1. Field of the Invention
The present invention generally relates to a sound reproduction system having enhanced directional control at low frequencies and, more particularly, to a sound reproduction system characterized by reduced off-axis sound levels.
2. Description of Related Art
Sound is a physical disturbance in the medium through which it propagates. For example, in air, sound consists of localized variations in pressure above and below normal atmospheric pressure. Accordingly, the vast majority of sound reproduction systems are comprised of electromagnetic transducers whereby an electrical signal is transformed into a mechanical vibration which, in turn, is transformed into an acoustic signal. Loudspeakers commonly included in such a sound reproduction system are typically comprised of a diaphragm, most commonly, a cone, a support system in which the cone diaphragm is mounted and a drive coil which vibrates the cone diaphragm in a desired fashion to produce sound waves.
While sound reproduction systems have been the subject of numerous innovations over the years, pattern control of sound projection within an auditorium or other listening area has remained a problem. For example, it would be desirable if sound levels produced by a sound reproduction system installed in an auditorium could be the same for all listeners, regardless of their location in the auditorium. One obstacle to achieving such a system is that, in general, the distance separating listeners from the sound reproduction system tends to vary dramatically based upon the listener's position within the auditorium. In many auditoriums, the distance from the sound reproduction system to the nearest listener is about 25-35 feet, the distance to a listener located in an extreme side seat is about 50-70 feet while the distance to the most distant listener is about 80-120 feet. Thus, as sound levels tend to drop-off as distance from the sound reproduction system is increased, sound level distribution within the auditorium seating tends to be uneven.
Another obstacle to achieving such a sound reproduction system is that sound reproduction systems tend to lack sufficient pattern control, particularly when generating low frequency sounds. By the term "low frequency", it is intended to refer to frequencies where the wavelength .delta. of the sound wave is large compared to the diameter of the cone of the loudspeaker(s) generating the sound wave, i.e., typically frequencies lower than 1500 Hz. At such low frequencies, when multiple low frequency devices are used, the directional characteristics, i.e., the amount of drop-off in sound levels, of sound waves tend to vary dramatically based upon the off-axis angle of the listener. Furthermore, the directional characteristics at any given off-axis angle tend to shift based upon the particular frequency of the sound wave. Thus, the amount of compensation required to compensate for off-angle drop-offs in sound levels will tend to vary based upon the frequency of the sound being generated.
Referring now to FIG. 1, a conventional sound reproduction system 10 characterized by poor pattern control at low frequencies will now be described in greater detail. The sound reproduction system 10 includes a plurality of loudspeakers, each having a cone diaphragm, mounted in a support structure 14. For example, the sound reproduction system 10 may include first, second, third and fourth loudspeakers 12a, 12b, 12c and 12d. The loudspeakers 12a-d are arranged in a vertical array, i.e., the general center of each loudspeaker 12a-d is located along a vertical axis and spaced a selected distance from a central axis "A" which is generally orthogonal to the vertical axis and extends outwardly from the front side surface of the support structure 14.
A single amplifier 16 is connected to the electrical input side of each one of the loudspeakers 12a-d. As is conventional in the art, a signal generator (not shown) transmits an electrical input to the amplifier 16 which, in turn, provides an amplified electrical signal to each of the loudspeakers 12a-d where the amplified electrical signal is converted into an acoustic signal and propagated into an auditorium or other listening area.
Referring next to FIG. 1B, a polar response plot which illustrates directional characteristics of sound generated by the sound reproduction system 10 may now be seen. More specifically, line 18 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at a frequency of 110 Hz, line 20 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at 130 Hz, line 22 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at 155 Hz, line 24 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at 220 Hz, line 26 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at 261 Hz and line 28 illustrates the directional characteristics of sound generated by the sound reproduction system 10 at 311 Hz.
As illustrated in FIG. 1B, while the amount of sound drop-off which occurs on-axis, i.e., along the axis "A" is negligible, for very low frequencies, significant drop-offs in sound levels can occur only 30 degrees off-axis. At higher frequencies, however, off-axis drop-off is of less concern. For example, at f=110 Hz, the sound level will go 20 dB down at 30 degrees off-axis while, at f=311 Hz, the sound level will only go down about 6 dB. Furthermore, at frequencies below 185 Hz, the drop-off begins to lobe, thereby producing a very irregular drop-off pattern characterized by very dramatic drop-offs at specific angle for the various frequencies. For example, a very sudden drop-off occurs at 90 degrees off-angle for f=185 Hz.
Referring next to FIG. 1C, a level plot which illustrates the drop-off in sound level at particular angles will now be described in greater detail. More specifically, line 32 illustrates that, for frequencies under f=300 Hz, negligible drop-off occurs on-axis. In contrast, however, line 34 illustrates that, for 45 degrees off-angle, a 30 dB drop-off will occur at about 275 Hz while line 36 illustrates that, for 90 degrees off-angle, the 30 dB drop-off will occur at about 195 Hz.
Prior attempts at improving pattern control for low frequency sounds have been directed towards the design of complicated sound reproduction systems intended to achieve even sound levels by selectively adding or canceling. Often, such systems include a sophisticated computer processing system to selectively adjust sound levels generated by various loudspeakers included in the sound reproduction system. Typically, such sound reproduction systems are custom built for a particular auditorium and have remained quite expensive to design and build.
It can be readily seen from the foregoing that it would be desirable to provide a simple, low cost sound reproduction system which achieves improved pattern control at low frequencies. It is, therefore, the object of this invention to provide such a sound reproduction system.