Nowadays optical fibers are used in many industries. In addition to the data transmission, many sensor applications with using some special optical fibers are expanding. Faraday's phenomenon is often used for detection of the magnetic field. The size of the deviation of the polarization plane of optical radiation passing through the optical fiber is evaluated in Faraday's phenomenon that occurs due to the influence of the external magnetic field. This paper describes our new approach to addressing this issue using standard telecommunication fibers, PDMS-based optical connections, specialty products and some magnetic field sensitive materials. Our detector of the magnetic field can find a use for simplified measurement of weak magnetic fields up to 0.3 T.
This paper deals with methods for processing signals from an optical interferometer to monitor vital signs (Respiration Rate and Heart Rate). Optical interferometer signals are contaminated by variety of technical and biological artifacts (motion artifacts, hospital/patient-generated noise, etc.). Tested optical sensors are very sensitive, it therefore crucial to reduce such unwanted signals. In this article, a complex application for processing the signals from optical interferometer based on virtual instrumentation was developed. The experiments were conducted on data sensed by optical interferometer using a National Instruments card NI USB-6216 BNC and application in the LabVIEW environment. Frequency selective filters were tested in the experiments. The results obtained by using optical interferometer were statistically compared with the ECG and PCG reference. According to the results, optical interferometers are able to measure both the Respiration and Heart Rate under the given conditions. Unfortunately, the measurement is very difficult to replicate in the hospital environment, which is the primary reason why these methods are not used in clinical practice.
Authors of this article focused on the analysis of the influence location of the fiber-optic sensor on the measurement and determination the heart rate of the human body. The sensor uses a Fiber Bragg Grating (FBG) and is encapsulated in the polymer polydimethylsiloxane (PDMS). The combination of fiber-optic technology and its encapsulation in a polymer PDMS allows the use of the sensor e.g. in magnetic resonance environments (MRI). Among currently solved doctors requirements belongs field focusing on the study of hyperventilation and panic attacks of patients during MRI examination due to their very frequent occurrence. Proposed FBG sensor can help doctors to predict (based on heart rate) hyperventilation and panic attacks of patients during MRI examinations. For the most accurate determination of the heart rate, it is necessary to know the influence location of the sensor on the human body. The sensor functionality and analysis of the sensor placement on the heart rate has been verified by a series of real experimental measurements of test subjects in laboratory environment.
The team of authors focused on analyzing of using fiber-optic sensor based on Fiber Bragg Grating (FBG) for the monitoring of heart rate (HR) of long-term ill patients with a minimum of physical movement load. During all experiments, test subjects were asked to simulate their natural behavior in the most accurate way (for instance, the focus was on the use of fine motor skills - not only movements of hands and arms, legs, and feet, coughing, changes in body positions, but also walking). All these external aspects are taken into account in the bellow-described results of the probe efficiency and show that it is very necessary to know the impact of these artifacts for the determine the heart rate of the human body. Final results were discussed with the senior doctor of the long-term care department.
This article focuses on the sensitivity of encapsulated interferometric probes. These probes are used mainly for BioMed and security applications. Fiber-optic sensors are interesting for these applications, as they are resistant to electromagnetic interference (EMI) and that also do not affect the surrounding medical and security equipment. Using a loop of the optical fiber with is not a suitable for these measurements. The optical fiber should be fixed to one position, and should not significantly bend. For these reasons, the optical fiber is encapsulated. Furthermore, it is necessary that the encapsulated measuring probes were flexible, inert, water resistant and not toxic. Fiber-optic sensors shouldn't be magnetically active, so they can be used for example, in magnetic resonance environments (MR). Probes meeting these requirements can be widely used in health care and security applications. Encapsulation of interferometric measuring arm brings changes in susceptibility of measurements in comparison with the optical fiber without encapsulation. To evaluate the properties of the encapsulated probes, series of probes made from different materials for encapsulation was generated, using two types of optical fibers with various degrees of protection. Comparison of the sensitivity of different encapsulated probes was performed using a series of measurements at various frequencies. The measurement results are statistically compared in the article and commented. Given the desired properties polydimethylsiloxane (PDMS) polymer has been proven the most interesting encapsulating material for further research.
