Design Solutions - Spring 2020 - 50

absorption in a fairly narrow
band of frequencies. Lower
frequencies can be absorbed
by drilling a larger diameter
hole in the back forming
a 'T-hole'. The frequency
bands of absorption are
determined by the thickness
of the wood, diameter of
the holes, the shape of the
hole, and spacing between
them. The material requires
supplemental absorption
behind the panel in order
to achieve the same effect
accomplished during the
panel testing, NRC range 0.5
- 0.90.

FIGURE 2

The noise reduction
coefficient (NRC) is typically
used to classify absorptive
materials. The NRC is the
numerical average of the
absorption coefficients in
the 250 Hz, 500 Hz, 1000 Hz
and 2000 Hz octave bands
rounded to the nearest 0.05.
Care must be exercised when
only specifying using the
NRC rating method as two
materials may have identical
ratings but completely
different absorption coefficient
curves (see figure 2).

FIGURE 3

50

I

designsolutions spring 2020

Some wood absorbers use
a hybrid Helmholtz process
to achieve absorption (see
figure 3). This process is
much like blowing across
a bottle to create a tone.
When air is forced into a
cavity, the pressure inside
increases. When the external
force pushing the air into the
cavity is removed, the higherpressure air inside will flow
out. The cavity will be left at
a pressure slightly lower than
the outside, causing air to be
drawn back in. This repeated
back and forth motion,
combined with supplemental
absorptive material behind
the cavity, causes energy
losses or absorption. Wood
absorbers with perforations
or slots use this process.

COMMON ABSORBERS

The most common wood
absorbers that use this
process are:
Perforated Materials wood panels or tongue and
groove planks that have
single diameter holes drilled
through (M-hole). M-hole
panels are characterized
by having considerable

Grooved Material - wood
panels or tongue and groove
planks that have linear
slots milled on the face and
holes drilled in the back.
The holes meet the slots
to form a passage for the
soundwaves. The acoustic
performance varies based on
the groove spacings and rear
hole diameters. The material
requires supplemental



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