GB2530565A

GB2530565A - Acoustic thermometry

Info

Publication number: GB2530565A
Application number: GB1417054.2A
Authority: GB
Inventors: Andrew Elliott
Original assignee: University of Salford Enterprises Ltd
Current assignee: University of Salford Enterprises Ltd
Priority date: 2014-09-26
Filing date: 2014-09-26
Publication date: 2016-03-30
Also published as: GB201417054D0; GB2545138A; GB201704542D0; WO2016046569A1

Abstract

A method of, and apparatus for, measuring the temperature of an environment by measuring the speed of sound or vibrational waves provided by background sound or vibration within the environment. The method may include measuring the frequency spectrum of sound or vibrational waves within a resonant cavity and using the obtained frequency spectrum to determine the speed of the sound or vibrational waves. The resonant cavity 4 may be tubular resonant cavity having opposing ends 6 , 8 and a sensor 10 may be located within an end of the cavity. The method may further comprise measuring at least one additional frequency spectrum corresponding to sound or vibrational waves outside of the resonant cavity using a sensor (110, fig 2), those waves also produced by background sound or vibration within the environment.

Description

I

Acoustic thennometn'

Technical Field of the Invention

The present invention relates to an apparatus and method for performing acoustic thennometry.

Background to the Invention

Acoustic thermometry is a technique of measuring temperature using the principle that the speed of sound through a medium changes with the temperature of the medium in a consistent and well known manner. Conventional acoustic thermometers generally work by transmitting sound waves along a gas-filled cavity which may be in the form of a tube, or a spherical cavity such as a Helmholtz resonator, for example. The waves are produced by a source placed at a first position within the cavity and a receiver is placed at a second position within the cavity. By measuring the time delay between the production of the sound wave from the source to the receipt of the sound wave at the receiver, or through the determination of a resonant frequency of the cavity, the speed of sound through the medium can be measured.

The temperature T of the gas can be calculated using the following: rM [Equation 1] Where M is the molar mass of the gas, R is the universal gas constant, y is adiabatic index of the gas and vs is the speed of sound. Given that the speed of sound through the medium is dependent on the temperature of the medium, the temperature can be inferred through measurement of the speed or sound through the gas which is calculated from the measured time dethy or the determined resonant frequency, along with the dimensions of the cavity.

Acoustic thermometry has a number of benefits over other means of measuring temperature such as thermocouples, thermistors and resistance temperature sensors.

Foremost, acoustic thermometers can detect small changes in temperature given their very high resolution. Furthermore, acoustic thermometers work over a very wide temperature range when compared with thermocouples and thermistors and are less sensitive to thermal radiation given that the temperature of the gas is measured rather than the sensor itself, Given its relative insensitivity to radiation, conventional acoustic Ihermometry is presently 1 0 used within hostile environments wherein high levels of radiation are present, such as nuclear reactors, for example. However, acoustic thermometry is not widely used in many applications given its current relative complexity and cost when compared with other forms of sensors.

It is therefore an aim of embodiments of the invention to provide an apparatus and method for performing acoustic thermometry which is less complex and at a lower cost than present apparatus and methods.

It is also an aim of embodiments of the invention to provide an apparatus and method for performing acoustic thermornetry in which costs are comparable to other forms of temperature sensors such as thermocouples, thermistors and resistance temperature sensors.

It is a ftmrther aim of embodiments of the invention to overcome or mitigate at least

one problem of the prior art disclosed herein.

Summary of the Invennon

According to a first aspect of the present invention there is provided a method of measuring the temperature of an environment comprising measuring the speed of sound or vibrational waves provided by background sound or vibration within the environment.

[he measurement of the speed of the sound or vibrational waves may be performed directly or indirectly.

The terni "sound waves" is intended to include waves of any frequency which includes, hut is not limited to, audible frequencies, ultrasound and infrasound and may be any subset of frequencies or individual frequency. The term "background sound or vibration" is intended to cover vibrational or sound waves generated by any form of vibration source which is normally present in the environment hut which has not been located therein specificaily for the measurement of the temperature of the environment.

