GB2184323A - Loudspeaker system - Google Patents

Abstract

A loudspeaker with omnidirectional radiation properties has a vertical enclosure (1) extending from a support, an electro-acoustic transducer having a diaphragm (2) closing an opening in one end of the enclosure distal from the support and a baffle (3) mounted beyond said one end opposite an outer surface of the diaphragm so as to define a continuous peripheral opening between the baffle and the enclosure which is less than one quarter of the shortest wavelength to be radiated. Preferably the baffle is profiled so that the distance between the diaphragm and the baffle is nowhere greater than one quarter of the shortest wavelength to be radiated, thus suppressing unwanted resonances in the opening. A second baffle can be mounted opposite a bass reflex opening (26) in the other end of the enclosure so as to define a second continuous peripheral opening. The transducer may be a low range unit, with a third baffle (15) mounted beyond baffle (3) to provide a third peripheral opening (24) from high range transducers (16) mounted opposite each other on the first (3) and third (15) baffles. <IMAGE>

Description

SPECIFICATION Loudspeaker system This invention relates to loudspeaker systems comprising drivers in the form of electro acoustic trans- ducerswithin an enclosure.

The function of a loudspeaker enclosure, apartfrom aesthetic considerations and the necessity to protect the drivers, isto couple the drivers acoustically to the surrounding environment, typically by providing acoustic separation between the two sides of a transducer diaphragm, and supplying a suitable air cavityto one side ofthatdiaphragm such asto provide acceptable electro-acousric efficiency in the lower portions of the audible frequency spectrum. The vibratory motion ofthe transducer diaphragm has the effect of generating pressure waves in the elastic medium surrounding it, i.e. the ambient air, and the effect of the enclosure is usually to confine this generating action essentially to the exposed or outer surface ofthe diaphragm.This necessarily imparts directional characteristics to the system.

The intensity ofthe pressure waves emitted by the diaphragm is always highest on the central axis ofthe diaphragm, and the polar response of such a transducerfalls off with increasing angular displacement from the central axis. It is well known and can be demonstrated mathematically that this fall off occurs more rapidly as the frequency radiated increases, and as a broad generalization, the largerthe diaphragm, the more concentrated its emission at any particular frequency will be towards the central axis. Since the dimensions ofthe diaphragm are fixed, the angle over which radiation intensity is reasonably well maintained varies in inverse proportion to the radiation frequency.

Within the conventionally accept-ed limits of the audio frequency spectrum, namely 20 Hz and 20KHz, the wave length of sound in air varies between 16.5 metres and 1.65 centimetres. This entails that any transducerwith a diaphragm of practical size will have a polar response ranging from practically spherical at the lower end ofthis rangeto narrowly directional at the upper end ofthe range.Since it is normal practice in practical loudspeakersystem to divide the range and assign itto two or more transducers, having typical diameters of 20 to 30 centimetresforthe lower end oftherangeand2.Scentimetresforthehigh end ofthe range, the effective radiation angle of a modern commercial system typically varies from 360" at Hz to about 60 at 20KHz. Effective stereophonic perception with standard 2 channel commercial systems requires a sufficiently wide radiation pattern across the whole audible range for effective generation of a stereophonic image, and it is usually pleaded that the radiation angles provided by conventional systems are sufficient for acceptable performance.However, the limited radiation angle at certain frequencies in fact requires that the listener be rather precisely located relative to the two systems, andthatthe acoustic environment is suitable, both of which requirements are difficult to meet in practic in domestic situations.

Published research has suggested that the nonuniform polar response of loudspeakers in conjunction with uncontrolled acoustic characteristics ofthe listening environment are a major source of deficien ciesinthetonal and image response of stereophonic systems.

It is believed that these problems are substantially alleviated if a loudspeaker system is provided with an essentially uniform omnidirectional radiation pattern.

Whilst a number of units have been marketed having allegedly omnidirectional radiation patterns, these either utilize arrays oftransducers aimed in different angular directions, or use reflective paddles to scatter the emission of a singletransducer. By and largethe polar response of such units has been by no means spherical, and has remained frequency dependent, and/orthe appearance of the units has been less than satisfactory.

It is an object ofthe present invention to provide a loudspeaker system which can provide a good approximation of a spherical radiation pattern, and also facilitate production of a unit having a satisfactory appearance.

Accordingly the present invention provides a loudspeaker system comprising an enclosure extending vertically into a room from a support, an electroacoustictransducerhaving a diaphragm closing an opening in one end ofthe enclosure distal from the support and a baffle mounted beyond said one end oppositean outersurfaceofthediaphragm soasto define a continuous peripheral opening between the baffle andthe enclosure which is less than one quarter ofthe shortest wavelength to be radiated. Preferably the baffle is profiled so that the distance between the diaphragm and the baffle is nowhere greater than one quarter ofthe shortest wavelength to be radiated, thus suppressing unwanted resonances transversely of the slot.

