10. Sound Flashcards
t or f, sound travels fastest through solids, then liquids, then gases. What is the speed of sound through air?
true! Through air, sound is roughly 340 m/s
t or f, sound waves are transverse waves.
False, sound waves are longitudinal waves which means the direction of oscillation of the medium is parallel with the direction of energy movement.
High pressure areas = compressions
Low pressure areas = rarefactions
Explain how you find the wavelength sound waves in an open pipe?
The open ends of a pipe always represent anti-nodes (crests). Thus, an open pipe has an anti-node at each side.
An open pipe behaves the same way as a transverse standing wave, thus
λ = 2L / n
where n is the number of half wavelengths
Explain how you find the wavelength sound waves in an closed pipe? How is the formula different than for an open pipe or transverse standing wave?
A closed pipe has one end closed to the atmosphere. The closed end of a pipe is always a displacement node (whereas the open end is always an anti-node/crest).
λ = 4L / n
where n is an odd number of quarter wavelengths
note: f = vn / 4L
A organ pipe closed at one end has a length of 3 meters. What is the second largest harmonic wavelength the pipe can have?
closed pipe therefore λ = 4L / n, and n can only be odd numbers. The second largest λ would have n = 3.
λ = 4 (3) / 3 = 4 meters
What is a beat frequency? How do you calculate it?
When two sounds of different frequencies interfere, it creates moments of no sound and then moments of sound called beats. To find this, we just subtract the initial frequencies
beat = | f1 - f2 |
Note: beats typically only occur when the frequencies are similar.
What is the Intensity (I) of a wave? What is it proportional to (radius and ?)?
Intensity of a wave describes the power of a wave (energy / time) spread out over an area.
Intensity is inversely proportional to the square of the radius.
Intensity is proportional to the square of the waves amplitude.
intensity = watts / m^2
What is the area of a sphere?
A = 4 x pi x r^2
This is the area a point source emits waves over.
The threshold of hearing is the lowest sound intensity a human can hear. What is it?
I-o = 10^-12 W / m^2
What is intensity level (B)?
B = 10 x log(I / I-o)
I-o = 10^-12 W / m^2
measured in Bels but more commonly called decibels (10^-1).
t or f, If we multiply intensity by 10, we add 10 to B. If we divide intensity by 10, we subtract 10 from B.
true, remember this.
Suppose someone moves 10 X further from a loud siren of constant power. What is the resultant change in sound level?
r –> 10r
intensity is inversely proportional to r^2, so intensity is reduced by 100. Two factors of 10 reduction is equal to 20 decibel drop in sound level.
Explain the Doppler effect when
A) source and detector are approaching each other
B) source and detector are moving further a part
A) Approaching: when they are approaching, perceived frequency is higher than emitted.
B) Receding: when they are moving a part, perceived frequency is lower than emitted.
Explain why frequency changes when
A) source is stationary, detector is moving
B) detector is stationary, source is moving
A) source is stationary, detector is moving. The detector is moving with or against the wave fronts of the source. Thus, the relative speed of the waves is increased or decreased respectively. If speed has changed but wave-fronts are constant, frequency must have changed.
B) detector is stationary, source is moving. When the source is moving, the waves become distorted as it moves. If speed of waves is constant, then frequency must change when wavelength changes.
What is the Doppler effect equation? Explain the + / - signs.
fd = fs (v +- vd / v -+ vs)
it goes + - - +. The first signs (top signs) are used for when the source and detector are approaching each other.
Think: If source and detector are approaching, we KNOW that perceived frequency is higher than emitted. So how do we maximize the fraction? Increase numerator (+) and decrease the denominator (-).
