Ultrasonic sensors can be very interesting. Sound waves are sent and received by transducers, the sensor automatically shows measurements, and it all happens without human intervention. You can select sensors according to range transducer type or even reviews or ratings. Sensors usually reflect their range and frequency simultaneously, which is somehow intriguing.
So why is an ultrasonic sensor's range always attached to its frequency? First off,
ultrasonic sensors utilize ultrasonic waves when measuring distance. The whole wisdom behind ultrasonic sensors is that distance is equal to time multiplied by velocity. For a certain combination of air temperature, humidity and other atmospheric conditions, sound velocity is known. Therefore, the time it takes for a sound wave to move from the transducer's sensor to its destination and back, must have a lot to do with the distance between them. But of course, that was clear from the start. For more information about ultrasonic sensors view here.
The question is, why are effective detection ranges shorter when it comes to higher frequency sensors?
When you talk about signal loss of any kind, attenuation is always to be blamed. This certainly applies to sound waves. Sound waves travel through the air (and everything else), which means air properties resisting that motion accumulate together. This is known as attenuation. As earlier mentioned, the speed of sound is influenced by atmospheric conditions, making them a substantial part of sound attenuation in air.
For sound waves between 50 kHz and 300 kHz, maximum attenuation can be estimated at ?(f) = 0.022 * f - 0.61, where ?(f) refers to the maximum attenuation in dB/ft, while f is the sound wave's kHz frequency. This equation may not seem to shed light on anything. After all, a factor of 0.022 is quite tiny. Attenuation is 0.5 dB/ft at 50 kHz, but it magically soars to 2.7 dB/ft at 150 kHz. Obviously, the decline in signal is not gradual, but sharp and abrupt.
Even then, that's not all there is to it. The above equation is only applicable to the full humidity range and leaves out changes in atmospheric pressure, although the effects are not very significant. Still, you have to note that the equation measures dB for every foot lost, but it is clueless as to the sensitivity of your sensor or the strength of your signal.
Hence, it is important to keep in mind that not all ultrasonic sensors are the same. Some are as basic as having little to zero configurable options, while others give you full control. At the end of the day, you still have to look for the sensor that is best aligned with your needs. Frequency is, without doubt, a big part of the picture, but it's still not everything.