primer on reverberation

Primer on reverberation

R.Narasimha SwamySenior consultant

narasimhaswamy@yahoo.com

Reverberation• This is due to multiple reflections of the sound waves in enclosed spaces.

• Wallace Sabine introduced the reverberation time (RT60) as a measure of acoustic characteristics of a enclosed spaces almost a century before.

• RT60 provides a rough thumb rule figure of the acoustic characteristics of the enclosed space.

• In past 20 years, several other parameters have been added to accurately characterize and compare the relative performance of the enclosed spaces.

• All acoustic parameters are derived from sound decay characteristics - signature of the enclosed spaces.

Decay characteristics – Signature of the enclosed spaces

The sound decays exponentially after the source ceases.

The build-up and decay of sound in a room

The direct sound arrives first at time = 0, reflected

components arriving later.

The sound pressure at H builds up stepwise.

Reverberation RT60

• Reverberation time RT60 is defined as that time taken for the sound in a room to decay to 60 dB below the source level, after the source is muted.

• In very rough human terms, it is the time required for a sound that is very loud to decay to inaudibility.

• Reverberation is a desirable effect for music, whereas they are highly undesirable for speech, as it adversely affects intelligibility of the spoken words.

Determining the RT60

• Can be computed from the dimension and the absorption characteristics of the material used for the inner surfaces of the enclosed spaces by using classical Sabine’s formula. Used for preliminary design.

• Can be measured using instrumentation.

Sabine’s formula

• Sabines formula (1900): T60 = 0.16 * V / Se

• V is a volume in m3

• Se is effective absorbing area in m2

• Se = a1*S1 + a2*S2 + a3*S3 + …– ai is the absorption coefficient (1 – β) for area Si

• Erving's formula: uses -ln(1-a) instead of a• Both do not account for air absorption

Measurement of RT60

• Classical Y-T recorders were used in earlier days to record the sound decay characteristics and deriving the RT60. This method is more or less obsolete now.

• Signal processing methods by exciting the enclosed space and then measuring the impulse response of the enclosed space and then deriving the RT60.

Impulse response

• In signal processing, the IR of a dynamic system is its output when its input is presented with an impulse signal.

• The impulse response describes the response of the system as a function of time that characterizes the dynamic behaviour of the system.

Impulse sound

• Physical stimuli like bursting of balloons, gun shots etc.

• Exciting the room with the help of electronically generated signals like Maximum length sequence [MLS], swept tone etc and recording the response of the room and analyzing it with the help of signal processing to obtain the complete acoustic characteristics of the enclosed spaces.

The Impulse response from a simple audio system. The original impulse, the response after high frequency boosting, and the response after low frequency boosting.

Room impulse response [RIR]

Most real rooms (at all frequencies) have exponential decay

Exponential decay produces a single-slope.

If the direct sound is strong enough the effective early decay can be short.

But, then there will be too few early reflections and the late reverberation will be weak.

If the direct sound is weak, there will be too much energy between 50 and 150ms, and the sound will be MUDDY.

Early and late reflections• Early reflections:– Strong and distinct– Provide spatial information– Should be modelled accurately

• Late reverberation:– Low intensity and high density of reflections– Provide room information– No longer depends on source position– Can be modelled statistically

Early reflections

• The direct sound reaches the listener in 20 to 200 ms, depending on the distance from the source to the listener.

• The first group of reflections from the walls and the ceiling reaching the listener within about 50 to 80 ms, is often called the early sound.

• If the total energy from lateral reflections is greater than the energy from overhead reflections, the hall takes on a desirable “spatial impression.”

Precedence effect• Rather remarkably, human auditory processor

deduces the direction of the sound source from the first sound that reaches our ears, ignoring reflections. This is called the precedence effect or “law of the first wave front.”

• The source is perceived to be in the direction from which the first sound arrives provided that: • Reflections arrive within 35 ms.

• Reflections that have spectra and envelopes similar to the first sound.

• A : Absolute threshold of audibility of reflection• B : Image shift/broadening threshold

• C : Lateral reflections perceived as a distance echoes.

Reverberant chambers

T60 > 1000 ms.Large, specially designed concert halls.Acoustically pleasant (for each seat!).Even decay of the reverberant response to

mask disturbing artefacts.Good for Western classical symphony orchestraMost unsuitable for speech.• Intelligibility is impaired by late reflections.• Try to talk in a large gym or in a pool…

Anechoic chamber

T60 < 100 msNo sound sources and no reflectionsUnnatural and often unpleasant feelingAnechoic chamber is used for simulating free

field conditions for testing of:– audio devices and equipment– Conducting hearing tests

Choice For a particular enclosed space, the choice is

always between clarity or reverberation.

Reverberation currently has the upper hand because it provides a pleasant aural experience.

Emphasis on reverberation is often misguided.

However, whenever an opportunity is provided to increase the clarity, the improvement is noticed immediately and appreciated by everyone.

Thank you