Authors of this article present the fiber-optic system based on fiber Bragg gratings (FBGs) which are used to secure the entrance areas such as buildings, halls, warehouses, etc. The system uses the specially encapsulated sensory array of fiber Bragg gratings which are implemented into the floor or on the floor and allows for monitoring the area of 1 m2 up to 100 m2 depending on the number of FBG sensors. The sensory array is characterized by immunity to electromagnetic interference (EMI), passivity regarding electrical power supply, the possibility of remote evaluation (up to units of km) and high sensitivity. Proposed sensor system has detection capability greater than 99 % and furthermore, provides information about the weight load to an accuracy of ± 5 kg. The concept has been tested in a real environment within the test polygon for several weeks. As the reference devices, we used the CCTV (Closed Circuit Television).
This article discusses the topic of the monitoring of the operating condition of the pressing machines. In the process of pressing there are a large acting of the forces of the press machine on the machined material. It is accompanied by an expression of deformation of parts of the pressing machines. Knowledge about the resulting loading is important for the final product quality and durability of these machines. In the case of multi-columned presses is important even load distribution among the individual columns. For this reason, it is necessary to monitor the load of the hydraulic pistons. Conventional approaches of load monitoring utilize piezoelectric sensors and strain gauges, which are sensitive to electromagnetic interference, require electrical power, subject to corrosion and action of moisture. A suitable alternative is the fiber Bragg gratings (FBGs) sensors, that those failures have. Their use for load monitoring production presses have been verified by the real measurement on a four-post hydraulic press ALPHA 4 from the company FORM + TEST Seidner. For measurements were used the FBG sensors and classic foil strain gauges (reference). Real measurement was performed during a load of press in the range from 0 to 2000 kN with a step of 500 kN.
Protection of persons and assets is the key challenge of Smart City safeguards technologies. Conventional security technologies are often outdated and easy to breach. Therefore, new technologies that could complement existing systems or replace them are developed. The use of optical fibers and their subsequent application in sensing is a trend of recent years. This article discusses the use of fiber-optic sensors in perimeter protection. The sensor consists of optical fibers and couplers only and being constructed without wires and metal parts bring many advantages. These include an absence of interference with electromagnetic waves, system presence can be difficult to detect as well as affect its operation. Testing installation of perimeter system was carried out under reinforced concrete structure. Subjects walked over the bridge at different speeds and over the different routes. The task for the system was an absolute detection of all subjects. The proposed system should find application mainly in areas with the presence of volatile substances, strong electromagnetic fields, or in explosive areas.
Distributed temperature sensing systems (DTS) are based on the principle of time-domain reflectometry where an optical fiber acts as a temperature sensor. DTS is capable of measuring the temperature along the optical fiber using the nonlinear phenomenon referred as Raman scattering. The biggest advantage of such sensing system is the use of an optical fiber itself as a sensor which gives the benefits of electromagnetic interference immunity, low sensor cost, measurement distances up to 10 kilometers and the safe use in flammable and corrosive environments. Fiber optic DTS can be therefore used in the environments and processes in which the application of conventional sensors is impossible. This article discusses the use of DTS for the moisture measurement in the masonry. In structures with built-in optical fiber, the immediate detection and location of moisture are possible. To perform the measurements an experimental brick wall has been built and between each wall layer the optical fiber was placed. The wall was built in stainless steel tub with a drain valve and was placed on a mobile trolley. The dimensions of the wall were 106 x 100 x 30 cm. The actual measurements were carried out in two stages. In the first, the tub was filled with water and the temperature change associated with the gradual increase of moisture inside the wall was measured. This measurement lasted until the saturation which was the time when the wall has no more moisture to adopt. The second stage then examined the evolution of the temperature inside the wall during gradual desiccation until the time when the temperature inside the wall was uniform between all layers.
Today interferometric sensors are among the most accurate available thanks to their inherent high sensitivity. These highly versatile sensors may be used to measure phenomena such as temperature, strain, fluid level, flow, vibration, stress, etc. This article concentrates on the composition of fiber-optic interferometers, in particular the Mach-Zehnder type. The Mach-Zehnder type is composed of two arms, one for measurement and a second serving as a reference. When light enters the interferometer, ideally the phase of the light is shifted only in the measurement arm while the phase in the second arm remains unchanged. Interference occurs when the light recombining at the output and the resulting light intensity is proportional to the measurand. A major issue in the application of fiber based sensors is laying and fixing the fibers effectively in real life environments. Different approaches are necessary for both arms. The reference arm should as far as possible be isolated from the measurand. In this paper, various isolating materials are considered, however there are almost unlimited materials that may be used for isolation purposes. Conventional construction methods and materials were used such as aluminum tubing, flexible PVC tubing, double sided tape, steel clinches, superglue, PVC strips and PVC strips filled by silicon.
