Electrical audio signal processing systems and devices – Loudspeaker feedback
Patent
1993-03-29
1995-10-24
Kuntz, Curtis
Electrical audio signal processing systems and devices
Loudspeaker feedback
381 89, 381159, H04R 300
Patent
active
054616760
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to sound reproduction systems with electrodynamic loudspeakers and closed housings. More particularly, the invention relates to a sound reproduction system for improved bass reproduction.
2. Prior Art
Conventional loudspeaker systems have an inferior bass reproduction if the housings are small. In small housings air compression forces will build up and hinder the movement of the radiating loudspeaker's membrane. These forces evolve from volume changes in the air inside the housing which are caused by the movement of the loudspeaker's membrane. The membrane compresses or decompresses the air and the resulting forces hinder the movement of the membrane. Being elastic forces they also increase the resonance frequency of the system.
To achieve a satisfying bass reproduction large, impractical housings are used, or different kinds of resonant boxes are employed. Often the driving signals are corrected in their frequency characteristic, or the loudspeakers are controlled by servo systems. All these solutions cause distortions or are impractical to use, or show a poor pulse response.
Another known method (Tiefenbrun, U.S. Pat. No. 4,008,374) uses a second loudspeaker incorporated into the housing to simulate a larger volume. However this method just transfers the problems from the outer to the inner loudspeaker. To achieve satisfying results large housings must be used once again. Additionally, problems arise from distortions caused by phase differences between the movements of the membranes'.
Price Shelton's invention (Goodman, appl. GB.821 5906) follows Tiefenbrun's principle of using an inner transducer to simulate a larger inner volume. In addition Shelton places a pressure sensor into the inner chamber of the housing to measure pressure changes. The signal produced by the sensor is amplified by an operational amplifier and drives the inner transducer. Optionally a feedback circuit can be inserted into the signal path between the sensor and the amplifier.
Sheltons disclosure fails to teach how the system should really work: In particular just conveying the signal produced by the sensor to the operational amplifier will result in oscillation of the system and distortions generated by the system. The function of the optional feedback circuit is not clearly defined either.
SUMMARY OF THE INVENTION
The inventions as defined by the claims improve the bass reproduction of loudspeaker systems with small housings and with large loudspeaker membranes. Neither a direct correction of the driving signals is used in the invented systems nor is a servo system for the radiating loudspeaker employed.
The above-mentioned results are achieved by the systems characterized by the claims. The invented systems are unique because of the fact that differences between the air pressure inside the housing and the time-averaged mean pressure outside the housing are almost eliminated by the use of a closed loop control system. The differences are measured by pressure sensors and the corresponding electrical signals are conveyed to a controller. The control system practically eliminates the differences. This reduction of pressure differences is achieved by the movement of the membrane of an electromechanical transducer inside the housing. The membrane adjoins the concerned air volume inside the housing. The transducer is incorporated into a closed loop control system. A controller receives the electrical signals produced by the pressure sensors. It calculates corresponding output signals, which are amplified by a power amplifier and which then drive the transducer. The signals are calculated in a way that the membrane of the transducer is forced to perform movements which eliminate the pressure differences.
For a fuller understanding of the nature of the invention, reference should be made to the following detailed description of the preferred embodiments of the invention, considered together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
REFERENCES:
patent: 3867996 (1975-02-01), Lou
patent: 4008374 (1977-02-01), Tiefenbrun
Kuntz Curtis
Lee Ping W.
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