Hydrophone
A hydrophone (from ancient Greek ὕδωρ hydor “water” and φωνή phoné “sound”, “voice”) is a device for converting water- borne sound into an electrical voltage corresponding to the sound pressure. Another name is “underwater microphone ”.
Hydrophones are used in the sea to record waterborne noise and, above all, in medicine for non-invasive medical diagnoses or for the positioning of lithotripters . These areas of application in the sea and in medicine differ according to the extent of their respective application medium also in the frequency range used (in the sea between approx. 10 Hz and 400 kHz, in medicine at 1 MHz to 40 MHz) and thus in construction.
Hydrophones are fundamentally designed differently than microphones. The reason for this is less the required insensitivity to moisture than the different acoustic "hardness" ( acoustic impedance ). While microphones are also available as fast receivers ( moving coil microphones, ribbon microphones), almost only pressure receivers are used as hydrophones. The piezoelectric effect is preferably used here.
In principle, hydrophones can also be used as underwater sound sources. These are more likely to be referred to as projectors or - if they are used both as sound transmitters and receivers - as sound transducers. Such sound sources are usually dimensioned differently, but are often also used as receivers, especially in active sonar systems. In this case, however, they are not referred to as hydrophones.
Use of conventional microphones in moisture and under water
To make microphones waterproof, you can pull a condom over the microphone body (small microphone). This trick is practically used by resourceful sound engineers when shooting outdoors in the rain or underwater in a swimming pool. The slack latex skin seals the microphone capsule watertight and the pressure changes are perfectly transmitted. However, this technique can only be used with pure pressure receivers (i.e. microphones with omnidirectional characteristics); Pressure gradient receiver - such as B. Microphones with a cardioid polar pattern - do not work with them.
Hydrophones for use in the sea
Basic construction of a hydrophone
Commercially available hydrophones contain one (or more) hollow cylinders or hollow spheres made of lead zirconate titanate (PZT) as an acoustically effective element . PZT has a greater impedance than water. For this reason, relatively thin-walled hollow cylinders or hollow spheres are used in order to achieve a favorable impedance matching. This makes cylinders made of the brittle PZT ceramic particularly prone to breakage. Therefore, despite the somewhat greater structural effort, hollow cylinders are now preferred. The inside of the cylinder or the ball is sound soft (air, cork or similar). The piezoelectric material is biased in such a way that circumferential changes occur due to the pressure, which are picked up inside and outside by a conductive coating across this direction.
The dimensions of the cylinders or spheres are such that they are below a wavelength. Some hydrophones for use in the sea therefore have a diameter of around 1 cm, which means that they meet this requirement at a speed of sound of 1480 to 1500 m / s in water up to around 100 kHz (wavelength around 1.5 cm). However, since this design does not provide enough level, the manufacturers of high-quality hydrophones (see Brühl & Kjaer, Reson, Sonar Surround) use special shapes with up to 12 cm long sensor surfaces in cylindrical or spherical form. These sensors are built to detect noise over long distances. Over long distances, however, the vertical propagation of sound can be neglected in the sea. For this reason, these hydrophones have an omnidirectional directional characteristic, especially in the horizontal plane.
The entire hydrophone is protected against the water by an elastomer coating .
The equivalent circuit of a piezoelectric hydrophone is essentially a capacitor . Only at the upper frequency end of its range of application does the mass of the material act as an inductance , causing a resonance . Below this resonance frequency the frequency response of a hydrophone is usually very linear, above it it drops off steeply.
However, together with the input resistance of the following amplifier, a high pass is formed. This is a very desirable effect because otherwise when the hydrophone is lowered into the water, a high voltage would be created by the hydrostatic pressure.
The inherent capacity of a hydrophone is usually in the range of a few nF . Together with the coaxial cables commonly used, this capacitance forms a capacitive voltage divider . This is why a preamplifier is usually placed near the hydrophone. This avoids electrical interference at the same time, because the high-resistance, sensitive line is completely under water, which provides excellent shielding .
Special forms
There are special forms mainly with regard to the special requirements of the installation, such as for the purpose of structure-borne noise decoupling or for installation in the oil hose of a line antenna ( Sonar LFAS). Because the hydrophones used in the sea are almost always small compared to the acoustic wavelength, they usually have no directional characteristics. Gradient receivers are an exception . They are, for example, in DIFAR- sonobuoy used. They can be used to determine the direction of dominant targets, but no interference signal suppression or directional separation of targets is achieved. A gradient receiver works very similarly to a fast receiver. However, the speed of sound and the amplitude of movement are very small because of the high impedance of the water, so that the pressure difference at a small distance, precisely the pressure gradient, is used more effectively. Two gradient receivers arranged orthogonally to one another are required to determine the full direction . Instead, especially in LFAS antennas, three individual receivers are used at a short distance ( triplet array).
