The field of this invention relates to sound reproduction equipment and more particularly to the construction of a loudspeaker cabinet so as to achieve a superior reproduction of the sound substantially duplicating the source from whence the sound came.
In the 1920's, discoveries were made which allowed sound waves to be recorded and reproduced by electrical means. Before that time, the record player worked on simple mechanical principles. As the pick-up of the record player traced the record, the mechanical vibrations of the needle were coupled to a flared horn mounted to top of the instrument and this produced the sounds directly. As the theory of electricity became more closely allied to acoustics, the tiny mechanical movements of the needle on the record could be transformed into an electrical signal. This electrical signal could be amplified by an amplifier, but it could not be fed through an acoustic horn. Hence, some means had to be found of converting an electrical signal back into sound waves. This conversion was achieved by the invention of the loudspeaker.
The loudspeaker consists of a light circular diaphragm which is suspended from a metal frame by springy suspensions both around the edge of the diaphragm and near the center of the diaphragm. Mounted forward of the diaphragm is a cone whose function is to direct the produced sound waves from the diaphragm into the ambient. A voice-coil is mounted aft of the diaphragm. When the signal is applied to the voice-coil, a force is exerted and since the voice-coil is rigidly attached to the diaphragm, the diaphragm is caused to move proportionately. The movement of the diaphragm closely follows variations in the electrical signal and sets up sound waves in the air.
The audible frequency range extends from about twenty to twenty thousand cycles per second. It was not long after the first loudspeakers that certain problems arose when radiating such a wide range of frequencies. First, the sound from the rear surface of the diaphragm had to be isolated from that of the front of the diaphragm, otherwise the sounds leaving the two surfaces would cancel each other at low frequencies. Second, sounds are not radiated in all directions, but become concentrated in a narrow beam at high frequencies. This second problem is only more noticeable with large diametered diaphragms so, therefore, it is not a problem when dealing with typical stereo speaker systems which is designed to be utilized within the home. However, to improve the reproduction and efficiency at bass frequencies, a loudspeaker is mounted in a cabinet.
Usually each cabinet will contain more than one speaker. In order to achieve a wide range of sound reproduction, there is utilized a bass type of loudspeaker and a treble type. The base type is frequently referred to as a "woofer" which covers the low or bass range of sound. To cover the high range of sound, there is utilized a smaller in size loudspeaker which is frequently termed a "tweeter".
It has been common in the past to mount these speakers into a single cabinet with the single cabinet containing one or more woofers and one or more tweeters. In order to eliminate the sound cancelling which occurred by the sound produced from the rear of the loudspeakers, it has been common, in the past, to mount the speakers within a cabinet which totally encloses the rear portion of the speakers and also includes a sound deadening material. This type of loudspeaker system is less efficient in that it inherently will require more power from the amplifier because of the significant amount of the sound that is lost in the rear portion of the loudspeakers.
In other cabinet designs, the rear sound waves are arranged to aid those from the front portion of the cabinet after being delayed slightly within the cabinet. The amount of the delay is to be such that the rear portion of the radiations are in phase with the sound waves that are produced from the front portion of the loudspeaker. This "in phase" relationship magnifies the sound waves produced from the front portion of the loudspeaker rather than cancelling the sound wave.