Sound in interiors

An enclosed space is a room or area bounded on every of its sides. The materials for enclosure may be classified into two

• Those that allow sound rays to pass through and
• Those that do not allow sound rays to pass through.

Areas bounded with materials that allow sound rays to pass through tend to enjoy good acoustic as the effect of indirect sound from reflection is reduced within the space. In this space, sound from external source can pass through the material into the enclosure as background noise. The vibration of some of the materials can be a source of noise within the space and this can be a bane to the achievement of clear and audible speech and music.

The effect of indirect sound may be pronounced in spaces enclosed with materials that do not allow the passage of sound rays through them. Adjustments to the material to aid diffusion will improve the acoustic of the space. Some of these materials can absorb sound, reducing the effect of indirect sound.

On encountering barriers posed by the enclosure, sound waves are likely to behave in the following ways

• Reflection
• Absorption
• Refraction
• Diffusion
• Diffraction
• Transmission

Reflection

This occurs when the wavelength of a sound wave is smaller than the surface of an obstacle. In the case of an enclosed space, the sound waves hit every side of the enclosure continuously until the sound energy reduces to zero. The amount of waves reflected depends on the smoothness, size, and softness of the materials of enclosure. The angle of incidence of sound rays is equal to that of the reflected rays only if the surface of the reflector is flat. But when it is curved, the angles are different.

Absorption

When sound waves hit the surface of an obstacle, some of its energy is reflected while some are lost through its transfer to the molecules of the barrier. The lost sound energy is said to have been absorbed by the barrier. The thickness and nature of the material as regards its softness and hardness influences the amount of sound energy absorbed.

Refraction

This is the bending of sound when it travels from one medium into another medium. The difference in the composition of the two different media bends the sound i.e. the angle of incidence changes into an angle of refraction as it travels into the new medium.

Diffusion

This is the scattering of waves from a surface. It occurs as a result of the texture and hardness of the obstacle is comparable to the wavelength of the sound. The direction of the incident ray changes when it strikes the surface of the obstacle. Satisfaction is achieved when sound is heard in all direction at equal level.

Diffraction

When the wavelength of a sound wave is smaller or equal to the size of the obstacle, the sound rays tend to bend round the edge of the obstacle thereby turning the edge to a sound source.

Transmission

In this phenomenon, sound wave is carried by molecules of the obstacle through vibration and re-emitted at the other side irrespective of the medium. It can be structure borne, air borne or impact sound.

Reverberation

This is the persistence of sound in an enclosed space as a result of continuous reflection or scattering of sound after the source has stopped. It is one the most prominent behaviours of sound in an enclosure. It occurs when sound waves hits a surface and are reflected toward another surface which also reflects it. Some of the sound is absorbed with this continuous reflection which gradually reduces the energy of the sound to zero. The phenomenon can affect the audibility of sound in an enclosure, especially if the reverberation time, which is the time taken for the sound pressure level to diminish to 60 dB below its initial value is considerably long.

Echo

This occurs when the reverberation time is long enough to cause a distinct repetition of the direct sound. This condition is an advanced form of reverberation where the sound is heard clearly and repeatedly after some time until it fades.