The denser the medium (solids > liquids > gases), the lesser the absorption, so sounds can propagate over much greater distances. As well, in any given medium, low frequency waves are less absorbed than high frequency waves and so can travel further. All this explains how the low frequency songs of the humpback whale can travel thousands of miles across the ocean.
Why do we hear echoes?
The property of sound waves responsible for this auditory phenomenon is reflection. Sound waves bounce off objects while travelling through the air, thereby creating echoes in spots surrounded by large rocks, such as caves and canyons.
Certain animals use echolocation to move about. Bats make high pitched sounds while flying, which bounce off objects in the form of echoes. This gives the bats information about the distance and direction of the objects, which in turn helps them navigate. Submarine sonar works in a way that is similar to echolocation.
Psychology has a seemingly different definition of what sound is. The psychological definition states that sound is based on an individual’s perception.
This explanation, however, is restrictive and does not account for frequencies that are above or below an individual’s hearing threshold. Sound waves, in the range of human hearing, vary from 20 to 20,000 Hz. Waves recorded below 20 Hz are called infrasonic waves, while higher frequencies above 20,000 Hz are known as ultrasonic waves.
In actuality, the perceptive features of sound - loudness, pitch, and timbre - are all influenced by the physical characteristics of sound waves.
What is loudness?
The average amount of energy passing through a unit area per unit of time in a specified direction is called the intensity of the wave. The amplitude of a sound wave in part determines its intensity, which in turn is perceived by the ear as loudness. As defined in physics, the intensity of a wave is proportional to the square of its amplitude:
Therefore, the greater the amplitude, the louder the sound perceived. Amplitude is measured in decibels.