Sound (Part 2)

The Speed of Sound

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

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

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

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

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

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

Review Question: What characteristic of a sound wave determines the pitch of the sound?

Natural Frequency 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.

Sound Wave Behavior: Natural 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: Natural 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: Natural Frequency

Harmonics The same note from different instruments has different qualities because the sounds from instruments are rarely pure notes, i.e. of one frequency. Rather they consist of one main note which is predominant and other smaller notes called overtones.

Compare the same note played on a flute and a violin Harmonics

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

Harmonics and instruments The same note sounds different when played on different instruments because the sound from an instrument is usually not a single pure frequency. The variation comes from the harmonics, multiples of the fundamental note. A C note played on a piano and played on a guitar:

The Doppler Effect 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.

Let's listen.... One more... The Doppler Effect

The Doppler Effect on The Big Bang Theory

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 Doppler Effect 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.

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

Sound wave Behavior: Reflection

Sound wave Behavior: Reflection An echo is a reflected sound wave.

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

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

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

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

Sound Wave Behavior: Diffraction 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.

Future test question: Why do sound waves diffract more than light waves?

Forced Vibration 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.

Resonance Be careful though! Forced vibration is not the same thing as resonance. Resonance occurs when something starts vibrating because of another vibrating object that isn’t touching it!