Omnidirectional sound system

BACKGROUND OF THE INVENTION

The present invention relates to omnidirectional sound systems, and more particularly to a system wherein low ^4r>frequency sounds are rendered omnidirectional by mounting a low frequency speaker with its central axis vertically disposed, and high frequency sounds are rendered omnidirectional by horizontally rotating the high frequency speaker. 50

It is well known that an omnidirectional field of sound can be closely approximated by an arrangement wherein a low frequency speaker is mounted with its central axis extending vertically and a high frequency speaker is mounted at right angles thereto and constantly rotated 55 about the low frequency speaker axis at approximately 38 to 60 revolutions per minute. The rotation of the high frequency speaker causes its radiations to fan out and thereby enables the high frequencies to sweep every location in the sound field. Speed of rotation is important, ^f,nhowever, in order to assure that there is no deterioration of sound quality due to the rotation. Specifically, the rotating speaker is directed at an individual listener for a relatively short portion of each revolution. If the speaker is rotated too slowly, pulsating intensity variations are discernible, a phenomena known as “wowing.” At speeds of 38 revolutions per minute and above, this wowing disappears and the intensity as well as the frequency distribution of the sound field become uniform. The reason for this may be analogized to the animation effect perceived 70 by the human eye upon viewing motion pictures at rates above twelve frames per second; that is, the ear fills in, or interpolates the sound during the portion of each revolution during which the speaker is directed away from the listener.

It is also known that the omnidirectional qualities of the rotating speaker are improved by employing a baffle having a thin vertical slot therein over the face of the rotating speaker. This expedient minimizes the tendency of a high frequency speaker to focus the field of sound somewhere in front of the speaker. Specifically, the baffle slot causes diffusion of the acoustic energy transmitted by the speaker in the horizontal plane by acting, as a line source radiator of sounds having wave lengths equal to or greater than the width of the slot, the diffusion effect becoming small at frequencies having wavelengths less than the slot width.

While dual speaker systems of the type described have tended to improve the omnidirectional qualities of sound production, such systems, as they exist in the prior art, have a few disadvantages. For example, in an effort to conserve space, one known prior art system mounts a motor for driving the rotating speaker within the basket volume of the stationary speaker. A defect of this arrangement is the impairment of the natural sound field produced by the low frequency stationary speaker. This impairment is produced by acoustic reflection from the motor and its associated components. Another disadvantage of known systems of the type under discussion resides in their utilization of a complex electrically conductive bearing arrangement to permit connection of the audio signals to the rotating loudspeaker. Still another disadvantage of the known stationary-rotating speaker system relates to the shape of the slot in the baffle covering the face of the rotating speaker. Specifically, a substantially rectangular or oblong slot has been heretofore employed in an attempt to obtain a line source of sound from the rotating speaker, which source would then theoretically provide a uniform angular dispersion of the sound from the rotating speaker. However, a slot of this type, having a substantially uniform width throughout its entire length, does not take into consideration the differential in relative amplitude of various portions of a sound wave radiated from the speaker. More particularly, a conical speaker diaphragm achieves a greater lateral displacement at its dome than at the rom of the diaphragm, the amount of displacement for a given sound impulse falling off as a function of the distance from the dome. Hence, the amplitude of a sound impulse produced by the diaphragm varies in accordance with distance from the speaker axis. Where a line source of sound is required, the amplitude transmitted from such source should ideally be uniform throughout the length of the source; but a slot of uniform length does not compensate for the non-uniform amplitude of the sound wave produced by the diaphragm. In order to provide truly a uniform angular dispersion in the horizontal plane therefor, it is necessary to provide some compensation in this regard.

SUMMARY OF THE INVENTION

The present invention employs a stationary-rotating speaker system of the type wherein the low frequency speaker is mounted in an enclosure with its central axis disposed substantially vertically. And the high frequency speaker is rotatably mounted on a shaft extending along the central axis of the low frequency speaker, the central axis of the high frequency speaker being disposed in a horizontal plane. The shaft about which the high frequency rotates extends entirely through the low frequency speaker to a motor which is mounted on the rear of the low frequency speaker basket. The shaft drive mechanism is thereby removed from a location from which it can adversely affect the field of sound produced by the low fre

3,483,945 quency speaker. In addition, the baffle employed for the high frequency speaker includes a section which extends fully into the low frequency speaker basket. As the high frequency speaker rotates this section tends to impart an angular dispersion effect to the low frequency sound produced by low frequency speaker.

