This is the 3044th most frequent Russian word.
звук
"Звук" means "sound" in English.
In this sentence, 'звук' is used to refer to a specific sound, indicating something auditory that caught attention.
Я услышал странный звук за дверью.
I heard a strange sound behind the door.
Here, 'звук' is used in a scientific context, referring to sound as a general concept studied in physics.
Он изучает физику звука.
He studies the physics of sound.
'Звук' in this sentence describes a quality of a sound, emphasizing its auditory pleasantness.