ELECTRO-ACOUSTIC TRANSDUCERS

Abstract: An electro-acoustic transducer of the compression driver kind has a diaphragm (1) which is substantially acoustically loaded by the provision of a compression (phasing) plug (8) on one side of the diaphragm and a loading cap (7) similarly distanced on the other; the geometry of the phasing plug (8) and loading cap (7) being such that substantially the same compression ratio is provided on both sides of the diaphragm (1) over all areas of the diaphragm.

Description: ELECTRO-ACOUSTIC TRANSDUCERS TECHNICAL FIELD The invention relates to electro-acoustic transducers, and more particularly to the class of loudspeakers known as compression drivers. BACKGROUND ART The elasticity resulting from the resistance of a gas to compression and rarefaction was first investigated scientifically by Robert Boyle in 1662. He showed that under isothermal conditions the pressure, p, of a given mass of gas varies inversely as to its volume, v, i.e. pv = c where c is a constant. This is a relationship for so called 'ideal' gases. The behaviour of air as a medium for sound propogation does not comply with isothermal conditions. The rapidity of successive compressions and rarefactions determine adiabatic conditions. S.D. Poisson showed that the relationship between pressure and volume of a gas under adiabatic compression or expansion is of the form pv Q = c where < is the ratio of specific heat of the gas at constant pressure to the specific heat at constant volume. For air this value is ϊf = 1.41 and c = 0.726. Poisson's relationship shows that equal positive and negative increments of pressure imposed upon equal masses of air cause unequal changes in volume, the volume change for positive pressure being less than that for negative. The propogation of waves of finite amplitude was treated mathematically by Poisson in 1808. As a consequence of the above relationship, he showed that in general, acoustic waves cannot be propogated without a change in form and as a result additional harmonic frequencies are parasitically generated from the energy present in the fundamental. The theoretical magnitudes of the energies of the harmonics can be obtained from the solution to the exact differential equation of wave propogation in air. With respect to loudspeakers, the problem was first treated in 1933 by M.Y. Rochard who investigated the production of harmonics in horns due to the non-linearity of air. In the theory and design of electro-acoustic compression drivers, the consideration of the linearity of volume displacement of the diaphragm for applied AC voltages has been a neglected factor. It has been assumed that for a sinusoidal electrical input, the displacement of the diaphragm is sinusoidal resulting in a sinusoidal change in the pressure of the air between itself and the phasing plug. (Lo Beranek, Acoustics P 272.) Compression drivers are known which have a radiating diaphragm which is acoustically loaded, at its side coupled acoustically to a horn, by a so-called 'phasing plug' known per se. The phasing plug is a structure having a ported surface, the port or ports opening into a duct or a plurality of ducts which expand to terminate in the proximity of the throat of the loading horn. The phasing plug also serves as a compression plug, and in high quality commercially available compression drivers, a compression ratio of about 10s 1 is a common design criterion which results in a practical tradeoff between distortion and efficiency. In some product examples the outer side of the diaphragm is loaded by a loading cap establishing a primary cavity which is either left uncoupled or coupled to a further cavity by means of a hole or leakage path. The volume of the primary cavity and the degree of coupling, if any, to said further cavity, determines the loading of the diaphragm, more particularly at the lower frequency region of the compression driver's power response range. Loading caps of this type are intended to reduce diaphragm excursion at the lower operating frequencies. DISCLOSURE OF INVENTION I have discovered that lower acoustic distortion can be achieved in a compression driver, if a loading cap is so configured as to provide substantially the same acoustic compression as the phasing plug. If non-linear asymmetrical loading is avoided in this manner, then the acoustic distortion, due to the departure of the behaviour of air from the ideal gas law, will be minimised. In this respect, although it has been generally believed that the acoustic distortion from horn/driver combinations is due to the horn loading of the driver and has been termed 'throat distortion' , it will now seem that the non-linear acoustic output from a compression driver/horn combination is primarily dependent on the compression plug and its loading on the diaphragm, taking into account that the compression in the throat is approximately 4:1 compared with 10:1 for the plug. Accordingly, my invention comprises an electro-acoustic transducer of the compression driver kind, having a diaphragm which is substantially symmetrically acoustically loaded by the provision of a compression (phasing) plug at one side of the diaphragm and a loading cap at the other side, the configuration of the loading cap and the compression plug being such that substantially the same compression ratio is provided by those said elements at either side of the diaphragm. Preferably, the configuration of the loading cap is such that it mirrors the surface/port geometry of the phasing plug about the contour of the diaphragm and is disposed at approximately the same distance from the diaphragm as is the phasing plug. The cap may have ports opening into a matrix of ducts, holes, radial slits or concentric slits. Practical measurements on an experimental symmetrically loaded diaphragm of 100mm and a 10:1 compression ratio have yielded improvements in total harmonic distortion of up to 6dB in the top octave of operation ( 1OKHz - 20KHz). The lowered energy of the harmonics is retained in the energy of the fundamental frequencies, and results in an increase in sensitivity. At the higher frequencies, experimental results reveal up to 3dB greater electro-acoustic conversion efficiency. It is notable that the reduced acoustic distortion enabled by the present invention makes it practicable to produce higher efficiency drive units than hitherto, by appropriately increasing the compression ratio. BRIEF DESCRIPTION OF THE DRAWINGS Following is a description of a preferred embodiment of the invention with reference to the drawings in which: Figure la, lb and lc is a plan view of loading caps to match known phasing plug types, and Figure 2 is a cross-section of a compression driver with a ratio of 20:1 at the diaphragm of 100 mm diameter and a 16:1 ratio at the throat. The driver has a phasing plug of five concentric slots which form a convex wavefront at the throat. The appropriate loading cap is also illustrated. Figure 3 is an schematic enlarged part sectional view of the loading cap diaphragm and phasing plug. BEST MODE FOR CARRYING OUT THE INVENTION Figures 2 and 3 show a compression driver having a diaphragm (1) mounted at its periphery by fixing means (2). The diaphragm (1) carries an annular voice coil (3) which is disposed in an annular gap (4) of a locating assembly (5) . At the side of the diaphragm (1) which faces the throat (6) of a loading horn (7) or of an element to which a loading horn can be coupled, a phasing plug (8) is disposed. The plug (8) has ducts or passageways (9) extending therethrough. The phasing plug acts as a compression plug. It may provide a compression ratio of about 20:1. Viewed in the axial direction of the horn (7), the ducts (9) are concentrically arranged. At the other side of the diaphragm (1), and equally spaced therefrom, a loading cap (11) is provided, which follows the contours of the diaphragm (1). The loading cap (11) has ports (12) so configured and spaced from the diaphragm that the loading cap provides the same geometric configuration outside the diaphragm (1) as is provided by the phasing plug (8) at the other side. As illustrated, the compression ratio of the loading cap is such that it mirrors the surface/port geometry of the phasing plug. Accordingly, symmetrical acoustic loading of the diaphragm is achieved over all areas of the diaphragm and at all frequencies, up to the high frequency cut-off of the phasing plug. There are numerous variations of diaphragm contours, both convex and concave, and of duct geometry used in phasing plugs ranging from the matrixed port variety shown in Figure la, and the radial port variety shown in Figure lb, to the concentric port variety shown in figure lc. In all cases the ports in the loading cap are designed to mirror on the outside of the diaphragm the port geometry of the phasing plug inside the diaphragm. It will be obvious to one skilled in the art that compression devices can be designed which vary in other geometrical particulars without departing from the general idea of this invention.