As a further feature of the present invention, the slot defined in the high frequency speaker baffle is of generally hour-glass configuration so as to more closely approximate a line source of sound in conjunction with the conical diaphragm of the high frequency speaker. More particularly, the widened portions of the hourglass slot are disposed in alignment with opposing rim portions of the speaker diaphragm; that is, the diaphragm area exposed by the slot is generally a function of the distance from the central axis of the speaker. In this way more of the lower amplitude portions of the speaker diaphragm is exposed and a closer approximation of a line source of sound is achieved.

In another feature of the present invention a drive shaft for the rotating speaker, which extends through the low frequency speaker, is divided into two mechanically coupled but electrically insulated sections. The electrically conductive metal sections of the shaft are then able to serve as independent electrical conductors to which the input terminals of the high frequency speaker can be connected for receipt of the audio input signal. In a preferred embodiment, an electrically conductive wall of the system enclosure and an electrically conductive bearing for one of the shaft sections comprise a portion of the signal path for the rotating speaker.

In still another feature of the present invention, an audio amplifier is provided in which an audio input signal is divided into high and low frequency channels. The bandwidths of the channels are adjustable so that the high frequency cutoff of the low frequency channel is at some lower frequency than the low frequency cutoff of the high frequency channel. The null band thus created between the channels has been found to result in noticeably better sound quality when used in conjunction with the speaker system described above than is present when the passbands overlap.

Other objects, fetaures, and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a view in perspective of a preferred embodiment of a speaker system constructed in accordance with the principles of the present invention wherein the speaker enclosure is partially cut away;

FIGURE 2 is a plan view in cross-section taken along lines 2—2 of FIGURE 1;

FIGURE 3 is a top view in section taken along lines 3—3 in FIGURE 2;

FIGURE 4 is a diagrammatic representation of the internal electrical connections for the speaker system of FIGURE 1;

FIGURE 5 is a side view in cross-section taken along the lines 5—5 in FIGURE 2;

FIGURE 6 is a view in perspective of the top portion of the enclosure of the speaker of FIGURE 1 as viewed from within the speaker;

FIGURE 7 is a detailed view in section taken along the lines 7—7 in FIGURE 2 and illustrating the manner in which the rotating speaker is mounted to the drive shaft;

FIGURE 8 is a view in perspective and partial section of the coupling between electrically insulated sections of the drive shaft; and

FIGURE 9 is a schematic diagram of the audio amplifier of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now specifically to FIGURES 1-5, there is illustrated a preferred embodiment of the dual speaker omnidirectional sound system of the present invention. The system is contained in an enclosure having a cylindrical wall 11 and a pair of oppositely disposed circular end walls 13 and 15. Circular wall 11 is a screen of electrically conductive metal such as copper or aluminum and end walls 13 and 15 are of solid electrically insulative material such as wood, plastic, or the like. The speaker system enclosure is divided into two distinct compartments by a circular baffle 17 having a centrally defined circular aperture defined therethrough. A cylindrical cardboard drum 19 is disposed concentrically within and flush against cylindrical screen 11 and is bounded at its ends by circular baffle 17 and bottom walls 15. A low frequency loudspeaker 21 has its basket rim mounted to the underside of circular baffle 17, the central axis of speaker 21 extending vertically in the orientation illustrated in the drawing. Speaker 21 is thereby arranged to direct acoustical energy through the circular aperture in baffle 17 in the general direction of top wall 13 of the system enclosure. To this end, the aperture in baffle 17 is made large enough to avoid so as to expose the entire cone of the speaker. A motor 23 is mounted on the rear of the speaker 21. A cylindrical metal shaft 25 arranged to be rotatably driven by motor 23 extends from the motor into the basket of speaker 21 and along the central axis of the latter. Shaft 25 terminates interiorly of the region defined between the cone of speaker 21 and baffle 17, there being a flexible cylindrical coupling member 27 secured to shaft 25 at its termination. Coupling member 27 may be made of plastic, rubber or other flexible electrically insulative material. Shaft 25 is snugly received through a centrally disposed circular aperture in one end wall of coupling member 27. The opposite end wall has a similar centrally disposed circular aperture for receiving in snug relation a second cylindrical shaft 29. Second cylindrical shaft 29 is of the same material as shaft 25 and is secured to coupling member 27 so as to rotate therewith. Shaft 29 is thus driven by shaft 25 via member 27 but the two shafts do not meet nor are they electrically connected. Shaft 29 extends generally coaxial with shaft 25 to end wall 13 where it is journaled in a bearing member 31 disposed in a suitable circular aperture in end wall 13. Bearing 31 is made of electrically conductive material and is generally conventional.

