Sound (Part 2)

The Speed of SoundThe Speed of Sound

The speed of sound The speed of sound depends depends onlyonly on the on the properties of the medium properties of the medium it’s travelling through.it’s travelling through.

The Speed of SoundThe Speed of Sound

In general, sound travels fastest In general, sound travels fastest through solids. This is because through solids. This is because molecules in a solid medium are molecules in a solid medium are much closer together than those much closer together than those in a liquid or gas, allowing in a liquid or gas, allowing sound waves to travel more sound waves to travel more quickly through it. In fact, quickly through it. In fact, sound waves travel over 17 sound waves travel over 17 times faster through steel than times faster through steel than through air. through air.

Speed of SoundSpeed of Sound

Sound also travels faster in Sound also travels faster in liquids than in gases because liquids than in gases because molecules are still more tightly molecules are still more tightly packed. In fresh water, sound packed. In fresh water, sound waves travel 4 times faster waves travel 4 times faster than in air!than in air!

Speed of SoundSpeed of Sound

The The speed of sound speed of sound in air is 343 in air is 343 meters per second (660 miles per meters per second (660 miles per hour) at one atmosphere of pressure hour) at one atmosphere of pressure and room temperature (21°C).and room temperature (21°C).

The speed of soundThe speed of sound

The speed of sound The speed of sound through a gas depends through a gas depends on the temperature.on the temperature.

When we look at the properties of a gas, we When we look at the properties of a gas, we see that only when molecules collide with see that only when molecules collide with each other can the compressions and each other can the compressions and rarefactions of a sound wave be passed rarefactions of a sound wave be passed along. So, it makes sense that the speed of along. So, it makes sense that the speed of sound depends on how often the particles sound depends on how often the particles collide. At higher temperatures, molecules collide. At higher temperatures, molecules collide more often, giving the sound wave collide more often, giving the sound wave more chances to move forward rapidly. more chances to move forward rapidly.

Speed of SoundSpeed of Sound

What characteristic of a What characteristic of a sound wave determines sound wave determines the pitch of the sound?the pitch of the sound?

Review Question:Review Question:

The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed is known as the natural frequency of the object.

Natural FrequencyNatural Frequency

Some objects tend to vibrate at a single frequency and they are often said to produce a pure tone. A flute tends to vibrate at a single frequency, producing a very pure tone. Other objects vibrate and produce more complex waves with a set of frequencies that have a whole number mathematical relationship between them; these are said to produce a rich sound. A tuba tends to vibrate at a set of frequencies that are mathematically related by whole number ratios; it produces a rich tone.

Sound Wave Behavior:Sound Wave Behavior:Natural FrequencyNatural Frequency

Still other objects will vibrate at a set of multiple frequencies that have no simple mathematical relationship between them. These objects are not musical at all and the sounds that they create could be described as noise.

Sound Wave Behavior:Sound Wave Behavior:Natural FrequencyNatural Frequency

Sound Wave Behavior:Sound Wave Behavior:Natural FrequencyNatural Frequency

The same note from different instruments The same note from different instruments has different qualities because the sounds has different qualities because the sounds from instruments are rarely pure notes, i.e. from instruments are rarely pure notes, i.e. of one frequency.of one frequency.

Rather they consist of one main note which Rather they consist of one main note which is predominant and other smaller notes is predominant and other smaller notes called overtones. called overtones.

HarmonicsHarmonics

Compare the same note played on a flute and a violin

HarmonicsHarmonics

The main note or fundamental note is also The main note or fundamental note is also referred to as the first harmonic and if it has a referred to as the first harmonic and if it has a frequency f, the overtone with frequency 2f is frequency f, the overtone with frequency 2f is called the second harmonic and the overtone with called the second harmonic and the overtone with frequency 3f is called the third harmonic and so frequency 3f is called the third harmonic and so on. The sum of all the harmonics is the waveform on. The sum of all the harmonics is the waveform and determines the quality of the sound. and determines the quality of the sound.

HarmonicsHarmonics

The same note sounds different when played on different The same note sounds different when played on different instruments because the sound from an instrument is usually instruments because the sound from an instrument is usually not a single pure frequency.not a single pure frequency.

The variation comes from the The variation comes from the harmonicsharmonics, multiples of the , multiples of the fundamental note.fundamental note.

A C note played on a piano and played on a guitar:A C note played on a piano and played on a guitar:

Harmonics and Harmonics and instrumentsinstruments

The Doppler effect is a phenomenon observed whenever the source of waves is moving with respect to an observer. The Doppler effect is an apparent upward shift in frequency for the observer when the source is approaching, and an apparent downward shift in frequency when the source is receding.

