PHYS341/2024/Project8

Reverberation

Reverberation is reflected sound waves that are perceived as separate or distinct from the original (direct) sound. It occurs when sound is reflected off of surrounding surfaces at varying rates in time. The quality (perceivability) and strength (loudness) of reverberation is dependent on the architecture / geometry, materials, air temperature, etc. of the environment in which the sound is produced. Too much reverberation (too little time in between reflection waves) interferes with speech/sound intelligibility.

Four Acoustic Features

Simple 2D Model of Reflected Sound Energy. When sound energy from a direct source is reflected off of surrounding boundaries, reflection occurs. The waves reflect off of each surface while decaying, some energy is absorbed by a surface while most is reflected. For smooth, hard surfaces almost all of the energy is reflected.

There are four acoustic features considered in a room's acoustic properties, reverberation is one of these features. Direct Sound: sound waves emanating directly from the source.

Early Reflection: sounds that arrive at the external ear after approximately 1/10th of a second after the direct sound; perceived as being within the direct sound. These early reflections allow an acoustic space to perceptually "hold" the sound, thus giving it a certain quality of life and enhancing the clarity and strength of the direct sound.

Reverberation: reflected sound waves that are perceived as separate to the direct sound. Reverberation is also referred to as late reflection and can interfere with sound intelligibility depending on the time between reflections. More or less overall reverberation is dependent on the absorption properties of the surrounding environmental surfaces and air; a larger space with more reflective walls will see higher reverberation.

Echo: discrete sound reflections that are perceived as distinct acoustic events. Echos interfere with a sound's intelligibility and are more common in larger spaces as they have a longer reflection tail.

• Slap echo: single sound return.
• Flutter echo: rapid series of decaying echoes.

Frequencies

Frequency ranges affect reverberation quality.

A study done by Traer & McDermott (2016) found that larger spaces with more reflective walls (hard & smooth materials) produced greater reverberation. Traer & McDermott (2016) also noted that the longest decay rates were within the Mid Frequencies (200 - 2000 Hz) Rougher or more porous materials will absorb frequencies around the mid - high range.

• Low Frequencies: 20 - 200 Hz. (average f0 of adult speech; non sibilant)
• Mid Frequencies: 200 - 2000 Hz. (sibilants)
• High Frequencies: 2000 - 20,000 Hz.

Frequency Chart

Frequency Summary	Frequency Range	Frequency Values
Low Frequencies	Sub-Bass	20 - 60 Hz.
	Bass	60 - 200 Hz.
Mid Frequencies	Low Midrange	200 - 500 Hz.
	Midrange	500 - 2000 Hz.
High Frequencies	Upper Midrange	2000 - 4000 Hz.
	Presence	4000 - 6000 Hz.
	Brilliance	6000 - 20,000 Hz.

Measuring Reverberation

When measuring reverberation in a given environment, a few considerations must be taken... Frequency Balance: the strength of the frequencies involved in reverberation. Strength: how loud is the reflected sound compared to the direct sound? If a reflected sound is louder than the direct sound, the brain will perceive it as unique to its source (direct sound) and it will interfere with the intelligibility of the direct sound.

Decay: how long it takes for the reflected sound (reverberation) to fade after the direct sound stops. Larger rooms produce fewer reflections per unit time, thus reverberation is slower to decay and has a longer tail.

• Lower frequencies give a ‘warm' reverberation tail.
• Higher frequencies give a ‘bright' reverberation tail.

D50: ratio of received early (0-50ms post arrival of direct sound) sound energy to total received energy.

• Early reflection is considered to have positive effects for sound intelligibility (0-50ms).
• Late (delayed) reflection is considered detrimental to sound intelligibility (>50ms).

Reverberation Time (RT, RT60).

Used to predict the sound that any source would produce in x environment and thus describe the quality of reverberation.

RT is a measurement of how quickly a sound decays to inaudibility in a given environment.

RT60 is the rate at which amplitude (sound volume) decreases by 60dB in a given environment.