In some embodiments the temperature of the environment is determined through measurement of the frequency or frequencies of sound or vibrational waves within the environment.

The frequency or frequencies of the sound or vibrational waves may be measured by at least one sensor located within the environment. In embodiments where the speed of sound waves is measured, the or each sensor may be a sound wave receiver operable to measure the frequency or frequencies of the received sound waves. The sensor may be a vibrometer or an accelerometer for example.

In some embodiments the method may comprise measuring the frequency of the waves using a plurality of sensors located within the environment.

The method may comprise measuring the speed of sound or vibrational waves in a Iluid medium located withrn the environment, said fluid medium comprising either a liquid or a gas, for example. In other embodiments the method may comprise measuring the speed of sound or vibrational waves in a solid medium located within the enviromnent.

In embodiments wherein the medium comprises a gas, the or each sensor may comprise either a microphone or a particle velocity sensor, for example. In embodiments wherein the medium comprises a liquid, the or each sensor may comprise either a hydrophone or a pressure transducer, for example. In embodiments wherein the medium comprises a solid, the or each sensor may comprise either an accelerometer or a laser vibrometer, for example.

In other embodiments the temperature of the environment is measured through measurement of the velocity of particles within the environment moving under the influence of the sound or vibrational waves. In such embodiments the sensor may comprise an air particle velocity sensor or a hot wire anemometer, for example.

I 5 In some embodiments the method comprises measuring the frequency or frequencies of sound or vibrational waves within a resonant cavity within the environment.

In such embodiments the method may further comprise identifying the or each resonant frequency of the resonant cavity from the received waves within the resonant cavity. The or each identified resonant frequency may be used to calculate the temperature of the environment.

The use of a resonant cavity acts to amplify sound or vibrational waves having a frequency or frequencies which are at a resonant frequency of the cavity. Therefore, the amplitude of the signal received by the sensor at these frequencies will be greater than at any other frequency. The resonant frequencies of the cavity are dependent on the shape and size of the cavity. For example, changes in the length or volume of the cavity, or changes in the size of an opening within the cavity all affect the resonant frequencies of thc cavity.

The or each frequency of the sound or vibrational waves within the resonant cavity may he measured using a sensor placed within or in the region of the resonant cavity. The sensor may be placed at an end of the resonant cavity or at an opening within the resonant cavity.

Tn other embodiments other factors may he used to measure the temperature of the environment, which may include acceleration or displacement of particles within the environment. Other factors may include measuring thc strain on a solid medium, or measuring pressure within a fluid medium.

In some embodiments the method comprises measuring the or each frequency of sound or vibrational waves propagating along a tubular resonant cavity having opposing ends. In such embodiments the sensor maybe located at a first end and the resonant cavity may comprise an opening at the opposing end. In other embodiments both ends of the tubular resonant cavity comprise openings, and the sensor may be located at any point along the cavity. The resonant cavity may comprise a substantially cylindrical configuration.

In some embodiments the spccd of sound may be measured directly by measuring the time taken for a sound wave to travel between two or more known points within the environment, in such embodiments, the method may comprise measuring sound waves which propagate along a waveguide, the waveguide being operable to ensure the sound waves only travel in a single direction along the waveguide.

The sound waves may he measured by at least two sound wave sensors located along the waveguidc. In such embodiments, the method may comprise sensing a sound wave at a first sensor and subsequently sensing the sound wave at a second sensor located downstream of the first sensor along the waveguide. The time delay between the signals received by the at least two sensors may be used to calculate the speed of the sound wave.

In some embodiments the time delay may be obtained through cross correlation of the signals received by the two sensors, the time delay being given by the maximum cross correlation of the signals as a function of time. The method may further comprise flirther discrimination of the received sound waves by amplitude and!or frequency.

