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Science
Physics
Acoustics Exam Two (Topic 5)
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Terms in this set (33)
Sound Field
any environment that contains sound waves
Free Field
An unbounded sound field without any obstacles
Attenuation of Sound Over Distance
Reduction of sound over distance
Inverse Square Law
Sound intensity is inversely proportional to the squared value of the distance from the sound source
Attenuation of Intensity in dB
-20 log (dx/dr)
Attenuation of Pressure in dB
-20 log (dx/dr)
Reflection
The phenomenon that sound wave is reflected back from the interface of two media
What types of waves exist in a sound field as reflection occurs?
Incident waves and reflected waves. Originally there was one sound wave but some energy is reflected back into the original medium (i.e. air) while the rest of the energy continues to travel through the second medium (i.e. object)
Is there any change in sound speed, frequency, and acoustic energy as a portion of a sound is reflected back from the interface between two media?
No change in speed and frequency of the reflected wave because all frequencies travel at the same speed and it depends on the natural frequency of the medium. The intensity of the reflected wave depends on the difference of characteristic impedance between the two mediums.
What factor does the magnitude of sound reflection depend on? How?
The greater the distance, the greater the energy that is reflected back. The amount of energy that is retained in the first medium because once the rest is reflected back there are two sound waves vs. one.
Reverberation
The phenomenon that a sound is heard more than once in a sound field due to reflection
Mechanism of Reverberation
Arrival time difference of the incident and reflected waves
Reverberation Time (RT)
The time required for a reflected sound wave to be attenuated by 60 dB relative to its original level
Anechoic Field and Reverberation
The field (or room) that is designed to minimize reverberation.
What should be done to create an anechoic field in a normal room?
Having furniture, decorations on the wall, carpet, etc will modify the RT by reducing the characteristic impedance
Absorption
Transmission of sound from one medium into another medium.
Relationship between Absorption and Reflection
Indirectly proportional. Increased absorption reduces reflection.
What will happen to the reflection and absorption of sound energy as you increase the absorption coefficient of the interface between two mediums? What about with reverberation?
If you increase the absorption then the reflection will be decreased. This will also decrease the reverberation time.
What should you do if you want to build a sound treated room for conducting audiological tests?
Use sound absorbing panels on the walls, the ceiling, and use carpet on the floor.
Interference
The interaction of sound waves in a sound field between the regions of condensation and rarefaction.
Does sound interference mean that a sound is eliminated or suppressed by other sounds in the same field? What does actually happen?
No. There is an interaction when the two waves meet, resulting in them combining and producing a new sound wave.
Constructive Interference
The condensation (or rarefaction) regions of two waves overlap at an instant of time, causing an increase in density (or pressure) of the air.
Destructive Interference
The condensation region of one wave and the rarefaction region of another wave overlap at an instant of time, causing a decrease in density (pressure) of the air.
Standing Wave
The wave patterns with fixed locations of nodes and antinodes caused by interference of two sound waves with the same frequency and amplitude when traveling in opposite directions.
Key Pre-requisites For a Standing Wave to Occur
1) Constructive and destructive interference at fixed locations at an instant of time.
2) Interactions of the two waves at all locations over time.
Nodes
The locations at which the amplitude is 0.
Antinodes
The locations at which the wave alternates from the maximal positive to the maximal negative displacement.
Formula to compute the fundamental frequency for a standing wave in a tube with a closed end?
f₀ = c/4L
f₀
fundamental frequency (Hz)
c
sound speed (m/s)
L
length of the tube
What is the significance of the fundamental frequency for a tube with a closed end?
It equals the resonant frequency.
Is the occurrence of standing wave in the ear canal good or not good? Why?
No. It results in the occlusion effect.
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