Once students understand how sound energy is transferred in waves, they explore, in this lesson, how sound waves interact with different kinds of matter as they observe how much sound energy is transmitted and absorbed by different materials.
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Renovating a Concert Hall
Jordan Hall is part of the New England Conservatory, located in Boston, Massachusetts. It is considered one of the United States’ best performance spaces because of its acoustics.
Acoustics are the properties of a space that determine how sound waves travel. It is important when designing buildings such as auditoriums, theaters, and libraries.
In 1995, Jordan Hall was renovated. To renovate means to make repairs. These renovations were done carefully to keep the acoustics of the hall. No structures of the room were changed that might affect how sound was carried.
But the first night musicians played after the renovations, the music sounded different. Many people said it sounded too “bright.” Some audience members covered their ears with their hands.
How Materials Absorb Sound
It turned out that the new paint may have had a role to play in the changed sound. The new paint wasn’t as porous as the old paint. This means it didn’t have a lot of spaces between its particles.
It’s possible the new paint was much more reflective than the old paint. As the musicians played, their instruments made vibrations. These vibrations moved through the air in sound waves. As the waves came into contact with the painted walls, they were reflected back.
The more porous paint absorbed more of the sound waves. To absorb means to take in. When a sound wave is fully absorbed into a material, all of the sound energy transfers to the material and is changed to another form, such as heat. When this happens, no sound is transmitted through the material.
Many materials absorb some, reflect some, and transmit some sound energy. It depends on the properties of the material, including how porous it is and how thick. When sound waves are partially absorbed and partially transmitted, some of the sound is carried through the material, but it isn’t as loud because some of the energy has been converted to another form.
Acoustics of a Room
The old paint of the concert hall absorbed more of the sound waves than the new paint did. This resulted in a softer sound. After a year, the concert hall added felt cloth to its walls. This returned the acoustics to the previous sound because felt is more absorbent than the new paint. In general, softer, rougher materials tend to be more absorbent. Harder, smoother materials tend to be more reflective.
Concert halls like Jordan Hall are designed so that everyone in the room can hear the music. One way these halls achieve this is their shape. They are often designed with many curves that move the sound around the room.
Concert halls also have carpet to absorb sound energy. This transfers the sound energy into heat and prevents echoes.
Libraries have a different acoustical design goal. Libraries are designed to keep sound from traveling to help them stay quiet.
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