Theory:

If we play two different musical instruments at the same time, the vibrations produced by the musical instruments travel through the same air medium, but we hear different sounds.

Travel of sound through air medium

A sound has three characteristics. They are
• Loudness
• Pitch or shrillness
• Quality
Loudness:

It is defined as a sound quality that allows us to distinguish between a feeble and a loud sound. The amplitude of a sound determines how loud it is. The sound will be louder as the amplitude increases, and vice versa. A weak sound is produced when a drum is lightly beaten. When it is beaten vigorously, however, a loud sound is produced. The $$decibel$$ is a measurement of sound volume ($$dB$$).
Example:
If we strike a bass drum with stick, the amplitude of the wave is high, so the sound is loud.

Drum

Pitch or shrillness:
• Pitch or shrillness of a sound is determined by its frequency.
• Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch.
• The faster the vibration of the source, the higher is the frequency and the higher is the pitch. Therefore, a high pitch sound produces more number of compressions and rarefactions, passing a fixed point per unit time.
• Sound with higher pitch is very clear to hear, but sound with low pitch is not clear.
Example:
If we blow a whistle, the frequency of the wave is high, so the pitch is high. Therefore the sound of whistle is very clear.

Whistle
Quality of sound:

The quality of sound, also known as timbre, is the property that allows us to distinguish between two sounds that have the same pitch and amplitude. In an orchestra, for example, the sounds produced by some musical instruments may be of the same pitch and volume. Nevertheless, each instrument's sound can be easily distinguished.
Reference:
https://www.flickr.com/photos/pablobd/3484668783
https://pxhere.com/en/photo/905672
https://commons.wikimedia.org/wiki/File:Fox-40-whistle.jpg