The effect of sound on the human auditory organs does not disappear immediately with the disappearance of the sound, but will stay for a while. If the time interval between the two sounds reaching the human ear is less than 50ms, then the human ear will not feel that the two sounds are intermittent.
However, when the time difference between the two exceeds 50ms, which is equivalent to a sound path difference of more than 17m, the human ear can distinguish that they are two independent sounds from different directions. Indoors, when a sound source emits a sound, people first hear the direct sound, and then successively hear the reflected sound through each interface.
It is generally believed that the reflected sound that arrives within about 50ms after the direct sound can enhance the direct sound, while the reflected sound that arrives after 50ms will not enhance the direct sound. If the time interval between the arrival of the reflected sound is longer and its intensity is more prominent, it will form the feeling of echo. Echo sensation can hinder the good hearing of language and music, so it needs to be controlled. The law of the human ear's perception of echo was first discovered by Hass, so it is also called the Hass effect.
Figure 2-15 shows the Haas effect. The abscissa in the figure is the time difference between the two sounds, and the ordinate represents the percentage of the total number of people who feel disturbed. Each curve represents the interference situation when the sound level difference between the two sounds is different. It can be seen that the smaller the time difference and the larger the sound level difference, the smaller the interference.