Height and loudness of the sound
When we often say that a sound is 'low' or 'high,' we are referring to the height of the sound. The height of the sound is completely different from the loudness of the sound.
The height of a sound is the number of times a sound wave vibrates per second. The higher the frequency, the higher the pitch is. The keys on the piano sound higher as you go to the right. In stringed instruments, the thinner and taut the string, the higher the pitch.
On the other hand, the loudness is related to the amplitude. To say that a sound is 'big' means that the amplitude is high. The loudness of the sound has nothing to do with the aforementioned frequency.
For example, if you fix both ends of a rubber band and bounce it in the middle, the rubber band vibrates and makes a sound. At this time, the harder you bounce the rubber band, the louder the sound becomes. If you bounce the rubber band hard, you won't get a high note. Also, when you hit the drum, the harder you hit the drum, the louder it vibrates, and the sound becomes louder. If you hit the drum hard, the sound gets louder, but the height doesn't change.
Audible frequency and ultrasound
There is a limit to the frequency of sounds that we can hear. Usually, people can hear sounds between 20Hz and 20kHz. The frequencies in this range are called the 'audible frequency.' Sounds outside this range are hard to hear.
We can't hear high notes as we get older. This is probably due to the aging of the auditory organs and the weakening of the elasticity.
Sounds higher than 20 kHz are called 'ultrasonic waves.' Usually, people cannot hear ultrasound. It is known that bats can hear ultrasonic waves well, and they can also make ultrasonic sounds. The bat emits an ultrasound, then hears the echoes coming back, and measures the distance to the cave wall. In the case of a car, an ultrasonic sensor is installed on the rear bumper, which measures the distance to the wall when reversing and sounds as a warning before hitting the wall.