Room Acoustics

Whether a room has "good" or "bad" acoustics is an ill-defined question. What needs to be determined is whether a room is good or bad for a particular use. No room is appropriate for every acoustical situation. The analysis of a room depends on several criteria. The listener views these criteria as subjective but many of these criteria may be linked to quantitative measurements. The following is a list of some common criteria.

Acoustical Criteria

Liveness or Reverberance
Clarity vs. Fullness
Loudness
Intimacy
Warmth vs. Brilliance
Uniformity and Diffusion
Texture or Smoothness
Envelopment
Balance or Blend
Sense of Ensemble or Performer Satisfaction

Liveness or Reverberance: Characterized by the reverberation time of primarily the high and medium frequency ranges. Determined by volume of room and effective surface area. Can be altered by changing area and nature of absorbing materials. Suitability depends on nature of sound or music. A room with too short a reverb time for a particular type of music may classified as "dry" or "dead" while one that is too alive or has too long a reverb time may be called "muddy" or "watery".

reverb
The above table was scanned from page 534 of Rossing, Moore and Wheeler's Science of Sound text published by Addison Wesley: ISBN 0-8053-8565-7.

Clarity vs. Fullness: Measure of intensity of the direct sound relative to the reverberant or reflected sound. The greater the reflected intensity, the more "full" the room is perceived and the less clear the spoken word or fast or highly articulated sections of music are perceived. Clarity is sacrificed in a "muddy" room but a lesser degree of clarity enhances slow passages of music from the romantic era. To achieve greater clarity or "definition" the entire audience should be close to the stage and have an unobstructed view. This can be accomplished by placing performers on a raised stage and perhaps a raked stage, and by placing the audience on a sloped floor or in balconies.

Loudness: A longer reverb time contributes to the "loudness" of the hall since loudness is a measure of the average power detected by a listener. The relative intensities of ambient and external noise levels are also a factor and should be minimized.

Intimacy: Connected to clarity. The time between the arrival of the direct sound and the first reflection determines the listener's perceived proximity to the performers. An intimate feeling occurs when the initial time delay is between 8 and 20ms. The more popular halls have a smaller seating capacity because they are generally louder and more intimate.

Warmth vs. Brilliance: This character is determined by the reverb time of the low frequencies relative to the medium and high frequency ranges. In general, the reverb time is a little longer for frequencies below 500Hz than those above. For a nearly constant reverb time as a function of frequency, the room may be classified as "bright" or "brilliant". The longer the lower frequency reverb time is the warmer the room. Yet too long and the room sounds muddy. Most highly regarded concert halls are classified as warm.

Uniformity and Diffusion: Uniform spatial distribution of both direct and reflected sounds throughout the audience. Uniformity in the direct sound can be achieved by minimizing the distance between first and last rows, e.g., a shallow hall with several balconies. Hot spots, caused by the focusing effect of curved walls or domed ceilings, reduce uniformity as well as do dead spots or "shadows" caused by physical obstructions or balcony openings. Objects that diffuse sounds of all wavelengths are necessary for good uniformity.

The above table was scanned from page 314 of Donald Hall's Musical Acoustics text published by Brooks/Cole, ISBN 0-534-13248-0.

Texture or Smoothness: For a smooth room there should be no more than 20-30ms between successive reflections reaching a point in the audience and the intensity of the reflections should smoothly die away in time. Loud echoes can be avoided by not using large flat highly reflecting surfaces and by using sound diffusers of different sizes.

Envelopment: Early reflections, those arriving in the first 100ms, should arrive from all directions in order to give the listener a sense of envelopment. Recent designs have given more attention to the importance of lateral reflections. Organ music requires an extreme sense of envelopment. Pipes may be placed above, behind, or to the side of the audience to mask the origin of the sound source. For other types of music, too much envelopment can blur the aural image of the stage.

Balance or Blend: Sound coming from different locations on the stage should have balanced intensities. Somewhat related to uniformity. This is generally a problem for seats close to a wide stage. This situation can be balanced with a low, irregularly shaped ceiling and appropriate onstage reflecting surfaces.

The above figure was scanned from page 313 of Donald Hall's Musical Acoustics text published by Brooks/Cole, ISBN 0-534-13248-0.

Sense of Ensemble or Performer Satisfaction: The performers must be able to hear themselves and the other performers. There should be many reflections strong enough to be heard by the performers but the reflections should decay uniformly and in a time shorter than the shortest time between notes. Flutter echoes reflecting from parallel side walls must be avoided. Acoustic shells increase the sense of ensemble and helps to project more of the sound toward the audience. Ensembles in which members are separated by more than approximately 5 meters easily lose synchronicity and need a conductor to give visual cues.

Acoustical Design Problems

Extreme values of reverberation time
Echoes
Focusing of sound or hot spots
Balcony and column shadows
External noise
Room resonances
Precedence effect