Masking, Upward Masking, And The Haas Effect; Reverberation - Toa BS-1030 User Manual

TOA Electronics Speaker Guide

Masking, Upward Masking, and the Haas Effect

Masking refers to one sound being obscured by another so it is unnoticed or indistinguishable
from the first sound. It is one of the main obstacles to speech intelligibility. Typical sound systems
include a number of potential sources of masking effects.
Background noise is the most obvious: the sound system's normal operating level should be at
least 15–25 dB above the background noise level.
High distortion levels in amplifiers, speakers, or other sound system components, is another
possible source of masking. Excessive distortion is easily avoided by using high-quality
equipment and following industry guidelines for proper gain structure. For a definitive discus-
sion of sound system gain structure, see Sound System Engineering.
Late reflections, or late-arriving sounds from distant speakers, can be especially troublesome
and can ruin both music quality and intelligibility. Good speaker layout avoids echoes from dis-
tant speakers. It is also important to properly place and aim speakers to avoid echoes from dis-
tant walls or other surfaces.
The Haas Effect refers to a characteristic of human hearing that perceives early reflections (i.e.,
from surfaces near the sound source or listener) as part of the original sound, while later reflec-
tions from more distant surfaces are perceived as discreet echoes. This characteristic can be
used to advantage in room and sound system design, but can also indicate conditions to avoid.
As a general rule, avoid strong reflections from surfaces more than 15 ft from either the sound
source or the listener. Field experience indicates that reflections from surfaces 7–10 ft from the
source or listener blend more smoothly with the direct sound. Very close reflections, within 4 ft
of the source or listener, cause audibly wide notches in frequency response due to phase can-
cellation and should be avoided or moderated using acoustically absorbent materials.
Upward masking refers to the characteristic of a sound to mask not only other sounds in the
same frequency range, but also sounds several octaves higher. This often overlooked aspect of
human hearing can result in a loss of intelligibility when the lower frequencies predominate in
a sound system—a common occurrence, especially with speakers with dispersion patterns that
get narrower with increasing frequency. For example, an eight-inch ceiling speaker is omni-
directional below 400 Hz, but has less than 60° coverage above 2 kHz, resulting in excessive
reflected and off-axis sound energy in the low frequencies.

Reverberation

Reverberation is another common source of masking-related intelligibility loss. Significant
reverberation occurs in a large room (i.e., church, gymnasium, or auditorium) where repeated
reflections merge into a seemingly continuous sound with a gradual rate of decay. Many in-
stalled sound systems are used in spaces where there is little or no significant reverberation. This
design guide is applicable in these situations. When designing a speech reinforcement system
for a large, reverberant room (RT60 > 2.5 s), we recommend consulting with a specialist in acous-
tical system design. More information on sound system designs for large rooms can be found
in Sound System Engineering, and in Handbook for Sound Engineers.
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Audio Basics