W AVE C HANGES Shaznay N. Lagura III-Galileo

R EFLECTION If you shout toward a large brick wall at least 30 feet (9 meters) away, you will hear an echo. The echo is produced when the sound waves are reflected from the wall to your ears. Generally, when sound waves in one medium strike a large object of another medium some of the sound is reflected.

If sound travels at about the same speed in both materials and both have about the same density, little sound will be reflected. Instead, most of the sound will be transmitted into the new medium. If the speed differs greatly in the two mediums and their densities are greatly different, most of the sound reflected. Sound waves travel much more slowly through air than through brick, and brick is much denser than air.

R EFRACTION When sound waves leave one medium and enter another in which the speed of the sound differs, the direction of the waves is altered. This change in direction results from a change in the speed of the waves is called refraction. If sound waves travel slower in the second medium, the waves will be refrected toward the normal. The normal is an imaginary line perpendicular to the boundary between the mediums.

Sound wave can also be refracted if the speed of the sound changes according to their position in medium. The waves bend toward the region of slower speed. You may have noticed that sounds carry farther at night than during a sunny day. During the day, air near the ground is warmer than the air above. Sound waves in the air are bent away from the ground into cooler air above, where the speed is slower. This bending of the waves results weaker sound near the ground. At night, air near the ground becomes cooler than te air above, enabling sound near the ground to be heard over longer distances.

DIFFRACTION Sound waves traveling along the side of a building spread out around the corner of the building. When sound waves pass through a doorway, they spread out around its edges. This spreading out of waves as they pass by the edge of an obstacle or through an opening is called diffraction. Diffraction occurs whenever a sound wave encounters an obstacle or opening. Diffraction enables you to hear a sound from around a corner, even though no straight path exists from the source of the sound to your ears.

RESONANCE Is the reinforcing of sound. It occurs when a small repeated force produces larger vibrations in an object. To produce resonance, the repeated force must e applied with the same frequency as the resonance frequency of the object.

R ESONANCE IN STRING INSTRUMENTS The vibration of the strings on stringed instruments have the form of standing waves which produce a fundamental frequency and all harmonics of that fundamental frequency simultaneously. These frequencies depend upon the tension, mass and length of the string. The harmonics make the sound timbre fuller and richer than the fundamental alone. The particular mix of harmonics present depends upon the method of excitation of the string.

R ESONANCE OF WIND INSTRUMENT Wind Instrument, musical instrument that is sounded by an airflow (the performer's breath) to make a column of air vibrate within a vented tube that resonates (intensifies and prolongs sound). The airflow sometimes activates a reed or reeds, flexible strips that vibrate to produce a sound. In wind instruments, the pitch of the note is controlled by the length of the column of air. Major types of wind instruments are the human voice; whistles, including the recorder and flute; reed instruments, such as the clarinet and bassoon; brass instruments, including horns; and free-reed instruments, such as the mouth organ.