Acoustics 101

Sounds are complex and many are made of low, middle and high frequencies. This is called Broadband sound.

Due to the fact that sounds are logarithmic, our ears don’t really hear 1 dB differences in sound level. 3 dB differences are just perceivable. 5 dB is louder and 10 dB is heard as twice as loud. Since sound is a vibration that travels in waves, everyone will perceive it differently. For this reason, criteria have been established to quantify sound in a space.

We are actively participating in seminars and acoustical sessions with leaders in the field to define these criteria for both the environment and interior spaces.

ACOUSTICAL METRICS

Reverberation

Reverberation Time is the amount of time it takes a sound to decay 60 dB, typically measured in seconds.

In performance halls and religious facilities, we can find reverb times of 1.2-1.5 seconds or even longer such as in a cathedral.

Reverberation is dependent on the room volume, the materiality (whether the materials are reflective or absorbent) and the shape of the room.

Alpha Bar

The alpha bar is a scale from 0 to 1 that is the weighted average sound absorption coefficient for a room that’s calculated by weighting the sound absorption coefficients of the individual surfaces in the room according to their respective areas and taking the arithmetic average. 0 being absolutely reflective, 1 being absolutely absorbent.

Noise Criterion

Noise Criterion were established for rating indoor noise like noise from HVAC equipment and similar. It’s used as a way to measure how loud sounds from the building equipment are, while the building is unoccupied. Meaning no other sounds are considered for this.

STC/IIC

STC: Sound Transmission Class ratings. “How much sound does your building assembly reduce as it moves through air?” IIC: Impact Insulation Class.

This refers to sounds from the impact of an object hitting a floor surface and being heard below.

OITC

OITC is a single number rating for transmission loss or noise reduction through the façade of a building.

These ratings are designed to correlate with subjective impressions of the ability of façade to reduce the noise from ground and air transportation, which has strong low-frequency sound.

DTC

Door Transmission Test is a field test to determine how much sound is reduced by a door. Doors and window are the weak point in an assembly. There are instances where it’s very important that a door actually reduce sound – such as those in sound lock vestibules in theaters, doors needed for secure or confidential speech, etc.

LDN

The Day-Night sound level, or LDN, takes an average of all the sounds that take place in a day and night, and gives you a single number. The night time is weighted by 10 dB.

An LDN can give an approximation of what happens in a day – but this values should be used with care since multiple sounds occur in a period of 24 hours.

LAeq

An LAeq, or A-weighed equivalent sound level is the single number value of the sound level, over a given period of time.

We can have a 5 hour, 1 hour, 30 minute, 1 minute, 30 second, 1 sec, 1ms LAeq. Each time period is going to average the sound level over that particular amount of time.

AGENCIES AND GUIDELINES