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A surface micromachined microphone is presented with 230 kHz bandwidth. The structure uses a 2.25 μm thick, 315 μm radius polysilicon diaphragm suspended above an 11 μm gap to form a variable parallel-plate capacitance. The back cavity of the microphone consists of the 11 μm thick air volume immediately behind the moving diaphragm and also an extended lateral cavity with a radius of 504 μm. The dynamic frequency response of the sensor in response to electrostatic signals is presented using laser Doppler vibrometry and indicates a system compliance of 0.4 nm/Pa in the flat-band of the response. The sensor is configured for acoustic signal detection using a charge amplifier, and signal-to-noise ratio measurements and simulations are presented. A resolution of 0.80 mPa/√Hz (32 dB sound pressure level in a 1 Hz bin) is achieved in the flat-band portion of the response extending from 10 kHz to 230 kHz. The proposed sensor design is motivated by defense and intelligence gathering applications that require broadband, airborne signal detection.

MEMS acoustic sensors based on the operating principle of the Ormia ochracea fly provide a unique way to determine the direction of sound. A typical sensor consists of two wings coupled by a bridge and the entire mechanical structure attached to a substrate using two torsional legs. The sensor mechanical structure has two different vibration modes which can be coupled by placing their resonant frequencies close to each other. The acoustic wave impinging on the sensor induces displacements which are detected using comb finger capacitors at the edges of the wings. Previously, this has been accomplished using the universal capacitive readout MS3110, which is prone to static variations of the comb finger capacitors. In this work, a charge amplifier circuit was developed to independently measure the displacements of the wings under sound excitation. The ability to simultaneously read these displacements is needed to determine the bearing of sound using a single MEMS sensor, in contrast to previously reported works that needed either a calibrated microphone or a second MEMS sensor to determine the incident sound pressure level. Measurements showed that both the phase difference and amplitude difference of oscillating wings were strongly dependent on the incident angle of sound.

A team of seniors from the University of Rhode Island were tasked with analyzing the acoustic data and evaluating the data acquisition systems used in Pacific Northwest National Laboratories’ (PNNL) study of blast dredging in the Columbia River. Throughout the semester, the students learned about the unique acoustic signatures of confined underwater blasts and the necessary specifications of systems used to record them. PNNL used two data acquisition systems. One was a tourmaline underwater blast sensor system created by PCB Piezotronics. The second was a hydrophone system using a Teledyne TC 4040 hydrophone, a Dytran inline charge amplifier, and a signal conditioner built for the blast sensor system. The students concluded that the data from the blast sensor system was reliable because the system was built by the company for this specific application and there were calibration sheets showing the system worked properly. The hydrophone data was deemed unreliable because components were orientated in an unu...

A SONAR array’s radiation pattern is affected by the acoustic interactions, which may limit the effectiveness of beamforming algorithms when transmitting. A negative feedback system with a velocity sense signal fed back to the power amplifier can mitigate the array interactions proportional to the loop gain, and be effective across a broad frequency range without requiring a priori knowledge of the input signals. The tradeoffs between motional current velocity control and accelerometer based velocity control are summarized. Class D switching amplifiers can achieve greater than 90% efficiency and are increasingly being used to drive SONAR arrays. When a velocity control system is used with a Class D amplifier, feedback stability becomes a significant concern due to obtaining the feedback signal after the amplifier’s LC output filter. The array equations are still obtained by converting the amplifier into a Thevenin equivalent force and output impedance, which now includes the amplifier’s output filter and the synthesized impedance due to the velocity control loop. Sample beampatterns both with and without velocity control are shown, concluding that velocity control is well suited for use with complex dynamic transmit beamforming.

Tourmaline hydrophones are well suited for measurement of underwater blasts and other impulsive type sounds having extremely fast rise times and high peak pressures. The physical characteristics of these small, lightweight sensors, however, make them an attractive choice for other applications. To determine their performance at low‐ and midfrequencies, substitution calibrations of two tourmaline sensors with a calibrated laboratory hydrophone were completed using a J‐ll underwater sound transducer in a 100‐gal tank. The calibration signals consisted of tone bursts containing 5–20 cycles at frequencies ranging from 250 to 10 000 Hz and having sound pressure levels from 110 to 180 dB re 1 μa. Results indicate that a tourmaline hydrophone coupled to an in‐line charge amplifier and preamplifier to provide fixed voltage gains ranging from 100× to 10 000× has excellent performance over this range of frequencies. [Work supported by the National Oceanographic Partnership Program (NOPP) through ONR Award No. N0001...

