Frequency (f)

Professional Definition

In the physical and technical sense, frequency is defined in a very precise way:

Frequency (f) – the inverse of the oscillation period, numerically equal to the number of oscillation periods per second.

The unit of frequency in the SI system is Hertz (Hz). One Hertz (1 Hz) means exactly one complete oscillation (one wave cycle) per second.

Acoustics in Simple Words

If in the previous article we compared acoustic pressure to the "force" or "power" of sound, then frequency is its "pitch" or "tone".

Every sound occurs as a result of vibrations. The thick string of a double bass vibrates slowly. The thin string of a violin vibrates incredibly fast. Our vocal cords vibrate just like a speaker's membrane. Frequency describes exactly how fast these vibrations occur.

• Low frequency = slow vibrations = low, deep sound (bass).

  • Example: The sound of a ship’s engine, the lowest piano keys (around 30-50 Hz).

• High frequency = fast vibrations = high, squeaky sound (soprano).

  • Example: Birdsong, the hiss of cymbals, squeal of brakes (often above 2000 Hz, i.e., 2 kHz).

The standard "A" sound, used for tuning instruments in an orchestra, has a frequency of exactly 440 Hz. This means that the string or air in the instrument makes 440 complete oscillations each second.

Limits of Human Hearing

Our ears are not able to perceive all possible frequencies. The theoretical hearing range of a healthy young person is from 20 Hz to 20,000 Hz (20 kHz).

• Below 20 Hz are infrasound. We cannot hear them, but with high energy, we can feel them as vibrations. They are generated by, for example, earthquakes or wind turbines.

• Above 20 kHz start ultrasound. They are inaudible to us, but can be heard and utilized by, for example, dogs, dolphins, or bats. They have also found wide applications in medicine (ultrasound) and industry.

It is worth knowing that as we age, our ability to hear high frequencies naturally decreases.

Summary

Frequency is, along with amplitude (acoustic pressure), a key parameter describing sound. It allows us to distinguish musical tones, understand speech (which consists of many different frequencies), and identify sound sources in our environment. Understanding frequency is the first step towards analyzing more complex phenomena, such as acoustic spectrum or timbre of sound.