Preferably also a second baffle is mounted opposite a bass reflex opening in the other end ofthe enclosure so asto define a second continuous peripheral opening between the enclosure and the second baffle, and preferably also the first electro-acoustictransducer is excited only by frequencies in a lower part of the audio frequency spectrum and a third baffle is mounted beyond the first baffle so asto define a third continuous peripheral opening extending from furth erelectro-acoustictransducers mounted facing each other on the first and third baffles.

Preferably means are provided within the openings between the enclosure and the first baffle, and between the first and third baffles so as to trap unwanted resonances occasioned by the radial increase in cross section of openings with respectto the axes ofthe transducers failing to comply with an exponential law.

Furtherfeatures of the invention will become apparent from the following description with refer ence to the accompanying drawings, in which: Figure lisa diagrammatic cross section through a loudspeaker system, illustrating certain basicfeatures ofthe invention; Figure 2 is a plan view ofthe system shown in Figure 1; Figure 3 and 4 are fragmentarvviews corresponding to Figure 1 and illustrating certain characteristics of the system; Figure 5 is a furtherfragmentaryvertical section of a modified embodiment illustrating a furtherfeature of the invention; Figure 6 is a corresponding view showing a further developed embodiment; Figure 7 is a vertical cross section illustrating an assembly utilized forthe reproduction of high frequencies;; Figure 8 is a vertical cross section through a preferred embodiment ofthe invention; Figure 9 illustrates a basic arrangement which may be used to drive the high frequency electro-acoustic transducers shown in Figures 7 and 8; and Figure 10 illustratesthe equalizing network utilized to drive these transducers.

Referring to Figure 1, a vertically extending enclo sure1 stands on or is supported from the floor of a room in which the unit isto be used. It will normally be most convenientforthe unitto befloor standing, but it should be understood that any other arrangement aliowing omnidirectional radiation at an appropriate level would be possible. For example, the unit could be inverted and suspended from a ceiling. Mounted in an opening in the top ofthe enclosure is an electroacoustic driver unit ortransducer having a diaphragm 2. Spaced from the top ofthe enclosure is a baffle 3 which cooperates with the enclosure to define a continuous peripheral opening for the emission of sound waves4 as seen in Figures 1 and 2.

Referring to Figure 3, the width ofthe opening 5 is selected to be less than a quarter ofthe wave length of the highestfrequencyto be handled bythe driver, with the result that, overthe entire frequency range to be handled, the sound will radiate from the opening over a wide angle 6. Obviously there will be reduced radiation in a vertically upward or downward direction, but in practical listening situations this will not constitute a significant deviation from a spherical radiation pattern.

A problem with the arrangement shown in Figures 1 to 3 is that with a conventional conical diaphragm 2 there will be varying distances 7 and 8 between different portions ofthe cone 2 and the baffle 3, which are likely at certain frequencies to besufficientto constitute a quarterwave length or an integ ral number of quarterwave lengths of a frequency being handled, thus setting up unwanted resonances in the opening between the enclosure and the baffle.In order to overcome this problem, the baffle may be configured as shown in Figure 5, by providing a conical downwardlydirected plug which projects into the cone of the diaphragm so as to maintain the distance 10 between the plug and the diaphragm less than one quarter ofthe wave length of the highestfrequencyto be handled.

Although this arrangement avoidsthe generation of unwanted resonances due to the spacing of the diaphragm and the baffle, unwanted and deliterious variations in the acoustic impedance ofthe opening at different frequencies will still arise since the rate of radial increase in the cross section ofthe opening, referredto the central axis of the diaphragm, will still departconsiderablyfrom the exponential low desir able for a smooth response.

In order to overcome this problem, the arrangement shown in Figure 6 may be employed. The hollow plug 9 of Figure 5 is changed to a hollowfrustum 11 of a cone surrounding a solid inner cone 12 so as to form a quarterwavetrap 13 dimmensioned so asto absorb the unwanted resonances. The O orfigure of merit of the trap may be adjusted by introducing damping material within the cavity 13.

Figure 7 illustrates how omnidirectional radiation may be achieved from conventional middle and high frequency or high frequency driver units. Two horizontal baffles 15 are located opposite one another, each baffle supporting a driver unit 16 (see Figure 8) having a domed diaphragm 17. The diaphragm 17 are arranged co-axiallyfacing each other in close proximity, and are driven so that the diaphragms move in phase opposition. This opposite motion 21 has the effect of generating radial pressure waves 22, whilst the radial length ofthe opening 24 is large compared with the wave length of the sound being radiated exceptatthebottom ofthefrequency range.The residual anomaly at that point may be corrected by the provision of a quarterwavetrap 19 consisting of a pipe closed at one end and tuned to the appropriate frequency, its acoustic properties being adjusted using a quantity of damping material 20. The asymmetric placement of the trap as shown in Figure 7 is immaterial, since the offset is short compared with the operating wave length.

Figure 8 shows a complete enclosure incorporating the features already described above. In this instance, the interior5 of the enclosure 1 is provided with a bottom vent27 in ordertoform a base reflex enclosure, the vent 27 communicating with the ambientairthrough afurtheropening26formed by spacing the enclosure 1 from a further baffl e 28. Th e entire assembly is held together by a number, typically 4, ofthreaded rods (not shown) which tension the top baffle 15 to the base baffle 28, using suitable spacers for maintaining the proper openings betweenthevarious baffles and the enclosure.