An amplifier system receives an input signal and generating therefrom an amplified output signal. The amplifier system is recited as comprising an input stage and an amplifier stage. The input stage is configured to receive the input signal and provide a level-shifted signal that has an average signal level that is shifted in regards a level shift value. The amplifier stage is configured to receive the level-shifted input signal from the input stage and generate therefrom the amplified output signal. The level shift value being selected to minimize a DC offset in the amplified output signal at least when the amplifier system is initially powered on. Since the amplified output signal has a minimal or zero DC offset, the amplifier system avoids generation of undesirable noises when it is initially powered on.

A system, circuit, and method of canceling DC offset errors in cascaded amplifiers comprises arranging a plurality of any of analog voltage and analog current amplifier stages in any of cascaded and parallel configurations; operatively connecting a feedback comparator and digital logic in a feedback path around a given amplifier, wherein the digital logic comprises a finite state machine implementing an adaptive search algorithm comprising fixed switching and modulated switching; operatively connecting a switch at a differential input of the amplifier to short both input terminals of the amplifier; performing fixed switching on binary weighted elements generating discrete analog steps used to vary any of DC offset voltage and current at the input of the amplifier; and performing modulated switching on at least one lower least significant bit (LSB) of all bits used to vary the any of the DC offset voltage and current.

An acoustic sensor for monitoring machining processes in machining tools, e.g. grinding machines, includes a stationary unit, or stator (21) and a rotating unit, or rotor (23), the latter being coupled, for instance, to the grinding wheel and rotating with it. The rotor includes a support (45;45') and a piezoelectric transducer (33). The position of the transducer is defined and locked without using glues by means of mechanical elements (48, 53, 54; 63, 64; 74, 78) that may include elastic elements such as springs. The rotor also includes a printed circuit board (55), connected to the piezoelectric transducer, with a charge amplifier (37) for amplifying the output signal of the piezoelectric transducer. The rotor is electrically coupled to the stator by means of transformer type couplings (30, 31).

This paper presents the results obtained in an international laboratory comparison among NIST/USA, CENAM/Mexico and INMETRO/Brazil which was focused on the electric calibration of a charge amplifier B & K model 2626 from 50 Hz to 5 kHz. One of the main concerns of this comparison was to conclude all measurements within the period of 1 month, in order to minimize the influence of long‐term temporal instabilities. Each participating laboratory carried out the measurements in accordance with a calibration protocol, which was established to avoid the effect of additional sources of uncertainty. The results are compared with the ones obtained during the former comparison SIM.AUV.V‐K1 and the conclusions are presented.

A low‐profile large‐area underwater force sensor was desired to measure quasistatic forces on a rigid structure. The design for a PVDF sensor (a low‐profile piezoelectric) intended for use in measuring sustained (10‐s) impact forces with minimal error is presented. The final sensor design integrates a PVDF bimorph piezoelectric element and long time constant charge amplifier electronics into a 1‐in. diam by 3/8‐in. waterproof package. This work addresses design issues including electronics design to maximize the useful time constant, piezoelectric material selection, thermal design and simulation to minimize pyroelectric effects and noise minimization efforts. [This work was supported by NSWC‐CD and ONR.]

A multi-band equalizer for receiving and processing an audio signal includes a plurality of operational amplifiers, each for processing a different frequency band of the audio signal to produce an output signal. Each operational amplifier includes an inverting input, a non-inverting input, a resistor means coupled to the non-inverting input for receiving the audio signal and supplying it to the non-inverting input, an output, a feedback circuit coupled between the output and the inverting input for determining the upper limits of the frequency band processed by that particular operational amplifier, and an input circuit coupled between the inverting input and ground potential for determining the lower limits of the frequency band processed by that operational amplifier. An additional operational amplifier is provided for processing a higher frequency band of the audio signal than those processed by the plurality of operational amplifiers, for producing an output signal. The additional operational amplifier includes the same elements as the plurality of operational amplifiers except the feedback circuit which is replaced by a resistor. A summing circuit is coupled to the outputs of all of the operational amplifiers for combining the output signals thereof to produce a resultant signal. The summing circuit includes a plurality of variable resistors, each coupled to the output of a different operational amplifier for selectively varying the magnitude of the output signal from the respective operational amplifiers.

