Basic knowledge of ultrasonic sensors: Properties of sound waves | Basic knowledge about ultrasonic sensors

Properties of sound waves
Speed of sound and acoustic impedance within a medium
Directionality, resolution, and frequency
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Properties of sound waves

The propagation speed of sound waves differs depending on the medium. In addition, sound waves have properties of reflection and transmission related to the acoustic impedance of the medium.

The higher the elasticity the faster sound travels.

○Sound travels more quickly through solids than through liquids, and through liquids more quickly than through gases.
Speed of sound = elasticity ÷ density

The speed of sound when traveling through the air is affected by the temperature.

○C = 331.5 + 0.6T [m/s]
C: Speed of sound traveling through air, T: Temperature (°C)

The greater the difference in acoustic impedance between media, the greater the reflection. When there is little difference in acoustic impedance between media, transmission occurs.

○Acoustic impedance＝ρ･Ｃ
ρ: Density of medium, Ｃ: Speed of sound traveling through medium

Speed of sound and acoustic impedance within a medium

Every object has its own characteristic acoustic impedance. Acoustic impedance is expressed as a numeric value indicating the ease of sound propagation. It can be calculated as density of medium × speed of sound traveling through medium.

Zo=ρc

Ultrasonic waves are reflected when the difference in acoustic impedance between media is large, and they are transmitted when the difference is small.
Murata ultrasonic sensors propagate ultrasonic waves through the air. Since there is a very large difference with the acoustic impedance of water ( 10⁶ ), reflection occurs at the boundary surface.

Speed of sound and acoustic impedance within various media

Propagation medium	Zo=ρc	ρ(kg/m3)	c(m/s)
Steel	39×10⁶	7700	5000
Aluminum	14×10⁶	2700	5200
Fused quartz	12×10⁶	2700	4400
Water	1.44×10⁶	1000	1440
Air	410	1.2	340

Zo: Acoustic impedance
ρ: Density
c: sound speed

Directionality, resolution, and frequency

Here we describe the directionality, resolution, and attenuation of ultrasonic waves with relation to frequency.

Directionality

The directionality of the ultrasonic sensor describes a cone shape radiating upward from the top surface, with directionality defined as −6 dB compared with the front surface sound pressure. The directionality is wide compared to optical sensors, and one of its features is that it becomes sharper the higher the frequency.