ie-Chemistry P11
Sound absorbing paint

designing inefficient molecules


Imagine painting walls with sound absorbing paint.  Private offices, connecting corridors, and room dividing walls coated with such paint could absorb the energy of sound waves rather than reflect them, quietly turning their energy to heat.  Private activities and communications could be absorbed rather funneled to others hearing.  Sleeping quarters would be quieter and have less of distracting conversations and other noises.

Humans, like other animals, have sound detecting ears because sounds and changes in sounds provide valuable clues about our environment.  Like many animals, humans have developed elaborate oral languages for communication.  But background noises can make communication oral communication difficult, obscure more valuable information about our environment, make sleep difficult, and even impair the long term health of otherwise normal people.  So there are situations and occasions when it is desirable to reduce the ambient, background sound levels.

Sound is a form of energy carried by the vibration of molecules.  Generally sound energy is carried by compression waves where the material is alternatively compressed and rarified.  While it can generally be transmitted well through solids and liquids, usually the sound we hear primarily travels through the air.  We often intentionally make sounds by mechanically compressing and rarifying the air in devices we call speakers.  But any vibrating materials generates sound.  If the frequency of the sound vibration (symbolized ν) is increased, we sense this as higher pitch.  Sound travels away from is source with a speed depending on temperature and the nature of the media, but typically close to 340 m/s in room temperature air.  As a result of its creation frequency and speed of sound (s), from one compression to the next is separated by a distance called the wavelength (symbol λ).  The three properties are related by s = λ ν.  Young human ears can detect sound frequencies from about 20 Hertz (compressions per second) to 20,000 Hz, corresponding roughly to wavelengths of 20 m to 2 cm.  (Exposure to loud sound and perhaps aging impairs hearing.)Alexander G. Bell

The degree of compression is described as the amplitude of the sound.  This is alternatively described as the sound's intensity, pressure, power, energy, and loudness.  The human ear can detect sound amplitudes over an enormous range of about 1012.  So an exponential unit of measure is convenient.  One such unit was created by Bell Telephone engineers to compare audio signal loss over 1 mile of telephone cable.  It was later renamed after the creative company founder, Alexander Graham Bell (b1847, d1922 at right).  While it has a wide variety of applications, for sound intensity the Bel is defined as IB ≡ log (I/Io) where Io is generally taken as the lower limit of human hearing, 10-12 W/m2.  To avoid needing fractional numbers when distinguishing sound loudness, units of 1/10 Bel are commonly used: decibels.

It might help to recall that individual air molecules simply act according to Newton's Laws of motion, traveling at constant velocity until they collide with another molecule, the room walls, or another object in the room.  If they encounter something that acts as a single massive, elastic object, the molecule rebounds retaining nearly all its initial speed and energy.  But if the molecules encounter slowly moving targets with about equal mass, a significant portion of their energy can be transfered.  While the molecules in a compressed sound wave are predominately moving as an air mass in the direction of the of the wave's travel, the energy transfer is essentially by separate, distinct lone molecules.

However if the molecules in the object or wall respond to the collective collisions by vibrating in unison, they may effectively rebroadcast the sound to the next molecules which collide with the object or wall.  So an effective sound absorber likely will need to rapidly randomize the motions into heat to maximize sound absorption.


Consider what should be the nature of a substance's structural formula which would make it suitable for absorbing sound waves and gently releasing that energy as mild warmth.  How would you test to see if your idea works?

One approach might be to learn as much as you can about existing sound absorbing materials, including the atomic or molecular structure which absorbs the energy.  Then consider the properties of the sound waves pondering if more effective sound absorbing paint might be made.

An alternate approach might be to start with first principles of physics to engineer a system to transfer the orderly motion of sound waves to the random motion of heat.  Then determine what sized molecules and what structure would best do that.  The final task would be to find a way to embody that in a surface coating.


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created 22 November 2006
revised 8 December 2006
by D Trapp
Mac made