EXPERIMENT 2
Psychoacoustics
|
Filtered square wave with augmented 3rd harmonic Filtered square wave with augmented 5th harmonic Filtered square wave with augmented 7th harmonic |
A square
wave spectrum has only odd
harmonics, but since the amplitudes of the upper
harmonics are very strong, this wave has been
low-pass filtered around 3 kHz which makes it more
mellow. In the 1960s, the Dutch psychoacoustician
Reiner Plomp conducted a more sophisticated
version of this experiment in order to determine
the ear’s resolving power, i.e. the critical
bandwidth, by asking subjects to compare a
waveform like this to two single frequencies, one
of which matched a harmonic in the reference wave,
and the other whose frequency was between two
harmonics. Beyond a certain point, the choices
became random as the harmonics involved were too
closely spaced together. In this simplified version you will hear five sounds that alternate between normal (N) and a boosted harmonic (B), so the sequence is N B N B N. The EQ used can be seen here. The idea is for you to try to hear the boosted harmonic (3rd, 5th and 7th) in the N version, after hearing it boosted in the B version. This takes time for you to focus analytical attention on the harmonic which will sound faint but present in the normal spectrum. However, be warned, the 7th is much more difficult to hear, compared with the 3rd and 5th, given how close the harmonic spacing is becoming. Also, headphones may be preferable for listening to this exercise. The point of the exercise is that “hearing out” a Fourier component in a spectrum is very difficult, and in this case requires time, a steady state sound, and a trigger to draw attention to it. In the past, organists and some other musicians, who are quite sensitive to adding harmonically related tones to the timbre of their notes, claimed to be able to hear high harmonics, but it now appears likely that the added pitches were so strong as to trigger pitch detection mechanisms, not harmonic components in the resulting timbre. |