As mentioned above,thetransducers 16should be driven so that their diaphragms move in phase opposition. Since the diaphragms are physicaily opposed, both transducers should be driven by the same electrical signal. and thiiiss is achieved by placing them in series. Since the operation ofthe transducers 16 in close proximity results in a three dB increase in efficiencyoverthat of a single unit, the series connection may be utilized whilst still maintaining the same efficiency. Resistor R1 is connected in series to bring the overall efficiency down to the level ofthatofthe low frequency unit. To quote a specific example, transducers 16 might be 6 Ohm units rated at 92 dB efficiency. The series connection results in 92 dB efficiency with a resulting impedance of 12 Ohm. If the efficiency of the low frequency unit is 87dub, resistor R1 may be 10 Ohm, the impedance of the system shown in Figure 9 thus being 22 Ohm.

One matter which requires to be taken into consideration in the design of a truly omnidirectional unit is thattransducer are normally designed to provide a flat frequency response on theircentral axis. Sincethe radiation pattern becomes more concentrated along the axis as the frequency increases, the actual overall power output ofthe transducer must decrease with frequency. Ifthe polar response is rendered substantiallyspherical as in the present invention, and conventional transducers are utilized, the power output in any particular direction will fall with frequen cy since the radiation atthe higherfrequencies is less concentrated.Typically, this fall off in power output as compared to the axial response of a conventional unit mayamountto about 15 dB at 20 KHz relative to the mid range response. One way of resolving this problem is illustrated in Figure 10, where a differential transformerTwith windings W1 and W2 running in opposite directions and presenting a ratio of 2:1 is provided, with the winding W1 in series with a circuit similarto that shown in Figure 9, and the winding W2, together with a compensation network is connected in parallel thereto.Assuming the effect of capacitors C1 and C2 to be negligible at middle frequencies, then if the current in winding W2 is twice that in winding W1, the net magnetization ofthe transformer core is zero; thusthewindingsW1 andW2presentnoinductance and provide no transformer action. In a specific example, if the combined impedance of R1, and transducers 16 and 17 is 22 Ohm,thenthevalue of resistor R2 should be Ohm.As thefrequency increases, the impedances ofthe capacitors C1 and C2 fall to levels which are low compared with those of the resistors R1 and R2, such thatatthetop ofthe audio frequency range, transformer action between the windingswillcausethevoltageattheoutputof winding W1 to beth reetimesthat at the input end, providing a top end boost of 9.5 dB. Atthe same time, the effect of R1 is reduced, providing a further 2.5 dB boost. By this means, compensation can be provided for the omnidirectional distribution of the high frequency radiation, whilst maintaining the impedance ofthe network as a whole at a satisfactory level, typically4Ohm at20 KHz.

Since the system described has an essentially box shape, and the only openings required are the three peripheral slots which will normally be parallel and horizontal, the appearance of the system can readily be made acceptable. The system need not of course have a rectangular plan as shown in Figure 2 and both round and other polygonal forms wouid be possible.

Claims (7)

CLAIMS:

1. A loudspeaker system comprising an enclosure extending vertically into a room from a support, an electro-acoustic transducer having a diaphragm closing an opening in one end of the enclosure distal from the support and a baffle mounted beyond said one end opposite an outersurface ofthe diaphragm so asto define a continuous peripheral opening between the baffle and the enclosure which is less than one quarter ofthe shortestwavelength to be radiated.

2. A loudspeaker system according to Claim 1, wherein the baffle is profiled sothatthe distance between the diaphragm and the baffle is nowhere greaterthan one quarter ofthe shortest wavelength to be radiated.

3. A loudspeaker system according to Claim 1 or2, wherein a second baffle is mounted opposite a bass reflex opening in the other end ofthe enclosure so as to define a second continuous peripheral opening between the enclosure and the second baffle.

4. A loudspeaker system according to Claim 1,2 or 3, wherein the first electro-acoustic transducer is excited only by frequencies in a lower part ofthe audio frequency spectrum and a third baffle is mounted beyondthefirstbaffle so asto defineathird continuous peripheral opening extending from furth erelectro-acoustictransducers mounted facing each other on the first and third baffles.

5. A loudspeaker system according to any of the preceding claims, wherein means are provided in the opening between the enclosure and the first baffle as to trap unwanted resonances occasioned by the radial increase in cross section of openings with respect to the axes ofthe transducers failing to comply with an exponential law.

6. A loudspeaker system according to Claim 4, wherein means are provided in the opening between the first and third baffles so as to trap unwanted resonances occasioned by the radial increase in cross section of openings with respect two the axes ofthe transducers failing to comply with an exponential law.

7. A loudspeaker system substantially as hereinbefore described with reference to, and as illustrated in, Figures 1 to 4, Figure 8, and Figures 1 to 4 as modified by Figure 5, Figure 6 and Figure 7 ofthe accompanying drawings.