The Doppler EffectThe Doppler Effect

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The Doppler EffectThe Doppler Effect

The Doppler Effect on The Big Bang Theory

The Doppler EffectThe Doppler Effect

As a car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - a shift in the apparent frequency for a sound wave produced by a moving source.

The source of sound always emits the same frequency. Therefore, for the same period of time, the same number of waves must fit between the source and the observer. If the distance is large, then the waves can be spread apart; but if the distance is small, the waves must be compressed into the smaller distance. For these reasons, if the source is moving towards the observer, the observer perceives sound waves reaching him or her at a more frequent rate (high pitch). And if the source is moving away from the observer, the observer perceives sound waves reaching him or her at a less frequent rate (low pitch). It is important to note that the effect does not result because of an actual change in the frequency of the source.

The Doppler EffectThe Doppler Effect

For the Doppler Effect to For the Doppler Effect to occur, the source may be occur, the source may be moving, the listener may be moving, the listener may be moving, or both may be moving, or both may be moving.moving.

Important NoteImportant Note

Sound wave Behavior:Sound wave Behavior:ReflectionReflection

Sound wave Behavior:Sound wave Behavior:ReflectionReflection

An echo is a reflected sound wave

Builders of auditoriums and concert halls avoid the use of hard, smooth materials in the construction of their halls. With a hard material such as concrete, most of the sound wave is reflected by the walls and little is absorbed. Walls and ceilings of concert halls are made softer materials such as fiberglass and acoustic tiles. These materials have a greater ability to absorb sound. This gives the room more pleasing acoustic properties.

Reflection of SoundReflection of Sound

You’re standing at the bottom of a You’re standing at the bottom of a canyon, and wondering how far canyon, and wondering how far away the other side is. To figure away the other side is. To figure out the distance across the canyon, out the distance across the canyon, you could yell, and clock the time you could yell, and clock the time until you heard your echo. Let's until you heard your echo. Let's say this took exactly 3.0 seconds. say this took exactly 3.0 seconds. Since you took physics, you know Since you took physics, you know that sound travels at about 0.2 that sound travels at about 0.2 miles per second. How far away is miles per second. How far away is the other wall of the canyon?the other wall of the canyon?

Reflection can also Reflection can also cause test questions:cause test questions:

A ship on the surface of the water A ship on the surface of the water sends a SONAR signal, and it sends a SONAR signal, and it bounces off a submarine in the bounces off a submarine in the water. Calculate the distance of the water. Calculate the distance of the submarine from the ship if the submarine from the ship if the signal is returned in 4.0 seconds. signal is returned in 4.0 seconds. (The speed of sound in water is (The speed of sound in water is 1450 meters/sec).1450 meters/sec).

Reflection can also Reflection can also cause test questions:cause test questions:

Sound Wave Behavior:Sound Wave Behavior:DiffractionDiffraction

Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path.

Diffraction: Light vs Diffraction: Light vs SoundSound

Imagine going to a baseball game, and you discover that Imagine going to a baseball game, and you discover that your seat is directly behind a wide post. You cannot see your seat is directly behind a wide post. You cannot see the game, of course, because the light waves from the field the game, of course, because the light waves from the field are blocked. But you have little trouble hearing the game, are blocked. But you have little trouble hearing the game, since sound waves simply diffract around the post. since sound waves simply diffract around the post.

Sound Wave Behavior:Sound Wave Behavior:DiffractionDiffraction

The reason for the difference—that is, why sound diffraction is more pronounced than light diffraction—is that sound waves are much, much larger than light waves. The amount of diffraction increases with increasing wavelength and decreases with decreasing wavelength.

Why do sound waves Why do sound waves diffract more than light diffract more than light waves?waves?

Future test question:Future test question:

If you were to take a guitar string and stretch it to a given length and a given tightness and have a friend pluck it, you would hear a noise; but the noise would not even be close in comparison to the loudness produced by an actual guitar. If the string is attached to the sound box of the guitar, the vibrating string is capable of forcing the sound box into vibrating at that same natural frequency. The sound box in turn forces air particles inside the box into vibrational motion at the same natural frequency as the string. The entire system (string, guitar, and enclosed air) begins vibrating and forces surrounding air particles into vibrational motion. The tendency of one object to force another adjoining object into vibrational motion is referred to as a forced vibration. This causes an increase in the amplitude and thus loudness of the sound.

Forced VibrationForced Vibration

• Be careful though!Be careful though!• Forced vibration is not the same thing as Forced vibration is not the same thing as

resonance.resonance.• Resonance occurs when something starts Resonance occurs when something starts

vibrating because of another vibrating vibrating because of another vibrating object object that isn’t touching itthat isn’t touching it!!

ResonanceResonance