur-. rent... In the case of a ‘moving coil’ speaker, it is the speech or music variations of: current that aré passed through ‘the wire, which is in the form of a coil’ Fastened’to. this coil of wire is the cone or diaphram of the speaker. Thus variations of current set the coil and cone in motion. The movement de-. pends upon the natute of the variations of current, and this movement or vibration .sets the ‘surrounding. air in vibration-producing sound "waves. . Mr. Wright went on to say that the explanation could be’ made: more comprehensive if’ the speaker was divided into sections suitable for individual dis-, cussion, :: To: provide a powerful magnetic: several methods are; ayail‘able: (1). Permanent magnets;. .(2), electro magnet operated from 6 V: storage battery; (3), electro magnet -oper--ated from’ 100-200 DC ‘supply; '(4) 6volt ‘winding’. (rectifier for use from AC‘ main). 8 To. obtain ‘the: degree’ of : magnetism required, «permanent magiiets. have. one or- two. drawbacks, namely, "they -can be-magnetised. only after’ assembling. | ‘The. use, of ‘an electro magnet with a 6-volt storage. battery is, perhaps, the. most: common. meéthod,. as. the. supply: ean.be: drawn from the. storage pattery operating: the: ‘receiver, or- from .a_battery charger...In the case’ of an AC. ‘ operated. : carnplifier- employing:-smooth-

ing. chokes ‘the ‘winding of a highe . resistance cdil can ‘be utilised ‘in ‘the place -of one of:'the’ chokes, For’ this method -MG. speaker ‘field. .coil ‘are "wound : to operate’ on 100:200 VY. .direct eurrent supplies. . >, One or two firms supply ‘some. form ‘of dry, rectifier,’ and mount the whole .as one unit. These can be plugged . into the lighting socket. The "methods.: enumerated are the usual. ones for the ‘production’ of a suits able magnetic density, and the matter: was: explained‘ very fully, by the aid of blackbdard ° diagrams. Mr. .Wright went on to ‘describe in- detail .the. section, comprising the cone and coil, The angle of the cone. plays a great part, in the... reproduction. Although’. much _ a

stronger, a, cone made with a more acute angle at; the apex. ‘gives. undesixable focusing, | especially of the high notes. A: shallow cone being mechanically weaker, ° ‘tends to give undesirable resonance peaks; "this -was .explained--by. studying the ‘movement of ‘a .cone at different frequencies. At,, low frequencies the cone moves as.a whole, but a point is reach--ed, depending upon ‘the :angle* and ma- ' terial of which the cone is made, when" the.cone does. not, vibrate: as a. whole, but splits, up. into ‘segments. At the point.on the frequency. scale where this ‘change occurs @ pronounced resonance is. noticeable, and manufacturers try to get. this undesirable point’ as high in the frequency range: as -possible. Centring devices for. the -suspension and ‘maintenance. of . the -coil-in the small air gap were ‘discussed and ex-, amples were. provided by the two speakers at the:meetingThe, last’ -section- comprised different forms. of baffies, and the modifications of. these to-regulate the low register, A large ‘baffle is necessary: to fully de-. velop and reproduce low notes, and a M.C. speaker without a-.baffle is little, if .any,. better. than -the flea pet, reed- a driven cones: on the market. -. At the conclusion of. the ‘lecture: sev-" eral members discussed with the speaker. different: points-relating to the: pute ject. "One ‘speaker,, ‘referring: to th operation -of. a) six-volt . accumulator model: being operated .from a: battery charger pointed out that ‘he: had by the: use of an ‘electrolytic condenser ‘so hunted’, across’. the terminals . of the tharger, all. trace 6f hum-Wsd been eliminated from his*speaker, -In .response ‘to- a". question as’ tu the functioning .of dynamic speakers,. Mr. Wright stated that the -best "could ' be obtained: from a MAC. : "speaker only when used with, a::power‘ amplifier. However, excellent: results.-could °:-hbe. obtained. with’ ‘less * expensive equipment. One speaker stated in. this rei "spect. that ‘his results : with *a" M.C, ‘speaker used with a erystal: and’ push-. pull amplifier, ‘convinced’ him. nothiiig. could be finer.’ . : Finally,’ Mr. wh ‘accorded a , very. liearty vote of thanks’ ‘for his able ‘an interesting lecture. +

. Exhibition visitors -will be able to see and hear the Amplifier described. on. pages 36 and 37. It, ‘will ‘be on view at the "Radio Record" stall.