Condenser Microphone Research Articles

Study of low frequency acoustic signals from superheated droplet detector

The bubble nucleation process in superheated droplet detector (SDD) is associated with the emission of an acoustic pulse that can be detected by an acoustic sensor. We have studied the neutron and gamma-ray induced nucleation events in a SDD with the active liquid R-12 (CCl2F2, b.p. −29.8°C) using a condenser microphone sensor. A comparative study in the low frequency region (∼0–10kHz) for the neutron and gamma-ray induced nucleation is presented here. From the analysis of the waveforms we observe a significant difference between the neutron and gamma-ray induced acoustic events.

Investigation of noise radiation from a swirl stabilized diffusion flame with an array of microphones

Next generation of combustors are expected to provide significant improvement on efficiency and reduced pollutants emission. In such combustors, the challenges of local flow, pressure, chemical composition and thermal signatures as well as their interactions will require detailed investigation for seeking optimum performance. Sensor networks with a large number of sensors will be employed in future smart combustors, which will allow one to obtain fast and comprehensive information on the various ongoing processes within the system. In this paper sensor networks with specific focus on an array of homogeneous microphones are used examine the spectral characteristics of combustion noise from a non-premixed combustor. A non-premixed double concentric swirl-flame burner was used. Noise spectra were determined experimentally for the non-premixed swirl flame at various fuel–air ratios using an array of homogeneous condenser microphones. Multiple microphones positioned at discrete locations around the turbulent diffusion flame, provided an understanding of the total sound power and their spectral characteristics. The growth and decay coefficients of total sound power were investigated at different test conditions. The signal coherence between different microphone pairs was also carried out to determine the acoustic behavior of a swirl stabilized turbulent diffusion flame. The localization of acoustic sources from the multiple microphones was examined using the noise spectra. The results revealed that integration of multiple sensors in combustors and reactors does allow one to obtain additional information which can be further tailored for the quest to develop advanced combustors.

Virtual prototyping of condenser microphones using the finite element method for detailed electric, mechanic, and acoustic characterization

Recent development and advances within numerical techniques and computers now enable the modeling, design, and optimization of many transducers using virtual prototypes. Here, we present such a virtual prototype of a Brüel & Kjær Type 4134 condenser microphone. The virtual prototype is implemented as a model using the finite element method with COMSOL Multiphysics and includes description of the electric, mechanic, and acoustic properties of the transducer. The acoustic description includes thermal and viscous losses explicitly solving the linearized continuity, Navier-Stokes, and energy equations. The mechanics of the diaphragm are modeled assuming a pre-stressed membrane, electrostatic attraction forces, and acoustic loads. The model includes electric description of the active and passive capacitances of the microphone cartridge as well as an external circuit model representing the preamplifier. Different modes of the system are studied, including the important first rocking mode of the membrane. The model has no free fitting parameters and results in the prediction of the frequency dependent sensitivity, capacitance, and mechanical impedance. The model results show good agreement with measured data.

Human echolocation system using a miniature dummy head

To promote understanding of sonar mechanisms in bats, we propose a novel tool that makes echolocation available for humans. In this method, ultrasonic echoes are captured by a miniature dummy head so that they can be converted to binaural audible sounds using time expansion.In order to examine the effectiveness of this technique, perceptual listening tests were conducted on human listeners with normal hearing.The sounds (white noise with frequencies between 5 and 90 kHz with 0.7-s duration, including 0.05-s rise/fall time) were recorded at a distance of 1 m from a loudspeaker using two condenser microphones that were placed in the ear canals of a 1/7 size miniature dummy head. The recorded ultrasounds were 1/7-times pitch converted, and then were presented to the listener through headphones. As a result, the listeners perceived correct directions of the pitch converted sounds, which were recorded using the miniature dummy head, although front-back error was occasionally observed. When the miniature dummy head was rotated during the recording, the listeners perceived the movement of the sounds as out-of-head sound localization. The miniature dummy head may provide humans with a tool to understand biosonar mechanisms.

Influence of classroom acoustics on the vocal behavior of teachers

Erroneous vocal behavior of teachers and their changes in the voice production due to poor acoustics in classrooms, can be investigated through recently developed voice-monitoring devices. These devices are portable analyzers that use a miniature contact microphone glued to the jugular notch in order to sense the skin acceleration level due to the vibration of the vocal folds. They estimate the Sound Pressure Level (SPL) at a certain distance from the speaker’s mouth, provided that a suitable calibration procedure is performed, the fundamental frequency and the time dose. Two different devices are compared in this work: the former is a commercial device, whose phonation sensor is a small accelerometer; the latter, recently developed by the authors, uses an electret condenser microphone to sense the skin acceleration level. SPL and fundamental frequency are estimated over 30 ms- and 50 ms-length frame and the results that refer to a sample of 40 primary school teachers and some university professors are analyzed. The length of the voice and pause frames is analyzed in order to detect the maximum of occurrence and accumulation in different conditions of noise and reverberation. A method for the detection and analysis of the emphatic speaking is also proposed.