According to a second aspect of the present invention there is provided aniethod olmeasuring the temperature olan environment comprising the steps of: a) measuring the Frequency spectrum of sound or vibrational waves within a resonant cavity; b) identiI'ing at least one resonant frequency within the measured spectrum; c) using the identified resonant frequency or frequencies to calculate the speed of the waves; and d) calculating the temperature of the environment based on the speed of the waves; wherein the sound or vibrational waves are provided by background sound or vibration within the environment.

The frequency spectrum may be measured by at least one wave sensor which may be located within or in the region of the resonant cavity. The or each sensor may be an elcctro acoustic transducer, a vibrometer, or an accelerometer, for example, In some embodiments, the signal from the or each sensor may be analysed using a computing means to form a digital spectrum of tile received waves. The computing means may subsequently be used to identify resonant frequencies within the digital spectrum and may be used to calculate the temperature of the environment through calculation of the speed of sound using the identified resonant frequencies.

The signal from the sensor may be proportional to the frequency of the wave received by the sensor. In such embodiments the computing means may interpret the signals received from the sensor by forming a digital frequency spectrum having peaks in amplitude of the signal at a frequency or frequencies corresponding to the resonant frequency/frequencies of the resonant cavity given that signals at these frequencies will be amplified by the presence of the cavity. These peaks may subsequently be identified by the computing means and. the temperature of the environment calculaled, In other embodiments the frequency or frequencies may be measured via the phase of each frequency or difference in phase of two or more frequeneies There may be multiple sensors within the resonant cavity such as two or more sensors located at different positions, or adjacent, within the cavity.

In some embodiments the method further comprises measuring at least one additional frequency spectrum corresponding to sound or vibrational waves outside of the resonant cavity, those waves also produced by background vibration within the environment. The at least one additional frequency spectrum may be compared with the spectrum obtained from tile waves within the resonant cavity in order to increase the accuracy of the determined resonant frequencies.

S

The or each additional frequency spectrum may be obtained using a sensor located outside of the resonant cavity. The or each additional sensor may be tile same sensor used to measure the spectrum of the sound or vibrational waves within the resonant cavity. In prefer red embodiments, the or each additional frequency spectrum is obtained using a separate sensor to the first such that no manila! tnanipulation of the apparatus is required between readings. the additiona! sensor or sensors may be the same type of sensor as the one located within or in the region of the resonant cavity, or may be different.

The signal from the or each additional sensor may be analysed using a computing means to Ibmi a digital spectrum of the waves received outside of the resonant cavity. The computing means may interpret the signals received from the or each additional sensor by forming a frequency spectrum in the form described above. By forming spectra from both the sensor within or in the region of the resonant cavity, and at least one located outside of the resonant cavity, a direct comparison can be made between them.

The use of an additional sensor is particularly uscflul in environments wherein there is a source or sources of background vibration which produce sound or vibrational waves at a generally constant frequency or frequencies and/or increased amplitude when compared with waves produced from other sources of background vibration, In such cases, the frequency or frequencies may not correspond to a resonant frequency of the cavity and may result in a false peak in the frequency spectrum obtained from the sensor located within the cavity. By comparing the two obtained speca, true resonant frequency peaks can be confirmed as they will only be present in the spectrum obtained from the sensor located within the cavity, whereas false peaks will be present in both spectra.

l'he method according to the invention may be employed in many situations including those wherein conventional method of measuring temperature are already employed, e.g. those which use thermocouples, thermistors or RTDs. Examples of which include climate control systems, mobile electronic devices, laboratories and weather stations.

According to a third aspect of the present invention there is provided an apparatus for measuring the temperature of an environment comprising a sound or vibrational wave sensor for measuring the speed of sound or vibrational waves provided by background sound or vibration within the environment.