During track maintenance operations, the early detection of oncoming rail vehicles is critical for the safety of maintenance personnel. In addition, the detection system should be simple to install at the trackside by minimally qualified personnel. Fibre optic based sensor systems have the inherent advantages of being passive, unaffected by radio frequency interference (RFI) and suffering very low signal attenuation. Such a system therefore represents a good alternative to conventional approaches such as ultrasonic based sensor systems. The proposed system consists of one or more passive fibre trackside sensors and an x86 processing unit located at the work site. The solid fibre connection between sensors and processing unit eliminates the risk of RFI. In addition, the detection system sensors are easy to install with no requirement for electrical power at the sensor site. The system was tested on a tram line in Ostrava with the results obtained indicating the successful detection of all the trams in the monitoring windows using a single sensor. However, the platform allows flexibility in configuring multiple sensors where required by system users.
This paper deals with problematic of Free Space Optical (FSO) Links. The theoretical part describes the effects of atmospheric transmission environment on these FSO connections. The practical part is focused on the creation of an appropriate experimental workplace for turbulences simulation (mechanical and thermal turbulences), fog effects and subsequent measurement of these effects. For definition how big impact these effects on the FSO system have is used the statistical analysis and simulation software Optiwave. Overall there were tested three optical light sources operating at wavelengths of 632.8 nm, 850 nm and 1550 nm respectively. Influences of simulated atmospheric effects on the signal attenuation were observed. Within the frame of simulation in Optiwave software there were studied influences of attenuation on given wavelengths in form of FSO link transmission parameters degradation. Also for the purposes of real measurements it was necessary to fabricate an experimental box. This box was constructed with sizes of 2.5 and 5 meters and was used for simulation of atmospheric environment.
Fiber Bragg gratings (FBGs) belongs to the single-point optical sensors used in many fields and applications where they often replace a standard sensors. They are easy to multiplex and the wavelength division multiplex is the most widely used method. FBGs in sensory branch are designed for a different Bragg wavelength which gives different measure and sensitivity coefficients. Existing algorithm is based on the determination of left and right boundaries of the measuring channel and the central Bragg wavelength. In this paper is presented the new mathematical model for calculation of Bragg wavelength, sensitivity coefficient and channel width of any FBG in the single step. The model takes into account the following input parameters: wavelength of the optical source, source bandwidth, the type of measured quantity, measuring ranges, width of the FBG reflected spectrum and the guard band between adjacent channels. The mathematical model is verified by using a simulation in software OptiSystem.
Fiber optic interferometers belong to a group of highly sensitive and precise devices enabling to measure small changes in the deformation shapes, changes in pressure, temperature, vibration and so on. The basis of their activity is to evaluate the number of fringes over time, not changes in the intensity of the optical signal. The methodology described in the article is based on using the interferometer to monitor traffic density. The base of the solution is a Mach-Zehnder interferometer operating with single-mode G.652 optical fiber at the wavelength of 1550 nm excited by a DFB laser. The power distribution of the laser light into the individual arms of the interferometer is in the ratio 1:1. Realized measuring scheme was terminated by an optical receiver including InGaAs PIN photodiode. Registered signal from the photodetector was through 8 Hz high pass filter fed to the measuring card that captures the analog input voltage using an application written in LabView development environment. The interferometer was stored in a waterproof box and placed at the side of the road. Here panned individual transit of cars in his environs. Vertically across the road was placed in contact removable belt simulating a retarder, which was used when passing cars to create sufficient vibration response detecting interferometer. The results demonstrated that the individual vehicles passing around boxing showed characteristic amplitude spectra, which was unique for each object, and had sufficient value signal to noise ratio (SNR). The signal was processed by applications developed for the amplitude-frequency spectrum. Evaluated was the maximum amplitude of the signal and compared to the noise. The results were verified by repeated transit of the different types of cars.
