Franssen effect

The Franssen effect , named after Nico Valentinus Franssen, describes the observation that in a reverberant environment a localization of sound sources is only possible if there are stronger changes in the sound signal and that once a certain direction is assumed as the direction of the auditory event until again direction information can be evaluated.

attempt

This effect can be illustrated by the following listening test:

Set up two loudspeakers in a larger, somewhat reverberant room and let the listeners sit down so that they are far enough outside the reverb radius of the loudspeakers.

You play signals with constant level and constant spectrum in the loudspeakers (narrowband noise or single tones).

To do this, let the sound in the right speaker set in hard. The listeners will correctly locate the sound at the position of the right speaker .

Then you fade very gently from the right to the left speaker. For the listener, the sound will still come from the direction of the right speaker. The changed direction of incidence of the direct sound and the changed reflection pattern do not seem to be able to be evaluated by the hearing.

Now, to confuse the audience, you can pull the connection cable from the right loudspeaker, nothing will change in the location of the “right loudspeaker”.

The left loudspeaker can only be localized if you now change the volume more strongly.

Explanation

Franssen effect: short 1 kHz pulse on the left, decaying, followed by 500 ms continuous 1 kHz signal on the right, then repetition of the short pulse on the left.

The reason for this behavior is that outside the reverberation radius, the volume of the sound reflected from the walls of the room is significantly greater than the direct sound from the loudspeaker. Thus, in the steady state of the room, sound from all possible spatial directions affects the listener at the same time; a direction cannot be determined from this.

Only at the beginning of the experiment, when the right loudspeaker is switched on, is there a short period of time in which the direction can be determined, namely when the direct sound has already reached the listener, but the sound from the wall reflections has not yet. This point in time seems to be used by the hearing to determine the direction of incidence of the sound. And the hearing seems to maintain this direction until direction information can be evaluated again.

During the gentle fading between the loudspeakers, the sound reflected from all directions outweighs the direct sound at all times; hearing has no way of determining a direction of incidence. Therefore, even after fading and unplugging the cable, the audio event remains at the position of the right speaker.

Only when there are greater signal differences again, so that the direct sound can briefly outweigh the reflected sound, the hearing can again determine a reliable direction of sound incidence and then assign the sound to this direction for the time being.