Sound Waves PHYS116 A01 111112 Lecture 35 Momchil

Sound Waves PHYS-116 A-01, 11/11/12, Lecture 35 Momchil Velkovsky

While a guitar string is vibrating, you gently touch the midpoint of the string to ensure that the string does not vibrate at that point. The lowest-frequency standing wave that could be present on the string A. vibrates at the fundamental frequency. B. vibrates at twice the fundamental frequency. C. vibrates at three times the fundamental frequency. D. vibrates at four times the fundamental frequency. E. not enough information given to decide

A problem

Longitudinal waves show the sinusoidal pattern A motion like the pulses of a speaker cone will create compressions and rarefactions in a medium like air. After the pulse patterns are seen, a sinusoidal pattern may be traced.

Sound waves may be graphed several ways

Speed of sound in liquids and solids 1240 km/h, 770 mi/h • The speed of sound will increase with the density of the material.

Standing sound waves and normal modes • Experiments often done in a first physics course laboratory will use common materials to reveal standing sound waves in resonance.

Cross-sectional views help us visualize the wave Nodes and antinodes will line up so that nodes are found where the resonator is closed antinodes at an open pipe. The crosssectional view helps to see the pattern.

Cross-sectional views reveal harmonic waves II

Cross-sectional views reveal harmonic waves III

The air in an organ pipe is replaced by helium (which has a lower molar mass than air) at the same temperature. How does this affect the normal-mode wavelengths of the pipe? • • A. The normal-mode wavelengths are unaffected. B. The normal-mode wavelengths increase. C. The normal-mode wavelengths decrease. D. The answer depends on whether the pipe is open or closed.

Different instruments give the same pitch different “favor” The same frequency, say middle c at 256 Hz, played on a piano, on a trumpet, on a clarinet, on a tuba … they will all be the same pitch but they will all sound different to the listener.

The speed of sound can be revealed by a resonant pipe • The frequency, speed of sound, and wavelength are all used to measure normal modes in a pipe

Sound intensity • Go beyond the wave on a string and visualize, say … a sound wave spreading from a speaker. That wave has intensity dropping as 1/r 2. . b= (10 d. B)log(I/I 0), I 0=10 -12 W/m

The logarithmic decibel scale of loudness Table 16. 2 shows examples for common sounds.

Reading for next time • The rest of Chapter 17