Directional characteristic
Since most hydrophones were developed for measuring purposes, they have an omnidirectional directional characteristic up to the highest transmittable frequency. Due to the high speed of sound ergo wavelength in water, acoustic directivity effects due to transit time elements such as ports or interference pipes are not practical. An exception here is the directional sphere from Sonar Surround, which uses a curved interface and a special material to give hydrophones the directional effect of a supercardioid.
The military, however, needs far greater directivity up to an accuracy of 0.2 degrees. Such directional effects can only be achieved with the largest possible acoustically sensitive surface. This is achieved by interconnecting a large number of hydrophones (cf. sonar dome, tow sonar). A computer can now analyze the wavefront on the basis of the minimal time difference and thus determine the direction of incidence. The disadvantage here is that by mixing together up to a thousand or more hydrophones, the sound is very falsified. Electronic directivity (at least 4 hydrophones in the tetrahedron) are therefore not suitable for aesthetic recordings.
For stereophonic recordings, only time-of-flight techniques or intensity techniques with the Sonar Surround DS come into question.
Hydrophones for medical diagnostics
Physical basics
Water-borne sound represents a spatial area with a density that changes periodically over time (density wave and pressure wave are equivalent models for the phenomenon of sound).
Impedance
For the proportion of sound energy that can pass from one propagation medium to another, it is crucial how the characteristic acoustic impedances of these media relate to one another. The characteristic acoustic impedance of a material is the product of the density and the speed of sound of this material. The ratio of the smaller impedance to the larger one at a media boundary is a dimensionless number in the interval [0… 1] and describes the acoustic coupling. With quotients close to 1 (corresponds to impedances of roughly the same size), the sound wave couples very well from one medium to the other. The speed of sound in water is about four times as great as in air. In addition, the densities of the two media differ by almost three orders of magnitude. An acoustic coupling of approx. 0.0003 results for a water-air interface. Such a situation with extremely poor impedance matching is also called quasi-decoupling. In accordance with the laws of refraction and the resulting proportions for reflection and transmission , a water-air interface almost completely throws the incident sound back into the water. In order to study sound phenomena in water, the sound signals are either converted into the desired signal form alternating voltage in the medium of water itself and without an air layer in front of the sensor, or one couples between the water and the sensor with a layer that adjusts the impedances (in sonography typically: strong water-based gel ). In order for a hydrophone to be able to convert useful acoustic signals with a high degree of sensitivity, the best possible acoustic coupling must be sought in addition to other properties.
Useful signal
The questions posed by hydrophones, which are physically different in principle, prioritize different features of the design and its properties. Depending on the topic of a measurement, the duration, the intensity and the frequency are of interest, but in rare cases also the phase and the scattering angle.
running time
For the determination of distances (echo sounder and similar applications) spatial resolution and linearity of the frequency response play a subordinate role. In contrast, high sensitivity and suitable damping are essential. Here hydrophones are mostly operated in the range of their natural frequency , which is selected to be adapted to the expected distances and structure sizes. The time that elapses between the start of the transmission pulse ( ping ) and the reception of the echo response results in the product with the water-borne sound velocity as a measure of the distance covered by the signal.
amplitude
Water-borne sound is weakened through obstacles on its way in the direction of propagation. Part of the sound energy is reflected in impedance jumps . Some of the sound energy can be absorbed in an obstacle . Part of the intensity is scattered on particles . With known properties of the original sound, information about the hydroacoustic properties of the obstacles can be calculated from the different intensity components of the reflection, transmission, scattering and absorption . Measurements of the reflection represent superimpositions of several partial reflections at different levels of impedance jumps at different times. Good echo sounders not only evaluate the first echo response, but also calculate an impedance profile in the direction of the beam from the chronological sequence of differently strong echo responses. Refinements of this method lead to the techniques of sonography in reflection. The amplitudes of the sound pressure measured by the hydrophone then lead to the calculated representation of the extinction properties in the examined spatial area. The intensities of the scattered components of an ultrasonic beam in cloudy water measured by hydrophones can be used to calculate the scattering characteristics. If the ultrasound wavelength is known from the theory concepts Rayleighs and Mies, which are also valid for sound, the size of the scattering particles results . Resolving power and linearity are essential requirements for the hydrophone in a system that determines extinction values from the sound intensity. This is particularly true in systems for sound transmission with incoherent sound. In these cases, a hydrophone is selected whose natural frequency is as clearly as possible above the high-frequency components of the useful signal. In mixed methods in which the extinction component in the reflected echo response is measured, i.e. a (coherent) pulse-echo system is used, good results are also achieved with selective receivers. Hydrophones convert pressure amplitudes. In order to obtain the sound intensity from the output voltage of the sensor, this primary stimulus response is squared.