A Hidden Markov Model based speaker identification system using mobile phone database of North Atlantic Treaty Organization words

This paper describes results of an experiment to conduct text independent speaker identification of large number of speakers (about 100) using a standard vocabulary of about 23 NATO words—such as Alfa, Bravo, etc. These words in isolation were spoken in a sound treated room by Hindi natives having very good education in English (both males and females) and recorded by a three channel data recording system—the cardioid microphone, electret condenser microphone, and a NOKIA mobile telephone. The pre-processed digitized database of isolated words was further processed to determine 39 MFCC's and their derivatives and used to build an HMM model for each speaker based on all the words. The HMM model was trained using an HTK tool kit to generate the model parameters and tested using Viterbi algorithm. The identification of speakers was done in a closed set manner, based on comparison of each NATO word in the model. In addition to correct identification, false acceptance and false rejection scores were also found. The results show varying performance due to variations in channels, male/female speakers. The overall identification scores vary between 60% and 70%. The paper gives detailed analysis of results.

Design Issues for a Portable Vocal Analyzer

The aim of this paper consists in identifying the main design specifications of a portable analyzer that allows the vocal signal of a person under monitoring to be acquired and stored. Suitable calibration procedures will be also defined for the microphone used in the portable device in order to ensure the traceability of the obtained measurements. The collected data will be used to identify voice disorders, to prevent an improper use of the voice, and to evaluate the vocal effort induced by noisy and/or reverberant environments. The vocal analyzer is based on a contact microphone worn by the person under monitoring and a portable microcontroller-based unit that hosts the conditioning circuitry, analog-to-digital converter, and storing memory. Two alternative sensors are investigated as contact microphones: an accelerometer and an electret condenser microphone. The former is expected to exhibit a negligible sensitivity to airborne effects, and the latter would provide a larger bandwidth, thus allowing parameters related to the vowel quality to be estimated. Particular attention will be paid toward traceability assurance and uncertainty estimation, and a calibration procedure for each of the estimated parameters will be proposed. In this paper, a data-acquisition-board-based setup has been used instead of a portable microcontroller unit in order to easily perform tests that allow the used microphones to be characterized and the calibration procedures for the parameters of interest to be assessed. Experimental results are reported that refer to the estimation of the parameters sound pressure level ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SPL</i> ) and fundamental frequency ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ).

Enhanced Photoacoustic Beam Profiling of Pulsed Lasers

An improved version of a photoacoustic beam profiler of pulsed lasers is presented. The new model resorts to high-bandwidth condenser microphones to register higher-order, excited acoustic modes, thus enabling more accurate profiling. In addition, Xe was used as a buffer gas since its high atomic weight further reduces the eigenfrequencies. Furthermore, a new gas-handling system makes up for some deficiencies found in the first model. The system was calibrated using the Airy pattern generated with a pinhole illuminated by a frequency-doubled Nd:YAG laser that excited $$\text{ NO }_{2}$$ traces. Once calibrated, the beam profile of a TEA $$\text{ CO }_{2}$$ laser was obtained, using ethylene as the absorbing species. This profiler returns more accurate profiles than thermal paper.

Heart Sound Monitor for Bio-Signal Learning

This research presents the design and development of the heart sound monitor for bio-signal learning which can be worked with a personal computer. The prototype will receive the heart sound via the condenser microphone built-in the stethoscope. The condenser microphone will be conversed the air pressure from heart beats to electrical signal that signal will transformed to computer via sound card. The sound card will be conversed the analog signal to digital signal for process by heart sound processing program developed by LabVIEW program. The signal will be analyzed with short time Fourier transforms in heart sound processing program by graphical user interface. The user is able to select a band pass of signal for filter and choose the power spectrum of heart sound for display. The output database from this prototype is necessary for Medical Education or Clinical Practice. The prototype was tested and it worked satisfactory.

