Passive Ultrasonic Sensor: A Comprehensive Guide

Passive ultrasonic sensors (PUS) are a type of sensor that detects and analyzes ultrasonic waves generated by other sources, rather than actively emitting waves like active ultrasonic sensors. PUS are often used in applications such as non-destructive testing (NDT) of materials, structural health monitoring (SHM), and acoustic emission (AE) testing, providing valuable information about the properties and behavior of various materials and structures.

Understanding Measurable Data from Passive Ultrasonic Sensors

PUS can provide a wealth of measurable, quantifiable data that can be used to analyze and understand the properties of the materials and processes being monitored. Some of the key data points that PUS can provide include:

Time of Flight (TOF)

TOF is the time it takes for an ultrasonic wave to travel through a medium and be detected by the sensor. This information can be used to calculate the distance to the source of the wave, as well as the velocity of sound in the medium. For example, in a concrete monitoring application, changes in TOF can be used to detect the onset of cracking or other structural damage.

Signal Amplitude

The amplitude of the detected ultrasonic signal can provide information about the strength of the source. This can be useful in applications where the intensity of the ultrasonic waves is an important factor, such as in NDT or AE testing.

Frequency Content

The frequency content of the detected ultrasonic waves can provide information about the type of material or process that is generating the waves. This can be useful in applications where the identification of specific materials or processes is important, such as in SHM or NDT.

Technical Specifications of Passive Ultrasonic Sensors

passive ultrasonic sensor

PUS can vary widely in their technical specifications, depending on the application and manufacturer. Some common specifications include:

Frequency Range

The frequency range of a PUS can vary from as low as 20 kHz to as high as 1 MHz, depending on the specific application. For example, PUS used in NDT of metals may have a higher frequency range (e.g., 500 kHz to 10 MHz) compared to those used in SHM of concrete structures (e.g., 20 kHz to 100 kHz).


The sensitivity of a PUS is typically expressed in decibels (dB) relative to 1 volt per micropascal (dB re 1 V/μPa). A higher sensitivity value (e.g., -60 dB) indicates a more sensitive sensor, which can detect weaker ultrasonic signals.

Dynamic Range

The dynamic range of a PUS is the difference between the maximum and minimum detectable signal levels, typically expressed in decibels (dB). A wider dynamic range (e.g., 120 dB) allows the sensor to detect a broader range of signal amplitudes, which can be important in applications with varying signal strengths.


PUS may also have specifications for the input impedance and output impedance, which can affect the sensor’s compatibility with other electronic components and the quality of the signal transmission.

Building Your Own Passive Ultrasonic Sensor

For those interested in building their own PUS, there are several resources available, particularly in the Arduino community. One example is an Arduino forum post that discusses testing the accuracy, precision, and resolution of the HC-SR04 ultrasonic sensor, a popular and affordable PUS module.

The author of the post provides detailed procedures and results, as well as the Arduino sketch used for the tests. The tests revealed that the HC-SR04 sensor has an accuracy of around 3 mm, a precision of about 1 mm, and a resolution of approximately 0.3 mm, making it a suitable choice for many DIY projects and applications.

Another Arduino forum post discusses how to modify the HC-SR04 sensor to measure within a specific range, rather than providing a percentage reading. The post includes a sketch that converts the distance measurement to a percentage based on a user-defined tank depth, which can be useful in applications where a specific measurement range is required.


Passive ultrasonic sensors are a versatile and powerful tool for a wide range of applications, from non-destructive testing to structural health monitoring. By providing measurable, quantifiable data such as time of flight, signal amplitude, and frequency content, PUS can help researchers, engineers, and DIY enthusiasts gain valuable insights into the properties and behavior of materials and structures.

Whether you’re working on a professional project or a DIY application, understanding the technical specifications and capabilities of PUS can help you select the right sensor for your needs and get the most out of your ultrasonic monitoring system.


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  5. Product Detection and Ranging Using Ultrasonic Sensors – DigiKey