Haas effect
Encyclopedia
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
Precedence effect
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...

 (or law of the first wavefront).

Experiments and findings

In 1951 Haas examined how the perception of speech is affected in the presence of a single, coherent sound reflection. To create anechoic
Anechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...

 conditions, the experiment was carried out on the rooftop of a freestanding building. Another test was carried out in a room with a reverberation time of 1.6 ms. The test signal (recorded speech) was emitted from two similar loudspeakers at locations 45° to the left and to the right in 3 m distance to the listener.

Haas found that humans localize
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...

 sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction. A single auditory event
Auditory event
Auditory events describe the subjective perception, when listening to a certain sound situation. This term was introduced by Jens Blauert in the year 1966, in order to distinguish clearly between the physical sound field and the auditory perception of the sound.Auditory events are the central...

 is perceived. A reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a secondary auditory event (echo
Echo (phenomenon)
In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...

). This time span varies with the reflection level. If the direct sound is coming from the same direction the listener is facing, the reflection's direction has no significant effect on the results. A reflection with attenuated higher frequencies expands the time span that echo suppression is active. Increased room reverberation time also expands the time span of echo suppression.

Sound reinforcement systems

Haas' findings can be applied to 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 and public address
Public address
A public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...

 systems. The signal for loudspeakers placed at distant locations from a stage may be delayed electronically by an amount equal to the time sound takes
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

 to travel through the air from the stage to the distant location, plus about 10 to 20 milliseconds. The first arrival of sound from the source on stage determines perceived localization whereas the slightly later sound from delayed loudspeakers simply increases the perceived sound level without negatively affecting localization.

Haas kicker

Many older LEDE
Lede
Lede is a municipality located in the Belgian province of East Flanders in the Denderstreek, nearby cities are Gent, Aalst and Dendermonde. The municipality comprises the towns of Impe, Lede proper, Oordegem, Smetlede, Wanzele and Papegem. In 2011, Lede has a total population of 17,882 . The total...

 ("live end, dead end") control room
Recording studio
A recording studio is a facility for sound recording and mixing. Ideally both the recording and monitoring spaces are specially designed by an acoustician to achieve optimum acoustic properties...

 designs featured so-called "Haas kickers" - reflective panels placed at the rear to create specular reflections which were thought to provide a wider stereo listening area or raise intelligibility. However, what is beneficial for one type of sound is detrimental to others, so Haas kickers just like compression ceilings are no longer commonly found in control rooms.

Further reading

  • Floyd Toole "Sound Reproduction", Focal Press (July 25, 2008), Chapter 6
  • Blauert "Spatial Hearing - Revised Edition: The Psychophysics of Human Sound Localization", The MIT Press; Rev Sub edition (October 2, 1996)
  • Litovsky et al. (1999), "The precedence effect" J. Acoustic. Soc. Am., Vol. 106, No. 4
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