Thermo-acoustic behavior of a swirl stabilized diffusion flame with heterogeneous sensors

Next generation combustors are expected to be significantly more efficient while reducing pollutants and eliminating carbon emissions. In such combustors, the challenges of local flow, pressure, chemical composition and thermal signatures as well as their interactions require understanding to seek for optimum performance of the system. The current practice of using a single sensor to measure certain parameters at a single location cannot provide sufficient information to achieve desirable and optimum overall performance of the combustor. A high density sensor network with a large number of sensors will be required in future smart combustors to obtain detailed information on the various ongoing processes within the system. As an initial step towards the development of such sensor networks, the effect of mean and fluctuating temperature distribution on the distribution of acoustic sources within the flame has been examined by using a thermocouple and condenser microphone using swirl stabilized diffusion flames. The measurement of high frequency temperature signal allowed observation of characteristic mean and fluctuating temperatures, and thermal stratification characteristics from within the flame. Specifically mean and fluctuating temperatures, integral and micro-thermal time scales have been determined at various spatial locations in the flame. Investigation of the thermal field and their effect on the localization of acoustic sources in the two flames formed at different equivalence ratios has been examined. The thermal characteristics data obtained provided a better insight on the thermal behavior of co-swirl diffusion flames. Noise spectra for varying air–fuel ratios were determined. Results of time average and fluctuating temperature and sound pressure level spectra showed noise emission in flames to lie near to the regions of high temperature which result in pressure fluctuations within the flame. The results are complemented with 3D CFD simulations that supported the localization of the acoustic sources within the turbulent diffusion flames.

Heart Sound Monitor for Biomedical Instrumentation

This research presents the design and development of the heart sound monitor for biomedical instrumentation which can be worked with a personal computer. The prototype will receive the heart sound via the condenser microphone built-in the stethoscope. The condenser microphone will be conversed the air pressure from heart beats to electrical signal that signal will transformed to computer via sound card. The sound card will be conversed the analog signal to digital signal for process by heart sound processing program developed by LabVIEW program. The signal will be analyzed with short time Fourier transforms in heart sound processing program by graphical user interface. The user is able to select a band pass of signal for filter and choose the frequency spectrum of heart sound for display. The output database from this prototype is necessary for Medical Education or Clinical Practice.

Ten-Micrometer-Thick Silicon Diaphragm Used in Condenser Microphone

Small condenser microphones with high performances are strongly demanded. This paper describes a silicon condenser microphone fabricated on silicon on insulator (SOI) substrate using only one photo mask with improved sensitivity performance. The improvements were due to reduction of the floating capacitance, thinning of the silicon diaphragm, and increasing the electrode area at the back plate of the microphone. This microphone consists of a diaphragm with the thickness of 10 μm and the diameter of 2 mm, a SiO2 insulative spacer (4-μm-thick buried oxide), and a 300-μm thick silicon back plate with the meshed structures having small (60 μm) hexagonal shaped acoustic holes. The gap between the 10-μm-thick silicon diaphragm and the back plate was 4 μm. This microphone was confirmed to function in the auditory frequency region with a sensitivity of –85 ~ -70 dB.

Design and Construction of Hearing Aid Device

Hearing aid device is a small electronic gadget that is fit in or behind the ear to improve one’s hearing and consequently communication ability.This research work involves the design and development of a hearing aid device withpre-amplifier; an acoustic signal picked-up using a condenser microphone. TDA 2822M IC is configured to produce an audio amplification which is converted to audio signal through a headphone. Design equations were employed to calculate the physical parameters of the circuit. After the design, the circuit was constructed and tested on 5 people with partial hearing problem. The result showed that there was a significant improvement in the hearing ability of all the patients tested. Recommendations were proposed for further improvement.

Fabrication and Evaluation of Electret Condensor Microphone with Film Lamination and Silica Spacers

The electret condenser microphones (FilmECMs) with film lamination and silica agglomerates were fabricated and the influence of the silica agglomerates on the characteristic of the FilmECM was investigated. The silica agglomerates was formed on PFA (perfluoroalkoxy) films by the application of colloidal silica using inkjet printing. The silica agglomerates for spacers formed the microgaps between the insulation films in the gap. Eventually, The silica agglomerates drastically enhanced the sensitivity of FilmECM while the resonance frequency was decreased. The size and the interval of the silica agglomerates influenced on the sensitivity and the resonance frequency. Such a influence can be explained by the density of the contact points of the films and silica agglomerates in the gap of FilmECM. Thereby the FilmECM which had the comparable sensitivity with commercial ECM was obtained. The resonant frequency was much higher than audible frequency (around 50 kHz). Hence, a wideband ECM could be obtained using FilmECM if the poor sealing of the gap and the diffraction of the sound is eliminated.

