0.6 Sound  (Page 2/4)

Speed of sound

The speed of sound depends on the medium the sound is travelling in. Sound travels faster in solids than in liquids, and faster in liquids than in gases. This is because the density of solids is higher than that of liquids which means that the particles are closer together. Sound can be transmitted more easily.

The speed of sound also depends on the temperature of the medium. The hotter the medium is, the faster its particles move and therefore the quicker the sound will travel through the medium. When we heat a substance, the particles in that substance have more kinetic energy and vibrate or move faster. Sound can therefore be transmitted more easily and quickly in hotter substances.

Sound waves are pressure waves. The speed of sound will therefore be influenced by the pressure of the medium through which it is travelling. At sea level the air pressure is higher than high up on a mountain. Sound will travel faster at sea level where the air pressure is higher than it would at places high above sea level.

Speed of sound

The speed of sound in air, at sea level, at a temperature of $21{}^{\circ }\mathrm{C}$ and under normal atmospheric conditions, is $344\mathrm{m}·\mathrm{s}{}^{-1}$ .

Sound frequency and amplitude

Study the following diagram representing a musical note. Redraw the diagram for a note

1. with a higher pitch
2. that is louder
3. that is softer

Physics of the ear and hearing

The human ear is divided into three main sections: the outer, middle, and inner ear. Let's follow the journey of a sound wave from the pinna (outermost part) to the auditory nerve (innermost part) which transmits a signal to the brain. The pinna is the part of the ear we typically think of when we refer to the ear. Its mainfunction is to collect and focus an incident sound wave. The wave then travels through the ear canal until it meets the eardrum. Thepressure fluctuations of the sound wave make the eardrum vibrate. The three very small bones of the middle ear, the malleus (hammer),the incus (anvil), and the stapes (stirrup), transmit the signal through to the elliptical window. The elliptical window is the beginning of theinner ear. From the elliptical window the sound waves are transmitted through the liquid in the inner ear and interpreted as sounds by the brain.The inner ear, made of the semicircular canals, the cochlea, and the auditory nerve, is filled with fluid. The fluid allows the body todetect quick movements and maintain balance. The snail-shaped cochlea is covered in nerve cells. There are more than 25 000 hairlikenerve cells. Different nerve cells vibrate with different frequencies. When a nerve cell vibrates, it releases electrical impulsesto the auditory nerve. The impulses are sent to the brain through the auditory nerve and understood as sound.

Ultrasound

Ultrasound is sound with a frequency that is higher than 20 kHz. Some animals, such as dogs, dolphins, and bats, have an upper limit that is greater than that of the human ear and can hear ultrasound.