Sound Radiator

Abstract

A series of at least three loud speakers is set one in each of at least three walls forming alternately oppositely opening dihedral angles with each other. The center-to-center distance between the loud speakers is less than 10 times the wavelength of the upper limiting frequency of the sound frequency band. The loud speakers are excited by the same signal source, and a stronger interference field is produced than if the sound emanated from a planar or spherical surface.

Description

The object of the invention is a sound radiating element and a sound radiator built up of such elements, in which the subjective sensation of the radiated sound is a better one than that of prior sound radiators.

Experts dealing with problems of electroacoustical transmission are well aware of the fact that no high sound power can be obtained from a loudspeaker consisting of only one single sound radiator of small dimensions. By sound radiator it is meant a loudspeaker (s) built in a box, horn, baffle etc. Further it is well known that the radiation properties, i.e., the characteristics of the sound field, the frequency response curve and the directivity pattern (1) (It is to be noted that the parenthetical numerals throughout this text relate to the bibliographical references at the end thereof.) of sound radiators consisting of a number of radiating elements are influenced by the arrangement of the construction. It is to be mentioned that in some cases even one of the sound radiating elements can be used as a sound radiator itself. By such arrangements nonhomogenity sound field, i.e., a sound field with strong interference is energized in the neighborhood of the sound radiator. Its extension, its nonhomogenity depends on the utilized radiators, their dimensions and their arrangement relative to each other (2). Investigating the different solutions already realized, a correlation can be observed between the nonhomogenity of the nearfield and the directivity pattern of the radiator. Theoretically the directivity pattern of the surface radiator becomes narrow at high frequencies. By this the sound pressure is diminished deviating from the axis of the radiator. In FIG. 1, 1 indicates the radiator from side-view, 2 the vector leading to the observer's point in the far-field, .alpha.(.degree.) is the angle between the vector of the observer's point and that of the axis of the radiator. FIG. 2 shows the idealized frequency response curve p= p(f) in the axis (.alpha.= 0.degree.) and in a certain angle .alpha. 0.degree. off the axis the directivity pattern gradually diminishes at high frequencies. These theoretical considerations are well proved by practice. It has been the practice to design convex radiators and concave ones too. In FIG. 3, 3 indicates a convex and 4 a concave sound radiator while 2 here also marks the vector of the observer's point in the far-field. In such radiators the former effect increases and even the frequency response belonging to the axis (.alpha. =0.degree.) is irregular. The corresponding frequency response curves p= p(f) of the usual aforementioned arrangements as a function of .alpha. (.degree.) are shown in FIGS. 4 and 5. In practice in both arrangements further irregularities occur. On convex surfaces between 2 and 5 kHz. a characteristic dip can be observed. On concave surfaces due to the focusing effect in the range of low and middle frequencies where the wavelength, the distance of the focus and that of the measuring microphone are about the same-- a maximum of sound pressure occurs. At high frequencies due to the diminishing of the wavelength a diminishing of the focal length can be observed. Hence, the observer or the microphone-- as in the case of convex surfaces-- is surrounded by a sound field in which the frequency response, as a function of frequency, is falling even in the axis, i.e., at .alpha. = 0.degree.. The fulfillment of the abovementioned considerations may be well observed even in line radiators that can be considered to be a limiting case of surface radiators.

Spherical radiators are to be mentioned particularly as being the most general forms of convex radiators. The classical type of this kind of radiators, the so called Kosters' radiator (3) was followed by a number of constructions--used in the industry extensively-- among which the patent descriptions (4), (5) and (6) are to be mentioned as examples. By these radiators a weak interference field is produced in free field, as the elemental radiation has a divergent character. In a living room, i.e., in the neighborhood of reflecting surfaces the interference of the sound field is increased by the reflections from the walls (7), (8), (9). Thus, the subjective sensation of the "sound image" produced by the sound field is enlarged, the origin of the sound cannot be determined exactly and the effect of the room surrounding the sound source is to be felt considerably.

Although these systems represented a great step forward as compared to prior systems having one single loudspeaker only-- which improvement is due to the increased nonhomogenity of the produced sound field-- for further progress the requirements set in connection with the radiator and the subjective experience have to be revised. In the course of revision efforts are to be directed toward finding which are the desirable characteristics of a sound radiator reproducing sound with remarkable fidelity and the why of it.

