Condenser Microphone Research Articles

Round-robin tests of the different types of sensors for floor impact sounds and vibrations

Recently, floor impact noise monitoring system was introduced for residents to control making noises spontaneously using sound and vibration sensors. The present study aims to investigate the performance and characteristics of various current using sound and vibration sensors which can be eventually compatible with the floor impact noise monitoring system. For measurements of sound sensors, 7 sound microphones were used including 2 condenser types, 4 dynamic types, and 1 electro condenser type. Also, 6 vibration meters were used including 2 MEMS types, 2 Piezo types, and 2 spring types. Sound levels were measured in a semi-anechoic chamber using white noise in the frequency range from 63 Hz to 12.5 kHz. Vibrations were measured using high density wooden panel with an impact ball in the frequency range from 16 Hz to 4 KHz. Round-robin tests were done to confirm the exact performances of all the sensors under same condition. As a result, it was found that dynamic type microphones show the most similar response characterization in comparison with the reference condenser microphone especially in the air-borne sound frequency range from 250 Hz to 2 KHz. Also, it was found that spring type vibration meters show the most appropriate characterization at 63 Hz, most critical structure-borne sound frequency. Thus, the results drawn from the round-robin tests could be used for floor impact noise monitoring systems with the benefit of both economy and performance.

Sensitivity of an optical feedback interferometer for acoustic waves measurements.

This work presents a sensitivity study on the use of an optical feedback interferometer to measure acoustic pressure from plane waves. The sensitivity is established by linearising the interferometer's governing equations. It is shown to be independent of the acoustic wave frequency but dependent on configuration parameters such as the optical feedback parameter or the length of the laser through which the acoustic wave passes. Experimental validation is carried out using three acoustic waveguides in the 0.5-18 kHz range. The sensitivity obtained enables broadband acoustic pressure measure with a low mean relative error in comparison with a reference condenser microphone.

Electronic Auscultation System: a Method for Measuring the Amplitude-Frequency Characteristics of Stethoscopes

The purpose of research is increasing the diversity of designs of electronic auscultation systems with measured characteristics and testing. A series of articles includes the development of a model of an electronic auscultation system, the development of an electronic stethoscope design, the manufacture of an experimental sample, the development of a method for measuring the amplitude-frequency characteristics of electronic and classical stethoscopes, testing the proposed models and methods, and analyzing auscultatory data. The article proposes a method for measuring the frequency response of classical and electronic stethoscopes. The proposed method was tested. The frequency response of classical and electronic stethoscopes was measured. The design of the head of the electronic stethoscope has been improved, the frequency response of the original and improved designs has been compared. Methods. The studies were based on methods for measuring the frequency characteristics of electroacoustic transducers proposed in interstate standards. On the basis of methods in interstate standards, a simplified method for measuring the amplitude-frequency characteristics of electronic stethoscopes has been developed. The following equipment was used in the study: Type 4227 "Artificial Mouth" emitter from Bruel & Kjaer; measuring condenser microphone "Artificial ear" Type 4146 with a 2CC reference chamber from Bruel & Kjaer; Type 2118 analyzer from Bruel & Kjaer. Results. The amplitude-frequency characteristics of classical and electronic stethoscopes are obtained. The design of the electronic stethoscope head has been improved, the amplitude-frequency characteristics of the original and improved designs have been compared. Conclusion. A method for measuring the parameters of the frequency characteristics of the sound pressure level of classical and electronic stethoscopes is proposed. The method is as close as possible to the standardized methods for monitoring acoustic parameters, while being sufficiently simple and cheap compared to the pressure measurement method, demonstrating the qualitative similarity of the measurement results.

Temperature-robust optical microphone with a compact grating interferometric module.

The high demand for advanced acoustic sensors has prompted optical microphones to become a current research hotspot; this is especially the case in light of the performance of existing electroacoustic microphones having reached the ceiling. In this work, a thermally stable optical microphone has been developed for sensitive detection of low-frequency acoustic signals. The microphone was prepared using a prestressed nickel diaphragm and a compact grating interferometric module. The adjacent surfaces of the diaphragm and grating form a short Fabry-Perot cavity, which makes the microphone robust to ambient temperature fluctuation due to the reduced thermal drift of its operating point relative to the quadrature point of the interferometer. The cavity length-operating wavelength relationship of the microphone operating at the quadrature point was obtained. The performance of the prepared microphone was tested using various methods. Experimental results show that the microphone enables stable operation at the quadrature point over a wide range of temperatures from 0°C to 60°C with low signal distortion and high sensitivity. The response of the prepared optical microphone to low-frequency drone noise was measured and compared with that obtained with a commercial electret condenser microphone.