Claims: CLAIMS 1. An electro-acoustic transducer of the compression driver kind having a diaphragm which is substantially symmetrically acoustically loaded by the provision of a ported compression (phasing) plug on one side of the diaphragm and a ported loading cap on the other side, the geometric configuration of the compression plug and loading cap being substantially the same, such that substantially the same compression ratio is achieved on both sides of the diaphragm. 2. The transducer of claim 1 in which the geometry of the loading cap substantially mirrors the surface/port geometry of the phasing plug about the contour of the diaphragm and is disposed at substantially the same distance from the diaphragm as the phasing plug. 3. The transducer of claim 2 in which the said loading cap geometry is of a concentric ring variety. 4. The transducer of claim 2 in which the said geometry is of a matrixed hole variety. 5. The transducer of claim 2 in which the said geometry is of a radial slot variety. 6. The transducer of claim 2 in which the said geometry is any combination of ring, matrixed hole and slot variety. 7. A transducer as hereinbefore described with reference to the accompanying drawings.

Inventors: GRUNBERG ROBERT MICHAEL, GRUNBERG, ROBERT, MICHAEL
Applicants: GRUNBERG ROBERT MICHAEL, GRUNBERG, ROBERT, MICHAEL
Application year: 1988
Main classification: H04R7
IPC codes: H04R1