In sound radiators the frequency response and the directivity pattern within the transmitted band ought to be smooth and as independent of the frequency as possible, or these conditions should be fulfilled at least within a broad band (10), (11). This is an especially important requirement when radiating perspective sound (12). Further to the abovementioned conditions, according to our practice, it is also very important that the frequency responses measured in free field and in a living room should be as equal as possible. In this case namely almost the same sound pressure-- and therefore the same subjective sensation-- can be obtained in small and in middle size rooms i.e., the reproduced sound image will be independent of the room. This statement can be explained by the equality of the radiated power when the aforementioned conditions are fulfilled.

There emerges, however, the question under what conditions a transmitted band can be considered to be a smooth one.

The program signal to be reproduced-- be it speech, music, natural or another sort of noise-- in fact, never is a vibration with a line spectrum i.e., a sinusoidal vibration, but always a signal with practically finite bandwidth. The natural sounds, namely, be they speech sounds or musical ones have needs a beginning and an end, i.e., they are of finite duration. Applying Fourier's theorem it is obvious that the ear almost never is excited by pure sinusoidal sounds. Furthermore, in the case of finite duration the spectrum of sounds that could be characterized at infinite duration with a single spectrum line will widen to the bandwidth .DELTA. f according to the equation .DELTA. f=2/.DELTA. t (13). Here .DELTA. t means the duration of the signal, .DELTA. f its bandwidth the value of which, in the case of t= 50 ms for example, is 40 Hz. The investigations of Winckel (14) show that the phonems of speech and the swift passages of music comprise signals with a duration of about 50 ms. Hence, the system is in fact mostly energized not by a pure sound but by sounds having spectra of considerable bandwidth with a configuration varying with time.

It is well known that the intensities of components within the critical band of hearing (15), at an excitation longer than 10 ms are added by the ear unmindful of the components frequencies within the critical band of hearing (16). The irregularities within the critical band, however, cannot be observed, which fact is proved by the following experience. Let us consider that in a direct hearing, i.e., free of electroacoustical chain, the acoustical surrounding of the listener is an interference sound field, due to reflections on the boundary surfaces and diffraction on objects including the human body (17). The nonhomogenity of this interference field-- although considerable-- according to practice cannot be observed, not even in an idealized surrounding, free of reflections, in the so-called anechoic chamber. If namely a natural sound source e.g., a speaking person is listened to in an anechoic chamber and the speaking person i.e., the sound source slightly turns away from the listener, the ear of the listener-- due to the changing of the geometrical arrangement-- is in a different sound field, which fact can be well measured physically, yet the turning of the sound source cannot be observed (18). This phenomenon is obviously due to the fact, that the signals of speech-- as it is well known-- are signals of finite bandwidth. A similar experience can be made when hearing natural sounds, or music, or noise. In a natural surrounding having reflecting surfaces this irregularity will occur even stronger. Let us mention for example the well known highly irregular character of the frequency response curve-- measured with the help of sinusoidal signals-- of sound radiators operating in a room having reverberation (19).

Similar conclusions can be drawn-- among others-- from the results of Flohrer (20): according to his experience a dip having a relative bandwidth .DELTA.f/f.ltoreq. 0.1 in the frequency response curve cannot be observed.

It follows from the aforementioned that there is no practical reason for taking into consideration the irregularities of the frequency response within the critical band or the narrow dips having an interference character of the directivity pattern. On the other hand the larger dips and peaks of the frequency response are worth noting, as, when reaching the critical bandwidth, their presence-- even if they are of interference origin-- may be heard and is of disturbing character (21).

Out of the usual specification for sound radiators let us consider the shape of the frequency response curve within the transmission band. Rather often an irregularity of .+-.5 db. can be observed here. According to Shorter's experience (11) irregularities of .+-.2 db. may be observed in some ranges of the frequency response curve.

He observed that for example the quality of the sound-- due to the irregularities of a frequency response curve of some db., having dips or peaks broader than the critical band-- becomes colorless, hard and metallic, and that a broad dip at frequencies of 2-5 Hz. provides the sensation of the sound source being far off while at the same place a small size peak gives the impression of the sound source being rather near the observer.

The requirements set in the aforementioned from the subjective side lead to very strict requirements in the technological realization of sound radiators, all the more so, if it is taken into consideration that during monitoring further irregularities are caused in the sound field surrounding the listener.

In the design of the prior sound radiators, on the basis of the aforementioned theoretical considerations, efforts had been directed toward reducing the interference field in order to possibly avoid nonhomogenities. As already mentioned in connection with FIG. 1-5 and when describing the different types of sound radiators, these efforts were only partly crowned by success and even the best loudspeakers have dips and peaks which can be measured objectively, and which can be observed by the listener subjectively. At the same time the task to eliminate the always existing nonhomogenity of the interference field seems to be rather hopeless.

