SOUND. Second, the energy is transferred from the source in the form of a longitudinal sound wave.

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1 SOUND - we can distinguish three aspects of any sound. First, there must be a source for a sound. As with any wave, the source of a sound wave is a vibrating object. Second, the energy is transferred from the source in the form of a longitudinal sound wave. Third, the sound is detected by an ear or an instrument. Longitudinal waves traveling in any material medium are often referred to as sound waves. Sound cannot travel in the absence of matter. The speed of sound is different in different materials. These values depend somewhat on the temperature, but this is mainly for gases. Ex. For air, the speed increases approximately 0.60 m/s for each Celsius degree increase in temperature. v = ( T) m/s

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3 Longitudinal waves have compressions and rarefactions. There are pressure variations - a compression has an area of higher air pressure and a rarefaction has an area of lower air pressure.

4 Ex. 1 Sound with a frequency of Hz travels through water at a speed of 1435 m/s. Find its wavelength in water. Ex. 2 Find the frequency of a sound wave moving in air at room temperature with a wavelength of m. Ex. 3 The human ear can detect sounds with frequencies between 20. Hz and 16 khz. Find the largest and smallest wavelengths the ear can detect, assuming the sound travels through air with a speed of 343 m/s at 20. o.

5 Sound characteristics are immediately evident to a human listener. Loudness - the energy in a sound wave; depends on the amplitude and pressure variation of a wave. Pitch - refers to whether the sound is high (piccolo) or low (bass drum); the frequency of the wave. Pitch can also be given the name of a note on a musical scale. Octave - The interval of eight diatonic degrees between two tones, one of which has twice as many vibrations per second as the other. Or, two notes with frequencies related by the ration 2:1 differ by an octave. An octave has sounds with double the frequency of the sounds in another frequency (same note but higher or lower pitch). Sound level - because longitudinal waves differ in air pressure (compressions and rarefactions), they are measured by a quantity called sound level with units in decibels (db). Humans hear frequencies in the Hz range.

6 EFFECTS OF HEARING The human ear is a remarkably sensitive detector of sound. The function of the ear is to efficiently transform the vibrational energy of sound waves into electrical signals which are carried to the brain by way of nerves. In the outer ear, sound waves from the outside travel down the ear canal to the eardrum, which vibrates in response to the impinging waves. The middle ear consists of three small bones known as the hammer, anvil, and stirrup, which transfer the vibrations of the eardrum to the inner ear at the oval window. The inner ear consists of the semicircular canals, which are important for controlling balance, and the liquidfilled cochlea, where the vibrational energy of sound waves is transformed into electrical energy and sent to the brain.

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8 DOPPLER EFFECT What is the Doppler Effect? Have you ever noticed the change in pitch of a siren or horn on a fast moving vehicle as it passes you? When the source of sound (ie. Siren) is moving toward the observer, the pitch is higher than when the source is at rest (fire truck is at rest). When the source is moving away from the observer, the pitch is lower. Pitch - The distinctive quality of a sound, dependent primarily on the frequency of the sound waves produced by its source.

9 Calculations based on the Doppler Effect f ' = f v v + v v o s f: frequency of the source v: speed of the sound wave v o : velocity of the observer v s : velocity of the source v o and v s are: + if moving towards - if moving away Ex. A person playing the sax down on the wharf emits a frequency of 1430 Hz. What frequency will be heard by an observer biking at a speed of 33 m/s A) away from the source B) toward the source

10 Reflection of Sound Waves Sonar - sound navigation and ranging The reflection of sound is used in many applications to determine distance. Sound waves are sent from a ship to the ocean floor. The time it takes for the sound to go to the bottom and return depends on the depth of the water. Echos - produced when sound is reflected by a hard surface. Observer and surface must be 17 + m apart for an echo to be heard.

11 REFLECTION reflection of sound is an echo. Energy reflected from a surface is large if the surface is rigid and smooth. The energy is less if the surface is soft and irregular. Sound energy that is not reflected is either transmitted or absorbed. when sound reflects from a smooth surface, the angle of incidence equals the angle of reflection. Sometimes when sound reflects from walls, ceilings, and floors of a room, the surfaces are too reflective and the sound becomes garbled. This is due to multiple reflections called reverberations. If the room is too absorbent, the sound level is low and the room may sound dull and lifeless. the study of sound properties is called acoustics.

12 When a narrow beam of light strikes a flat surface, we define the angle of incidence, θi, to be the angle an incident ray makes with the normal to the surface (perpendicular) and the angle of reflection, θr, to be the angle the reflected ray makes with the normal. The law of reflection: the angle of incidence equals the angle of reflection (for flat surfaces) θ i = θ r

13 Sound of Music Resonance: when the frequency of forced vibrations imposed on an object matches the object's natural frequency. A dramatic increase in amplitude occurs. Ex. A swing we pump in natural rhythm with the natural frequency of the swing. More important than the force is the timing. Ex. Tacoma Narrow Bridge Washington State 1940s

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15 Resonance in Closed Air Columns when a longitudinal sound wave is emitted by a tuning fork, some of them travel down the closed air column. The end of the tube reflects the sound waves back. A node is formed at the bottom of the column. resonance first occurs when the column is 1/4 λ in length. The next possible lengths with a node at one end are 3/4 λ, 5/4 λ, etc.

16 Ex. 1 A vibrating tuning fork is held near the mouth of a column filled with water. The water level is lowered, and the first loud sound is heard when the air column is 9.0 cm long. Calculate: A) wavelength of sound from tuning fork. B)length of air column for the second resonance. Ex. 2 The first resonant length of a closed air column occurs when the length is 16 cm. A) what is the wavelength of the sound? B) If the frequency of the source is 512 Hz, what is the speed of sound?

17 Resonance in Open Air Columns if a standing wave interference pattern is created by reflection at a free end, an antinode occurs at the free end. Therefore, antinodes occur at both ends. The first length in which resonance occurs is 1/2 λ. the next possible lengths are λ, 3/2 λ, etc.

18 Ex. 1 An organ pipe, 3.6 m long and open at both ends, produces a musical note at its fundamental frequency. A) what is the wavelength of the note produced? B) What is the frequency if velocity is 346 m/s? Ex. 2 A tuning fork with a frequency of 392 Hz is found to cause resonance in an open air column at 21.0 cm and 65.3 cm. The air temperature is 27 o C. Find the velocity of sound in air at that temperature.