The invention provides an equalizing circuit for a loudspeaker system. The circuit includes an output amplifier connected to the circuit output, and a feedback amplifier connected to the circuit input. A plurality of filter elements are connected in series to the output of the feedback amplifier and the outputs of each of the filter elements are connected to inputs of both the output amplifier and the feedback amplifier, the amplifier inputs defining scaling factors for achieving a desired target response function for the loudspeaker system. The circuit generates a circuit transfer function having a numerator and a denominator. The numerator defines sufficient zeroes for cancelling all poles of the known transfer function of the loudspeaker system. As a result, when the circuit is connected in cascade to the loudspeaker system, the desired response function can be obtained.

The smart patch includes two piezoceramic actuators which operate on both sides of a structural member at the same fixed distance from the neutral axis to produce a bending method. The piezoceramic actuators are configured such that when subjected to a voltage, one shrinks and one expands. This provides the moment. One piezoceramic could also be used to provide half the moment provided that it was the same distance from the neutral axis of the structural member. There is a piezoceramic sensor that senses the vibrational mode to be suppressed. Finally there are control electronics that consist of a charge amplifier, a two pole low pass filter and a bridge amplifier. All of these elements are integrated into one entity.

Apparatus for powering a loudspeaker amplifier (5) in a telephone set fed with a line current (ILO) from a telephone line (3). An incomming speech signal (S) is amplified (8) and sent to a loudspeaker (4) via the amplifier (5), which is powered via its feed connections (19, 20) from a reservoir capacitor (6). By the capacitor feed there are avoided variations in the line current (ILO) on amplification of the speech signal (S). The capacitor is recharged with the aid of the speech circuit (13), which has an amplifier means (16, 17) sensing a line voltage (UO) via a voltage sensing circuit (18). The amplifier means (16, 17) steers the line current (ILO) to a constant value responsive to the line voltage (UO) by sending a current (16) to the line (3) via a resisitor (R1), thus keeping constant a current (14) through the resistor (R1), this current being sensed by the ampifier means. The reservoir capacitor (6) is connected via a controllable current generator (22) to the resistor (R1) and a circuit (24), which senses the voltage across the resistor (R1) and controls the generator (22). The reservoir capacitor (6) is charged by a current (15) through the current generator (22) and the amplifier means (16, 17) controls its output current (16) such that the current (14 = 15 + 16) through the resistor (R1) is kept constant. The apparatus utilizes the existing speech circuit (13) to keep the line current (ILO) constant at the power supply.

To compensate for the effect of external parameters on piezoelectric sensors connected to a charge amplifier, a small portion of this sensor is used to form the feedback capacitor of the amplifier. These parameters then have the same effect on the sensor and on the capacitor, thus making it possible to provide compensation to hydrophones made of piezoelectric polymer material.

A method of harmonizing the impedances of an audio amplifier and a loudspeaker interconnected therewith, the audio amplifier including a line amplifier and a positive feedback circuit for compensating for the counter electromotive force of the loudspeaker. A voice coil of the loudspeaker is mechanically driven for generating an electric signal at input terminals of the loudspeaker. Then, the amplification degree of the line amplifier or the positive feedback circuit is adjusted to reduce the signal voltage between the input terminals of the loudespeaker to a minimum, during the circulation of the generated signal through the positive feedback circuit and the line amplifier.

A negative-feedback circuit for use with a moving-coil loudspeaker and a host amplifier modifies the feedback signal from the output of the host amplifier in such a say that the velocity of the voice coil of the speaker more closely follows the signal voltage input to the host amplifier. The power output of the power amplifier is delivered to the voice coil through a low resistance, and a high resistance feedback path is provided from each side of this low resistance to the input of the host amplifier. Other reactive, resistive and non-linear elements may also be used with the high resistance feedback paths for improved operation.

Audio frequency converter apparatus, in particular for treating subjects suffering from audio-phonatory disorders, the apparatus being suitable for generating, on the basis of an input audio frequency signal, a parametric signal for application via an electro-acoustic transducer to a person being treated. Included in the apparatus are an input amplifier, a first threshold detector connected to the output from the input amplifier and subtracting a threshold voltage from the voltage supplied from the input amplifier, after rectification, a second threshold detector, likewise connected to the output from the input amplifier, and adding a voltage threshold to the signal from the input amplifier, after rectification, and having a time constant which is not less than the time constant to the first detector, a voltage controlled oscillator circuit connected to the output from the first threshold detector, and a (balanced) modulator whose two inputs are respectively connected to the outputs from the second threshold detector and from the oscillator circuit, and whose output provides the parametric signal.