The 2010 Benjamin Franklin Medal in Electrical Engineering is Presented to Dr. rer. nat. Gerhard M. Sessler, Darmstadt University of Technology, Darmstadt, Germany and James E. West, The Johns Hopkins University Baltimore, Maryland, USA

For the invention and development of the electret microphone, a permanently biased condenser microphone which can operate in normal environments indefinitely without the need for high voltage supplies, thereby allowing it to be miniaturized.

Multi-time delay, multi-point linear stochastic estimation of a cavity shear layer velocity from wall-pressure measurements

In this paper, the recently introduced multi-time-delay linear stochastic estimation technique is thoroughly described, focusing on its fundamental properties and potentialities. In the multi-time-delay approach, the estimate of the temporal evolution of the velocity at a given location in the flow field is obtained from multiple past samples of the unconditional sources. The technique is applied to estimate the velocity in a cavity shear layer flow, based on wall-pressure measurements from multiple sensors. The cavity flow was investigated by performing simultaneous measurements of a single hot-wire probe, traversed on a fine grid in the shear layer, and of multiple wall-mounted condenser microphones in the cavity region. The paper compares classical high-order single-time-delay estimation approaches with the multi-time-delay technique, which is significantly more accurate as it produces a much lower mean-square estimation error, thus providing a faithful reconstruction of the time-evolution of the velocity field. This improved accuracy is strongly dependent on the number n of past wall-pressure samples used in the estimate. In this paper, we also demonstrate that the estimated velocity field only contains the signature of the relevant flow mechanisms which correlate well with the wall-pressure, while incoherent components are filtered out. The multi-time-delay approach successfully captures the spatio-temporal dynamics of the velocity fluctuation distribution in the shear layer, as it clearly resolves the dynamics of the relevant flow structures. The effect of the number of sensors used in the estimate was also considered. In general, it was evident that use of more sensors leads to better accuracy, but as the number n of past values increases, the gain becomes marginal.

Comparison of acoustic sensor systems for quality analysis of asparagus using scanning laser vibrometry for visualization

Automatic detection of hollow white asparagus spears (Asparagus officinalis L.) is a severe issue. One promising approach is the computerized acoustic resonance analysis. The principal components of newly developed acoustic sensor systems were investigated for their technical usability to stimulate and measure asparagus vibrations. Each tested measurement setup included a sample holder with an integrated sensor (piezo film or condenser microphone) and a mechanical excitation device (piezo actuator, midget solenoid, tiny hammer). Non-contacting scanning laser vibrometry was used as an appropriate tool to measure characteristic properties of the experimental setups. Without altering vibration conditions, this technology allowed to visualize vibration response of test specimens (asparagus spears and wooden rods) as well as to evaluate coupling properties between sample and sample holder and the sensitivities of sensors.Each setup enabled free vibration of samples investigated. In addition, reference sensors were proved to be placed at suitable positions for signal detection away from nodal points of oscillation. From the presented results, it turned out that, compared to piezo film sensors, specifically the chosen microphones have the better adapted frequency characteristic considering the vibration behavior of asparagus spears.The measured frequency characteristic of the piezo film sensors suggests that attention has to be focused on the coupling conditions between sample, sample holder and machinery.

The 2010 Benjamin Franklin Medal in Electrical Engineering is presented to Gerhard M. Sessler and James E. West for the invention and development of the electret microphone

For the invention and development of the electret microphone, a permanently biased condenser microphone which can operate in normal environments indefinitely without the need for high voltage supplies, thereby allowing it to be miniaturized.

A simple electronic stethoscope for recording and playback of heart sounds

undergraduate teaching of cardiovascular physiology often involves lectures on heart sounds. To complement these lectures, online resources are commonly used. The audio files of heart sounds are available as free downloads on the internet from websites of stethoscope manufacturers such as Thinklabs

Development of Unconstrained Heartbeat and Respiration Measurement System With Pneumatic Flow

The management of health through daily monitoring of heartbeat and respiration signals is of major importance for early diagnosis to prevent diseases of the respiratory and circulatory system. However, such daily health monitoring is possible only if the monitoring system is physically and psychologically noninvasive. In this paper, an unconstrained method of measuring heartbeat and respiration signals, by using a thermistor to measure the air flows from the air mattress to an air tube accompanying the subject's heartbeat and respiration, is proposed. The SN ratio with interference by opening and closing of a door as environmental noise was compared with that obtained by the conventional condenser microphone method. As a result, the SN ratios with the condenser microphone method were 26.6 ± 4.2 dB for heartbeat and 27.8 ± 3.0 dB for respiration, whereas with the proposed method they were 34.9 ± 3.1 dB and 42.1 ± 2.5 dB, respectively.