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C U R I O S I T Y - D R I V E N
R E S E A R C H @ H K U S
Each resonant unit
has a low frequency
resonance of
400Hz and some
higher resonance
frequencies.
Resonant units
assembled as a sonic
crystal – for example,
a cubic structure,
as shown. Sonic
transmission exhibits
a sonic gap at 400Hz.
Locally resonant sonic materials, designed
to reflect sound waves at pre-specified
frequencies by local resonance.
The HKUST team came at these problems
by employing the physics law of local reso-
nance and trialing rudimentary novel ma-
terials that might have the same impact on
sound waves. Eventually, they created an
exciting new composite that did just that.
The first HKUST publication on the
findings appeared in
Science
in 2000. It
was met with acclaim by the media. But
not so readily accepted by those in the
acoustics field. “Many people thought we
were wrong: ‘How can someone come
from nowhere and think of something
new’,” Prof Sheng said. Despite the skep-
ticism of some in the acoustics world,
the HKUST team continued their studies
and experimentation. Gradually, through
continuous research, major strides were
achieved.
In place of a wall of extremely heavy
material to bounce back sound, the
researchers created a thin, lightweight
membrane-type acoustic metamaterial,
capable of breaking the mass density
law for frequencies of 150Hz to 1,000Hz
by around 200 times. While it seemed
counterintuitive to think of stopping low
frequency sound with a thin membrane,
it was in fact due to the membrane’s
“flimsiness” that this could be achieved
at low frequencies. Even in a small finite
sample with boundaries defined by a
rigid grid, there could be low-frequency
oscillation patterns. With the addition of
a small mass, or “button”, at the center
of the membrane sample, the vibrational
eigenfrequencies could be tuned, and at
the frequency between two eigenmodes
where the vibration amplitude was zero,
the result was almost complete reflection.
Stage One:Wave Reflection
Blocking Low Frequency Sound
The question that set the HKUST team on
its way was: is it possible to find an acous-
tic material with properties that enabled
it to shield any kind of sound wave, in-
cluding the difficult low frequency ranges
of 400Hz and below? Metal was known
to play a special role in manipulating
low frequency electromagnetic waves.
But no such class of materials existed for
acoustics.
Low frequency noise is stubbornly hard
to shield from due to its long wavelength,
which gives it greater penetration than
high frequency noise. Using conventional
mass density laws, this means that it takes
four times the mass to screen out a low
frequency sound of 200Hz than to attenu-
ate an 800Hz noise to the same degree.
Now think of trying to block a noise. To
stop noise going from one area to another,
several materials would usually be
combined to reflect or absorb different
frequencies. However, using conventional
methods (reflection-based solid materials),
it would take five centimeters of materials
(aluminum, mass density 2,700kg/m) to
attenuate 100Hz by 40dB. Such methods
would thus require materials too thick
to be practical for low-frequency appli-
cations. To absorb low frequency sound
would require structure as shown to the
right. They are bulky and not practical for
everyday applications.
Conventional sound
absorbing materials
Push the tube against
the membrane
Membrane tightened
Circular mass is attached
to center of membrane
These sonic materials demonstrated they
were tunable when different materials,
sizes and geometry of the basic unit were
used. They could screen out specific
frequencies over a range of 150Hz to
1,000Hz.
The thickness of materials could also be
reduced. Acoustic metamaterial units are
a factor of 10 to 100+ smaller than the
relevant wavelengths. Thus, the
metamaterial could be thin and small,
and still manipulate and attenuate low
frequency sound.
