A Little Demo of the Franssen Effect (Precedence Effect)

[NTK]

The Franssen effect is an illusion due to the "law of the first wavefront" (aka precedence effect). When both of our ears receive coherent sounds, we lock on to its first few milliseconds (the first wavefront) to determine its direction, and our brains "fix" the sound to this direction. Our brains fix what we subsequently hear (what our brains think is a continuation of the initial sound) to that initial location regardless of the directions they come from. The hypothesis is that when we were cave dwellers, with strong sound reflections coming from every direction, that's how our hearing system evolved to tell us where sounds inside the cave came from.

Below are plots of the demo waveform. The right channel starts strong with a decaying amplitude envelop, and the left channel is the reverse. Both channels sum to a rectangular envelop. The signals end similarly to how they start.

Because the right channel started first, the sound source appears to be from the right, even though the vast majority of the sound is from the left. In my tests, the effect seemed stronger when listening with my laptop speakers in the nearfield than with headphones.

This demo shows how our hearing sometimes acts in highly unexpected ways, and why we haven't had much success finding ways to measure speaker spatial qualities — how we hear spatially is a very complicated and nonintuitive subject. WAV file is in the ZIP.

 

[LearningToSmile]

Very interesting demonstration - with(closed back) headphones the effect didn't work for me at all, but as soon as I switched to speakers it became apparent. It's was so striking if I didn't read your description beforehand, I'd think my speakers were connected in reverse. Great food for thought.

 

[ebslo]

Doesn't work at all for me on headphones, I hear a tone on L with clicks at the start and end on R. But on speakers it sounds like it all comes from the R, even if I stand directly in front of the R speaker. Only if I stand directly in front of the L speaker can I tell there is sound coming from it. It was so convincing I adjusted the balance to each extreme to verify it was playing correctly (it was).