Haas effect: what it is and how it affects the perception of sounds

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

Although he has received the last name of Helmut Haas, he was not the only one who investigated this particular sound effect. Let's take a deeper look at what it is, who investigated it, and some examples of applications of this effect in daily life.

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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 they are 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.

Since the emission of the second sound is so fast, the human ear does not perceive it as if it were an echo, but as if the two sounds were one, despite the fact that they have been issued at different times.

This effect becomes important when determining the location of objects by the sound they emit. When two sounds are perceived as if they were one by having a minimum temporal separation, spatial location is determined by the first sound that reaches the ear, which dominates over the second. The sounds that come after the first one will cause the brain to 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 after 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 et al. discussed the precedence effect at greater length, showing how if two sounds were presented almost at the same time they would be heard as one. They were able to observe that after 40 milliseconds apart in the emission of both sounds, the echo effect began to occur.

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

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 rapidly repeated sound. Haas discovered that humans locate based on the direction of the first sound heard, regardless of whether or not there is a repetition of it.

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Conditions of appearance when perceiving sounds

the haas effect occurs if the second sound comes between 2 and 50 milliseconds after. It should be said, 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 elapse, however, for music, the Haas effect disappears after more than 100 ms.

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

1. summative localization

In case the second sound is given after less than 2 ms, the listener will only perceive a 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 proximity of the object based on the first sound.

3. Delay affects discrimination

The less time passes between the first and second sounds, less ability to be aware that two sounds are being heard.

Applications

The Haas effect is important for hearing 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 sound.

Next we will look at some situations in which this effect is used in a deliberate way.

1. sound reinforcement systems

These types of systems are used in public events. By placing multiple speakers, there may be a risk of echoes occurring.

If you take the Haas effect into account and set up your speakers in such a way as to ensure that are retransmitted with delays of less than 50 ms, it will ensure that two or more sounds.

2. Dolby surround

This effect has been taken into account when developing devices with dolby surround. Whether they are televisions or music devices, it is well kept in mind the importance of both or more speakers delivering sound at the same time or with very little delay each other.

3. ambient 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 you listen to it gives the sensation of being in a hairdresser's and the barber cutting your hair.

In this particular case, a lot is played with the precedence effect, in addition to making the sound heard in a headset. louder sound than in the other, giving the sensation of depth and coming to think that we have scissors near.

4. DJ's

Many DJs use this effect to add depth to their mixes., in addition to playing with the echoes and the volume of his compositions.

Thus, they can make the melody they are playing feel closer or further away, in addition to playing with the perceived position of the speakers.

Bibliographic references:

• Cremer, L. (1948): "Die wissenschaftlichen Grundlagen der Raumakustik", Bd. 1. Hirzel-Verlag Stuttgart.
• Haas, H. (1951). "Uber den Einfluss eines Einfachechos auf die Horsamkeit von Sprache," Acustica, 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.