Sound Carl Wozniak Northern Michigan University

Some sound facts?  Sound is a mechanical wave which is created by vibrating objects and propagated through a medium from one location to another.  A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location.  The medium is simply the material through which the disturbance is moving;

Parts of a wave  The crest of a wave is the point on the medium which exhibits the maximum amount of positive or upwards displacement from the rest position. Points D and I on the diagram represent the troughs of this wave.  The trough of a wave is the point on the medium which exhibits the maximum amount of negative or downwards displacement from the rest position.  The amplitude of a wave refers to the maximum amount of displacement of a a particle on the medium from its rest position.  The wavelength of a wave is simply the length of one complete wave cycle.  The frequency is the number of complete waves that pass a stationary point in one second.

Pitch  The pitch of a sound refers to whether is is high, like the sound of a piccolo, or low, like a bass drum. The lower the frequency, the lower the pitch The higher the frequency, the higher the pitch  Sound waves whose frequencies are outside the audible frequencies are called ultrasonic (if higher) and infrasonic (if lower).

Sound level  Loudness is related to the energy in a sound wave. We measure sound in units called decibels (db) Ticking of a Watch20 Whisper30 Normal Speech50-60 Car Traffic70 Alarm Clock80 Lawn Mower95 Chain Saw110 Jackhammer120 Jet Engine130

Speed of sound  At a temperature of 68°F (20°C), sound travels at 1,125 feet/sec (343 meters/sec) through air. This is the same as traveling at 756 miles/hr (1,217 km/hr).  As the temperature rises, the speed of sound increases. V ≈ ( T) m/s, where T = temperature °C  As density increases, the speed of sound increases.

Speed of sound  When a jet flies faster than the speed of sound it creates shock waves at the front of the plane (bow shock) and tail of the plane (tail shock).  Shock waves are abrupt jumps in the pressure, temperature, speed, and density of the fluid in which they propagate  A sonic boom, is our perception of these waves as they pass by us.

Speed of sound  The most common misconception about sonic boom is that it is the sound of the object "crashing through the sound barrier".

Speed of sound  If an object or aircraft entered the atmosphere from the vacuum of space and never decelerated to a speed below the sound speed of the atmosphere, the sound barrier would never be broken, yet observers on the ground would all hear a sonic boom, provided that they were close enough to the aircraft.

The sound barrier  In the transsonic region, condensation clouds frequently form around the object. The clouds appear for the same reason that clouds always form, namely, that the air has cooled to the point that the ambient water vapor condenses.

Near the sound barrier

How we hear  Our ears detect and amplify the vibrations in the medium (i.e. air or water) around us.  The vibrations are measures in hertz, cycles per second.  We can hear vibrations between 20 and 20,000 hertz (Low numbers are low pitches, high numbers are high pitches.)

How we hear  Vibrations strike the eardrum  The vibrations are transferred and amplified by the three middle ear bones: hammer, anvil, stirrup.  The vibrations are passed on to the cochlea. Inside the fluid-filled cochlea are many small hair- like structures that detect the vibrations.  The vibrations are turned into an electrical impulse which is passed on to the auditory nerve, which connects with the part of the brain that perceives sound.

Hearing in different media  We can tell the direction a sound is coming from because the sounds don’t reach our ears simultaneously.

Hearing in different media  When we are under water, the speed of sound so fast (3200 mph, 1400m/s) that our ears cannot separate the difference  We can’t tell the direction of a sound underwater.

Doppler effect  If a noisy object is stationary, sound waves propagate out in all directions evenly.

Doppler effect  If that same object is moving, the sound waves are compressed ahead of the object and stretched behind it. Direction of motion

Doppler effect  The wavelengths of the compressed waves are shorter, resulting in an increased pitch, and the wavelengths of the stretched waves are longer, resulting in a decreased pitch.

Doppler effect

 When an object is moving supersonic, it is moving in front of the compression waves.  The sound comes from behind the object.