Basic course in acoustics

Sound arises from small pressure variations in air, water, or some other substance. Sound waves are thus kinetic energy in the form of vibrations, which for us humans are usually experienced through the air. And the sound source itself, which causes the air to vibrate, is often a vibrating surface. For example, our voices are the result of vocal cords vibrating thanks to air from the lungs. Similarly, strings on a guitar or a diaphragm in a speaker work in the same way. Sound can also be created through a rapid combustion, such as an explosion. When the sound waves reach our ears, they are converted into signals that are sent to the brain, and then we perceive them as sound.

Sound frequency

The frequency tells us what tone the sound has. A high-frequency sound is squeaky or sharp, while a low-frequency sound is rumbling or muffled. The frequency is measured in hertz (Hz) and indicates how many oscillations per second a sound wave has. 1 Hz means one oscillation per second. The human ear can, at best, perceive sounds ranging from 20 to 20,000 Hz. Above 20,000 Hz is ultrasound, below 20 Hz is called infrasound. Many animals, however, can perceive a much wider frequency range. For example, dogs can perceive high-frequency sounds up to 60,000 Hz. That's why they react to special whistles that are completely silent to us.

There are sound enthusiasts (audiophiles) who claim that tones outside the audible range still matter in film and music contexts. They believe that infrasound is good for bass reproduction because it makes the room "breathe," while ultrasound adds "spatial information," such as the guitarist in a live recording standing behind the singer. Quite advanced, in other words.

While we are on the topic of music and HiFi, we might as well mention how sound frequencies are handled in a speaker. Most expensive speakers are divided into at least three components that manage different frequency ranges: bass (about 20–200 Hz), midrange (about 200–2,000 Hz), and treble (about 2,000–20,000 Hz). In English, the bass component is named after a dog's bark (woofer), and the treble is named after bird chirping (tweeter). Voices lie in the midrange, and it is also where our ears are most sensitive, i.e., best at perceiving nuances in sound.

The speed of sound

The denser a material is, the faster sound travels through it. In air, sound waves travel at approximately 340 m/s, which corresponds to 1,224 km/h or mach 1. In water, it is about 1,500 m/s and in metal, it can be up to about 6,300 m/s depending on the type of metal. When an aircraft exceeds the speed of sound, it breaks the sound barrier. A pressure wave is created, and when it hits the ground, it results in an explosion-like sonic boom that can shatter windows and damage entire buildings. Therefore, it has long been prohibited to operate civil aviation at supersonic speeds where there are built-up areas.

Another somewhat magical technique based on sound knowledge is the active noise cancellation found in some headphones. For it to work, a microphone is used to capture sound waves from background noise or disturbances. The exact same sound waves, but completely inverted, are then sent out through the headphones. The result is silence, and you don't have to hear the noise in the office, on the bus, or whatever it may be. This technology is also used in some cars and airplanes.