In this blog, we’ll cover laser vibrometers and how they employ the Doppler Effect. These devices are commonly known as Laser Doppler Vibrometers. Applications of Laser Doppler Vibrometer include testing manufactured parts to detect anomalies, flaws, and other nonconformance to specifications.

What Is the Doppler Effect?

Light reflected from a moving object is subject to a change in frequency proportional to the object’s velocity. This is also known as the Doppler Effect. Named after Austrian physicist Christian Doppler, this phenomenon applies to all types of waves, including sound and light.

Let’s get into the physics behind the Doppler Effect. And who better to turn to for this than NASA? NASA explains that the Doppler Effect is observable when a source of sound (like a siren) moves toward or away from an observer. As the source approaches, the sound waves are compressed, raising the pitch. When the source moves away, the waves stretch, lowering the pitch. This also applies to light waves in astronomy. For instance, NASA uses the Doppler Effect principle to measure the velocity of distant galaxies, where light appears redshifted as galaxies move away from Earth.

This paper titled “Doppler Effect” from NASA’s Glenn Research Center presents the equations behind the science.  As a moving source approaches our ear, the wavelength is shorter, the frequency is higher, and we hear a higher pitch. If we call the approaching frequency fa, the speed of sound a, the velocity of the approaching souce u, and the frequency of the sound at the source f, then

fa = [f * a] / [a – u]

As the source moves away from us, the wavelength is longer, the frequency is lower, and the pitch is lower. Again. if the leaving frequency is called fl, then

fl = [f * a] / [a + u]

Understanding the Doppler effect explains how sound, light, and other waves change in frequency based on movement. Aside from product quality inspection, this phenomenon has practical applications in astronomy, radar, medical imaging, and everyday technologies like speed detection and weather forecasting, improving our ability to observe and measure dynamic systems.

How Laser Vibrometers Use the Doppler Effect

Laser Doppler Vibrometers are often used to ensure the quality of manufactured parts. These devices measure the velocity of a vibrating surface. A laser beam is directed at the surface, and as it moves, the reflected light experiences a frequency shift. If the surface moves toward the vibrometer, the frequency increases; if it moves away, the frequency decreases. This frequency shift, proportional to the velocity of the surface, is detected and analyzed by the vibrometer, allowing precise, non-contact measurement of vibrations in a variety of materials and systems.

Learn more by reading this blog “Vibrometers: How they Work” from Signalysis and watching this video “Basic Principles of Laser Doppler Vibrometry” from Polytec.

Laser Vibrometers: The Key to Vibration Testing

Laser vibrometers are widely used in Noise, Vibration, and Harshness (NVH) testing to measure vibrations and surface velocities without contact. In automotive and aerospace industries, they help assess how various components, like engines or panels, contribute to unwanted noise and vibrations. The laser vibrometer directs a beam onto the surface of a vibrating part, capturing velocity data via the Doppler effect. This data is crucial for identifying sources of noise and vibration, optimizing designs, and enhancing product comfort and durability.

Signalysis incorporates Laser Doppler Vibrometers in our quality inspection systems. Their versatility, high precision, and non-intrusive nature make them ideal for evaluating dynamic behavior in a wide variety of NVH applications.

Learn More

Contact us to learn more about our quality inspection systems. We also offer training on Laser Vibrometers, NVH, Non-Destructive Testing (NDT), and Laser Vibrometer use.