PS991 thank you very much, it's perfect!
We changed our Arduino Uno to an Arduino MEGA2560 and adjusted the FFT_N to 256 bits.
This is much more accurate!
A new question:
for lines
val = analogRead(A0);
data[i] = val / 4 - 128;
What is the point of dividing val by 4 and subtract it 128 ?
Thank you all for your help