Feasibility of Using a Mobile Device to Identify the Axle Whine Noise in a Vehicle

This article investigates the feasibility of using the built-in microphones of mobile devices to diagnose axle whine noise in a vehicle. Three sound pressure measuring sensors or devices are selected. The first one is a high-end condenser microphone, which is calibrated during the measurements; this data is taken as a reference for the other devices. Then two different types of mobile devices are selected and relative calibrations are carried out. Measurements are first performed inside a hemi-anechoic room; artificially generated sinusoidal and random sound signals are played through a calibrated sound source and data is acquired with all devices. This setup is placed inside a vehicle cabin (for a non-running engine) and measurements are repeated. The onboard entertainment system of the vehicle is used as well in addition to the calibrated sound source. Measurements reveal that one of the mobile devices has superior performance when compared to the other one though calibration improves the performance of the latter. Finally, measurements are performed inside the vehicle cabin while the vehicle is being driven along a certain path to assess the diagnostic performance. The axle whine noise problem in two vehicles is successfully detected by employing an empirical mode decomposition process.

Parylene as an electret for condenser microphones

Electrets in condenser microphones are typically made from varieties of Teflon. This work evaluates different formulations and thicknesses of vapor deposited Parylene electrets as potential alternatives to Teflon. Parylene-coated condenser backplates used in this study were obtained from two different suppliers, as a third variable in addition to formulation and thickness. Long-term charge stability of the electrets was tested with a thermally stimulated discharge (TSD) method. Charge stability was also evaluated by building the charged backplates into condenser microphones and running them through environmental accelerated-life testing. Standard PTFE and FEP Teflon backplates were used as controls in these tests. The experiments showed clear differences in the long-term charge stability of the Parylene electrets due to thickness, formulation, and supplier. Several combinations of thickness, formulation, and supplier were found to perform comparably to traditional Teflon electrets and appear suitable for use in high-end condenser microphones.

Microphones: A bit of history

The earliest commercially successful microphone design was the carbon microphone, whose original patent was filed by Emile Berliner in 1877. An interesting variant of the basic carbon microphone was a dual diaphragm microphone patented by Granville Woods in 1883. The basic carbon microphone provided adequate performance at low cost and remained in service in the telephone system for at least the next century. Other microphone technologies can also be found in the early patent literature, though their commercial acceptance was delayed until electronic amplification was possible. The eventual development of vacuum tube amplifiers enabled the development of the condenser microphone, whose initial patent was filed in 1916. Condenser microphones provide the improvement of wide bandwidth and flat response compared to carbon microphones. The invention in the 1960s of high quality electret materials enabled the manufacture of low cost electret microphones that quickly displaced carbon mics in telephones and most commercial devices for at least the next 40 years. Then, in the early years of the 21st century, condenser microphones implemented in silicon, also known as MEMS microphones, took over in high all volume applications. This paper will discuss this history and the current state of the art.

Interferometric measurements of laser-induced shockwaves in air: Variable laser energy

When focused to a small spot size in air, a sufficiently energetic laser pulse initiates a rapidly expanding plasma. After a delay, a shockwave detaches from the plasma boundary and propagates. General features of the shockwaves can be deduced from condenser microphone measurements. However, the minimum range is limited by damage thresholds, and the presence of the microphone introduces a number of measurement artifacts. Distortion of the signal is caused by diffraction around the sensor, and the limited bandwidth does not allow rise times to be correctly quantified. In contrast, optical interferometry is a nonintrusive diagnostic for quantifying shockwave characteristics. In this study, a Nd:YAG laser is focused through a converging lens in order to generate laser-induced shockwaves. By using a variable attenuator, four laser energy outputs are examined: 25, 50, 75, and 100% of the maximum energy transmission. Heterodyne Mach–Zehnder interferometer measurements are made from 10 mm to 200 mm from the focal point of the lens. Virtual velocity signals, proportional to the time derivative of optical phase differences, are used to estimate density and pressure time histories, along with peak pressure as a function of distance.