The sensor may be operable to indirectly measure the speed of the sound or vibrational waves through measurement of at least one frequency of waves present in the environment. The sensor may comprise an electroacoustie transducer, a vibrometer or an accelerometer, Pr example.

In some embodiments the sensor may he operable to measure the speed of particles within the environment moving under the influence of sound or vibrational waves provided by background vibration within the environment. In such embodiments the sensor may comprise an air particle velocüty sensor, or a hot wire anemometer, for example.

In some embodiments (he apparatus comprises a plurality of sensors for measuring the speed of sound or vibrational waves provided by background sound or vibration within the environment.

In some embodiments the apparatus further comprises a cavity; and the or each sensor may be located within the cavity. The cavity may be a resonant cavity and the or each sensor may he operable to measure the frequency of the sound or vibrational waves provided by background sound or vibration within the environment which propagate through the cavity. There may he multiple sensors within the cavity.

in oilier embodiments the apparatus comprises a first and second sensor located within tile cavity. In such embodiments, the apparatus may further comprise a means to analyse the signals from the two sensors and calculate the time delay between the signals.

With knowledge of the distance between the sensors and the time delay between signals, the speed of the wave can he calculated.

According to a fourth aspect of the present invention there is provided an apparatus for measuring the temperature of an environment comprising: a cavity; and a sound or vibrational wave sensor loeated within the cavity for measurement of the speed of sound or vibrational waves within the cavity provided by background sound or vibration within the environment.

The sensor may he operable to indirectly measure the speed of the sound or vibrational waves through measurement of the frequency or frequencies of the waves. The apparatus may further comprise a means to form and analyse a frequency spectrum from the measured frequencies in order to calculate the temperature of the environment.

The cavity may comprise a resonant cavity. In such embodiments, sound or vibrational waves having a frequency at a resonant frequency of the cavity will be amplified. The amplified signal received by the sensor may he used to calculate the temperature of the environment. In alternative embodiments the phase of each frequency or the difference in phase, may be used to identi& each frequency and thus calculate the temperature.

The cavity may comprise an opening therein, the sensor being located within the cavity at a position away from the opening. In some embodiments the sensor may be located at the opening within the cavity.

In sonic embodiments the cavity comprises a tubular configuration with an opening at a first end and the sound wave sensor at an opposing end. The cavity may comprise a substantially cylindrical shape having a tirst and second end and in such embodiments the first end may comprise the sound wave sensor and the second end may comprise the opening within the cavity. In other embodiments the cavity may have a spherical or cuboidal configuration.

The cavity may be configured depending on the optimal frequency range for the processing of the signals from the sensor, in embodiments wherein the resonant cavity comprises a tubular configuration, the length of the cavity may be chosen depending on the optimal range of frequencies of the waves for processing. For example, in environments wherein the optimal range includes waves of an audible frequency, the length of the cavity must he greater than around 20mm (corresponding to the approximate shortest wavelength sound wave which is audible to humans at standard temperature and pressure (STP)). In other embodiments wherein the optimal range is generally of an ultrasonic frequency, the length of the cavity may be much shorter. In other embodiments the volume or diameter of the cavity may be chosen depending on the optimal range of frequencies to he measured.

In some embodiments the sensor may comprise an elcctroacoustic transducer operable to convert the sound or vibrational waves into an electrical signal, which may be a microphone such as a condenser microphone, a dynamic microphone or a piezoelectric microphone, for example. In such embodiments the means to analyse the frequency spectrum may comprise a computing means operable in use to form and analyse a digital frequency spectrum from the signals from the elcctroacoustic transducer. The computing means may further be operable to calculate the temperature of the environment from the analysed frequency spectrum through the identification of peaks within the spectrum which correspond to the resonant frequencies of the cavily. In other embodiments the sensor may comprise an accelerometer or a vibrornctcr, for example.