frequency
If the reflective boundary layer moves relative to the hydrophone in the direction of propagation of the wave, the received signal appears shifted according to the rules of the Doppler effect . In echocardiography , for example, this frequency shift represents a significant portion of the useful signal. For high accuracy of such measurements, hydrophones with a largely linear frequency response are preferred.
phase
As amplitude receivers , hydrophones integrate the local, elementary sound pressures acting on their measuring surface in the correct phase. A plane sound wave of any intensity incident at an angle with respect to the acoustic axis of the hydrophone ( normal vector of the sensor plate) does not trigger an output voltage if the pressure with all phase angles evenly distributed from the sensor calls for responses that are canceled out in pairs. The smallest angle at which this zero crossing occurs for the first time results from the diameter of the measuring sensor surface and the sound wavelength. This property of the hydrophones is used to locate foreign sound sources . This property is quantified in the directional characteristic of a hydrophone. The phase sensitivity of a hydrophone increases in direct proportion to the sensor diameter and also in direct proportion to the frequency of the detection signal. Thin sensor plates with large diameters resolve angles of incidence extremely steeply.
Water-borne sound preservation
In order to study the details of water-borne noise caused by an external source, one wants to save an image of the course of the sound that is as true to the original as possible. In these applications the hydrophone has the same role as a microphone in zoological behavior research. High-quality hydrophones for broadband and distortion-free recording of water-borne noise from external sources usually receive with small sensor surfaces and thin sensor thicknesses.
Thick Swingers
A plate that transforms periodic signals from a physical energy carrier into changes in its thickness that are equal in frequency and proportional to amplitude is called a thickness oscillator . Thickness vibrations are replaced to a certain extent as sound waves. In this operating mode, a thickness transducer generates sound ( sound transducer ). Reciprocally, a thickness oscillator generates a proportional, non-acoustic signal from sound hitting it. The natural frequency of a thickness oscillator results from the speed of sound in the medium of the oscillator and its neutral thickness, which corresponds to half a wavelength in the case of resonance. The spatial resolution of an ultrasound signal in water is limited by diffraction in the range of one wavelength of the scanning wave , analogous to Ernst Abbe's argument for light microscopes . From the claims that
- the acoustic coupling is close to 1 and
- the natural frequency is clearly greater than the uppermost useful signal frequency
the rule of thumb for a hydrophone is that its sensor should be thinner than the desired spatial resolution.
Piezoelectric effect
A piezoelectric thickness oscillator varies its thickness in the transmission mode periodically with the frequency of the alternating voltage that is applied to its electrodes. In (reciprocal) sensor operation, the same oscillator emits an amplitude-proportional alternating voltage of the frequency with which it is exposed to periodic pressure fluctuations in the direction of its thickness. In most medical applications, hydrophones record ultrasonic signals in the frequency range between 1 MHz and 20 MHz. In order to be able to distinguish 1 mm structures in water with ultrasound, a frequency of at least around 2 MHz must be used (the speed of sound is the product of wavelength and frequency). A piezo sensor for a hydrophone that is used in this spectral range must therefore be thinner than 1 mm. Suitable materials are
- PZT ceramic and
- PVDF .
PZT ceramics
have densities around 7.5 g / cm³ and sound speeds around 4600 m / s. Their acoustic coupling to water is only sufficient (0.042). In applications for measuring foreign water-borne noise, the quality of around 90 , which is too high for broadband receivers, is also a problem. The sintered material is brittle and very fragile if it is thin. This limits the possibilities for producing panels with very high natural frequencies. The thinnest, mechanically sufficiently stable PZT sensors reach their resonance increase at around 20 MHz. The combination of these three properties results in a very highly selective, spectral sensitivity. In quasi-static operation, satisfactory results can only be achieved with suitable electronic signal processing. In devices that work according to the pulse-echo method , the sensor receives reciprocally piezoelectrically the echo reaction of a signal pulse that it had emitted itself shortly before in piezoelectric transmission mode. This method minimizes the harmful effects of the fundamental, sensory deficits of PZT on the measurement results through the intrinsically optimal adaptation of the sensor to its own signal. The use of a single PZT plate as a combined transmitter and receiver in a pulse-echo system uses the excellent transmission properties of the ceramic and suppresses the objectively rather poor reception properties. According to the state of the art, PZT is the preferred material for hydrophones in pulse-echo systems.
