The Haas effect, widely used in the recording industry , is a psychoacoustic effect which occurs when two sounds are perceived as only one, even though they were not emitted at exactly the same time.

Although he has received the surname of Helmut Haas, he was not the only one who researched this particular sound effect. Let’s look more closely at what it is, who researched it and some examples of applications in everyday life of this effect.

What is the Haas effect?

The Haas effect, also known as the precedence effect or priority effect, is an effect that occurs when two sounds are heard but emitted almost at the same time . The second sound is emitted after a very short time, less than 50 milliseconds after the first sound was emitted.

Because the emission of the second sound is so fast, the human ear does not perceive it as an echo, but as if the two sounds were one , even though they have been emitted at different times.

This effect becomes important when determining the location of objects by the sound they emit. When two sounds are perceived as being one because they have a minimum temporal separation, the spatial location is determined by the first sound that reaches the ear , which dominates over the second one. The sounds that come after the first one will make the brain interpret that there is a certain depth, relating them to the first one rather than interpreting them as independent sounds.

Historical background

Actually, and although this binaural psychoacoustic effect has been named in honor of Dr. Helmut Haas, who discovered it in 1949, the truth is that before him research and findings were made on this same effect.

In fact, in 1948, Lothar Cremer discovered this effect, referring to it as the “law of the first frontal wave” . A year later, Wallach and collaborators spoke more extensively about the precedence effect, showing how if two sounds were presented almost at the same time they would be heard as one. They were able to observe that from 40 milliseconds of difference in the emission of both sounds the echo effect began to occur.

Later, Wallach’s group discovered that when two sounds from different locations are perceived as one or merged, the brain interpreted the object as being located in a particular place from the first sound heard , not from the second. Based on this, they saw that this explained why sometimes, when the sound bounces off a wall or furniture in a room, the human ear can interpret the object that emits it as being in a different place than it really is.

However, despite the great discoveries made by Cremer and Wallach, the reason why this effect is known as the Haas effect is due to a publication by Helmut Haas in 1951 . In this publication, Haas studied how speech perception is affected by the presence of a rapid repetition of a sound. Haas found that humans locate based on the direction of the first sound heard, regardless of whether or not there is repetition of the sound.

Conditions of appearance when perceiving sounds

The Haas effect occurs if the second sound comes after 2 to 50 milliseconds . It should be noted, however, that the precedence effect varies depending on the type of sound. For example, in the case of speech this effect disappears if more than 50 milliseconds pass, however, for music, the Haas effect disappears after more than 100 ms.

In the various experiments that have been carried out in order to deepen this effect, it has been seen that the location depends on several aspects:

1. Summative location

If the second sound occurs after less than 2 ms , the listener will only perceive one sound.

2. Location dominance

If the second sound occurs after 2 to 5 ms , the listener will also hear a single sound, and the person will interpret the closeness of the object based on the first sound.

3. Delay affects discrimination

The less time passes between the first and second sound, the less capacity you have to be aware that you are hearing two sounds .

Applications

The Haas effect is important for listening in closed rooms, making it possible to determine with this effect the location of an object that emits the sound, although it can be said that the presence of walls can confuse the person since they reflect the sound .

Here are some situations where this effect is used deliberately.

1. Sound reinforcement systems

This type of system is used in public events. When placing several speakers, there may be a risk of echoes .

By taking the Haas effect into account and setting your speakers to ensure that they broadcast with delays of less than 50 ms, you will ensure that two or more sounds will not be heard.

2. Dolby surround

This effect has been taken into account when designing the devices with Dolby Surround. Whether it’s a TV or a music device, the importance of both or more speakers delivering sound at the same time or with very little delay from each other is well taken into account .

3. Environmental sound

The Haas effect can be used to increase the feeling of being immersed in a certain environment , whether natural, urban or of any kind.

One of the best known cases is that of the barber’s audio, a reproduction that if listened to gives the sensation of being in a hairdresser’s and the barber cuts our hair.

In this particular case, the effect of precedence is very much at stake, as well as making the sound in one earpiece louder than in the other, giving the sensation of depth and making you think that you have a pair of scissors nearby.

4. DJs

Many DJs use this effect to give depth to their mixes , as well as playing with the echoes and the volume of their compositions.

Thus, they can make the melody they are playing feel closer or farther away, as well as play with the perceived position of the speakers.

Bibliographic references:

  • Cremer, L. (1948): “Die wissenschaftlichen Grundlagen der Raumakustik”, Bd. 1.
  • Haas, H. (1951). “Uber den Einfluss eines Einfachechos auf die Horsamkeit von Sprache,” Acoustics, 1, 49-58.
  • Litovsky, R.Y.; Colburn, H.S.; Yost, W.A.; Guzman, S.J. (1999). The precedence effect. The Journal of the Acoustical Society of America. 106 (4 Pt 1): 1633-16.
  • Wallach, H., Newman, E. B., & Rosenzweig, M. R. (1949). “The precedence effect in sound localization,” The American Journal of Psychology, 62, 315-336.