Miniaturized nonresonant photoacoustic gas analyzer for CO2 detection

AbstractIn this letter, a portable trace carbon dioxide (CO2) measurement instrument based on photoacoustic spectroscopy (PAS) was proposed and designed. The nonresonance PA cell (PAC) including a cooper tube with 251.2 μl was designed. Two condenser microphones were used to measure the PA signal simultaneously. The center wavelength of the distributed feedback laser was chosen to 2004 nm. Within 1 s integral time, the detection limit for CO2 reached 12.7 ppm. The normalized noise‐equivalent absorption coefficient was calculated to be 4.8468 × 10−9 cm−1W/Hz1/2. With Allan‐Werle deviation analysis, the measurement detection limit for CO2 improved to 4 ppm with 30 s integration time. As a result, the trace CO2 instrument based on PAS shows a good performance in long‐term detection.

Evaluation of the results of treatment of patients with functional dysphonia using a cepstral test

In order to evaluate the effectiveness of the treatment in patients with functional dysphonia, the Cepstral Peak Prominence (CPP) test was used. Twenty dysphonic women aged from 18 to 47 years were under observation. The control group consisted of 20 healthy women of close age. Patients underwent 5-7 sessions electrostimulation of laryngeal muscles and phonopedic treatment, after which a complete restoration of the voice was noted. The Praat clinical program was used, installed on a Hewlett-Packard 630 laptop (Pentium B960, 2.2 GHz). A SHURE SM94 condenser microphone was used as well. In the control group, the results were as follows: M=7.49 (SD=1.26) dB. In the main group before treatment: M=5.00 (SD=1.07) dB, after treatment: M=7.95 (SD=1.34) dB. Differences in KT values in the main group before and after treatment (5.00 dB and 7.95 dB, respectively) were significant at p<0.0001. Differences in KT values in the main group before treatment (5.00 dB) and in the control group (7.49 dB) were significant at p<0.0001. Differences in KT values in the main group after treatment (7.95 dB) and in the control group (7.49 dB) were not significant at p>0.05. The study showed high sensitivity of the method. The CPP data after treatment were higher than those before treatment and did not differ from the control ones. It is concluded that CPP is a highly sensitive method for evaluating the degree of periodicity of an acoustic signal and can be used to evaluate the effectiveness of treatment in patients with functional dysphonia.

Acoustic Noise-Based Detection of Ferroresonance Events in Isolated Neutral Power Systems with Inductive Voltage Transformers

Power-quality events and operation transients in power systems (PS) with isolated neutral can saturate inductive voltage transformers (IVT), which, when interacting with the overhead and underground cable capacitances, can cause ferroresonance events in the local PS. This abnormal operating mode can partially or totally damage the transformers and switchgears within the affected PS. Distribution system operators (DSO) can minimize these effects by detecting ferroresonance events accurately and fast enough and changing the mode of operation accordingly. Direct detection methods, i.e., based on voltage measurements, are reliable, but the massive deployment of this solution is relatively expensive; i.e., power quality analyzers cost thousands of USD. Alternatively, indirect detection methods are also available, e.g., IVT vibration measurements with accelerometers costing hundreds of USD, but their reliability depends on the installation method used. This manuscript proposes using the acoustic noise caused by magnetostriction forces within the IVT core during ferroresonance events to detect their occurrence. Compared to other indirect methods, electret condenser microphones with preamplifying stage cost less than USD 10 and are less sensitive to the installation procedure. The proposed method is validated experimentally, and its performance compared to IVT vibration measurements one by using the same detection methodology.

On the validity of numerical models for viscothermal losses in structural optimization for micro-acoustics

The increasing interest in miniaturizing acoustic devices has made accurate and efficient models of acoustic viscous and thermal losses progressively more important. This is especially the case in micro-acoustic devices such as hearing aids, condenser microphones and MEMS devices. Using the full linearized Navier Stokes equations to numerically model losses comes at a high computational cost. An approximate boundary layer impedance boundary condition representing acoustic losses has therefore become popular due to its high computational efficiency. This is especially true in the context of optimization where an efficient numerical method is required due to the many repeated analyses needed. However, the boundary layer impedance is only valid in the computational region where boundary layers are non-overlapping. Applying the boundary layer impedance can therefore lead to poor optimization results or limit the possible design space if the optimization violates this limitation. Therefore, the benefit of losses in narrow regions cannot be exploited if the boundary layer impedance is used. This work investigates two shape optimization test cases for maximizing the absorption properties of Helmholtz-like geometries based on the Boundary Element Method. The test cases are used to compare and validate the boundary layer impedance against a full viscothermal implementation revealing the benefits of the boundary layer impedance but also its limitations in a structural optimization setting. Based on the numerical experiments it is recommend to avoid the use of the boundary layer impedance in cases where any theoretical boundary layer overlap exists or at least verify simulation and optimization results with a full-losses implementation.