The apparatus may further comprise at least one additional sensor operable in use to measure the frequency or frequencies of sound or vibrational waves outside of the cavity, said waves also being provided by background vibration within the environment. The or each additional sensor may also comprise an electroacoustic transducer operable to convert the waves into an electrical signal, and may too be connected to a computing means operable in use to ibrm and analyse a digital frequency spectrum of the received waves. In such embodiments the computing means may be operable to analyse and compare the spectra obtained from the sensor measuring the frequency spectrum of waves within the cavity and the or each additional sensor.

In other embodiments the apparatus may comprise a first and a second sensor located within the cavity, both the first and second sensors being connected to the computing means. In such embodiments, the computing means may he operable to compare the signals received by the Iwo sensors to calculate the time delay between the signals in each sensor relating to the same wave. The comparison may comprise cross correlation of the signals received by both sensors.

the computing means may further be operable to calculate the speed of waves propagating between the two sensors, through the cavity, and subsequently calculate the temperature of the environment.

The apparatus in accordance with the invention may be employed in many situations including those wherein conventional method of measuring temperature are already employed, e.g. those which use thermocouples, thermistors or RTDs. Examples of which include climate control systems, mobile electronic devices, laboratories and weather stations.

In fact, the apparatus may be used in any situation wherein there is sufficient computation power to analyse the signals received from the or each sensor.

Detailed Description of the Invention

In order that the invention may be more clear]y understood an embodiments thereof will now be described, by way of exanipic only, with reference to the accompanying drawings, of which: Figure 1 is a perspcctk'c view of first embodiment of an apparatus in accordance with the present invention.

Figure 2 is a perspective view of a second embodiment an apparatus in accordance with tile present invention.

Figure 3 is a graphical illustration of a spectrum obtained using the apparatus as illustrated in Figure 2.

I 5 Figure 4 is an illustration of resonant modes of sound waves within the resonant cavity of the present invention.

Figure 1 illustrates an embodiment of an apparatus 2 of the present invention. The apparatus 2 includes a resonant cavity 4 having a length L, a first end 6 and a second end 8. The first end 6 is open and a sound wave sensor in the form of microphone 10 is located at the second end 8. The microphone 10 is connected via a connection 12 to a processing means in the fbrrn of computer 14 which is operable in use to analyse the signal from the microphone 10.

Figure 2 ii ustrates a second embodiment of an apparatus of the present invention.

The apparatus 102 shown in Figure 2 differs from the apparatus 2 shown in Figure 1 by the inclusion of a second sound wave sensor in the form of microphone 110 which is also connected to the computer 14. The second microphone 110 is connected to the computer 14 by connection means 112. In contrast to microphone 10, the second microphone 110 is not located at an end of a resonant cavity, rather, tile second microphone 110 is positioned indcpcndcnt of the othcr components of the apparatus 102.

It should be understood, however, that the position of each sound wave sensor wilL depend on the type of sensor used. For example, in embodiments wherein the sensors comprise a particle velocily sensor, both sensors may be located within the cavity 4, i.e. one at each end 6, 8 of the cavity.

The operational use of the apparatus 102 as illustrated in Figure 2 will now be described with reference to Figures 3 and 4.

Tn use, the apparatus 102 may be placed within an environmeni wherein there are 1 5 no dedicated sound sources, but there aTe sufficient levels of background noise. The background noise maybe generated by any form of vibration which is normally present in the environment but which has not been located therein specifically for use with the apparatus 102 Sound waves generated from by the background sources are received by microphones 10, 110. The signal from the microphones 10, 110 is relayed to the computer 14 along respective connection means 12, 112. The computer 14 is operable to interpret the signals received from the microphones 10, 110 to form digital spectra illustrating the frequencies of the sound waves received.