A negative-feedback circuit for use with a moving-coil loudspeaker and a host amplifier modifies the feedback signal from the output of the host amplifier in such a way that the velocity of the voice coil of the speaker more closely follows the signal voltage input to the host amplifier. The power output of the power amplifier is delivered to the voice coil through a low resistance, and a high resistance feedback path is provided from each side of this low resistance to the input of the host amplifier. Other reactive and resistive elements may also be used with the high resistance feedback paths for improved operation.

An amplifier system comprising an input transducer, an amplifier and an output transducer connected in cascade, and an automatic gain control circuit comprising a variable resistor for taking off a voltage which varies with the signal from the input transducer and supplies said voltage to a rectifier and an RC filter. The filter output is connected to the base of a first transistor having a collector that directly drives the base of a second transistor which is connected to the input of the amplifier in such a way that it bypasses the input signal of said amplifier to a greater extent as the signal across the variable resistor increases.

The decrease in signal‐to‐noise ratio caused by electronic noise is an important factor in the design of an acoustic receiving system. Normally, this loss is dominated by the hydrophone, cable, and proamplifier which make up the “front‐end” of the system. In this paper, the signal‐to‐noise degradation factor (SND) for these components is defined. It is shown that the SND can be minimized by proper impedance matching of components. This can be accomplished either by use of a transformer or by the choice of a particular interconnection scheme for a multielement hydrophone. The minimum value of SND which can be achieved is determined by the product of figures of merit (FOM) of the hydrophone and preamplifier. For piezoelectric hydrophones, the FOM is a function of fundamental material parameters and the volume of the ceramic element. The amplifier FOM depends on the equivalent voltage and current noise sources of the device. Both charge amplifier and voltage amplifier configurations have been considered. The...

A differential charge amplifier design for a hydrophone pre-amplifier inces a calibration circuit for in situ measurement of the hydrophone/pre-amplifier gain. The hydrophone signal is input to a pair of amplifier stages which form a balanced differential input charge amplifier. The signal from the input charge amplifier is amplified by a second pre-amplifier stage and then passed to a balanced output cable driver. The calibration circuit is enabled by an external signal to inject a calibration signal into the input charge amplifier front end.

An electronic musical instrument wherein depression of a key entails the discharge of a primary capacitor which is connected to the inverting input of an operational amplifier whose output is connected with a second capacitor serving to supply envelope control voltage signals to an analog switch which connects a tone signal generator with a tone processing unit. The extent to which the primary capacitor discharges depends on the speed of movement of the key from non-depressed to depressed position, and the intensity of residual voltage of the partially discharged primary capacitor determines the intensity of the envelope control voltage signal. The speed at which the second capacitor discharges can be regulated to produce sustain or banjo effects. The second capacitor is in series with a resistor in a feedback conduit connecting the output and the input of the operational amplifier. A second feedback connection contains a diode which ensures that a reference potential is applied to the input of the operational amplifier while the primary capacitor is charged in non-depressed position of the key.

A moving-coil loudspeaker system incorporating motional feedback operable at high frequencies, i.e. above about 1000 Hz. The feedback signal is developed by a tiny piezo-electric accelerometer mounted together with a charge amplifier directly on the loudspeaker coil, in alignment with the turns of the coil. The coil comprises two layers of rectangular, anodized aluminum wire wound tightly to form an effectively integral mass. The system includes a suitable stabilizing frequency compensation network.

An electronic reverberation device comprising an input amplifier and an output amplifier connected in a circuit in which a delay device is connected between an output of the input amplifier and an input of the output amplifier, and a feedback path connects an output of the output amplifier to an input of the input amplifier. In order to render the operation of the device substantially frequency-independent, the delay line is a variable delay line the delay of which can be regulated by a regulator controlled by a control device which derives a control voltage from the output of the input amplifier. Preferably, the delay device is a bucket memory, the regulator a clock, and the control device comprises a diode having one terminal connected to the output of the input amplifier and another that is earth-connected through a capacitor and connected through a resistor to a control input of the clock.