When realizing the object of the invention the basic consideration is not to eliminate the interference field but to create a rather strong one in which dips and peaks occur so frequently that the subjective sensation of their existence is no more possible. Thus, in spite of the fact that these said irregularities of the sound field can be measured objectively, the observer has the subjective impression of being in a homogenous field. Hence, the object of this invention is a sound radiator producing a soundfield the interference nonhomogenity of which is rather strong. If namely the irregularities of the sound field-- which is changing in space and time-- actually are falling within the critical bands of hearing, these irregularities cannot be observed any more and the subjective sound impression will be a good one. For this purpose-- on the basis of the aforementioned-- such a strong interference field has to be produced at which the condition .DELTA.f/f.ltoreq. 0.1 is satisfied. In this case even the disturbing factors of the room or the surroundings are compensated by the strong inhomogenity of the field, i.e., the electroacoustical transmission becomes almost independent of the surroundings. For better comparison between the subjective sensation and the parameters, measured objectively, it is advantageous to carry out measurements by help of signals having statistical properties like program signals have instead of the common sinusoidal signals. This condition is fairly satisfied by a noise of one third octave bandwidth.

Obviously the abovementioned requirements can only be met with by a compound sound radiating element comprising a plurality of loudspeakers, respectively a sound radiator built up of such elements.

In this invention the strong interference sound field of the element comprising a plurality of loudspeakers (from now: sound radiating element) which is a constituent part of the sound radiator, is produced immediately at the aperture of the sound radiating element. By aperture we means that real or fictive boundary surface of the sound radiating element through which the produced sound field is immediately passed on to the medium outside the sound radiator. In most of the possible arrangements the aperture in the solid angle of the main radiation of the loudspeakers is at one or more points in contact with the wall elements carrying the individual loudspeakers. On the surface of the aperture, being in the nearfield of the sound radiating element, the amplitude and the phase of the excitation of the air particles is rapidly changing from point to point. By this strong nonuniform phase distribution produced along the surface of the aperture as a function of place it is made possible for the sound radiating element to show superdirective properties (22), i.e., to prevent the actual narrowing of the main lobe of the directivity pattern at increasing frequency.

Utilizing this sound radiating element arranged according to the invention as a building block, sound radiators can be composed variable in a wide range with a view toward the actual acoustical task. The sound radiating element may be composed of a plurality or at least three single unit and/or multiple unit loudspeakers arranged in such a manner that the planar surfaces through the radiating surfaces of the loudspeakers have an angle different from 180.degree. and their intersecting lines are parallel, while in the planar surface perpendicular to these intersecting lines the intersecting line is a broken one, so that the radiating (main) axes of the individual neighboring loudspeakers, respectively their projection on the aforementioned perpendicular planar surface are intersecting each other before and behind the broken line alternately, and the radiating centers of the neighboring loudspeakers are at a distance from one another less than three times the wavelength of a sound wave belonging to the upper limiting frequency of the transmitted frequency band.

The upper limiting frequency-- according to the draft recommendation of the International Electrotechnical Commission (IEC)-- is that frequency at which the frequency response of the loudspeaker, measured on the reference axis, has decreased a stated amount (normally 10 db.) below the mean response averaged over a bandwidth of one octave in the region of maximum sensitivity. Sharp peaks and troughs in the response curve narrower than 1/8 octave shall be neglected for both the upper and the lower limits (23). If over an even number of planar surfaces identical loudspeakers are arranged symmetrically, a radiation with symmetrical directional effect will be obtained, if, however, an odd number of planar surfaces is utilized a special directional effect will result, i.e., the main lobe of the directivity pattern can be tilted. This tilting effect of the sound radiating element utilizing a plurality of loudspeakers is rather weak and is increasing with diminishing the number of loudspeakers building up the sound radiating element and its most expressed form is obtained at an element comprising three loudspeakers.

In fact, the loudspeakers may be arranged asymmetrically too, they can have different dimensions and different forms by which further desirable effects can be attained.

Moreover, to the planar surface over the radiating opening of a loudspeaker-- as to aperture plane-- more than one loudspeaker may be assigned.

The object of the invention shall be described in detail by way of example with preferred embodiments using the following drawings:

FIG. 6 gives a diagrammatic view of an element arranged according to the invention comprising five loudspeakers and suitable for the explanation of the concept of the invention.

FIG. 7 is the perspective view of an elemental one-line sound radiating element comprising three components.

