This invention relates to audio reproduction systems and in particular, to systems utilizing conventional speakers for the creation of sound waves which are reproductions of originally recorded audio signals.
Conventional electro-acoustic transducer systems for the reproduction of audio signals, generally referred to as loudspeakers, are designed to output a close approximation of the sound pressure waves generated by the original audio signal. Generally, the assumption is made during the design that the playback and amplification systems are linear with respect to the relationship between the input and output waveforms. A further assumption is generally made that the loudspeakers themselves exist in a perfect acoustic environment. In fact, they are tested in anechoic chambers during the design and production quality assurance processes. Some manufacturers provide for frequency spectrum control at the loudspeaker to compensate for the actual listening environment's effects on the sound waves after they have left the loudspeaker. Very small adjustments are feasible this way before distortion is objectionable to most listeners.
Most loudspeakers designers depend on the listener's application of audio frequency band equalization controls on the pre-amplifier to accommodate the large range of possible listening environments. This is less than desirable means of solving the problems of variable listening environments due to the side effects of conventional equalization (EQ) systems, leading to distortion of the signal, and the dynamic nature of many listening environments over time; as in a room that may have 1 or 25 persons in it, or re-arrangement of furniture.
Solutions have been proposed to the environment coupling problem previously described. The most commonly employed prior method has been to utilize a carefully placed microphone, near the "average" listening zone of a room, a signal generator or a test signal tape or disk, and a spectrum analyzer. Typically, the test tones were played back through the loudspeakers while the received spectral distribution of the signal was compared to the desired flat response curve. The difference identified was either automatically or manually applied to control the equalization settings of the audio reproduction system. Using this test procedure, environments that, for example absorb bass, could easily be detected and the proper amount of boost to such lower frequency regions of the audio spectrum could be applied.
More sophisticated methods have also been employed to measure the signal in the time domain. To correct for echoing environments, the departure and return times of a brief audio impulse are measured and appropriate compensation taken. The microphone typically would have to be moved many times to locate so-called hot spots, which can then be deadened with acoustically absorbent material. Acoustically "Dead" rooms are generally measured in the same way, with reflective materials properly placed to compensate for absorption. This process can be tedious, and generally requires a highly skilled acoustician. Even after repeated trials, some rooms remain acoustically problematical. Combining loudspeakers, environments and people has presented a hitherto unsolved problem.