As the hand slides across the surface of the aluminum rod, slip-stick friction between the hand and the rod produces vibrations of the aluminum. Then with great enthusiasm, he/she slowly slides her hand across the length of the aluminum rod, causing it to sound out with a loud sound. Being a trained musician, teacher reaches in a rosin bag to prepare for the event. A long hollow aluminum rod is held at its center. The result of resonance is always a big vibration - that is, a loud sound.Īnother common physics demonstration that serves as an excellent model of resonance is the famous "singing rod" demonstration. When the match is achieved, the tuning fork forces the air column inside of the resonance tube to vibrate at its own natural frequency and resonance is achieved. So by raising and lowering the water level, the natural frequency of the air in the tube could be matched to the frequency at which the tuning fork vibrates. Conversely, a decrease in the length of a vibrational system decreases the wavelength and increases the natural frequency. As we have learned earlier, an increase in the length of a vibrational system (here, the air in the tube) increases the wavelength and decreases the natural frequency of that system. But the location of the water level can be altered by raising and lowering a reservoir of water, thus decreasing or increasing the length of the air column. So if the frequency at which the tuning fork vibrates is not identical to one of the natural frequencies of the air column inside the resonance tube, resonance will not occur and the two objects will not sound out together with a loud sound. Resonance only occurs when the first object is vibrating at the natural frequency of the second object. Yet, in the absence of resonance, the sound of these vibrations is not loud enough to discern. These impinging sound waves produced by the tuning fork force air inside of the resonance tube to vibrate at the same frequency. As the tines of the tuning fork vibrate at their own natural frequency, they created sound waves that impinge upon the opening of the resonance tube. The tuning fork is the object that forced the air inside of the resonance tube into resonance. One of our best models of resonance in a musical instrument is a resonance tube (a hollow cylindrical tube) partially filled with water and forced into vibration by a tuning fork. Resonance is a common cause of sound production in musical instruments. The word resonance comes from Latin and means to "resound" - to sound out together with a loud sound. This is known as resonance - when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion. An instrument can be forced into vibrating at one of its harmonics (with one of its standing wave patterns) if another interconnected object pushes it with one of those frequencies. The natural frequencies of a musical instrument are sometimes referred to as the harmonics of the instrument. Each natural frequency of the object is associated with one of the many standing wave patterns by which that object could vibrate. These instrument categories may be unusual to some they are based upon the commonalities among their standing wave patterns and the mathematical relationships between the frequencies that the instruments produce.Īs was mentioned in Lesson 4, musical instruments are set into vibrational motion at their natural frequency when a person hits, strikes, strums, plucks or somehow disturbs the object. A fourth category - vibrating mechanical systems (which includes all the percussion instruments) - will not be discussed. Three general categories of instruments will be investigated: instruments with vibrating strings (which would include guitar strings, violin strings, and piano strings), open-end air column instruments (which would include the brass instruments such as the trombone and woodwinds such as the flute and the recorder), and closed-end air column instruments (which would include some organ pipe and the bottles of a pop bottle orchestra). In Lesson 5, the focus will be upon the application of mathematical relationships and standing wave concepts to musical instruments. Thus far in this unit, applications of sound wave principles have been made towards a discussion of beats, musical intervals, concert hall acoustics, the distinctions between noise and music, and sound production by musical instruments. The goal of Unit 11 of The Physics Classroom Tutorial is to develop an understanding of the nature, properties, behavior, and mathematics of sound and to apply this understanding to the analysis of music and musical instruments.
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