# Vern J. Ostdiek Donald J. Bord Chapter 6 Waves and Sound (Section 5)

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Vern J. Ostdiek Donald J. Bord Chapter 6 Waves and Sound (Section 5)

6.5 Propagation of Sound Once a sound has been produced, what factors affect the sound as it travels to our ears? The general aspects of wave propagation of course apply to sound waves. Of these, reflection, diffraction, and the reduction of amplitude with distance from the sound source are most important in influencing the sound that actually reaches us.

6.5 Propagation of Sound The simplest situation is a single source of sound in an open space—such as a person talking in an empty field. The sound travels in three dimensions, and its amplitude decreases as the wave fronts expand. In particular, the amplitude is inversely proportional to the distance from the sound source. When you move to twice as far away from a steady sound source, the amplitude of the sound is decreased by one-half. The sound becomes quieter as you move away from the sound source.

6.5 Propagation of Sound Sound propagation is more complicated inside rooms and other enclosures. First, diffraction and reflection of sound allow you to hear sound from sources that you can’t see because they are around a corner. We are so accustomed to this phenomenon that it doesn’t seem mysterious. Second, even when the source is inside the room with you, most of the sound that you hear has been reflected one or more times off the walls, ceiling, floor, and any objects in the room. This has a large effect on the sound that you hear.

6.5 Propagation of Sound The figure shows that sound emitted by a source in a room can reach your ears in countless ways.

6.5 Propagation of Sound Consider a single sound pulse like a hand clap. In an open field, you would hear only a single, momentary sound as the pulse moves by you. A similar pulse produced in a room is heard repeatedly: you hear the sound that travels directly to your ears; then sound that reflects off the ceiling, floor, or a wall before reaching your ears; then sound that is reflected twice, three times, and so on

6.5 Propagation of Sound These reflected sound waves travel greater and greater distances before reaching your ears and are heard successively later than the direct sound. This process of repeated reflections of sound in an enclosure is called reverberation. The single hand clap is heard as a continuous sound that fades quickly.

6.5 Propagation of Sound The graphs below compare the sound from a hand clap as it is heard in an open field and in a room.

6.5 Propagation of Sound Each graph shows the amplitude of the sound that is heard versus the time after the sound pulse is produced. The reverberation causes the sound to “linger” in the room. The indirect sound that one hears after the initial direct pulse is called the reverberant sound. The amount of time it takes for the reverberant sound to fade out depends on the size of the room and the materials that cover the walls, ceiling, and floor.

6.5 Propagation of Sound Sound is never completely reflected by a surface: Some percentage of the energy in an incoming wave is absorbed by the surface, leaving the reflected wave with a reduced amplitude. Concrete absorbs only about 2% of the incident sound’s energy, whereas carpeting and acoustical ceiling tile can absorb around 90%. A room with a large amount of sound absorbing materials in it will have little reverberation. After a few reflections, the sound loses most of its energy and cannot be heard.

6.5 Propagation of Sound The reverberation time is used to compare the amount of reverberation in different rooms. It is the time it takes for the amplitude of the reverberant sound to decrease by a factor of 1,000. It varies from a small fraction of a second for small rooms with high sound absorption to several seconds for large, brick-walled gymnasiums and similar enclosures.

6.5 Propagation of Sound The Taj Mahal, a breathtakingly beautiful mausoleum in Agra, India, is made of solid marble, which absorbs very little sound. The reverberation time of its central dome is more than 10 seconds.

6.5 Propagation of Sound When a steady sound is produced in a room, such as a trumpet playing a long note in an auditorium, the sound that one hears is affected by reverberation in a number of ways: 1. The sound is louder than it would be if you had heard it at the same distance in an open field. 2. The sound “surrounds” you; it comes from all directions, not just straight from the source.

6.5 Propagation of Sound 3. Beyond a short distance from the sound source, the loudness does not decrease as rapidly with distance as it would in an open field. That is why one can often hear as well near the back of an auditorium as in the middle. 4. Not only does the sound fade gradually when the source stops, the sound also “builds” when the source starts.

6.5 Propagation of Sound Moderate reverberation has an overall positive effect on the sound that we hear, particularly music.

6.5 Propagation of Sound However, excessive reverberation adversely affects the clarity of both speech and music. Speech and music are a series of short, steady sounds interspersed with short moments of silence. Each note, word, or syllable is followed by a brief pause. If we again graph the amplitude of sound versus time, we can see the effect of reverberation.

6.5 Propagation of Sound In an open field, one hears each syllable or note as a distinct, separate sound. In a room, the individual sounds begin to merge. As a new note is played or a new word is spoken, the reverberant sound from the preceding one can still be heard. The longer the reverberation time, the more the sounds overlap each other and the harder it is to understand speech.

6.5 Propagation of Sound Racquetball courts have hard, smooth walls and very high reverberation times; That is why it is very difficult for players to converse unless they are close to each other. It is recommended that the reverberation time of rooms used for oral presentations and lectures should be around 0.5–1 second. For concert halls, it should be from 1 to 3 seconds, depending on the type of music being performed.

6.5 Propagation of Sound Many other factors besides reverberation time must be taken into account by architects and building designers. For example, a balcony must be high above the main floor and not extend out too far, or little reverberant sound will reach the seats below it. Also, sound is focused by concave walls and ceilings. Building an elliptical auditorium could result in the sound being concentrated in a small area of the room.

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