How is Sound Measured?

Sound is an aural sensation caused by pressure variations in the air.  The variation is always produced by some source of vibration for example a solid object or from turbulence in a liquid or gas.  These pressure fluctuations may take place very slowly, such as those caused by atmospheric changes, or very rapidly like the crack of lightening.  The velocity of sound is independent primarily on temperature.

Frequency is the number of vibrations of pressure fluctuations per second.  The unit is the hertz (Hz).

Wavelength.  This is the distance travelled by the sound during the period of one complete cycle.  Low frequencies have long wavelengths and high frequencies have short wavelengths.

Velocity of Sound in air under normal conditions is around 340m per second.

Waves in air are longitudinal, where the vibrations are in the direction of the motion.  The pressure fluctuations are like a spring that returns to its normal state once the excitation has ceased.  Air particles don’t speed thru the air like a bullet but rather vibrate causing changes in pressure.  Once the vibration has stopped, so does the movement of the air.

We measure sound intensity in units called Decibels (dB) which is named in honour of Alexander Graham Bell who invented the audiometer and telephone.  The Decibel scale is logarithmic.  This better replicates the sound intensity change that we feel in our ears.  If we add 10 decibels of sound it will sound twice as loud.

How does the ear work?

Sound waves enter the outer ear and travels through a narrow passageway called the ear canal, which leads to the eardrum.

The incoming sound waves vibrate in the eardrum.  These vibrations are sent to three tiny bones in the middle ear. The malleus, incus, and stapes.

These bones in the middle ear amplify the sound vibrations and send them to the cochlea which is a snail-shaped structure filled with fluid, situated in the inner ear.  The cochlea is split into an upper and lower part by an elastic partition which runs from the beginning to the end. The elastic partition is called the basilar membrane and serves as the base on which key hearing structures sit.

Vibrations in the cochlea cause the fluid inside the cochlea to ripple resulting in a traveling wave which forms along the basilar membrane. Hair cells which are sensory cells sitting on top of the basilar membrane ride the wave. Hair cells near the wide end of the snail-shaped cochlea detect higher-pitched sounds, such as a bird chirping. Those closer to the center detect lower-pitched sounds, such as a horse neighing.

As the hair cells move up and down, microscopic hair-like projections, known as stereocilia,  which sit on top of the hair cells bump against an overlying structure and bend.  The bending causes channels (which are at the end of the stereocilia)  to open up. When that happens, chemicals enter the cells, creating an electrical signal.

The electric signal is carried by the auditory nerve to the brain, which processes it into a sound that we recognize and understand.