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Ultrasound

Ultrasound uses high-frequency sound waves above the 20,000 Hz range of human hearing to detect motion and measure speed through the Doppler effect. By analyzing how reflected wave frequencies shift, ultrasonic sensors can determine whether an object is approaching or moving away and calculate its velocity.

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About this video

Ultrasound occupies the portion of the sound spectrum above 20,000 Hz, the upper boundary of normal human hearing. Just as ultraviolet light exists beyond the blue end of the visible spectrum, ultrasound exists beyond the frequencies the human ear can detect. Sound waves are mechanical waves, which distinguishes them from the electromagnetic waves used in technologies like infrared and radar, but both types follow similar wave principles, making many of the same detection techniques applicable across both domains. Motion detection using ultrasound relies on the Doppler effect. A sensor emits a continuous signal at a fixed frequency, which travels outward and reflects off objects in its path. If the object is stationary, the reflected frequency matches what was originally transmitted. If the object is moving toward the sensor, it intercepts the outgoing waves sooner than it would at rest, compressing the return signal into a higher frequency. If the object is moving away, each successive wave has farther to travel before reflecting back, spreading the return signal into a lower frequency. The size of that frequency shift is directly proportional to the speed of the object. A small shift indicates slow movement, while a large shift indicates fast movement, and the direction of the shift reveals whether the object is approaching or receding. This allows ultrasonic sensors to function effectively as speed-measuring devices, providing the same core capability as Doppler radar but using mechanical rather than electromagnetic waves. These systems are widely applied in security, industrial automation, and traffic monitoring environments.

What you'll learn

What's covered

Ultrasound Motion Detection

Key terms

Ultrasonic Sensor
A device that emits ultrasound waves and detects their reflections to measure the distance, motion, or speed of objects.
Doppler Effect
The observed change in frequency of a wave caused by relative motion between the wave source and the observer, used in radar to detect the speed and direction of moving objects.
Ultrasound
Sound waves with frequencies above the human hearing range, typically above 20,000 hertz, used in sensing and imaging applications.
Frequency
The number of wave cycles that occur per unit of time, measured in hertz; changes in reflected frequency are used by radar to calculate object speed.
Mechanical Wave
A wave that requires a physical medium to travel through, such as sound, as distinguished from electromagnetic waves.

Topics

Ultrasound Doppler Effect Ultrasonic Sensors Motion Detection Signal Processing Sensor Technology

Transcript

Another technology that we can use to detect motion is ultrasound.

Where Ultrasound Sits on the Spectrum

We talked about visible light. This is anything from red to blue. And then the next stage of that is ultraviolet. Ultraviolet our eyes don't pick up, but it exists. We also have infrared, which is below the red spectrum. So infrared is below and ultra is above.

Well, what is ultrasound? Ultrasound is above sound waves. So sound comes in waves, just like what we've been talking about, except sound waves are what's called a mechanical wave, and what we've been talking about is actually electromagnetic waves. The two are different. However, in a lot of aspects, these waves behave very similarly.

So ultrasound is a sound wave, but it's just above our being able to hear it. Sound waves, waves that we can actually hear, these mechanical waves, are anywhere from 20 hertz, that means 20 cycles per second, to 20,000 hertz, which is 20,000 cycles per second. Those are really high highs, and when you get up into this range, a lot of people don't necessarily hear that high of a pitch, especially older people, because as we get older our sensitivity to those higher ranges actually goes down. But essentially what we're talking about when we're talking about ultrasound is what's above those sound waves. So above the 20,000 hertz range, then we get into ultrasound.

Using the Doppler Effect to Measure Speed

So how do we use ultrasound to gauge motion, to be able to measure speed? We use the Doppler once again. So if we've got the source of a frequency that's coming out, and let's say that frequency is a wave, so it bounces off this car, and if it's bouncing back in the same frequency that it got sent out at, and that's what gets registered, now we know that a car is standing still, or an object is standing still.

But let's say it's traveling towards the source, then it's bouncing back. It's moving forward, hitting those waves sooner, bouncing back, and then gets measured, and it says, oh yeah, this is coming towards this source. And then it can measure the speed at which it's coming at it based off of how close these waves are.

Then we've got a car that's moving away. So the first wave hits, and then by the time the second wave hits, it's further away, and by the third time the wave hits, it's further away yet. And so that comes in, and now this source, this radar detector, realizes that this car is moving away, because the frequency is less than what was sent out. And not only that, it can tell the speed at which it's moving away based off how far apart these waves are. So that's how we use the ultrasound to gauge what speed something is going and figure out motion.

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