|MadSci Network: Physics|
Aron, There are several ways to answer your question. A speaker is designed to be a broadband device, it can vibrate at many different frequencies. A device like a tuning fork is narrow band, it vibrates at a single fixed frequency. The flexibility of the speaker cone is what allows the speaker to have an increased broadband capability. Like a tuning fork, the speaker is most efficient at generating sound waves at or near to its natural resonance frequency. Different sizes of speakers have different resonant frequencies, which is why a range of speaker sizes are used with a divider network to produce sounds over the full audible frequency range. That is how a speaker can produce sounds at different frequencies. How does it produce multiple frequencies at the same time? The electrical signal which drives the speaker is the sum of all of the amplitudes of all the frequencies at any given instant in time. The motion of the speaker cone is complex in the way it varies with time when driven at multiple frequencies. The amplitude of the motion is also the sum of the amplitudes of all the driving frequencies in the electrical signal which drives the speaker. You might try an experiment with standing waves in a string. Take a length of string and tie one end to a fixed object. Pull the string tight and move the free end up and down at some frequency, say 15 times per minute. Try to keep the amplitude of the motion constant. A series of peaks and valleys will develop in the string. If you change the frequency of the motion, to say 20 or 30 times per minute, the number and location of the peaks and valleys will change. The peaks are the locations where the string is in resonance. Now, have someone else hold the fixed end of the string. Move one end 20 times per minute and the other end 30 times per minute. The resonance peaks will still form, however, now they will be the sum of both of the frequencies applied to the string. This is the same way the speaker produces sound at multiple frequencies at the same time. Bob Novak Specialist, Process R&D Carpenter Technology
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