Airborne acoustic emission of an abrasive waterjet cutting system as means for monitoring the jet cutting capability

Abrasive waterjet cutting is a manufacturing technology making use of a high-speed waterjet with abrasive particles in suspension, for cutting materials with different mechanical properties. Product quality requirements are pushing towards an improvement of tracking and stabilization methods of the relevant process variables. Amongst those, the jet kinetic power defines the cutting capability and has a significant impact on the final cut features. This variable is subject to relevant fluctuations versus time. Besides, the current state of the art does not provide means for its in-line monitoring. The aim of this contribution is to monitor the airborne acoustic emission of an abrasive waterjet cutting head and investigate its correlation with the jet kinetic power. The investigation is carried out by means of factorial studies, in which the jet is fired at various water pressures and abrasive feed rates, providing different kinetic powers. The acoustic emission is synchronously monitored by means of a condenser microphone, installed on the cutting head. Data at frequencies above 40 kHz is found to constitute a robust and selective acoustic signature of the airborne jet. The acoustic signature is proven to be an effective in-line indicator of the jet kinetic power and its pressure-induced variations, whilst abrasive-induced variations remain undetected. A calibration procedure is presented, for translating the acoustic data into a jet kinetic power. The method is validated by means of further experiments that envisage its deployment in a real scenario. Overall, the presented method constitutes a robust tool for monitoring pressure-induced variations of the jet cutting capability.

Pengaruh Pemberian Agen Kontras Pewarna Sintetik pada Jaringan Biologis terhadap Hasil Pencitraan Fotoakustik

An experiment to investigate the influence of synthetic dye contrast agent in photoacoustic imaging of biological tissue was conducted in this study. This study uses a simple phocoacoustic imaging system consists of three main components, i.e., a diode laser, condenser microphone, and a custom-build X-Y stage. Characterization was performed on the main components of the system to obtain the appropriate settings on imaging the biological tissue in this study. The results of the optimal frequency and duty cycle for laser modulation in this study were 19000 Hz and duty cyle of 40%, respectively. The addition of a contrast agent aims to improve the quality of the image by comparing the sample with methylene blue contrast agent, the sample with methyl red and the sample without contrast agent. The increases of acoustic intensity level is in proportion with the contrast agent concentration. The difference in the photoacoustic images reveal that the sample with methylene blue contrast agent has the highest acoustic intensity level compared to both sample with methyl red contrast agent and the sample without contrast agent. Therefore, this research proves that a photoacoustic imaging system can be developed to image biological tissue with a contrast agent and methylene blue has greater potential than methyl red to be used as a contrast agent in photoacoustic imaging.

PVDF based piezoelectric condenser loudspeaker and microphone

The piezocapacive transducers (PCTs) or condenser transducers (CTs) are helpful to transmit and record acoustic signals with better performance. The only drawback in this technology is high power consumption with low fidelity. Therefore in this study we have fabricated the two-in-one (TIO) acoustic transducers to transmit and record the sound signals with a single unit. For this purpose the CTs were made by simple technique of sandwiched layered structures with 3 cm2 radius. In this study we reported that due to the unique antiferroelectric properties of k = 10 and 20 blends we have attained loudness of 98 dBSPL with the improvement of + 5 dBSPL at 65 Volts. For the reversed transduction i.e. condenser microphone unit we have attained 1 V/Pa at 1 Hz and 1 mV/Pa at 50 Hz with clear sound recognition between 23.5 and 440 Hz of input sound signal at 0 V dc bias. The enhancement in transduction properties were observed due to the combined piezoelectric and antiferroelectric properties of blends because of its quick-charge and quick-discharge (QCQD) characteristics. The low powered robust CTs with high fidelity are much needed for the futuristic applications.

Development of a multi-modal sensor network to detect and monitor knee joint condition

Osteoarthritis is a major cause of mobility problems in older people and is a particular problem in former sportspeople. The objective of this study was to develop and characterise a new system for the detection, monitoring and analysis of acoustic emissions from knee joints. 15 adult volunteers participated in the study. The participants performed six sets of three sit-stand-sit cycles. Reflective markers were placed at specific body landmarks recorded by 3D cameras. The exercise was performed with one foot on a force platform. A sensitive condenser microphone with a wide frequency response was connected to a dedicated acoustic analysis unit. Preliminary results provide clear acoustic signals showing a distinctive sequence of impulse-decay forms occurring naturally during each sit-stand-sit cycle. There are distinct differences between the acoustic signals emitted from younger healthy knees and those from aged knees. This work demonstrates the potential for this system to be used as an indication of the state of health of a human knee during movement.

