Acoustics 101; A Glossary of Terms

1. Sound Pressure Level

The pressure level of a sound, as measured in decibels, conveying how loud or soft a sound is

The volume of a sound can be subjective, depending on how each individual perceives it, which is why we need an objective measure of how loud or soft a sound is. This measure is called sound pressure level, it is measured in decibels.

For instance, if you are in an open field, with birds chirping all around you and a soothing wind blowing, the SPL of that area would be about 35 to 40 dB. On the contrary, a normal conversation indoors would be around the range of 50 dB.

2. Ambient Noise

The sound present in the background of a given environment, excluding any sound generated by the primary observer

Ambient noise, or background noise, is a basic concept that every individual is familiar with. Like SPL, it can be measured in decibels. For example, let’s say you are out getting lunch at a restaurant, all of the sound around you, from scattered conversations, clinking cutlery and background music, forms the ambient noise of that space. Every space will have some background noise when it is operational.

3. Frequency

The number of vibrations a sound wave undergoes in a second is called frequency, it is measured in hertz (Hz)

When you get down to the physics of it, each sound that we hear or make is made up of vibrations. These vibrations decide the frequency of a sound. Each sound has a different frequency which is measured in hertz (Hz). Human ears can only detect sounds within the range of 20 to 20,000 Hz.

As a rule, high-frequency sounds are high-pitched, such as a shrill scream or a whistle, whereas low-frequency sounds are low pitched, such as the sound of thunder.

Frequency should not be confused with sound pressure level, SPL measures the volume of a sound in decibels whereas frequency is a measure of the number of vibrations per second, denoted in hertz.

4. Octave Band

A band of frequencies spanning one octave, where the upper limit of the band is twice the frequency of the lower limit

In the building material industry, and in most indoor spaces, the sounds that we hear, no matter of what volume, broadly fall under the frequency spectrum of 63Hz–8000Hz. As per the definition of octave band frequencies, below are the 8 frequencies according to which building material properties are measured and communicated.

63Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz

As there are 8 frequencies in the table above, it is known as Octave Band.

Every frequency is twice the previous frequency. For example, 125 Hz is twice that of 63Hz, following which, 250 Hz is twice that of 125 Hz. These bands are used to segment and analyze sound data by frequency levels.

5. Insulation

Sound insulation is a measure of how effectively a surface can obstruct sound waves from passing through it

In simple terms, it measures the sound-proofing ability of a room. As sound waves travel through the air, they hit many surfaces which either reflect or absorb the sound. Hard surfaces that reflect sound tend to create a noisy atmosphere, which is why we use surfaces that absorb sound for soundproofing in building construction. The sound insulation index is measured numerically, the higher the value, the more effective the insulation is.

It can be measured under two settings, under ideal conditions in a laboratory, or on-field, in a practical setting.

In laboratory

• STC (Sound Transmission Class) It is a numerical rating of how well a building partition reduces airborne sound, measured in single, whole numbers. It is widely used to rate the sound-proofing ability of interior partitions, ceilings, floors, doors, windows and exterior wall configurations.
• Rw (Weighted Sound Reduction Index) Like STC, Rw also measures the airborne sound insulating properties of a building element using a single-number rating. The difference lies in the range of frequencies covered, Rw can measure sounds at lower frequencies than STC.

Note- The weighted sound reduction index is used to ascertain the insulation of building material that has been measured in a laboratory setting.

In Situ (on-field)

• FSTC (Field Sound Insulation Class) The sound isolation performance of specific elements in a partition, as measured in the field and achieved by suppressing the effects of sound flanking paths. This can be useful for comparing walls with doors, when you are interested in removing the influence of the door on the measured field STC.
• Dw (Weighted Sound Level Difference) Single number quantity that characterizes airborne sound insulation between rooms, but which is not adjusted to reference conditions.

Note- Weighted sound level difference is used to ascertain the insulation between rooms in a building as they are; for the sake of accuracy, these values cannot be compared with measurements made under other conditions.

6. Absorption

When a sound wave strikes one of the surfaces of a room, some of the sound energy is reflected back into the room while the rest penetrates through the surface. The tendency of a material to absorb a sound instead of reflecting it back is called absorption. It is measured and indicated through different values, such as-

• NRC (Noise Reduction Coefficient)
  The ability of a material to absorb sound is measured in NRC.
  It is the arithmetic mean of the sound absorption coefficients at the 250, 500, 1000 and 2000 Hz mark, rounded off to the nearest multiple of 0.05. NRC values fall in the range of 0 (0%) to 1 (100%).
  Hence, NRC of 0.6 means 60% absorption and 40% reflection.

7. Reverberation Time

Time that would be required for the sound pressure level to decrease by 60 dB after the source of the sound has stopped.

Reverberation time is usually measured in octave or third-octave bands. This decay does not have to be measured over the full 60 dB range. The values ranging from 5 dB to 35 dB below the initial level are denoted by T30 and 5 dB to 25 dB below the initial level are denoted by T20.