Page 93 - Master Handbook of Acoustics

FIGURE 4-18 The precedence effect (or Haas effect) in the human auditory system describes
temporal fusion. In the 5- to 35-msec region, the echo level must be about 10 dB higher than the

direct sound to be discernible as an echo. In this region, reflected components arriving from many
directions are integrated by the ear. The resulting sound appears to come from the direct source and
seems louder because of the reflections. For delays 50 to 100 msec and longer, reflections are
perceived as discrete echoes. (Haas.)

Moreover, the level of the delayed sound had to be increased more than 10 dB over the undelayed
sound before its location was heard as being separate. In a room, reflected energy arriving at the ear
within 35 msec is spatially integrated with the direct sound and is perceived as being spatially part of

the direct sound as opposed to reverberant sound. This is sometimes called the fusion zone or Haas
zone. These integrated early reflections increase the loudness of the direct sound and can change its
timbre. As Haas said, they 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 spatially discrete is gradual, and therefore somewhat indefinite. Some

researchers set the dividing line at 62 msec (1/16 sec), some at 80 msec, and some at 100 msec
beyond which there is no question about the discrete location of the delayed sound. If the delayed
sound is attenuated, the fusion zone is extended. For example, if the delayed sound is −3 dB relative
to the first sound, integration extends to about 80 msec. Room reflections are lower in level than
direct sound, so we expect that integration will extend over a longer time. However, with very long

delays of perhaps 250 msec or more, the delayed sound is clearly heard as a discrete echo.
Other researchers had previously found that very short delays (<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.
The precedence effect is easily demonstrated. Stand 100 ft from a concrete wall and clap your