### Re: How does the single cone on a speakerreproduce multiple freq

Date: Thu Aug 19 16:22:40 1999
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 933832867.Ph
Message:
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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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