In general, it is expected that the amplitude of the signals received by the microphones 10, 110 will be roughly constant on average for all frequencies given that the sound waves received by the microphones 10, 110 come from random sources within the environment, However, by placing resonant cavity 4 within the environment and having the microphone 10 placed at the end 8 of the cavity 4, the formed spectrum corresponding to microphone 10 comprises peaks in amplitude at certain frequencies corresponding to the resonant frequencies of resonant cavity 4. The resonant cavity 4 acts to amplify any signals rcccivcd by the microphone I 0 at frequencies corresponding to the resonant frcquencies of the cavity 4 resulting in amplitude peaks within the formed spectrum.

Figure 4 illustrates sound pressure waves 320, 330 within the cavity 4 which correspond to resonant frequencies of the cavity 4. Resonant signals within a cavity which has an open end and a closed end, such as resonant cavity 4, correspond to pressure waves which have a pressure maximum at the closed end 8 and a minimum at the open end. The resonant waves 320, 330 can therefore only have a set wavelength depending on the length L of the cavity 4. This wavelength corresponds to a frequency using the formula v = [A [Equation 2], where v is the speed of sound through the cavity, f is the frequency of the sound wave and 2. is the wavelength of the sound wave. Given the requirements for a wave to be resonant within the cavity 4, it can be shown to good approximation that: = (2n-1) Equation3] where n=l corresponds to the pressure wave 320 in which L=X/4, n=2 corresponds to the pressure wave 330 in which L=3k/4 and soon in increasing half integer wavelengths (e.g. n=3, L=5X/4 etc.). Therefore, by obtaining values for the resonant frequencies, f,, the speed of sound, v, can be calculated. The calculated speed of sound may then be used to calculate the temperature of the environment using [Equation 1].

It should be appreciated that the invention is not limited to apparatus having a tubular resonant cavity such as cavity 4. Rather, any form of resonant cavity may be used, an example of which is a Helmholtz resonator which comprises a spherical configuration having a neck portion. The fundamental resonant frequency of the Helmholtz resonator can he calculated by: f = [Equation 4] where Scross sectional area of the neck of the resonator, V=volume of the resonator and L'length of the neck. By measuring the resonant frequency of the resonator, the speed of soui d cam be calculated.

Other forms of cavity include a euhoidal cavity having dimensions x, y and z. The resonant frequencies of the cuhoidal cavity can he calculated by: = 2 + (n)2 + (2 lhquation 5] where nx,y,z arc integcrs. By measuring the resonant frequency of the resonator using the above formula, the speed of sound can be calculated.

It should be appreciated that many other forms of resonant cavity could be used, each of which having a separate formula for the calculation of the resonant frequencies.

It is expected that the spectrum formed from the signals received by microphone should he generally constant over the complete range of frequencies, and the spectrum obtained from the signals received by microphone I C) will he similar with thc exception of peaks corresponding to the resonant frequencies of the resonant cavity 4, given that the amplitude of the signals received by the microphones 10, 110 should be similar on average for all frequencies. Therefore in most cases, the apparatus 2 as shown in Figure 1 is sufficient to calculate. the speed of sound, and hence the temperature of the environmenl However, in some enviromrients there may be a background source which generates sound waves at a relatively constant frequency and at a large enough amplitude to result in a false peak within the spectrum. the second microphone 110 is used to overcome this potential problem. By comparing the spectrum obtained from microphone 10, to that obtained from the signal from microphone 11 0, any false peaks within the two spectra due to background vibration of a constant frequency should be accounted for given that they should he present in both spectra. Any true resonant peaks should only bc present in the spectrum formed from the signal from microphone 10.

The above comparison may be achieved by dividing the spectrum obtained from microphone 1 0 by the spectrum obtained from microphone 110 to form a combined spectrum, An example of a combined spectrum is shown in Figure 3. The spectrum contains peaks 220, and 230 which correspond to the frequencies of the pressure waves 320, 330 illustrated in Figure 4, The majority of the other frequencies in the spectrum have a value of 1 which is to he expected after combining the two spectra from microphones 10. 110.

The above embodiments are described by way of example only. Many variations arc possible without departing from the scope of the invention as defined in the appended claims.