Page 99 - The Master Handbook Of Acoustics

74 CHAPTER THREE

source, even though reflections from other directions are involved.
The sound energy integrated over this period also gives an impression
of added loudness.
It should not be too surprising that the human ear fuses all sounds
arriving during a certain time window. After all, our eyes fuse a series
of still pictures at the cinema, giving us the impression of continuous
movement. The rate of presentation of the still pictures is important;
there must be at least 16 pictures per second (62-millisecond interval)
to avoid seeing a series of still pictures or a flicker. Auditory fusion
works best during the first 20 or 30 milliseconds; beyond 50 to 80 mil-
liseconds discrete echoes dominate.
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Haas set his subjects 3 meters from two loudspeakers arranged so
that they subtended an angle of 45 degrees, the observer’s line of sym-
metry splitting this angle. The conditions were approximately anechoic.
The observers were called upon to adjust an attenuator until the sound
from the “direct” loudspeaker equaled that of the “delayed” loudspeaker.
He then proceeded to study the effects of varying the delay.
A number of researchers had previously found that very short
delays (less then 1 msec) were involved in our discerning the direction
of a source by slightly different times of arrival at our two ears. Delays
greater than this do not affect our directional sense.
As shown in Fig. 3-19, Haas found that in the 5 to 35 msec delay
range the sound from the delayed loudspeaker has to be increased
more than 10 dB over the direct before it sounded like an echo. This
is the precedence effect, or Haas effect. In a room, reflected energy
arriving at the ear within 35 msec is integrated with the direct sound
and is perceived as part of the direct sound as opposed to reverber-
ant sound. These early reflections increase the loudness of the
sound, and as Haas said, result in “...a pleasant modification of the
sound impression in the sense of broadening of the primary sound
source while the echo source is not perceived acoustically.”
The transition zone between the integrating effect for delays less
than 35 msec and the perception of delayed sound as discrete echo is
gradual, and therefore, somewhat indefinite. Some peg the dividing
line at a convenient ⁄16 second (62 msec), some at 80 msec, and some at
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100 msec beyond which there is no question about the discreteness of
the echo. In this book we will consider the first 30 msec as in Fig. 3-19,
the region of definite integration.