PVDF films
PVDF films can be made far thinner than PZT and have become the preferred material for high-performance hydrophones in broadband applications. Due to the high attenuation, the soft material only achieves quality factors below 15. This means that the spectral sensitivity of a PVDF hydrophone is similar to the characteristic of a low-pass filter, which is desired for broadband receivers . The acoustic coupling is good (0.36), since both the density (1.78 g / cm³) and the speed of sound (2260 m / s) of PVDF are close enough to the corresponding values for water. PVDF remains mechanically sufficiently stable up to thicknesses in the range of a few micrometers, so that sensors can be produced from it with a natural frequency of up to 1000 MHz. A thin PVDF film only minimally disturbs the wave field it is supposed to measure.
calibration
Piezoelectric sound transducers have to be polarized during manufacture and age over time in the sense that their sensitivity and possibly also their frequency response change. In applications in which the absolute characteristics of a water-borne wave field are determined with a hydrophone, the sensor must be calibrated regularly. To do this, the output voltage of the hydrophone is compared with the absolute acoustic excitation known as precisely as possible. However, determining the absolute waterborne sound pressure in a measuring arrangement is not trivial. A sophisticated method for calibrating hydrophones uses the acousto-optical effect .
Acousto-optical effect
Hendrik Antoon Lorentz recognized in 1880 that the refractive index of a medium for light varies with the density of the medium. Therefore, a monofrequency, plane water- borne sound wave represents a thick phase grating for light that traverses this water-borne sound field in the direction of the wave fronts (i.e. perpendicular to the direction of propagation of the sound). The analysis of the diffraction pattern of a laser beam after passing through a water-borne sound wave field enables the determination of the parameters of the diffractive sound and thus enables an absolute value of the sound pressure to be determined which is independent of material parameters. A measuring arrangement for the diffraction of laser light by water-borne sound is itself a non-contact hydrophone and can serve as a calibration standard.
Several fiber optic methods for detecting waterborne sound have been developed since the late 1970s. A laser beam is guided through an optical fiber in the water . After passing the optical fiber, the change in amplitude, frequency, polarization or phase of the light is measured. The latter is particularly suitable for achieving sufficient sensitivity. The phase shift is measured using an interferometer , in which the phase-shifted light beam is superimposed on the light beam of a reference measurement .
Another fiber optic hydrophone measures the reflection of a laser beam at the end of a glass fiber that is inserted into the water. The proportion of reflected light changes with the refractive index of the water and thus with the pressure. Since the fiber can be made very thin and no moving parts are involved, a high spatial and time resolution is achieved.
Directional characteristic
Both piezoelectric and acousto-optic hydrophones are coherent, phase-sensitive amplitude receivers and therefore have a highly directional reception characteristic . If a receiving plate is hit by a plane wave at such an angle that the phase position of its relative to the greatest tilt relative to the opposite edges of the wave front is just shifted by one wavelength, the integral of all local excitations over the entire plate is zero. For a given sensor, the steepness of its characteristic also obviously correlates negatively with the wavelength of the received signal. In comparison to the wavelength, large sensor areas always lead to very steep directional characteristics in amplitude receivers. In order to obtain wide reception lobes, i.e. also to be sensitive to obliquely incident waves, the sensor area must be selected as small as possible.
The reception characteristics of a hydrophone can be designed variably if several small sensors are positioned in a suitable arrangement and their output signals are electronically coupled or offset with one another. Depending on whether the addition of the received signals takes their phase angle into account, or whether the individual signals are z. B. be squared (intensity corresponds to the amplitude square), the result is a steeper or a flatter and thus wider directional characteristic. The coherence and phase fidelity of the hydrophones must be taken into account when comparing analogies with the very highly developed, imaging light optics, since there only incoherent intensity receivers are known as passive sensors. The overriding physical principle for quantifying the directional characteristic of a hydrophone can be found in the theory of diffraction with the sensor surface as the aperture .
