Acoustically Soft vs. Acoustically Hard
- Acoustic properties limit the detection range of certain targets
- Understanding a target’s acoustic properties helps with sensor selection
- Targets that absorb more sound are more difficult to detect with ultrasonic sensors
The Reflection of Sound
All targets reflect sound to a varying degree. To better understand this, a parallel can be drawn between the reflection of light and the reflection of sound. Every object reflects different amounts and colors of light. The light that reflects off a target creates the object’s visual appearance. This encompasses the object’s color, sheen, luminosity, and any other visible features. Just as we see the difference in the way objects reflect light, we hear a difference in the way objects reflect sound. For example, a sheer cliff face or canyon wall create clear rolling echoes while thick foam absorbs sound to the point where you may never hear an echo. This demonstrates, that just as all objects have a set of visual characteristics, they also have acoustic ones.
It would be convenient if sound reflected off all targets equally, but this is not the case. Further, we can not assume that all hard targets reflect sound well and all soft targets reflect sound poorly. Instead, one of the key variables in acoustics is whether or not the target is porous in nature. Targets that are porous or not densely packed such as cloth, people, snow, or dust do not reflect sound well. The small pockets of air within these targets allow for the sound to dissipate. Even a hard target, like a pumice stone, does not reflect sound well. The thousands of air bubbles that allow pumice to float also make it a poor reflector of sound. In some cases, even physically hard targets like pumice fall into the acoustically soft category. Rather than reflecting sound, acoustically soft targets tend to absorb it.
Objects that form nonporous and smooth surfaces reflect sound well.1 Glass panes, sheet metal, finished wooden surfaces, and even water are a few examples of smooth non-porous surfaces that reflect sound well. While water is not a solid, it reflects sound well because it is non-porous and forms smooth flat surfaces. Such targets that reflect sound well are considered acoustically hard.
How Sound Reflection Affects Our Sensors
Ultrasonic sensors use the speed of sound to calculate distance based on the time it takes for an echo to return from a target. More simply put, our sensors detect distance much like a bat or dolphin does. Our sensors also produce a high-energy, high-frequency sound. That sound travels out from the sensor and hits the targets in front of it. The sound then reflects off of these objects and returns to the sensor as an echo. If any of the echoes are loud enough for the sensor to detect, the sensor calculates how much time passed between the echo and the release of the initial sound. Then the sensor uses the elapsed time and the speed-of-sound to calculate the distance to the target.
As reflected sound is key to the operation of our sensors, the target’s acoustic properties play heavily into an application’s success. As seen in Figure 2, sound can interact with targets in different ways. All objects reflect some sound even if it is only a negligible amount. This means our sensors detect many acoustically soft targets, but the distance at which soft targets are detected is limited. Because acoustically hard targets reflect more sound, our sensors can detect them more easily to greater distances.
Our datasheets and beam patterns provide a visual representation of how target acoustics affect detection zones. Targets that are acoustically soft fall closer to the detection zone of Beam Pattern A. Acoustically hard targets fall closer to the detection zone of Beam Pattern C. The amount of sound reflected will fall along a continuous scale. Thus, some discrimination should be used to determine if the detection zone for your target will be most similar to Beam Pattern A, B, C, or even fall beyond these options. To clarify a point, the size of the target is an additional factor in the amount of sound returned to the sensor. Large targets reflect more sound by sheer merit of their size. More surface area means more reflected sound. This means both size and the acoustic properties of the target affect detection. For additional help in understanding our beam patterns please review our beam pattern article.
Understanding the acoustic properties of your target is an important part of the sensor selection process. You can better select a sensor to detect your target if you understand how well your target reflects sound. Matching your sensor to your application and target helps ensure a successful application. Please contact our technical support team to guide you through sensor selection if you need help in selecting a sensor or in identifying the acoustic properties of your target. As a final note, our people detection article is an excellent resource for more information about detecting acoustically soft targets.
1There are non-porous absorbers which are also known as absorbing resonators. These are commonly used in industry to help absorb sound in some industrial settings.
2Pumpice Stone image found here: http://www.jei.is/index.html