Topics for today:
- Wave motion in the air (cont.)
- Waves with the same frequency from separate sources (or from reflections) can interfere
with each other, causing phase effects: constructive or destructive interference.
- Waves with different frequencies can interact to cause “beating,” which is an audible
amplitude modulation as the waveforms come into and out of being in phase.
- Besides pure tones, most “musical” signals have overtones and more complicated combinations
of frequencies. The frequency content of a signal is known as its spectrum.
- Sound levels and the decibel
- The “Sound Pressure Level” (SPL) is a way to express the amplitude of sounds on a
scale from zero to about 100.
- The “bel” is a logarithmic unit: the base-10 logarithm of the ratio of two quantities
expressed in the units of power (watts) or intensity (watts/square meter).
- The “decibel” is 1/10 of a bel. It is convenient to specify these quantities as decibel
rather than bel, and this is the customary way to do it.
- For human listeners, a sound level change of 10 dB is judged to be “twice as loud.”
A small change of between 1 and 3 dB is judged to be “just barely noticeable.”
Topics for the next lecture:
- Finish reading Section 1 material from the textbook.
- Harmonics and the analysis of complicated periodic vibrations.
Robert C. Maher, Ph.D., P.E.
Montana State University
P.O. Box 173780
Bozeman, MT 59717-3780
Office: 616 Cobleigh
Tel: +1 (406) 994-7759
ECE Main Office: (406) 994-2505
Fax: (406) 994-5958
rmaher@montana.edu
Lab:
Cobleigh Hall 410