Ultrasonic Transducer

Right. You need something rewritten. Let’s get this over with. Don’t expect hand-holding.

Acoustic sensor

An acoustic sensor is a device that does one of two things: it either converts sound waves into an electrical signal or does the reverse. The term encompasses a range of devices including microphones, hydrophones, and various sensors that operate outside the range of human hearing. The focus here, however, is on devices that traffic in the silent scream of ultrasound.

A curvilinear array ultrasonic transducer, the kind they use in medical ultrasonography to peer inside you. Below that, the guts of a Philips C5-2 curved array ultrasound sensor. A reminder that even the most sophisticated technology is just a collection of parts.

Ultrasonic transducers and sensors are fundamentally instruments that generate or perceive ultrasound energy. It’s a simple concept, so try to keep up. They can be crudely sorted into three categories, a taxonomy for the easily confused: transmitters, receivers, and transceivers. Transmitters take electrical signals and convert them into ultrasound , effectively shouting into a medium that can’t talk back. Receivers do the opposite; they listen for the returning ultrasound and translate it back into an electrical signal. Transceivers, in a stunning display of efficiency that other systems could learn from, are capable of both transmitting and receiving ultrasound. [1]

Applications and performance

The applications for this technology are as varied as they are mundane, a testament to its utility. Ultrasound can be used for measuring wind speed and direction, turning an invisible force into data via an anemometer . It’s used to determine the fluid level in a tank or channel, or even the sea level with a tide gauge , constantly checking how close we are to the metaphorical edge. For measuring speed or direction, a device employs multiple detectors, calculating velocity from the relative distances to particulates suspended in the air or water—a sophisticated way of watching dust motes. To measure a liquid level , the sensor simply measures the distance (ranging ) to the fluid’s surface. It’s a glorified tape measure, but without the flimsy metal.

Other applications include humidifiers , sonar (the moody, atmospheric original), medical ultrasonography , the crude shock of burglar alarms , and the meticulous work of non-destructive testing .

The typical system uses a transducer that generates sound waves in the ultrasonic range—anything above the 20 kHz threshold of human hearing. It does this by converting electrical energy into sound. Then, it waits. Upon receiving the echo, it reverses the process, turning the returned sound waves back into electrical energy that can be measured, analyzed, and displayed on a screen for someone to worry about.

This technology can also be used to detect approaching objects and track their positions with unnerving accuracy. [2]

Furthermore, ultrasound can facilitate point-to-point distance measurements by transmitting and receiving discrete, controlled bursts of sound between transducers. This technique is known as Sonomicrometry . Here, the transit-time of the ultrasound signal—the time-of-flight—is measured with digital precision and then mathematically converted to the distance between the transducers. This calculation, of course, hinges on the assumption that the speed of sound in the medium is a known quantity. It’s like shouting in a canyon and timing the echo, but with more math and less existential reflection. This method can be extraordinarily precise, both temporally and spatially, because the measurement can be derived from tracking the exact same point on the received waveform, either by a reference level or by its zero crossing. This allows the measurement resolution to be far finer than the wavelength of the sound frequency itself, a neat trick of engineering. [1]

Transducers

A look at the sound field of a non-focusing 4 MHz ultrasonic transducer in water, with a near field length of 67 mm. The sound pressure is shown on a logarithmic dB-scale, a chaotic bloom of energy. Contrast that with the sound pressure field of the same transducer, but with its surface spherically curved. The focus is sharp, concentrated. A visual metaphor for your attention span, perhaps.

Ultrasonic transducers are the heart of the system, converting alternating current (AC) into ultrasound and then back again. Most commonly, these transducers rely on piezoelectric transducers [3] or their more delicate cousins, capacitive transducers , to perform this conversion. [4] Piezoelectric crystals possess the useful property of changing their size and shape when a voltage is applied to them—a direct, physical response to an electrical impulse. [3] Capacitive transducers, on the other hand, operate on a different principle, utilizing the electrostatic fields that exist between a conductive diaphragm and a backing plate.

The beam pattern of a transducer—its shape and focus—is not arbitrary. It is dictated by the active area and geometry of the transducer, the wavelength of the ultrasound it produces, and the sound velocity of the medium through which the sound propagates. The provided diagrams illustrate this clearly, showing the sound fields of an unfocused and a focusing ultrasonic transducer in water, revealing starkly different energy distributions.

Because piezoelectric materials also generate a voltage when force is applied to them, they can function as ultrasonic detectors as well as emitters. It’s an elegant symmetry. Some systems dedicate separate components for transmitting and receiving, while others consolidate both functions into a single piezoelectric transceiver.

Ultrasound transmitters aren’t limited to piezoelectric principles. They can also use magnetostriction. Materials with this property change their size slightly when exposed to a magnetic field, which is another practical, if less common, way to create a transducer.

A capacitor microphone, sometimes called a condenser microphone, employs a thin diaphragm that vibrates in response to ultrasound waves. The subsequent changes in the electric field between this diaphragm and a closely spaced backing plate convert the sound signals into electric currents, which can then be amplified.

This diaphragm principle is the foundation for the relatively new micro-machined ultrasonic transducers (MUTs). These devices are fabricated using silicon micro-machining technology (MEMS technology), a method particularly suited for creating dense transducer arrays. The vibration of the diaphragm can be measured or induced electronically. This is achieved either by using the capacitance between the diaphragm and a backing plate (CMUT ) or by integrating a thin layer of piezo-electric material directly onto the diaphragm (PMUT ). Alternatively, recent research has demonstrated that the diaphragm’s vibration can be measured by a minuscule optical ring resonator built inside the diaphragm itself (OMUS), a far more elegant solution. [5] [6]

And yes, ultrasonic transducers can be used for acoustic levitation . Making objects float on sound waves. It’s one of the few applications with a shred of theatricality. [7]

Use in depth sounding

A diagram showing the basic principle of echo sounding . Send a signal, wait for the echo, do the math. It’s not complicated.

