QUESTIONS — FACT SHEET
1. How do Acousta-Wal
Unlike acoustical ceilings, Acousta-Wal blocks do not
operate on the principle of porosity. The basic operating
principle is the Helmholtz resonator. Every cavity has a frequency
to which it is sympathetic (called the resonant frequency).
Examples of this would be an opera singer breaking a glass
by singing a note at its resonant frequency or a bell ringing
at its resonant frequency when it is struck. It is as if the
cavity captures and absorbs the sound waves at or near its
resonant frequency. These sounds are then dissipated in the
structure. The Helmholtz resonator principle is not new. It
is used in internal combustion engine mufflers and was used
in ancient amphitheaters where bottles were built into the
walls to absorb unwanted sound.
2. What does NRC stand
NRC stands for Noise Reduction Coefficient. NRC is
the average of absorption readings obtained at four frequencies:
250, 500, 1000 and 2000 Hertz (Hz). For example, a reading
of .85 at 250 Hz. means that 85% of the sound striking the
Acousta-Wal surface at this frequency (250 Hz.) is absorbed
and, therefore, not reflected back into the room as unwanted
noise. It is customary and useful to take readings at 125
and 4000 Hz. even though they are not used to calculate NRC.
Although NRC is a convenient measure of a product’s
acoustical performance, it is something of a “shotgun”
approach and should not be depended upon when choosing sound
absorbing materials for a specific application.
3. Is it structural?
Yes. Acousta-Wal blocks are manufactured to meet ASTM
C90 or C129. The slots are more than compensated for by the
added strength and lateral stability provided by the closed
4. What is the fire rating?
Comparable sound absorbing blocks gave the following
results when tested.
- 6" Type I 2 hours
- 6" Type IV 2 hours
- 8" Type IRF 2.5 hours
- 8" Type IVRF 4 hours
5. What do the fillers
do? Are they for insulation?
No, they are not for insulation. They absorb high frequency
sound without diminishing the low frequency absorption and,
therefore, broaden the response of the units.
6. Does Acousta-Wal cause
a maintenance problem?
This is one of the most commonly asked questions. Walls
constructed with Acousta-Wal blocks actually require much
less maintenance than walls faced with other acoustical materials.
This is particularly true when Astra-Glaze-SW®+/Acousta-Wal
glazed acoustical units are used. When dust or dirt settles
in the slots, it can be removed easily with a vacuum cleaner.
Neither the fibrous filler nor the individual fibers will be
7. If water gets into
the block cavities, how can it be removed?
Because the Acousta-Wal blocks are laid with the open
ends of the slot down and the closed tops of the blocks up,
any water that enters the cavities quickly drains out.
8. Will moisture affect
the fibrous fillers or cause any adverse effects, such as
deterioration of the filler or a loss in the acoustical properties?
No, because fibrous glass is an inorganic material
it will not deteriorate, rot or mildew. The sound absorption
of Acousta-Wal blocks would be adversely affected only if
water completely filled the fibrous glass. This is unlikely
because the pad must be immersed completely in water and worked
like a sponge to reach the saturation limit. The filler can
hold no more than 20% of its own weight. Since the filler
used in an 8" Type IV Acousta-Wal block cavity weighs
only about 6/100 of a pound, the water it could hold, if saturated,
would amount to about 1/8 of cubic inch per cavity and would
evaporate to dryness in a relatively short time.
9. Can the blocks be
painted without disturbing their sound absorption value?
Yes. However, if the units are left unpainted, the
sound absorption at the high frequencies (above 1000 Hz) would
be slightly improved depending upon the surface texture of
the unit (porosity). The sound absorption at the low frequency
would be unchanged since the units function on the Helmholtz
resonator principle, as described in Question 1.
10. What type of unit
should I use on my job?
The type of application controls the choice. In installations
where there will be adequate high frequency absorption provided
by other materials such as an acoustical ceiling, upholstered
seats or draperies, Type I would be adequate. In applications,
where both low and high frequency absorption are desired,
e.g. multi-purpose rooms, gymnasiums, and music rooms, Type
IV units provide the best results. In specialized applications
such as electrical generating stations and transformer noise
screens, 8" Type I is particularly effective because
its peak absorption occurs at the same frequency as the noise
generated by these devices.
Whenever possible get an octave band analysis
of the noise. Then it is a relatively simple matter to select
the unit giving the greatest absorption (See Page TR7705,
Sound Absorption Coefficients, behind this tab) at the frequencies
with the highest decibel readings. For complicated applications,
contact Trenwyth Industries. Rule of thumb:
- Low Frequency Absorption Only Type I or Type IV
- Low and High Absorption Type IV
11. What is the Sound
Transmission Loss of Acousta-Wal blocks?
It is critically important to remember the difference
between sound absorption and sound transmission loss. Sound
absorption is a measure of the sound absorbed in the same
room in which the sound is generated. Sound transmission is
a measure of sound transmitted from one room into an adjacent
room. Sound transmission loss is measured in decibels. The
sound transmission class of a material is determined by comparing
the measured sound transmission loss of the material at various
frequencies to the response of the human ear at the same frequencies.
Sound Transmission Class (STC)
6" ordinary 2-core block
6" Type I Acousta-Wal
6" Type IV Acousta-Wal
As you can see, Acousta-Wal blocks give superior
sound transmission loss performance when compared to ordinary
hollow concrete masonry units of the same thickness and composition.
To ensure good sound transmission loss performance, (1) the
unslotted side of the units must be painted or parged to seal
the porosity of the aggregate, and (2) the units must be laid
in a full horizontal bed of mortar. This same procedure must
also be followed when using ordinary concrete masonry units
when good sound transmission loss is important.
12. Does it matter where
I place the units in the room?
In schools and public buildings, the units should be
kept above door height. The slots must always face the noise
source. In an auditorium, sound absorption is most desirable
at the rear. When partitioning off an office or a control
room from an existing noisy plant area, the slots should face
into the office. In addition to the sound transmission loss
provided by the units (when properly installed in a full horizontal
bed of mortar), noise which enters the room through the doors,
windows or ceiling will then be subject to absorption by the
13. Are there any special
instructions I should give the masons regarding the laying
up of the units?
The units should be laid in a full horizontal bed of
mortar. They should be laid with the closed tops up.
The slots should face toward the noise source (with the exception
of partitioning a small office off from a large noisy plant
area). Under no circumstances should the units be laid upside
down, that is; with the closed tops down. First of all, it
would be impossible to lay the units in a full horizontal
bed of mortar if they are laid upside down. Second, excess
mortar would drip into the slots giving an unsightly appearance.
Third, if the cavity volume were reduced by more than 5 to
10% due to mortar dripping inside, the acoustical performance
of the units would be altered.
14. What is the delivery
time for Acousta-Wal® blocks?
In most cases, grey block will be available within
two to three weeks. For finished block, call your nearest
Trenwyth plant for lead time.
15. How can you have
a Sound Absorption Coefficient of greater than 1.00 e.g. 6"
Type IV Acousta-Wal® shows a coefficient of 1.23 at 500
This is an anomaly that occurs when the resonant frequency
of the Acousta-Wal® block cavities occur at or near the
test frequencies. The standard used in ASTM C423 is that 100%
of sound is absorbed by an open window (and the test equipment
is calibrated accordingly). ASTM also requires that the readings
shown on the test instruments be recorded exactly -- even
though common sense dictates that more than 100% of any sound
cannot be absorbed.