MadSci Network: Physics

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


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 

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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