Frequency and intensity of sound are related but distinct properties of a sound wave.
Frequency refers to the number of oscillations or cycles of the wave that occur per unit of time. It is measured in hertz (Hz). In the context of sound, frequency determines the pitch or perceived musical note of the sound. Higher frequency sound waves are perceived as higher-pitched sounds, while lower frequency sound waves are perceived as lower-pitched sounds. The human audible range typically extends from around 20 Hz to 20,000 Hz.
Intensity, on the other hand, refers to the amount or level of energy carried by a sound wave. It is often associated with the perceived loudness or volume of the sound. Intensity is typically measured in decibels (dB). Higher intensity sound waves are perceived as louder sounds, while lower intensity sound waves are perceived as softer sounds.
Now, regarding your question, if a sound has a frequency lower than 20 Hz (below the lower limit of human hearing), it will not be perceived as a distinct pitch by the human ear. However, the intensity or energy level of the sound can still be perceived if it is sufficiently high. Very low-frequency sounds below the audible range, but with high intensity, can still be felt as vibrations or pressure sensations in the body rather than being heard as distinct sounds.
For example, infrasound refers to sound waves with frequencies below the audible range of human hearing (less than 20 Hz). In some cases, intense infrasound can be perceived as vibrations or pressure waves, and people may feel its effects rather than hearing it directly. Some animals, such as elephants, can detect and communicate using infrasound.
In summary, frequency and intensity of sound are related but distinct properties. Frequency determines the pitch of a sound, while intensity determines its loudness or volume. Sounds with frequencies below 20 Hz are typically not perceived as distinct pitches by the human ear, but if they have high intensity, they may still be felt as vibrations or pressure waves.