Sound Flashcards
What is sound?
A longitudinal mechanical wave which travels though an elastic medium. Sound is thus produced by vibrating matter. There is no sound in a vacuum because it contains no matter.
What are compressions and rarefactions? (sound)
Compressions (also known as condensations) are regions where particles of matter are close together, they are also high pressure regions.
Rarefactions are regions where particles are sparse, they are low pressures regions of sound waves.
What two characteristics of matter effect the velocity of sound?
- Higher velocity in more dense matter (solids>liquid>gas)
- Higher velocity in increased temperature
The velocity of sound is proportional to the square root of the elastic restoring force and inversely proportional to the square root of the inertia of the particles (density is a measure of inertia, explaining the above)
What are the physical correlates to the following sensory features of sound:
Loudness
Pitch
Quality
Loudness: intensity (rate of energy propagation through space = power/area)
Pitch: frequency
Quality: waveform
What are infrasonic/ultrasonic frequencies?
Infrasonic: below 20 Hz
Ultrasonic: above 20,000 Hz
The human ear can hear between 20-20,000 Hz
What are beats?
When sounds of different frequencies are heard together, they interfere. Constructive interference results in beats.
What is the doppler effect?
The effect upon the observed frequency caused by the relative motion of the observer (o) and the source (s)
If the distance is decreasing between them, there is a shift to higher frequencies and shorter wavelengths (to higher pitch for sound and toward blue violet for light)
If the distance is increasing between them, there is a shift to longer wavelengths and lower frequencies (to lower pitch for sound and toward red for light).
A sound of a known frequency, wavelength, intensity and speed travels through air and bounces off an imperfect reflector, which is moving towards the source. Which of the properties of the sound remains the same before and after reflection?
Speed
Within still air, the speed of sound remains constant.
In order to determine the relative speed of approach of a sound source by Doppler measurements, three of the following items of data are necessary, which one is NOT REQUIRED?
A. Speed of sound in medium
B. Frequency of emitted sound
C. Frequency of observed sound
D. Distance between source and observer
D. Distance between source and observer
Given that v is the speed of sound in the medium, the Doppler equation for a source that is approaching (receding from) an observer can be written as:
f’ = fV/(V +/- Vsource)
How does the speed of sound differ between air and steel?
Much faster in steel than in air. Stiffer materials transmit sound much quicker than materials which are not as stiff.
What is the intensity of a wave? How is it calculated?
Intensity of a wave is a measure of the amount of energy a wave transports.
ΔE = IAΔT
Convert 70 decibels to intensity (W/m^2)
Io = 10^-12 W/m^2
70 = 10log(I/Io)
7 = log(I/Io)
10^7 = I/10^-12
I = 10^-5 W/m^2
Consider a pipe that is closed at one end. Describe the fundamental harmonic in this pipe.
The open end is an antinode and the closed end is a node, there are no additional nodes in the middle. Thus, the fundamental contains one fourth of a full wave in a closed ended pipe.
The full wavelength is four times the length of the pipe.
Each successive harmonic adds 1 NODE (and only a node!). This means that the second harmonic will be 3/4s the length of a full wavelength (draw it out if you don’t get it..)
Describe harmonics for pipes with openings at both ends.
Is an open end of a pipe a node or antinode for pressure variation?
The first harmonic has one node (and each opening is an antinode). Each successive harmonic adds a node.
Each opening is a node for pressure variation (closed ends are antinodes for pressure variation).
What are beats? Describe what you are hearing when you hear a 220 Hz sound played concurrently with a 222 Hz sound.
A phenomenon you get when two sound sources are very close in frequency but not quite, it makes a pulsating noise (similar to the sound of binaural beats)
Beat frequency: |Frequency 1 - frequency 2| = Number of beats heard per second (beats/s = Hz)