Distributed optical fiber sensors monitor the measured variables over the entire fiber length. Distributed strain and temperature system (DSTS) scans the frequency change of Brillouin scattering which depends on the measured temperature and mechanical stress of the fiber. This paper deals with the effect of fiber geometric deformations on the Brillouin scattering. The points of maximum and minimum deformations were searched using the Brillouin frequency change. The optical fiber was installed into base geometric shapes and deformed by mechanical stress. Standard single-mode fiber G.652.D was used. The aim of this study was to verify if the standard optical fiber originally designed for telecommunication transmissions are suitable for sensor applications with DSTS. It turned out that these fibers are applicable for the deformation measurement and geometrical arrangement has great influence on the measurement sensitivity too.
KEYWORDS: Connectors, Temperature metrology, Signal attenuation, Polishing, Single mode fibers, Statistical analysis, Optical fibers, Interfaces, Thermography, Data modeling
High requirements in terms of purity, quality, mechanical design and method of grinding are placed on the optical
connectors. Connectors are mostly exhibited to the negative effects, such as temperature, humidity, dusty environments,
etc. The effects of temperature on the optical fiber are already known and well described. However, the way in which the
temperature change has effect on different types of connectors is not yet well understood and validated. Therefore it was
necessary to find the answers within this article to the questions whether a change in temperature has the effect on the
connector return loss and if so how big. The dependence of the attenuation coefficient on temperature change for the
specific connector is also studied within the article. This allows determining how significant influence the choice of the
type of ferrule on the return loss is. The three most commonly used types of connectors SC, FC and E-2000 with PC and
APC polishing were selected. To describe the negative effects the dependence of return loss on temperature was chosen.
The return loss was chosen because it is one of the key parameters that describes connectors and is measured in practice.
Measuring instruments from EXFO model AXS-200/350 were used for the measurements. Measurements were carried
out on single-mode fiber SMF 652.D at wavelengths of 1310 nm and 1550 nm. The transmission method was selected
for the measurement. Measurements on each connector were performed 30 times and statistical model was compiled
from the measured return loss.
Next-generation passive optical access networks come to the fore nowadays. These optical next-generation networks are
the response to the increasing qualitative requirements from end users. Technologies using Time Division Multiplexing
include NG-PON (XG-PON 1 and XG-PON 2) and 10GEPON. Their advantage is the applicability to older topologies,
which are operated by the original technology of passive optical access networks. Wavelength Division Multiplexing
Passive Optical Network (WDM-PON) is an alternative also belonging to next-generation networks. Time Division
Multiplexing is in this case replaced by Wavelength Division Multiplexing. Certain variants of WDM-PON use a
combination of broadband light source, optical circulator, optical phased array and tunable FP laser. Construction of the
terminal units (ONU) is advantageous because it can always tune in to the appropriate wavelength in the given optical
DWDM channel (100 GHz). The disadvantage is the increased security risk on the primary layer due to channel crosstalk
in an optical phased array (AWG). The aim of this paper is to assess the degree of security risk in real conditions. The
article includes both simulation and real measurements in C + L bands with 100 GHz DWDM spacing.
Fiber optic distributed temperature sensing systems (DTS) are based on the principle of reflectometer and allow us to measure the temperature along the optical fiber. Optical fiber in these systems is used as a temperature sensor which can measure up to thousands of points simultaneously. DTS sensors use nonlinear phenomenon known as Raman scattering for temperature measurement. The advantages of this system include immunity to electromagnetic radiation, low cost of optical fiber, the possibility of measurement to a distance of 10 km and safe use in flammable or corrosive environments. The small size of optical fiber allows using in applications where the dimensions of the other sensors were problematic. A typical example of the DTS application is the fire detection in tunnels and buildings at risk, detection of water leaks on dikes and dams or monitoring of temperature in mine shafts. This article deals with the measurement of temperature transmission over various insulation layers using the DTS system. One of the problems of temperature transmission is that most of the sensors cannot measure the entire temperature profile but only allows a point measurement. This problem is solved by DTS systems with optical fibers. Optical fiber, due to its small size, can be applied among various insulation layers that were formed by rock wool. Three sensory layers formed by rings of multimode optical tightbuffered fiber with 50/125 micron core/cladding dimension were applied. The layers were linked together allowing a direct comparison of measured temperature. Rows of rings were placed on the margins and one was in the middle. Individual rings were linked together into the horizontal lines. Thus we were able to cover the whole surface of the insulation layers. Measurement was carried out in a closed air-conditioned room for 37 hours. Graphs with the progress of temperature at time and place were compiled from the measured data.