A high frequency speaker 33 has a baffle 35 disposed over its face and secured to its basket rim. Baffle 35 which may be made of wood, plastic, or a similar material, is secured at its top and bottom to shaft 29 by a respective pair of metal screws 37, 38 and lock nuts 39, 40 (as best illustrated in FIGURE 7). Screw 37 extends through shaft 29, through a piece of shock absorbent material disposed between baffle 35 and shaft 29, through the baffle 35, through an additional piece of shock absorbent material 43, through a washer 45, through a terminal lug 47 and is engaged by lock nut 39. Screw 37 thereby provides an electrical conducting path between shaft 29 and terminal lug 47, the latter being connected by means of electrical lead 49 to input terminal 51 of high frequency speaker 33. The other input terminal 53 of speaker 33 is connected by means of electrical lead 55, a terminal lug 57 and a screw (not illustrated) to shaft 25.

Baffle 35 is substantially rectangular in shape with a bottom edge E that extends toward speaker 21 and is contoured to slope in the same general direction as the contour of the cone in speaker 21. As speaker 33 and baffle 35 rotate, this bottom edge of the baffle serves to enhance the horizontal dispersion of the low frequency sound emitted from stationary speaker 21. Thus the slight tremolo or vibrato effect produced on the high frequency sounds by the rotation of high frequency speaker 33, is

3,483,945 also imparted to the low frequency signals produced by speaker 21.

An electrical receptacle 61 is mounted on cylindrical wall 11 and extends through an appropriately provided hole in both the cylindrical wall 11 and drum 19. Receptacle 61 mates with plug 62 of an audio amplifier 70, the latter providing the high and low frequency input signals for the respective speakers. Receptacle 61 receives all electrical input signals supplied to the speaker system and distributes these signals from respective terminal pins 64, 66, 68, 70 and 72, extending from the receptacle to the system interior. As best illustrated in FIGURE 4, a pair of electrical leads 63 extend from pins 64 and 66 to input terminals at motor 23 for supplying constant drive power to the motor. A system ground line 65 is connected between pin 68 and the frame of motor 23 so as to ground the motor frame and shaft 25. Lead 67 connects the grounded motor frame to input terminal 74 of speaker 21 thereby providing a ground reference at the speaker for the low frequency signal, the latter being supplied by lead 69 which connects pin 70 to input terminal 76 of speaker 21. The high frequency input signal is supplied at pin 72 and is connected to electrically conductive cylindrical wall 11 via lead 71. A further lead 73 is imbedded in a generally rectangular shaped channel 75 formed in the inner surface of end wall 13 and extending radially from bearing 31 to cylindrical wall 11. Lead 73 provides electrical contact between wall 11 and metal bearing 31, the latter as described above being in electrical contact with one of the input terminals of speaker 33 via the metal shaft 29, screw 37, terminal lug 47 and lead wire 49. Thus the high frequency input signal is connected to input terminal 51 of high frequency speaker 33 through a conductive path comprising the cylindrical wall 11 and shaft 29. Terminal 53 of high frequency speaker 33 is grounded by means of lead wire 55 which as described above, is connected directly to shaft 25.