FIG. 8 illustrates the perspective view of a one-line sound radiating element consisting of four components.

FIG. 9 shows the perspective view of a one-line sound radiating element comprising five components.

FIG. 10 is the perspective view of a sound radiator comprising two times four components. At this embodiment the same planar surface can be laid over the opening surfaces of the loudspeakers placed two over two other ones.

FIG. 11 shows the perspective view of a sound radiator comprising two times three components. In this embodiment the same planar surface can be laid over the opening surfaces of the loudspeakers placed two over two other ones.

FIG. 12 also illustrates the perspective view of a sound radiator comprising two times three components with the difference as compared to the embodiment shown in FIG. 11 that the components of the sound radiating element are turned away from one another.

FIG. 13 as embodiment is similar to the embodiment shown in FIG. 12 with the difference that the sound radiator is built up of two times four sound radiating elements.

FIG. 14 as embodiment showing a high power sound radiator system. The individual loudspeakers are special surface or line radiators which may also be used independently, the directional effect of which is-- at the arrangement according to the invention-- modified according to the advantages to be mentioned later.

FIG. 15 illustrates the perspective view of a sound radiator in which more than one loudspeakers belong to the planar surface-- as aperture surface-- through the individual loudspeakers of the sound radiating element. In this case and with these components wide band transmission is made possible by a multichannel system with the ability of radiating independently. For reasons of construction the two components in the middle of the embodiment shown in the FIG. are individual widerange loudspeakers each.

The sound radiating element to be seen in FIG. 6 is built up of five either single or multiple loudspeakers 5 which are arranged so that the planar surfaces 6 over the radiating surfaces have an angle 7 different from 180.degree. and the intersecting lines 8 of the said surfaces 6 are parallel to each other while the intersecting line 10 between the planar surface 9 perpendicular to the parallel intersecting lines 8 and the planar surfaces 6 is a broken one, the loudspeakers 5 are placed so that the main radiating (symmetry) axis 11 of the neighboring loudspeakers 5 respectively their projection 12 on the planar surface 9 intersect each other alternately before the broken line 10 in the intersecting point 13, respectively behind the broken line 10 in the intersecting point 14, and the radiating centers 15 of the neighboring loudspeakers 5 are at a distance from each other shorter than 10 times the wavelength of the sound wave belonging to the upper limiting frequency of the already defined sound frequency band.

FIGS. 7-15 show different embodiments the building up of which-- in consideration of FIG. 6-- needs no further explanation than already given for the said figures. Suffice it to say, for FIG. 7 as for the others of FIGS. 8-15, that the loudspeakers 5 are set in flat front panels 6a that meet perpendicular upper and low flat panels 9a and 9b in zigzag lines 10.

The sound radiators are excited by the same signal source. By the sound radiating element, respectively sound radiator arranged according to the invention a stronger interference field is produced than by prior sound radiators having a planar or a spherical surface. Its special advantage is that as compared to other systems-- for example the 1/8 spherical radiator mentioned in reference (9)-- its frequency response does not comprise any dip between 2,000 and 5,000 Hz. and the directivity pattern becomes more independent of frequency. For comparison, FIG. 16 shows frequency response curves under similar measuring conditions and using similar types of parallel connected loudspeakers, eight pieces each. In FIG. 16 curve 16 relates to the sound radiator arranged according to the invention shown in FIG. 10, while 17 is the frequency response curve of the 1/8 spherical sound radiator described in reference (9), both measured with a noise of one third octave bandwidth. Both measured sound radiators were built in boxes of equal width so that two loudspeakers were placed under each other subsequently. The boxes were damped inside with cotton wool. The angle between the loudspeakers was in the arrangement according to the invention .beta.=145.degree. --where .beta. is the angle 7 in FIG. 6-- and .beta.=135.degree. in the prior arrangement. The width of the box was 0.6 m. in both cases. The volume of the room used for measurement was 125 m..sup.3 having a reverberation time of 0.45 .+-. 0.05 s between 100 to 10,000 Hz. A Bruel and Kjaer measuring microphone Type 4135 was used. It was verified by the comparing measurements of the chosen two different sound radiators that taking similar loudspeakers and similar dimensions the arrangement according to the invention shows considerable advantages as compared to the prior arrangement imitating a spherical radiator. The same was verified also by the directivity patterns measured in free field. This time the sound radiators already measured in a living room were measured afresh-- for comparison-- in a field free of reflections. The measuring microphone was placed in a distance of 2 m. from the sound radiators. The exciting signal was a noise with a bandwidth of one third octave. The directivity patterns obtained with the arrangement according to the invention with a midfrequency of the noise of 1, 2, 4, 8, 16 Hz. are shown in FIG. 17 a, b, c, d, e. In FIG. 18 a, b, c, d, e the directivity patterns of the prior arrangement are to be seen. In connection with FIGS. 17 the directivity patterns gradually become steady as a function of frequency and the connecting superdirectivity may be well observed in the arrangement according to the invention.