There is an increasing pressure nowadays on the efficient use of existing ICT infrastructure in order to provide the latest services for corporate customers or end users. With the increase in number of services, requirements for optical networks of all hierarchies are increasing as well. This increase in the requirements, however, involves risks which must be faced by Internet service providers. These include the maximum use of spectral range, bandwidth and reachable distance, suppression of dispersion effect, route planning efficiency, CAPEX and OPEX costs management, or successful combination of technologies of deployed networks. The aim of this article is to present the problems associated with interconnection of WDM-PON and ver.2 EPON (IEEE 802.3ah standard). The entire simulation is based on real parameters, which were provided by the manufacturers of the technologies and then measured in the laboratory. Then we were able to perform simulations based on more realistic features of these technologies.
Researcher’s teams were dealing with the microwave emitter’s inhomogeneity problem since the microwaves were used. One possible way, how to measure electromagnetic field is the measurement on inhomogeneous temperature distribution on the irradiated sample, which can cause problems as in other material processing, so in the undesirable change of properties and even security. Inhomogeneity of electromagnetic field is specific by creating spots with higher or lower temperature called “hot spots”. This inhomogeneity strongly affects the temperature distribution in the cross section of the material and its resultant heating. Given the impossibility of using classical electronic devices with metal temperature sensors were various indirect methods used in the past. This paper deals with experimental measurement of the microwave emitter’s inhomogeneity (2.45 GHz) using the optical fiber DTS. The greatest advantage of this sensor system is just in using of the optical fiber (electromagnetic resistance, small size, safety using in inflammable and explosive area, easy installation). Due to these properties of the optical fiber sensor it’s possible to measure the temperature of the sample in real time. These sensor are able to measure the temperature along the fiber, in some cases they use nonlinear effect in optical fiber (Raman nonlinear effect). The verification of non-homogeneity consists in experimental measuring of the temperature distribution within the wooden sample. The method is based on heat exchange in an isolated system where wooden sample serves as an absorber of the irradiated energy. To identify locations with different power density was used DTS system, based on nonlinear phenomena in optical fibers.
Nowadays, the interferometric sensors belong to the one of the most accurate, thanks to its great sensitivity. With their
help we are able to measure temperature, strain, level, flow, vibration, stress, etc. For its operation the Michelson
interferometer consist of the two arms terminated by mirrors, by which is possible to measure generated phase shift in
the individual arms. Furthermore, polarization maintaining fibers were used. With this setup we will examine the effects
of vibration and also how is this sensor influenced by the different setup arrangement and how it will manifest its
frequency response. It is important to isolate the reference arm to increase the sensitivity of the measurement and the
subsequent effect on the maximum phase shift and maximum frequencies response. In this work, we are going to
describe various combinations of the arrangement of the measuring and reference arm and their effect on the sensitivity
of different measured phenomena. Subsequently obtained frequency bands are evaluated for these various configurations
and materials.
Nowadays the trends in the construction industry are changing at an incredible speed. The new technologies are still emerging on the market. Sphere of building insulation is not an exception as well. One of the major problems in building insulation is usually its failure, whether caused by unwanted mechanical intervention or improper installation. The localization of these faults is quite difficult, often impossible without large intervention into the construction. As a proper solution for this problem might be utilization of Optical-Fiber DTS system based on stimulated Raman scattering. Used DTS system is primary designed for continuous measurement of the temperature along the optical fiber. This system is using standard optical fiber as a sensor, which brings several advantages in its application. First, the optical fiber is relatively inexpensive, which allows to cover a quite large area for a small cost. The other main advantages of the optical fiber are electromagnetic resistance, small size, safety operation in inflammable or explosive area, easy installation, etc. This article is dealing with the detection and localization of building insulation faults using mentioned system.
Fiber-optic sensors based on phase shift measurements are one of the most sensitive sensors at all. In general they are capable to measure various phenomena, for example displacement, rotation, temperature, acoustic pressure, liquid flow and level, strain etc. In our paper we have used interferometer configuration based on the Mach–Zehnder principle with polarization maintaining components and narrowband DFB laser operating at a wavelength of 1550 nm. In this configuration it is important to isolate the reference arm against measured phenomena and on the other hand to increase the sensitivity of the measuring arm to maximize phase shift induced by the measured phenomenon. The paper describes various measurement arrangements of measuring and reference arm and their influence on the measurement sensitivity. The obtained frequency ranges are evaluated for all mentioned combinations.
