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Technical Library

Sound Control

In architectural acoustics, we are concerned with controlling the amplitude and/or the duration of sound. In walls and partitions, this is done by controlling sound transmission loss and sound absorption.

When sound waves strike a partition, some are reflected from the surface, staying in the same room as the source of the sound. Some are absorbed by the material of the partition, being converted to heat energy. And some are transmitted to the other side.

Sound Transmission Loss (STL) is the effectiveness of a barrier at preventing sound from getting from one side to the other. It is measured in decibels (dB), the same as amplitude. To determine STL, one measures the sound level on the side of the barrier closer to the sound source (the source room), the level on the opposite side (the receive room), and the reverberation or absorption of the receive room. The result is given by:

STL = L1-L2 + 10log (S/A) where
L1 = source room sound level
L2 = receive room sound level
10 log (S/A) = correction for absorption

Sound absorption is the effectiveness of a surface or material at preventing the reflection of sound. It does this by converting sound energy to heat. The more sound absorption, the less echoing will exist. The absorption of a material is measured in Sabines and is found by the equation:

A = .921 Vd/c where
V = the room volume
d = the measured rate of decay in decibels per second
c = the speed of sound

It is important to note the difference between a barrier and an absorber. Typical barriers made of hard, dense material may actually increase the echoes in the room, while absorbent batts of insulation allow sound to pass through as if they weren't there. Generally speaking, you can't use a barrier to absorb sound, and you can't use an absorber to block sound.

Other terms are also valuable to understand. Please refer to the Glossary provided at the end of this text. It includes acoustical words and phrases not previously discussed, as well as the ones we've already seen.

Getting the Sound Control You Need - Checklist

Now that you've been exposed to the basics of sound control and testing, it should be apparent that acoustics is a science. It's not guesswork. The same applies to selecting acoustical movable partitions. Here's a basic checklist that should help you get an installation suitable to the occupants and building management.

  1. Determine the use of the surrounding areas and the ambient noise of each.

  2. Hire an independent acoustician when sound control is critical or the existing construction is in question.

  3. Take the necessary steps in design and construction to avoid flanking paths.

  4. If flanking paths are unavoidable, specify a realistic STC for the partitions, keeping it in balance with the surrounding construction.

  5. Specify the STC needed, realizing that your actual installed NIC will be 5 to 10 dB lower.
  6. Select the appropriate type of partition, keeping in mind the application, construction quality, ease of operation, seals, and guarantee.

  7. Ask for references and published tests to ensure that the partition supplier has achieved the needed level of sound control in past installations.

  8. Make sure the partitions are installed by qualified, professional installers.

  9. In large, important installations, demand an ASTM E336 field test to make sure the desired NIC is achieved.

  10. Have operating personnel fully trained to move, store and adjust partitions properly.

  11. Utilize top and bottom mechanical seals to ensure ease of partition movement and a positive acoustical seal.

Achieving freedom from distraction requires teamwork among the acoustical consultant, the architect, the general contractor, the partition installer and all other trades. When approached as a team effort, the results can be spectacular.

Glossary of Acoustical Terms

ABSORPTION - The reduction of reverberating sound by the use of porous, non-dense materials.

AMBIENT NOISE - The ongoing regular noise of a given environment. Also known as background noise.

DECIBEL (dB) - Common unit of loudness, actually a logarithmic ratio of sound pressure level to a reference level.

FLANKING PATH - Leaks in the surrounding construction of a movable partition in which sound can travel. Shoddy construction, customary construction practices, or poor installation of the partition can all contribute to the leaks.

FREQUENCY - The rate of vibration, determining how high or low the pitch is. Frequency is measured in cycles per second, or Hertz (Hz).

HERTZ - Unit of frequency. One Hertz equals one cycle per second. Abbreviated Hz.

NIC (Noise Isolation Class) - This is a number describing the performance of ALL building elements in isolating one room from another. Perhaps the most practical way to state the acoustical performance of movable partitions already installed.

NR (Noise Reduction) - difference between the sound levels in the source and receive rooms.

NRC (Noise Reduction Coefficient) - average of absorption coefficients at four key frequencies. Rating of the absorptive characteristic of a surface.

PINK NOISE - Broadband noise with equal power at each constant-percentage bandwidth, often used for acoustic testing.

RECEIVING ROOM - Room opposite the room with the sound source, in acoustic measurement.

REVERBERATION - The reflection of sound from hard surfaces. Contributes to loudness.

SOURCE ROOM - In architectural acoustic measurements, the room that contains the sound source.

STC (Sound Transmission Class) - the most widely accepted standard for ranking the acoustical performance of accordion and operable partitions.

STL (Sound Transmission Loss) - is the effectiveness of a barrier at preventing sound from getting from one side to the other. It is measured in decibels (dB), the same as amplitude.

ASTM (Formerly American Society for Testing and Materials) - Organization which establishes standards for testing and application in many areas including acoustics.