Applications
Imaging procedures
In addition to the established sonography systems, which all work in pulse-echo mode and are described in detail in the corresponding main article, the quasi-optical ultrasound imaging method is particularly noteworthy as it provoked the development of the ultrasound matrix hydrophone. In this ultrasonic transmission camera , a dipping in a water body area of the patient is irradiated with incoherent, level ultrasound through the object passes and is imaged on the other side with ultrasonic lenses in the focal plane of an ultrasonic lens. The printed image in the focal plane is scanned over a large area by hydrophones and converted into a television image. Such a system was manufactured at the GSF in Neuherberg near Munich in 1980 and operated until 1989. In the first few years of this ultrasonic transmission camera, a PDP11, which was specially integrated into the system for this purpose, converted the pressure amplitude signals and positions of the measurement segments of the hydrophone line into a television image.
sonar
Hydrophones are used in the field of passive sonar, which is used in particular by the military, to detect underwater noises and other acoustic signals. This is particularly the case with submarines and surface units that are entrusted with submarine hunting .
Since 1950, NATO has been operating the SOSUS underwater eavesdropping system , which consists of a network of stationary hydrophones that have been sunk across the board on the sea floor.
Designs
PZT hydrophone
PZT plates are polarized over the entire surface and coated with silver electrodes on both sides. The two electrodes of the plate must be very well insulated from one another electrically. Suitable housing shapes therefore only expose one electrode surface and encase the edge and the second surface in a watertight manner.
PVDF hydrophone
PVDF foils are partly polarized over the entire surface, partly only in the spot. With broadband high-performance hydrophones, the polarized part often takes up less than 1% of the total film area. The remaining foil serves as a water-borne suspension. Gold serves as the contact and also protects the foil.
Another design is the needle probe from Müller and Platte. In this model, molten PVDF is applied to a needle tip and then polarized. An approximately hemispherical sensitive area of less than 0.5 mm in diameter is created. This design has a very low directional sensitivity and is also very robust for applications in shock wave measurements.
Hydrophone line
The sound image from the GSF's ultrasonic transmission camera is directed to the water surface via a sound mirror inclined at 45 °. There sits a linear hydrophone array made up of 240 individual sensors, arranged in three rows offset from one another. During a recording, the array is moved perpendicular to its length and parallel to the water surface. The squared sensor output amplitudes are read into an image memory at addresses that correspond to their respective sensor positions within the rectangular scan area. In this way, the area distribution of the transmitted sound is determined and displayed as a gray image, similar to an X-ray image. A serious disadvantage of this arrangement results from the mechanical movement of the sensor strip. Even at moderately fast propulsion, water eddies detach from the hydrophone line, triggering disruptive artifact signals.
Matrix hydrophone
In 1989 a development group from Siemens presented a newly developed area sensor for diagnostic water ultrasound. A rectangular PVDF film seals the front wall of the transmission basin on the image side in a watertight manner. The transmitted ultrasound is imaged directly onto this matrix hydrophone with the ultrasound lens. The foil is divided into rows and columns and each segment is operated as a self-sufficient, broadband and flat-directed individual hydrophone, electrically isolated. The outer side of the film carries the complete, highly integrated electronics for each sensory matrix element for squaring, amplifying and A / D converting the signal.
See also
literature
- Sutilov, Vladimir A .: Ultrasound Physics, Basics. Translated from Russian and edited by Peter Hauptmann. Vienna; New York: Springer, 1984, ISBN 3-211-81798-0
- Scruby, CB: Laser ultrasonics, Techniques and Applications. Scruby CB and Drain LE Bristol; Philadelphia; New York: Adam Hilger, 1990, ISBN 0-7503-0050-7
- Krestel, Erich [Hrsg.]: Imaging systems for medical diagnostics, 2nd revised and expanded edition. Berlin; Munich: Siemens Aktiengesellschaft, [Publishing Department], 1988, ISBN 3-8009-1505-7
- Müller, Michael and Platte, Michael: Use of a broadband pressure probe based on PVDF for the investigation of converging shock waves in water, Acustica Vol. 58, 1985, pp. 215-222
- Heinz G. Urban, Handbook of Water-borne Sound Technology, 2nd edition; STN ATLAS Electronics GmbH; Kiel 2002
Individual evidence
- ^ Wilhelm Gemoll: Greek-German School and Handbook , Munich / Vienna 1965
- ↑ Waidelich, Wilhelm et al. [Ed.]: Lasers in Medicine: Lectures of the 10th International Laser Congress 1991 - Berlin; Heidelberg; New York: Springer, 1992, ISBN 3-540-54934-X . Page 368 ff Wimmer, M. and Weidelich, W .: A laser sound pressure sensor.
- ↑ Shizhuo Yin, Paul B. Ruffin, Francis TS Yu (Eds.): Fiber Optic Sensors . 2nd Edition. CRC Press, 2008, ISBN 1-4200-5366-3 , pp. 367 ff . ( limited preview in Google Book search).
- ↑ Archive link ( Memento of the original dated November 13, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
