Precedence effect
Encyclopedia
The precedence effect or law of the first wavefront is a binaural
psychoacoustic
effect. It means: If the same sound signal arrives time delayed at a listener from different directions, only the direction of the first arriving sound signal is perceived
. The delayed sound signals are localized
from the direction of the first arriving signal (the first wave front).
For time delays below 2 ms sum localization appears. Then a direction between both sound source directions is localized.
An application for sum localization is the intensity stereo
phony, where 2 loudspeakers emit the same signal with different levels, resulting in a the localized sound direction between both loudspeakers. The localized direction depends on the level difference between both loudspeakers.
For time delays above 50 ms (for speech) or some 100 ms (for music) the delayed sound is perceived as an echo of the first arriving sound. Both sound directions are localized correctly. The time delay for perceiving echoes depends on the signal characteristics. For signals with impulse characteristics echoes are perceived for delays above 50 ms. For signals with a nearly constant amplitude the echo threshold can be enhanced up to time differences of 1 to 2 seconds.
A special appearance of the precedence effect is the so called Haas effect
. Haas showed that the precedence effect appears even if the level of the delayed sound is up to 10 dB higher than the level of the first wave front. In this case the range of delays, where the precedence effect works, is reduced to delays between 10 ms and 30 ms.
.
Sound reinforcement system
s in big rooms use the Haas effect, in order to enhance the sound level for distant locations. For this purpose the loudspeakers at distant locations emit the same sound signal as the direct sound at the listener's location, but the loudspeaker signals are enhanced by up to 10 dB and are emitted 20 ms after the direct sound arrives at the listener. In this configuration the listener will localize all sound from the direction of the direct sound, but he will benefit from the higher sound level, which has been enhanced by the loudspeakers.
The "precedence effect" was described and named in 1949 by Wallach et al.
The "Hass effect" derives from a 1951 paper by Helmut Haas.
Binaural
Binaural literally means "having or relating to two ears." Binaural hearing, along with frequency cues, lets humans and other animals determine direction of origin of sounds...
psychoacoustic
Psychoacoustics
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...
effect. It means: If the same sound signal arrives time delayed at a listener from different directions, only the direction of the first arriving sound signal is perceived
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...
. The delayed sound signals are localized
Sound localization
Sound localization refers to a listener's ability to identify the location or origin of a detected sound in direction and distance. It may also refer to the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space .The sound localization mechanisms of the...
from the direction of the first arriving signal (the first wave front).
Appearance
The precedence effect appears, if the subsequent wave fronts arrive between 2 ms and about 50 ms later than the first wave front. This range is signal dependent. For speech the precedence effect disappears for delays above 50 ms, but for music the precedence effect can also appear for delays of some 100 ms.For time delays below 2 ms sum localization appears. Then a direction between both sound source directions is localized.
An application for sum localization is the intensity stereo
Intensity stereo
Intensity stereo or Intensity stereophony is the technique used by a stereo sound image that is produced only by level differences in between the left and right loudspeakers, rather than arrival time differences. Also known as a set up of two microphones from across from each other so you could...
phony, where 2 loudspeakers emit the same signal with different levels, resulting in a the localized sound direction between both loudspeakers. The localized direction depends on the level difference between both loudspeakers.
For time delays above 50 ms (for speech) or some 100 ms (for music) the delayed sound is perceived as an echo of the first arriving sound. Both sound directions are localized correctly. The time delay for perceiving echoes depends on the signal characteristics. For signals with impulse characteristics echoes are perceived for delays above 50 ms. For signals with a nearly constant amplitude the echo threshold can be enhanced up to time differences of 1 to 2 seconds.
A special appearance of the precedence effect is the so called Haas effect
Haas effect
The Haas effect is a psychoacoustic effect, described in 1949 by Helmut Haas in his Ph.D. thesis. It is often equated with the underlying precedence effect .- Experiments and findings :...
. Haas showed that the precedence effect appears even if the level of the delayed sound is up to 10 dB higher than the level of the first wave front. In this case the range of delays, where the precedence effect works, is reduced to delays between 10 ms and 30 ms.
Applications
The precedence effect is important for the hearing in enclosed rooms. With the help of this effect it remains possible to determine the direction of a sound source (e.g. the direction of a speaker) even in the presence of wall reflectionsReflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
.
Sound reinforcement system
Sound reinforcement system
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
s in big rooms use the Haas effect, in order to enhance the sound level for distant locations. For this purpose the loudspeakers at distant locations emit the same sound signal as the direct sound at the listener's location, but the loudspeaker signals are enhanced by up to 10 dB and are emitted 20 ms after the direct sound arrives at the listener. In this configuration the listener will localize all sound from the direction of the direct sound, but he will benefit from the higher sound level, which has been enhanced by the loudspeakers.
History
The "law of the first wavefront" was described and named in 1948 by Lothar Cremer.The "precedence effect" was described and named in 1949 by Wallach et al.
The "Hass effect" derives from a 1951 paper by Helmut Haas.