Cornell University Ergonomics Web
DEA3500: Ambient Environment: Room Acoustics
Behavior of Sound in an Enclosed Space
Shape, dimensions, construction, and contents of any room will determine how sound is transmitted, reflected and absorbed. The way in which sound behaves in an enclosed space depends on the following factors:
- Attenuation due to distance.
- Audience absorption of direct sound.
- Surface absorption of direct and reflected sound.
- 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.
- Dispersion of modeled surface - Reflections can be reduced by making one surface dispersive i.e. not at a right angle.
- Edge diffraction - Edge diffraction results in the curvature of part of a sound wave around the edge of a barrier.
- 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.
- Primary reflection - Angle of incidence = angle of reflection, plus nature of sound reflector is important.
- 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
Basically 3 types:
- for speech
- for music
Acoustics for speech
Unamplified speech sounds, at a distance of 3 m:
30 dBA - whispering
60 dBA - lecture voice
70 dBA - loud actor (down to 50 dB after 30 rows of seats)
Speech intelligibility = power and clarity
Acoustics for Music
From middle of an auditorium sounds can vary from quiet music (30 dBA) to loud music > 80 dBA.
Basic design criteria:
Power - volume
- direct sound
- primary reflections
Clarity - direct sound
- primary reflections
Blend - grouping of players
- platform reflectors
- arrangement of audience
Ensemble - tiered platform
(aesthetics) - platform reflectors
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