Haas effect, Optimum reverberation time, Anechoic chamber
In audio reinforcement, a kind of propogation delay is used to compensate for the passage of sound through
air. A delayed signal is sent to loudspeakers so that the speakers reinforce the stage sound at the same time
or slightly later than the acoustic sound from the stage: approx. 1 millisec. of delay per foot of air or 3
millisec. per meter
Haas effect occurs when arrival times of the sounds differ by up to 30–40 milliseconds. As the arrival time
(in respect to the listener) of the two audio sources increasingly differ beyond 40 ms, the sounds will begin
to be heard as distinct; in audio-engineering terms the increasing time difference is described as a ‘delay’.
The Haas effect is often used in public address systems to ensure that the ‘perceived location and/or
direction of the original signal’ viz. localization remains unchanged.
The effect allows audio engineers to use additional speaker systems placed away from the stage and still
give the illusion that all sound originates from the stage. The purpose is to deliver sufficient sound volume
to the back of the venue without resorting to excessive sound volumes near the front.
In some instances, usually when serving large areas and/or large numbers of listeners, loudspeakers are
required to be placed at some distance from a stage or other area of sound origination. The signal to these
loudspeakers may be delayed electronically or by some other means for a time equal to or slightly greater
than the time taken for the original sound to travel to the remote location. This serves to ensure that the
sound is perceived as coming from the point of origin rather than from a loudspeaker that may be physically
nearer the listener.
The level of the delayed signal may be up to 10 dB louder than the original signal at the ears of the listener
without disturbing the localization.
OPTIMUM REVERBERATION TIME
1. In a room or auditorium designed for speech, the reverberation time that provides the highest speech
intelligibility consistent with other requirements.
2. In a room or auditorium designed for music, the reverberation time that provides optimum conditions
for playing and listening to music.
3. These optimum values depend on the use of the room, its volume, and may depend on frequency.
(Matter below compiled partly from Fundamentals of Acoustics by L E Kinsler and A R Frey)
Optimum reverberation time is a function of both volume of the enclosure and its use e.g. in small to
medium-sized rooms or offices it is possible to carry on conversations and routine work with little
interference when the reverberation time is about 0.5 sec. We expect that an enclosure designed for speech
should have a short reverberation time, to avoid overlap of syllables and to provide good articulation.
Nonetheless, the speaker does not prefer high absorbing conditions, as the enclosure then appears dead and
unresponsive to him. The most acceptable conditions for a speaker and audience seem to correspond to a
reverberation time of about 0.8 sec for a small auditorium of 50,000 ft3
volume, increasing to about 1.5 sec
or more for large auditoriums whose volume exceeds 1,000,000 ft3
The optimum reverberation time of an enclosure may even vary with type of music and the effect desired. In
general, enclosures designed as music rooms should be more reverberant than similar-sized rooms designed
primarily for speech. The optimum reverberation time is found to range from 1.0 sec in small rooms used for
solo performances to about 2.5 sec for organ music in auditoriums.
One factor that must be considered in auditorium design is the effect on its reverberation time due to the
audience. Since the sound absorption of an average human being is about 4.5 sabins, variations in the size of
the audience may produce significant changes in reverberation time.
1. Anechoic chambers are commonly used to conduct experiments in "free field" conditions.
2. All sound energy travels away from the source with almost none reflected.
3. Common anechoic chamber experiments include measuring the transfer function of a loudspeaker or
the directivity of noise radiated from industrial machinery.
4. In general, the interior of an anechoic chamber is very quiet, with typical noise levels in the 10-20
1. Full anechoic chambers aim to absorb energy in all directions whereas Semi-anechoic chambers have
a solid floor that acts as a work surface for supporting heavier equipment, such as cars, washing
machines, or industrial machinery,
2. Rather than the mesh floor grille found over absorbent tiles present in full anechoic chambers.
3. This floor is damped and floating on absorbent buffers to isolate it from outside vibration or
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