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Acousta-Wal sound-absorbing masonry units

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1. How do Acousta-Wal blocks work?
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 for?
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 tops.

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 disturbed.

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

STC 43

6" Type I Acousta-Wal

STC 49
6" Type IV Acousta-Wal
STC 52

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 Acousta-Wal blocks.

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 Hz.?
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.

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