Referring now specifically to FIGURE 9, there is illustrated a schematic diagram of audio amplifier 70. A pair of input jacks 78 and 80 receive the respective pair of input signals, one or both of which may be provided at any time. Input signal #1 is applied across a voltage divider 75 to the grid of triode amplifier 77 where it is amplified and coupled through an adjustable resistor 79 to the grid of amplifier-mixer triode 81. Input signal #2 is applied across a voltage divider 83 to the grid of triode amplifier 85 and where it is amplified and coupled through a variable resistor 87 to the grid of mixer amplifier tube 81. Amplified input signals #1 and #2 are thus mixed at amplifier-mixer 81 in proportions determined by the settings of adjustable resistors 79 and 87. The mixed signals are coupled to the grid of triode amplifier 85 through variable resistor 83, the latter serving as a gain adjustment. The amplified output signal from amplifier 85 is split into two distinct parallel paths, one of which comprises a low pass tone control filter 87 and the other of which comprises a high pass tone control filter 89. The filters are per se conventional in character. Low pass filter 87 comprises a resistor 91 connected in series with a variable resistor 93, the latter being shunted by a pair of series connected capacitors 97 and 99. The parallel combination of variable resistor 93 and series capacitors 97 and 99 is referenced to ground through a series resistor 95. The slider arm of variable resistor 93 is connected to one end of a further variable resistor 101, the other end of which is grounded and the slider arm of which is connected to the grid of tetrode amplifier 103. The function of the variable resistor 93 is to enable adjustment of the cut-off frequency for the low pass section of amplifier 70¹. Variable resistor 101 serves as a gain adjustment for amplifier 103. The amplified output signal from tetrode 103 is applied to the grid of phase-splitter triode 105. The plate of phase splitter triode 105 is AC coupled to the grid electrode of tetrode 107 which forms one half of a push-pull amplifier 110. The cathode electrode of phase splitter 105 is AC coupled to the grid of tetrode 109, which comprises the other half of push-pull amplifier 110. The cathode of tetrodes 107 and 109 are connected to ground and their plates are connected to opposite ends of primary coil 113 of transformer 115, the center tap of which is connected to a source of B+ voltage. Push-pull amplifier 110 operates in a conventional manner upon the oppositely phased input signals from phase splitter 105 to produce an output signal across secondary coil 117 of transformer 115, one end of which is grounded. The signal appearing across secondary coil 117 is the low frequency audio signal, which is applied via connector 62 to pin 70 of receptacle 61 of the speaker assembly described above.

The amplified output signal at the plate of amplifier 85 is AC coupled to high pass tone control filter 89 comprising the series combination of capacitor 119, variable resistor 121 and capacitor 123, the latter being connected to ground. The adjustable arm of variable resistor 121 is connected via variable resistor 125 to the grid electrode of tetrode amplifier 127. Variable resistor 121 permits adjustment of the low frequency cut-off point in the high pass section of amplifier 70, and variable resistor 125 provides a gain adjustment for tetrode amplifier 127. The amplified output signal at the plate of amplifier 127 is applied to the grid electrode of phase-splitter triode 129. The plate of phase splitter 129 is AC coupled to the grid electrode of tetrode 131 which comprises one-half of pushpull amplifier 130. The cathode of phase splitter 129 is AC coupled to the grid electrode of tetrode 133 which comprises the second half of push-pull amplifier 130. The cathodes of tetrodes 131 and 133 are grounded and their plates are connected to opposite ends of primary coil 135 of transformer 137, the center tap of which is connected to B-}-. Push-pull amplifier 130 amplifies the split phase signal received from phase splitter 129 to produce a signal across secondary coil 139 corresponding to the high frequency audio signal applied to speaker 33. Secondary coil 139 has one end thereof grounded, the opposite end being connected to an appropriate pin of connector 62 for ultimate connection to pin 72 of receptacle 61.

In addition to the above described components, amplifier 70 employs various biasing and coupling elements as required for the various tubes, such elements being entirely conventional and therefore need not be described in detail.

A primary important feature of amplifier 70 concerns the adjustability of the cut-off frequencies of low pass filter 87 and high pass filter 89 by means of respective variable resistors 93 and 121. Specifically, it has been found that when amplifier 70 is used in conjunction with the speaker system described above, a marked improvement in reproduced sound quality is achieved when a gap of adjustable width and position is provided in the overall pass band of the amplifier. More particularly, if the upper cut-off frequency of low pass filter 87 is adjusted to be below the lower cut-off frequency of high pass filter 89, the resultant field of sound produced by a low frequency stationary speaker 21 and high frequency rotating speaker 83 is more uniform.

It is found that when a rotating pattern of sound is provided in a reflecting room, resonances are removed largely but not entirely. The provision of a gap of adjustable width and position in the spectrum of the radiated sound, enables certain resonances to be eliminated by eliminating the frequencies involved. This is accomplished empirically in each public address installation. This is especially necessary where plural microphones are employed, variously located and which may be simultaneously on. The system simplifies amplifier design for the base and treble bands, reduces inter-modulation distortion between highs and lows, and provides separate volume control for the highs and lows, which enables optimum adjustment of the system for each installation.

While the present invention has been shown and de

3,483,945 scribed with reference to certain preferred embodiments thereof, it will be clear that various changes and modifications may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.