The Fiber Bragg Grating (FBG) sensors are nowadays used in many applications. Thanks to its quite big sensitivity to a surrounding environment, they can be used for sensing of temperature, strain, vibration or pressure. A fiber Bragg grating vibration sensor, which is interrogated by a distributed feedback laser diode (DFB) is demonstrated in this article. The system is based on the intensity modulation of the narrow spectral bandwidth of the DFB laser, when the reflection spectrum of the FBG sensor is shifted due to the strain that is applied on it in form of vibrations caused by acoustic wave pressure from loud speaker. The sensor’s response in frequency domain and strain is measured; also the factor of sensor pre-strain impact on its sensitivity is discussed.
The interferometers composed of optical fibers are due to its high sensitivity capable of to measure various influences affecting the fiber. These influences may be bending or different sorts of fiber deformations, vibration, temperature, etc. In this case the vibration is the measured quantity, which is evaluated by analyzing the interference fringes representing changes in the fiber. Was used a Mach-Zehnder interferometer composed of the polarization maintaining elements. The polarization maintaining elements were used because of high sensitivity to polarization state inside the interferometer. The light was splitted into the two optical paths, where the first one is the reference fiber and it is separated from the actual phenomenon, and the second one is measuring fiber, which is directly exposed to vibration transmission from the underlying surface. The light source was narrowband DFB laser serating at a wavelength of 1550nm and as a detector an InGaAs PIN photodiode were used in this measurement. The electrical signal from the photodiode was amplified and fed into the measuring card. On the incoming signal the FFT was applied, which performs the transformation into the frequency domain and the results were further evaluated by software. We were evaluating the characteristic frequencies and their amplitude ratios. The frequency responses are unique for a given phenomenon, thus it is possible to identify recurring events by the characteristic frequencies and their amplitude ratios. The frequency range was limited by the properties of the used speaker, by the frequency characteristics of the filter in the amplifier and used resonant element. For the experiment evaluation the repeated impact of the various spherical objects on the surface board was performed and measured. The stability of amplitude and frequency and also the frequency range was verified in this measurement.
Fiber optical interferometers belong to highly sensitive equipments that are able to measure slight changes like distortion
of shape, temperature and electric field variation and etc. Their great advantage is that they are insensitive on ageing
component, from which they are composed of. It is in virtue of herewith, that there are evaluated no changes in optical
signal intensity but number interference fringes. To monitor the movement of persons, eventually to analyze the changes
in state of motion we developed method based on analysis the dynamic changes in interferometric pattern. We have used
Mach- Zehnder interferometer with conventional SM fibers excited with the DFB laser at wavelength of 1550 nm. It was
terminated with optical receiver containing InGaAs PIN photodiode. Its output was brought into measuring card module
that performs on FFT of the received interferometer signal. The signal rises with the composition of two waves passing
through single interferometer arm. The optical fiber SMF 28e in one arm is referential; the second one is positioned on
measuring slab at dimensions of 1x2m. A movement of persons around the slab was monitored, signal processed with
FFT and frequency spectra were evaluated. They rose owing to dynamic changes of interferometric pattern. The results
reflect that the individual subjects passing through slab embody characteristic frequency spectra, which are individual for
particular persons. The scope of measuring frequencies proceeded from zero to 10 kHz. It was also displayed in
experiments that the experimental subjects, who walked around the slab and at the same time they have had changed
their state of motion (knee joint fixation), embodied characteristic changes in their frequency spectra. At experiments the
stability of interferometric patterns was evaluated as from time aspects, so from the view of repeated identical
experiments. Two kinds of balls (tennis and ping-pong) were used to plot the repeatability measurements and the gained
spectra at repeated drops of balls were compared. Those stroked upon the same place and from the same elevation and
dispersion of the obtained frequency spectra was evaluated. These experiments were performed on the series of 20
repeated drops from highs of 0,5 and 1m. The evaluation of experiments displayed that the dispersion of measured
values is lower than 4%. Frequency response has been verified with the loudspeaker connected to signal generator and
amplifier. Various slabs have been measured and frequency ranges were compared for particular slab designs.
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