Sound Acoustics is the study of sound. All sounds are waves produced by vibrating objects - tuning forks, vocal chords, reeds, lips, columns of air, strings, cricket legs Demo – tuning forks - water

Sound

Sound Waves Sound is a longitudinal wave with compressions and rarefactions.

Sound Waves page 272

Sound Waves Notice that the air molecules move in a direction parallel to the direction of the wave. Demo – Slinky on the floor.

Speed of Sound The speed of propagation MediumSpeed of sound Air343 m/s Steel5100 m/s Water1370 m/s Vacuum0 m/s

Speed vs Temp As temperature increases, the speed of sound increases. 0.6 m/s per o C

Speed of Sound vs Speed of Light Sound = 343 m/s Light = 300, 000, 000 m/s Notice that light travels much faster.

Lightning & Thunder Light reaches you in an extremely short period of time. Sound reaches you at a much slower rate. It takes about 5 seconds to travel 1 mile.

The delayed sound reaching your ear after the light. Other examples include – starters pistol, chopping wood You see the event(light), count the number of seconds until the sound arrives. 5 seconds = 1 mile 10 seconds = 2 miles

Figure Problem 14-59

Path that sound travels in your ear Sequence of vibrations from the source To the ear drum To the bones in the middle ear To the oval window in the middle ear To the fluid in the inner ear To the hairs in the cochlea in the inner ear To the nerves which go to the brain on the auditory nerve.

Pitch The pitch is determined by the frequency of the sound. Units are Hz or vibrations per sec. Humans hear 20 to 20,000 Hz

Loudness of Sound How loud a sound seems is determined by the wave’s amplitude. This is proportional to its energy. We use a decibel scale to measure loudness.

Sound Loud noises can damage your hearing. This usually lowers your upper limit. The tiny hairs in the inner ear may fall out.

Loudness SoundDecibels Hearing threshold 0 Rustle of leaves 10 Conversation60 Rock Concert110 Pain Threshold120 Jet Engine130

Reflection of Sound Reflection of sound is called an echo. Sound waves reflect off of hard smooth surfaces. Curtains and rugs results in most of the sound being absorbed

Doppler effect A change in frequency (pitch) of sound due to the motion of the source or the receiver Johaan Christian Doppler

Doppler Effect Doppler: Source Doppler: Observer Approaching, the frequency is higher because the wavefronts are closer together in time. Departing, the frequency is lower.

Sound - resonance Sound is produced by vibrating systems. All systems have one or more natural frequencies. A natural frequency is the frequency at which a system tends to vibrate in the absence of any driving or damping force.

Sound - resonance If a system is exposed to a vibration that matches its natural frequency, it will vibrate with an increased amplitude. This results in the amplification (increase in amplitude). of that frequency This phenomenon is called resonance.

Sound - resonance When resonance occurs in systems standing waves are formed.

Sound - resonance In order for standing waves to form in a closed pipe (closed at one end), the length of the pipe L must be an odd multiple of one fourth of the wavelength. The necessary condition is that there is a node at the closed end, and an antinode at the open end.

Sound - resonance

Standing Waves in a Pipe That Is Open at One End (Closed Pipe)

Sound - resonance

In order for standing waves to form in an open pipe (open at both ends), the length of the pipe L must be a whole number multiple of one half of the wavelength. The necessary condition is that there are antinodes at both ends.

Sound - resonance

Figure Standing Waves in a Pipe That Is Open at Both Ends

Sound - resonance Because the speed of sound in air is constant, we can only vary pipe length or frequency to obtain conditions needed for resonance.

Sound - resonance Example: A tuning fork with a frequency of 392 Hz is found to cause resonance in an air column spaced by 44.3 cm. the air temperature is 27 o C. Find the velocity of sound in air at that temperature.

Sound - resonance

Terminology – specifically for vibrating air columns.(pipes) Fundamental frequency – (first harmonic) - the frequency of the longest standing sound wave that can form in a pipe. Second harmonic – two times the frequency of the longest standing sound wave that can form in a pipe. Third harmonic – three times the frequency of the longest standing sound wave that can form in a pipe.

Sound - resonance Beats – Beats occur when two waves of slightly different frequencies are superimposed. A pulsating variation in loudness is heard.

Sound - resonance Waves on a string – the necessary condition for standing waves on a string, is that a node exist at either end.

Sound - resonance

Figure 14-24a Harmonics

Figure 14-24b Harmonics

Figure 14-24c Harmonics

Sound - resonance Example: One of the harmonics on a string 1.3 m long has a frequency of 15.6 Hz. The next higher harmonic has a frequency of 23.4Hz. Find (a) the fundamental frequency, and (b) the speed of the waves on this string.

Sound - resonance