What is Sound? Sounds are vibrations of the air that people can hear. Sound spreads out in all directions from the source. It is reflected, absorbed, and transmitted when it strikes various surfaces.

In outdoor conditions, (away from reflecting surfaces) sound spreads out somewhat spherically. Each time the distance from the source doubles, the sound level attenuates (reduces) by approximately 6 decibels (dB) ^[1]. In indoor spaces,sound propagation is more complicated because it includes reflected and diffracted sound; sounds do not attenuate as much indoors.

Frequency is a measure of the number of vibrations per second. It refers to the pitch of the sound (treble or bass) and is measured in Hertz (Hz). Most natural sounds are a combination of frequencies.

Decibel (dB) represents a relative measurement of sound level. Loud sounds relate to higher levels and quiet sounds relate to lower levels. A change of 3 dB is barely noticeable to the human ear; a change of 5 dB is a small difference; a change of 10 dB sounds like double the quietness or loudness. ^[2]

A-weighted decibels (dB(A)) ^[3] are also a measure of sound level, but are weighted according to the sensitivity of human hearing. We are more sensitive to some frequencies than to others and the weighting system approximates our sensitivities. Other weighting curves also exist but are rarely used for office acoustics. Sound Propagation in an Open-plan Office As sound spreads out in an open-plan office, it meets obstacles: floor, ceiling, partitions (partial-height screens), light fixtures, furniture, etc. These obstacles change the path of the sound. When designing an open-plan office to block sound, designers must consider all of these paths.

Direct Propagation (Red): Sound propagates outward in spheres. It spreads best when the line of sight is clear. Placing partitions between source and receiver blocks direct propagation paths.

Reflection (Green): When propagating sound meets a barrier, some of the sound energy is reflected off the barrier (like light reflection). Sound waves can reflect off multiple surfaces, travelling all around the office. However, with each reflection the sound is attenuated because some energy is absorbed into each surface.

Some surfaces are more sound reflective than others. Sound absorbent surfaces can reduce reflected sounds in the office.

Diffraction (Blue): Sound waves are able to bend over and around obstacles. However, the sound level attenuates as the sound waves bend. As the sharpness of the angle increases, the attenuation increases. For this reason, high, wide barriers block diffracted sound better than low, narrow barriers.

Sound Transmission (Orange): If there are any barriers between the source and receiver, the sound will propagate through the material of the barrier. Sound transmits more easily through some materials than through others. The denser and heavier the material, the more the sound will be attenuated. (If there are any holes in the barrier, the sound will travel directly through them.)

Attenuation: As sound waves propagate through any medium, the sound energy diminishes due to spreading, scattering, absorption, and sound transmission loss.

• Spreading occurs as the sound expands in spherical waves. The sound level decreases as the distance from the source becomes larger;
• Scattering occurs as the wave direction changes through diffraction or reflection;
• Absorption occurs as the sound enters a porous material and gets trapped in the air pockets. The trapped sound energy is converted to other forms of energy;
• Sound transmission loss occurs when sound energy is converted into vibrational energy within a material. Heavier, more massive materials attenuate sound more and, hence, have greater sound transmission loss values.

Acoustical Measures

There are three measures used to gauge the acoustical environment: Speech Intelligibility Index (SII), Sound Absorption Average (SAA), and Sound Transmission Class (STC).

The Speech Intelligibility Index ^[4] is a weighted speech-to-noise ratio that indicates how well speech can be understood in the presence of noise. The index ranges from 0 to 1, where 0 is perfect privacy and 1 is perfect intelligibility. Theatres and auditoriums need high SII values, but offices and other private locations need low SII values to satisfy occupants.

SII replaces the older Articulation Index (AI) ^[5] . AI is a little less complex to determine, but SII is more accurate. SII numbers are slightly higher than AI values; however, the indexes signify the same thing.
  Confidential privacy: SII 0.1 and AI 0.05. ^[6]
  Normal privacy: SII 0.2 and AI 0.15.

SAA: The Sound Absorption Average ^[7] is a measurement of sound absorption calculated by measuring the decay rate of the sound in a reverberation room. It indicates how well a material will absorb, and therefore attenuate, a sound. Highly absorbent materials have values close to 1; non-absorbent materials have SAA values close to 0. To stop sound reflection in open-plan offices, large surfaces need to be covered in materials with higher SAA values.

SAA replaces the older Noise Reduction Coefficient (NRC). SAA is a little more complex to calculate, but SAA values are almost the same as the NRC values for the same material: a material with SAA 0.9 has NRC 0.9.

STC: The Sound Transmission Class ^[8] is a measure of sound transmission loss and indicates to how loud the transmitted sound would seem to a listener. It combines transmission loss values at different frequencies in a manner intended to reflect their relative importance to listeners. Transmission loss varies considerably between frequencies. STC values start at 0 and can get very high. Most office partitions have STC values between 15 and 25. In comparison, full-height walls have STC values between 30 and 50.

COPE-CALC Software Users interested in acoustical open-plan office design or in evaluating a proposed design can model sample cubicles with the COPE-CALC software. The cubicles will be evaluated according to most of the principles listed above. The user can listen to the acoustical environment, read suggestions and make changes to and compare their designs.

References:

1: Crocker, M.J., ed. (1998). Handbook of Acoustics. Toronto, Canada: John Wiley & Sons, Inc. Pg. 305-306.

2: Crocker, M.J., ed. (1998). Handbook of Acoustics. Toronto, Canada: John Wiley & Sons, Inc. Pg. 10, 1182.

3: Crocker, M.J., ed. (1998). Handbook of Acoustics. Toronto, Canada: John Wiley & Sons, Inc. Pg. 985-986.

4: Acoustical Society of America. (1997). "Methods for Calculation of the Speech Intelligibility Index" (ANSI S3.5-1997). New York, USA: American National Standard, Standards Secretariat, Acoustical Society of America.

5: Acoustical Society of America. (1969). "American National Standard Methods for Calculation of the Articulation Index" (ANSI S3.5-1969). New York, USA: American National Standard, Standards Secretariat, Acoustical Society of America.

6: Bradley, J.S. (2003). "The Acoustical design of conventional open plan offices." NRCC-46274. Ottawa, Canada: Institute for Research in Construction/National Research Council (IRC/NRC).

7: ASTM C423-02a, "Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method", American Society for Testing and Materials, Philadelphia, USA.

8: Crocker, M.J., ed. (1998). Handbook of Acoustics. Toronto, Canada: John Wiley & Sons, Inc. Pg. 996.