A symmetrical directivity pattern results from the sound radiating element built up according to the invention symmetrically and utilizing an even number of loudspeakers which may be considered to be equal. If, however, an asymmetrical arrangement or an odd number of loudspeakers it utilized, or an even number but differing in sensitivity, the main lobe of the directivity pattern will tilt the direction of the more numerous loudspeakers or in that of the loudspeakers of a higher sensitivity. Applying the arrangement according to the invention-- as already mentioned-- a sound radiating element with a tilted main lobe and with the appropriate combination of such elements a sound radiator having the same properties may be built up.

Measurements were carried out with the embodiment shown in FIG. 11 and the obtained directivity patterns, to be seen in FIGS. 19 a. . .e clearly show the tilting of the main lobe by which a special effect can be obtained. The measuring conditions and the used mid-frequencies-- here too-- were equal to those mentioned in connection with FIGS. 17 and 18.

If the sound radiating elements are built up so that the angles between the elements are different from 180.degree., the symmetry of the directivity pattern, respectively its asymmetry may be produced even on planar surfaces perpendicular to each other.

In order to increase the nonhomogenity of the interference field, i.e., to influence the directivity pattern of the sound radiating element as a function of frequency it is suitable to place in the area in front of the loudspeakers obstacles the dimensions of which are comparable to the wavelength. By the diffraction occuring along the surfaces of such obstacles the phase variation at the aperture of the sound radiating element is further increased. If the dimensions of the obstacle placed in the area in front of the loudspeakers are half the wavelength of the sound belonging to the upper limiting frequency of the transmitted sound frequency band, the described effect will follow. For verifying this statement FIGS. 21 a, b, c, d and e show the directivity patterns obtained when measuring the arrangement according to FIGS. 20a and 20b, and using cylinders 18 of wood built in the sound radiator 19 comprising loudspeakers 5. When comparing FIGS. 17 and 21 it is well to be seen that after building in the obstacles the superdirective character of the directivity pattern increased more and more and-- due to the diffraction of the sound waves at the cylinders-- the directivity pattern widens from the axis. The character of the effect is also depending-- as a function of frequency-- on the wave impedance of the built in obstacle as compared to that of the air.

FIGS. 22, 23 and 24 show free field frequency responses. For the measurements the same geometrical arrangement and the same noise signal of one third octave bandwidth were applied as for the measurements of the directivity patterns. In FIG. 22 the results of the measurements with the arrangement according to the invention, shown in FIG. 10, are to be seen, while in FIG. 23 those of the prior spherical radiator. FIG. 24 shows the measuring results of the arrangement according to FIG. 10, embodied according to FIG. 10. When comparing FIGS. 16 and 22 it is obvious that the arrangement according to the invention provides the properties that the frequency response curves in the free field and in the living room are equal, that the directivity patterns are independent of frequency and even have a superdirective character. Comparing the measured results with the subjective experiences, dealt with in this description, and with the resulting technological task it is obvious, that by the use of the sound radiating elements arranged according to the invention a good subjective effect can be obtained which at the same time-- by objective parameters - can be firmly kept in hand technologically.

It is apparent that those skilled in the art may make numerous departures from the specific arrangement described herein without departing from the inventive concepts.

Claims

What we claim is:

1. A sound radiator comprising at least three loudspeakers excited by the same signal source and each having a flat front panel perpendicular to the radiating axis of the speaker, the panels being on a common level and meeting each other at angles substantially different from 180.degree. along lines that are parallel to each other, a flat panel perpendicular to and below and common to all said front panels, said front panels meeting said perpendicular panel in a zigzag line, said axes of adjacent speakers intersecting each other alternately before and behind said zigzag line, adjacent speakers having radiating centers that are at a distance from each other less than 10 times the wave length of the upper limiting frequency of the speakers.

2. A sound radiator as claimed in claim 1, and a second flat panel perpendicular to and above and common to all said front panels and parallel to the first-mentioned perpendicular panel, said front panels meeting said second perpendicular panel in a zigzag line congruent to the first-mentioned zigzag line.