Photoacoustic Imaging for Periodontal Disease Examination.

Introduction: After caries, periodontal tissue inflammation (periodontitis) is the most common oral health problem. Photoacoustic imaging (PAI) is a new technique that uses simple components such as a diode laser and a condenser microphone. This study aimed to evaluate the performance of a simple PAI system in periodontal disease imaging by using an animal model. Methods: Normal periodontal and periodontitis tissues were obtained from Sprague-Dawley rats categorized as the control group, treatment group 1 (7 days of periodontitis induction), treatment group 2 (11 days of periodontitis induction), and treatment group 3 (14 days of periodontitis induction). The PAI system was controlled by LabVIEW and Arduino IDE software from a personal computer. Results: Results revealed that the optimal frequency of laser modulation for periodontal tissue imaging was 19 kHz with a duty cycle of 50%. The photoacoustic (PA) intensity of periodontal tissues was -68.71 dB for treatment group 3, -70.34 dB for treatment group 2, -71.69 dB for treatment group 1, and --73.07 dB for the control group. PA image analysis showed that the PA intensity from periodontal disease groups was higher than the control group. Conclusion: This study indicates the feasibility of using a simple PAI system to differentiate normal periodontal tissues from periodontitis tissues.

Sex differences in frequencies in a species with modest sexual size dimorphism

ABSTRACT Sex differences in vocalizations are found across the animal kingdom, which may be due to different vocal apparatus, call function, and context. Rock hyraxes (Procavia capensis) of both sexes are vocal, but the sexes differ in repertoire size, call usage, and amplitude. In this study we examined sex differences in vocalization frequencies and predicted that they will be low and that frequency ranges will overlap since sexual size dimorphism in this species is modest (~ 15%). We utilized two datasets: recordings of captive hyraxes using a condenser microphone; and audio files of wild-living individuals recorded via a miniature recorder mounted on a collar. With the exclusion of two proximate call types recorded in the wild, all calls featured an ultrasonic component. However, in females there was an effect of duration on minimal frequency. Warning trills, which are heard by humans far away, featured the highest maximal visible harmonic in both datasets. No relationship was found between calling distance and the maximal harmonic in males, while in wild females, distant calls featured higher frequencies. Our results show sex differences in hyrax vocalization frequencies. Exploring the information encoded in frequencies beyond the human hearing range may expand our understanding of animal communication.

Active nonreciprocal metamaterial using a spatiotemporal modulation control strategy

A class of active nonreciprocal metamaterial (ANMM) is presented which consists of an acoustic duct with periodically placed active diaphragms that are controlled by a spatiotemporal modulation strategy. The acoustic nonreciprocities can be realized by modulating a system's properties spatiotemporally. Such an approach has been extensively employed by many investigators to break the reciprocity in acoustic and elastic metamaterials. However, our proposed ANMM distinguishes itself from the above-mentioned methods by introducing actively tunable space-time modulated feedback gain of the controllers. The controller is implemented in an analog manner to enable fast response at high modulation frequencies. By discretizing a 1D acoustic duct into multiple acoustic unit cavities, we introduced a time-varying gain with a phase difference between adjacent acoustic cavities. Directional band gaps of the modulated system are numerically analyzed as the asymmetric acoustic wave propagation can be realized by converting the acoustical energy from the fundamental mode to higher order modes. In addition, nonreciprocal behavior of the proposed ANMM was experimentally demonstrated using a waveguide with periodically placed condenser microphones (sensors) and speakers (actuators).

MmPhone: Sound Recovery Using Millimeter-Wave Radios With Adaptive Fusion Enhanced Vibration Sensing

Although electret condenser microphone (ECM) has been widely used in sound acquisition, sound capture techniques still attract growing interests. To this end, the emerging techniques of laser microphone and visual microphone have been developed, but they suffer from numerous fundamental problems. In this article, a novel concept, called millimeter-wave microphone (mmPhone), is proposed, which can recover sound signals with high quality via millimeter-wave vibration perception. In mmPhone, with acoustic excitation, the vibration response of multiple light objects can be perceived, but different objects have different frequency response bandwidths. Thus, we propose a sound recovery method of multitarget adaptive fusion enhancement (MAFE), allowing reconstruction of the real full band sound signals. The overview of the mmPhone system and principle is first depicted. The detailed procedures including object selection, denoise, and adaptively fusion enhancement for implementing the MAFE method are then illustrated. Finally, experimental results with different scenarios are presented to validate the performance of mmPhone, showing that the proposed method is promising for sound acquisition in the Internet of Things (IoT).