Definitions (Theory)
a) What are some different types of interference?
b) What would you hear if you were located in a point of each type of interference (using sound waves)?
a) Constructive and destructive interference
b) Constructive = amplifies the amplitude, hence sound would be louder than normal. Destructive = cancels out the amplitude, hence sound would be quieter than normal.
The least energy required to produce forced vibration in an object occurs
a) above its natural frequency.
b) at its natural frequency.
c) below its natural frequency.
b) at its natural frequency
Two tuning forks produce sound waves with frequencies of 450 Hz and 452 Hz. Calculate the beat frequency when both pure tones are played at the same time.
2Hz
A railroad locomotive is at rest with its whistle shrieking, then starts moving toward you.
a) Does the frequency of sound that you hear increase, decrease, or stay the same?
b) How about the wavelength reaching your ear?
c) How about the speed of sound in the air between you and the locomotive?
a) Increases
b) Decreases
c) Stays the same
What can sound travel through (and what can it not travel through)?
Sound can travel through any kind of matter, but not through a vacuum.
The speed of sound is different in different materials; in general, it is slowest in gases, faster in liquids, and fastest in solids.
The wavelengths of sound that carry farther in air are relatively
a) ultrasonic.
b) short.
c) long.
c) long.
If two speakers emit sound waves at frequencies of 680 Hz and 685 Hz simultaneously, calculate the beat frequency heard by an observer standing nearby.
5Hz
In an ultrasound examination suppose that you want to “see” a baby’s finger which has a thickness of 2 mm.
a) What should be the wavelength that you would use?
b) Given that we are largely made up of water, and that the speed of sound in water is about 1500 m/s, what frequency would you need for the ultrasound?
a) 2mm
b) 750kHz
Describe the Doppler effect in terms of:
1. What happens when the source of waves recedes?
2. What happens when the source of waves approaches?
1. Red shift: Longer wavelength and lower frequency
2. Blue shift: Shorter wavelength and higher frequency
The natural frequency of an object depends on its
a) shape and elasticity.
b) size and elasticity.
c) size and shape.
d) size, shape and elasticity.
d) size, shape, and elasticity.
A musician plucks two guitar strings simultaneously. One string has a frequency of 250 Hz, and the beat frequency is 6 Hz. Calculate the two possible frequencies that could be generated by the other guitar string.
244Hz and 256Hz
Suppose that using ultrasound imaging you wanted to be able to identify things down to a size of 0.75 mm.
a) What wavelength of ultrasound would you need to use?
b) If the speed of sound in soft tissue is 1500 m/s then what frequency of sound would you use?
a) 0.75mm
b) 2MHz
Why do low pitched noises tend to travel further than high pitched noises?
As waves travel through a medium their higher frequencies tend to be absorbed more readily than lower frequencies.
The explanation for refraction must involve a change in
a) frequency.
b) speed.
c) wavelength.
d) All of the above choices are true.
e) None of the above choices are true.
b) speed.
An amplifier increases the intensity of a signal by a factor of 1,000. If the original sound level was 80 dB, calculate the new intensity (in dB) after amplification.
110dB, 30dB more intense.
If the speed of sound in our body is 1500 m/s, we’re using ultrasound at a frequency of 1.5×106Hz to measure the speed of blood in the femoral artery, and the blood is travelling at a velocity of 0.5m/s, what is the frequency shift for waves reflected from this blood? (Assuming the probe is parallel to the blood vessel, hence no angle is relevant in this scenario)
1000Hz (or 1kHz)
Give an example of an inverse square law from last week's lecture.
I ∝ 1/r2
As distance increases the radiated power is spread out over a larger area → Intensity decreases.
Double the distance → four times the area → ¼ times the intensity
Surprise! Not really a multiple-choice question...
How much more intense is a 50 dB sound compared to a…
a) …40 dB sound?
b) …30 dB sound?
c) …20 dB sound?
d) …10 dB sound?
a) Increases by a factor of 10 (101)
b) Increases by a factor of 100 (102)
c) Increases by a factor of 1000 (103)
d) Increases by a factor of 10,000 (104)
We hear a sound as twice as loud if the intensity increases by a FACTOR of 10. i.e. by 10 dB on the decibel scale.
The intensity of a sound is 10-5 W/m2. Calculate the sound intensity in dB.
70dB
If the speed of sound in our body is 1500 m/s, and we use ultrasound at a frequency of 5 MHz to measure a frequency shift of 5 kHz for waves reflected from the blood in an aorta, then how fast is the blood going? (Assuming the probe is parallel to the blood vessel, hence no angle is relevant in this scenario)
0.75m/s