Doppler and Decibels

Which of the following are examples of the Doppler Effect? (Check all that apply)

• The pitch of a sound can change depending on whether the sound source is moving towards you, moving away from you, or is stationary.

• The color an object is can change depending on whether the light source is moving towards you, moving away from your, or is stationary.

• The further a sound source is away from you, the quieter the sound is.

• Sunlight striking millions of raindrops in the air causing a rainbow to appear.

Answer explanation

The Doppler Effect describes how the wavelength of a wave can change based on the motion of the wave source. A change in wavelength will produce a proportional change in the frequency but in the opposite direction.

When a wave source moves away (spacing increases), the wavelength will increase. That means the frequency decreases.

When a wave source moves closer (spacing decreases), the wavelength will decrease and the frequency increase.

A sound source is moving to the right as in the image shown. Compared to the sound made when the source is stationary, the sound you would hear at Position #2 would be ...

• higher in pitch.

• lower in pitch.

• louder.

• the same.

Answer explanation

As shown in the image, the wavelength of the sound gets shorter in front of a moving source. If the value of the wavelength decreases, the frequency will increase.

A sound source is moving to the right as in the image shown. Compared to the sound made when the source is stationary, the sound you would hear at Position #1 would be ...

• higher in pitch.

• lower in pitch.

• quieter.

• the same.

Answer explanation

As shown in the image, the wavelength of the sound gets longer behind a moving source. If the value of the wavelength increases, the frequency will decrease.

When observing distant galaxies, astronomers have found that the light they see has been "redshifted" -- the wavelengths are longer than they should be. According to the Doppler Effect, this means:

• those galaxies are moving away from us.

• those galaxies are moving towards us.

• those galaxies are dimmer than the Milky Way.

• those galaxies are very old.

Answer explanation

The galactic redshift happens because of the Doppler Effect. Shifting light towards redder colors means making its wavelength longer. Longer wavelengths mean the source is moving away from you.

The discovery of the redshift is one piece of evidence that led to the Big Bang Theory. Seeing the light from distant galaxies redshifted no matter which direction we look in the sky is an indication that our universe is expanding.

According to the Doppler Effect, the change in pitch of a sound is the same if (a) the source is moving away from the listener or (b) the listener is the one that is moving away.

• True

• False

Answer explanation

For the Doppler Effect, it does not matter who is moving. The source can move, the observer can move, or both can move.

If the two are getting closer to each other, the frequency will increase and wavelength will decrease.

If the two are getting father apart, the frequency with decrease and the wavelength will increase.

Which of the following statements are true about the decibel scale (check all that apply):

• Decibels are used to measure sound intensity or how loud a sound is.

• The closer you are to a sound source, the higher that sound will be on the decibel scale.

• The numeric values of the decibel scale are based on powers of ten -- it's a logarithmic scale.

• Decibels can be used for other types of waves as well, such as seismic waves and radio waves.

Answer explanation

Decibels refer specifically to how loud sounds are, which is related to the intensity of the sound energy.

Because the loudness of a sound decreases the further away you are from the source, the Decibel (dB) rating for the sound changes with distance.

Decibels are based on the logarithm of the sound intensity. That means one Bel equals one power of ten. Ten decibels is the same as one Bel or one power of ten.

Since decibels are a measure of sound loudness (what we perceive of sound intensity), they only can be used to measure sound.

A sound has a loudness of 60 dB. What would its loudness be if the intensity were increased by a factor of 1000?

• 30 dB

• 63 dB

• 90 dB

• It would stay the same. Loudness and intensity are different, unrelated things.

Answer explanation

1000 expressed as powers of 10 is 10³. That means a change of 30 decibels. Since you are increasing the intensity by 1000, you add 30 decibels to the original value.

A sound has a loudness of 120 dB. What would its loudness be if the intensity were decreased by a factor of 100?

• 20 dB

• 100 dB

• 140 dB

• It would stay the same. Loudness and intensity are different, unrelated things.

Answer explanation

100 expressed as powers of 10 is 10². That means a change of 20 decibels. Since you are decreasing the intensity by 100, you subtract 20 decibels from the original value.

A sound has a loudness of 60 dB. What would its loudness be if the intensity were increased by a factor of 100,000?

• 10 dB

• 65 dB

• 110 dB

• It would stay the same. Loudness and intensity are different, unrelated things.

Answer explanation

100,000 expressed as powers of 10 is 10⁵. That means a change of 50 decibels. Since you are increasing the intensity by 100,000, you add 50 decibels to the original value.

A sound has a loudness of 60 dB. What would its loudness be if the intensity were decreased by a factor of 1,000,000?

• 0 dB

• 120 dB

• It's impossible to change the intensity of a sound by that much.

• Since there still is some sound, it would have to have a value of at least 1 dB.

Answer explanation

1,000,000 expressed as powers of 10 is 10⁶. That means a change of 60 decibels. Since you are decreasing the intensity by 1,000,000, you subtract 60 decibels from the original value.

Decibels, like temperature, is a relative scale. Just like 0 F or 0 C does not mean there is no heat, 0 dB does not mean there is no sound.