This process, a core function of sonar , involves transmitting acoustic waves into water and meticulously recording the time interval between the emission of a pulse and the return of its echo. The resulting time of flight , combined with knowledge of the speed of sound in water, allows for a direct calculation of the distance between the sonar device and the target. This information is then typically used for navigation or to map the seabed for charting purposes.

The distance is calculated by multiplying half the travel time by the speed of sound in water , which is roughly 1.5 kilometers per second. The formula is simple: [T÷2 × (1.5 km/s)]. For high-precision applications like hydrography , one cannot simply rely on an approximate speed of sound. It must be measured directly, usually by deploying a sound velocity probe into the water. Echo sounding is, at its core, a specialized form of sonar used to find the bottom.

Since a traditional pre-SI unit for water depth was the fathom , an instrument used for determining water depth is sometimes called a fathometer. The first practical fathometer was invented by Herbert Grove Dorsey and patented in 1928, giving humanity a new and improved way to measure just how far down the darkness goes. [8]

Use in medicine

A 3D ultrasonography image. A fleeting, ghostly picture of life before it’s even begun.

In the medical theater, ultrasonic transducers, or probes, are our primary tools for non-invasive voyeurism. They come in a variety of shapes and sizes, each designed for making cross-sectional images of different parts of the body. The transducer might be used in direct contact with the skin, as in fetal ultrasound imaging, or inserted into a body opening such as the rectum or vagina for a more intimate view. Clinicians performing ultrasound-guided procedures often use a probe positioning system to hold the transducer steady, because human hands are fallible. [9]

Compared to other medical imaging modalities, ultrasound has several distinct advantages. It provides images in real-time, a live feed of the biological drama. It is portable and can be brought to a patient’s bedside, because crises rarely occur in convenient, well-equipped rooms. It is substantially lower in cost than other imaging strategies, a depressingly practical consideration. And it does not use harmful ionizing radiation , which is a low bar for a medical device, but here we are.

The drawbacks, however, are significant. Ultrasound has a limited field of view. It requires patient cooperation, which is often asking too much. Its effectiveness is dependent on the patient’s physique. It has difficulty imaging structures obscured by bone , air, or other gases, the body’s natural shields against prying eyes. [note 1] And most critically, its utility is entirely dependent on a skilled operator, usually a professional with extensive training. The tool is only as good as the hand that wields it.

Given these limitations, novel wearable ultrasound implementations are gaining traction. These are miniature devices that continuously monitor vital signs, designed to alert someone at the first sign of abnormality. A persistent, silent guardian. [10] [11]

Use in industry

An ultrasonic rangefinder as an electronic component. Small, unassuming, and relentlessly functional. A circular ultrasonic parking sensor mounted on a vehicle bumper . And the vehicle’s infotainment screen showing the obstacles detected by that sensor. A visual aid for the spatially challenged.

In the sterile, logical world of automated factories and process plants , ultrasonic sensors are the unblinking eyes of the machine. They detect the movement of targets and measure the distance to them, enabling a level of automation that makes human intervention obsolete. These sensors can have a simple on/off digital output for detecting the presence of an object, or an analog output that is proportional to its distance. They can sense the edge of a material as part of a web guiding system, keeping processes perfectly aligned.

Ultrasonic sensors are widely used in cars as parking sensors , aiding drivers in the delicate art of reversing into a confined space. They are also being tested for other automotive uses, including detecting people and assisting in autonomous UAV navigation. [citation needed ]

Because ultrasonic sensors use sound rather than light for detection, they work in applications where photoelectric sensors might fail. Ultrasound is an excellent solution for clear object detection and for measuring liquid levels—applications where photoelectrics struggle with target translucence. Furthermore, target color and reflectivity do not affect ultrasonic sensors, which can operate reliably even in high-glare environments. They are, in short, blessedly unconcerned with superficial appearances.

Passive ultrasonic sensors can be used to detect high-pressure gas or liquid leaks, or other hazardous conditions that generate ultrasonic sound. In these devices, the ultrasound captured by the transducer (a microphone, in this case) is converted down to the human hearing range (20 Hz to 20 kHz), making the inaudible audible.

High-power ultrasonic emitters are used in commercially available ultrasonic cleaning devices. An ultrasonic transducer is affixed to a stainless steel pan filled with a solvent (often water or isopropanol ). An electrical square wave feeds the transducer, creating sound in the solvent powerful enough to cause cavitation —the violent formation and collapse of microscopic bubbles. It’s a microscopic scrubbing for things you can’t, or won’t, clean by hand. This technology has been used for multiple cleaning purposes, one of which, gaining traction recently, is ultrasonic gun cleaning.

In ultrasonic welding and ultrasonic wire bonding , plastics and metals are joined using intense vibrations created by power ultrasonic transducers.

Finally, Ultrasonic testing is widely used in metallurgy and engineering to evaluate corrosion, welds, and material defects using various types of scans. It is a way of looking for the flaws beneath the surface, a task with which I am intimately familiar.

Notes

• ^ It is for this reason that a person subjected to an ultrasound of organs that can contain air or gas—the stomach, intestine, bladder—must follow a preparation protocol designed to reduce their quantity. This involves a specific diet, supplements for the intestine, and intake of non-carbonated water to fill the bladder. Sometimes, during the examination, you may be required to fill your stomach with non-carbonated water. It’s a small indignity in the service of a clearer picture.