Sounds are something we listen to, but more effective is the absence of sound that is being enforced through the silent interludes. Some silent interludes are intentionally created by the musicians, singers or the speakers due to hesitation, stuttering, or self-correction. Some pauses reinforce the delivery, while allowing time for the absorption of the message or information.
In 1952, Cage, a musician composed a piece that became his best-known and most controversial creation: 4’33”. The score instructs the performer not to play the instrument during the entire duration of the piece—four minutes, thirty-three seconds -and is meant to be perceived as consisting of the sounds of the environment that the listeners hear while it is performed.
‘Music inherently depends on silence in some form or another to distinguish other periods of sound and allow dynamics, melodies and rhythms to have greater impact. For example, most music scores features rests denoting periods of silence. In addition, silence in music can be seen as a time for contemplation to reflect on the piece. The audience feels the effects of the previous notes and can reflect on that moment intentionally. Silence doesn’t hinder musical excellence but can enhance the sounds of instruments and vocals within the piece’.
Whatever type of environment, enclosed or open, one is required to speak, the sound will be absorbed, reflected, or diffracted. The absorption occurs: in air and in materials. Sound as energy is lost in air due to the friction with air molecules. Moving or hot air with agile molecules will increase the absorption. Sound energy is lost in materials through its conversion into heat energy. Sound energy may be transmitted through induction of vibration in thin body materials.
Speakers and musician also know the craft of exploiting the effects of sound reverberation in a space. Reverberation is ‘lingering’ of sound after the main source stops, but its reflections continue within a space with gradual decrease in amplitude, until they can no longer be heard. The decrease in amplitude or gradual decay of sound occurs due to absorption in materials that form the spatial boundaries. The length of this sound decay is reverberation time. Excessive reverberation, if excessive makes the sound becomes muddy or garbled.
The time taken for a sound pressure level to diminish 1/1000 of its original level (-60dB), is called the reverberation time. It has major effect on the quality and intelligibility of speech. Some reverberation is needed to give a body and fullness to music. Ideal reverberation time depends on the type of music but lies between 1.75 and 2.5 seconds. Speech needs much less time for clarity. However, a little reverberation improves the subjective effect, as for example in sermons or similar addresses. For speech reverberation between 0.5 to 0.75 seconds could be adequate.
Most hard surfaces like concrete, stone, plastered masonry absorbs very little sound and are called sound reflective surfaces. Porous materials absorb the sound energy, whereas thin body materials convert the sound energy into a vibration.
Reverberation is not the same in all sections of a hall or for all frequencies. This is reason why acoustics’ quality differs from hall to hall and from seat to seat. Modifications are needed for different types of music by introducing absorbent panels and resonators or reflectors at various points.
Some spaces have long reverberation times at low frequencies, thus imparting a warm sonorous tone to the music, while others have longer treble times giving a more brilliant effect. Usually a balance is sought. Most halls with PA systems have longer reverberation times than required for speech resulting in loss of clarity. This can be slightly improved by use of column speakers angled towards the audience and away from the reflective walls of the hall. One of the most effective way is to provide a heavy curtain across the rear of the stage. This could be drawn back for live musical events to provide longer reverberation time.