*U.S. Patent No. 3,554,313 , 01/12/1971 (app. 11/20/1968) [DjVu110KB]
LOUDSPEAKER SYSTEMS (class 181/31)
Young, Richard Owen
Abstract: Loudspeaker systems which include a loudspeaker assembly and a thin, slack, easily movable, essentially nonresilient backing member located behind the loudspeaker assembly. The movable member acts to avoid the effects on the cone of the loudspeaker assembly of compression and rarefaction of the air mass behind the assembly caused by rearwardly directed sound waves emanating therefrom, thereby to provide substantially unimpeded movement of the loudspeaker cone of the assembly.

* U.S. Patent No. 3,624,298, 11/30/1971 [ DjVu 256 KB ]
Sound Improving Means and Method
Davis, Arthur C.; Davis, Donald B.; Noble, James J.
Assignee: LTV Ling Altec, Inc.
Abstract: A method for eliminating undesired effects produced by reverberations from structure on the quality of sound produced by a sound system which is operated in association with the structure by smoothing the sound pressure-level versus frequency characteristics of the sound; and apparatus for practicing the method in the form of a filter network for use in combination with the sound system whereby electrical signals are attenuated by different amounts within selected ones of discrete bands of audiofrequencies to shape the sound pressure-level versus frequency characteristic of the sound.

*U.S. Patent No. 3,681,618 , 08/01/1972 [ DjVu 110 KB ]
RMS Circuits with Bipolar Logarithmic Converter
Blackmer, David E.
Abstract: A logarithmic converter having an operational amplifier input stage including a pair of diode junction feedback loops of opposite polarities. The converter includes a pair of fixed gain amplifiers connected to amplify the respective outputs of opposite polarities from the operational amplifier, and a corresponding pair of rectifiers for respectively half-wave rectifying the outputs of the fixed gain amplifiers. The outputs of the rectifiers are summed with one another. Lastly, the converter includes a source of constant current and a capacitor coupled to the source and to the rectifier outputs so that the voltage across the capacitor will tend to bring the long term average sum of the output currents from the rectifiers into equality with the current from the source.

*U.S. Patent No. 3,732,370 , 02/24/1971 [ DjVu 207 KB ]
Equalizer Using a Comb of Spectral Frequencies as the Test Signal
Sacks, Jack
Assignee: United Recording Electronic Industries (U.R.E.I.)
Abstract: A system and method is described for electronically equalizing the composite transfer function of a loud speaker system and room which receives the sound generated by the speaker system. A test signal source is broadcast into a room through the normal loud speaker amplifier system. a substantially flat microphone and preamplifier is used to detect the system and feed the detected audio signal through a bank of audio filters substantially covering the low, middle and high
range audio spectrum from approximately 30 cycles to 20 kilohertz (KHZ) by using approximately three narrow band peaking filters per octave. the power in the bendwidth of each filter is detected and measured against a known reference and the individual gain of each of the filters is adjusted to obtain a substantially flat acoustic response. The normal conventional input sound source, be it a tuner or tape deck, is fed through the adjusted bank of narrow band filters directly into the audio system for broadcast into the room in the conventional manner.

U.S. Patent No. 3,714,462, 01/30/1973: [ DjVu 82 KB ]
Multiplier Circuits
Blackmer, David E.
Abstract: A gain control or multiplier circuit in which an input operational amplifier has a pair of feedback paths through respective collector-emitter circuts of oposite conductivity type transistors to form a first bipolar circuit for converting an input signal to a log form by virtue of the log-linear transfer characteristics of the transistors. Each transistor of the first circuit has connected to it another transistor for converting the log signal to its antilog. A second operational amplifier is used as an output buffer for the resulting combined output signals from the antilog transistors. One version employs a bias circuit connected between the emitters of the first bipolar circuit transistors to adjust quiescent current. Another version uses a neutralization circuit to pump currents into the input summing junctions of both operational amplifiers to adjust for capacitive storage effests. In all cases, a control voltage is summed with the log signal by applying the voltage to the bases of the log and antilog converting transistors, thereby controlling the gain between the two operational amplifiers.

*U.S. Patent No. 3,755,749 , 08/28/1973 (app. 11/22/1971) [DjVu112KB]
Sound Reenforcement Equalization System
Van Ryswyk, Carl Wayne and Boner, Charles Paul (assignee White Instruments, Inc.)
Abstract: An active filter in combination with a sound reenforcement equalization system is provided. The filter permits variation of the notch frequency over a wide range of frequencies and adjustment of the notch depth from zero to any desired value independent of frequency control. Active filters may be cascaded without additional loss in the pass band.

*U.S. Patent No. 3,838,215 , 09/24/74: [ DjVu 108 KB ]
Speakers and crossover circuit
Haynes, Edward N. Jr.

Abstract: A basic woofer, tweeter and crossover circuit apparatus is provided for directing lower frequencies to the woofer and higher frequencies to the tweeter with good circuit impedance constancy and with good woofer and tweeter frequency response characteristics. In a modification of the basic circuit an additional circuit is added to boost lower tweeter frequency signals to the tweeter and to further attenuate mid-range tweeter frequency signals to the tweeter. Also provided is a woofer, mid-range tweeter, high-range tweeter and crossover circuit apparatus for directing lower frequencies to the woofer, mid-range frequencies to the mid-range tweeter and high range frequencies to the high-range tweeter with good circuit impedance constancy and with good woofer, mid-range tweeter and high-range tweeter frequency response characteristics.

U.S. Patent No. 4,088,835, 05/09/1975 [ DjVu 216 KB ]
Comprehensive feedback elimination system employing notch filter
Thurmond, George R and Heinz, Harro K Assignee: Rauland-Borg Corporation
Abstract: A feedback elimination apparatus for a sound system which employs a notch filter assembly comprising a plurality of active notch filters, each capable of producing a narrow notch in the frequency characteristic of the system. Means are provided for temporarily switching each notch filter to an oscillating mode to produce a reference signal at notch frequency which, for adjustment purposes, is zero beat with an acoustic feedback signal in the system, so the feedback signal is canceled when the notch filter is returned to its normal mode. A compressor is temporarily interposed in the sound system for limiting and controlling the level of the acoustic feedback signal to facilitate separation of acoustic feedback signals of different frequency and to prevent overload. An equalizer is also interposed in the system and which, in addition to performing its equalization function, is capable of producing a relatively wide notch in the frequency characteristic of the system for canceling the points of acoustical feedback grouped in frequency.