Cornell University Ergonomics Web

DEA3500: Ambient Environment: Room Acoustics

ROOM ACOUSTICS

Behavior of Sound in an Enclosed Space

1. Attenuation due to distance.
2. Audience absorption of direct sound.
3. Surface absorption of direct and reflected sound.
4. Reflection from re-entrant angle - Sound entering right-angled corner of room will be reflected back towards source if surfaces are acoustically reflective. This can produce echoes in large spaces.

1. Dispersion of modeled surface - Reflections can be reduced by making one surface dispersive i.e. not at a right angle.
2. Edge diffraction - Edge diffraction results in the curvature of part of a sound wave around the edge of a barrier.
3. Sound shadow - Any barrier interrupting a sound wave will create a shadow (light). However, because of edge diffraction some sound will creep into this but such penetration is frequency dependent - high frequencies are less diffracted than low frequencies. Such problems can occur in auditorium with balconies.
4. Primary reflection - Angle of incidence = angle of reflection, plus nature of sound reflector is important.
5. Panel resonance - Sound waves can propagate "through" a solid material by panel vibration. The sound does not actually penetrate the material but rather causes this to vibrate and act as a sound source itself. The panel will be vibrated by both direct and reflected sound waves.

2 factors affect sound transmission: 1. increased weight per unit area of panel decreases sound transmission 2. increased frequency of incident sound decreases sound transmission. 

Apart from just mass of the panel other factors can affect sound transmission:

• Panel stiffness - at very low frequencies the stiffness (i.e. resistance to deformation) may have more effect than its weight. In this part of the frequency range insulation is termed stiffness controlled.
• Rigid panels - if a rigid panel is struck it will continue to vibrate at frequencies determined by its size, shape, and thickness - this is its "natural frequency" (natural mode of vibration).

All subsequent frequencies which produce such vibration are called resonant frequencies and sound insulation will be reduced. This is termed resonance controlled insulation.

Coincident sound - Not all sound striking a panel will strike this at right angles but some will strike this obliquely and these will produce a forced motion in the panel (the trace wavelength). When the bending wavelength of the panel is equal to the trace wavelength then transmission increases and insulation is decreased. In this frequency region sound transmission is said to be coincidence controlled

• Inter-reflection, standing waves and reverberation - Standing waves are stationary fluctuations in pressure due to the superposition of sound waves moving in the opposite direction.
• Sound Transmission

Types of Auditorium

1. for speech
2. for music
3. multi-purpose

Acoustics for speech

Speech intelligibility = power and clarity 

Acoustics for Music

Basic design criteria:
Power - volume
- direct sound
- primary reflections
- reverberation
Clarity - direct sound
- primary reflections
- reverberation
- echoes
Blend - grouping of players
- platform reflectors
- arrangement of audience
Ensemble - tiered platform
(aesthetics) - platform reflectors

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