ASA/CAA '05 Meeting, Vancouver, BC
Identifying Musical Instruments Through their Microrhythms
Rolf Bader R_Bader@t-online.de
Institute of Musicology, University of Hamburg,
Neue Rabenstr. 13, 20259 Hamburg, Germany
Popular version of paper 3aMU11
Presented Wendsday morning at 8 am, May 18, 2005
149th ASA Meeting, Vancouver, BC
If we listen to music, normally we do not have difficulty to recognize the
instruments that are playing. The violin is different from the oboe; the guitar
does not sound like a drum. We take this for granted, but what enables us to
distinguish between these instruments? We propose an astonishing answer: the
human ear may be able to hear the an instrument's distinct "microrhythm,"
its initial burst of sound (the "initial transient phase"), about
50 milliseconds (.05 seconds) from the beginning of the tone. How do we know?
If we record an instrument sound and cut its initial transient, a saxophone
may not be distinguishable from a piano, or a guitar from a flute. This is because
the so-called "quasi-steady state" of the remaining sound is very
similar among the instruments. But their tone beginning is very different, complicated
and so very individual. But how do we perceive this initial phase?
The ear has three integration times in terms of musical sound. After about 5
milliseconds (ms), we are able to perceive more than just a click. This is related
to the human inner ear capacity of building up the critical bandwidth for frequency
discrimination. The second important time is about 50ms. Here, we begin to hear
distinct frequencies, not very accurately, but our ear gives us the chance to
perceive a pitch. Then after about 250ms, the whole sound is perceived very
well. The pitch can be heard clearly and we have the possibility to discriminate
fine structures within the sound. So how do we perceive the first 50ms?
The very beginning of a sound can just be perceived as a whole. We are able
to analyze very brief sounds by using modern signal processing tools in the
computer that break them down into their different frequencies. But still the
ear is not really able to reach that level of precision. Here, we may ask, does
the ear have other ways to distinguish musical instruments without making frequency
calculations?
Taking 144 sounds of six different musical instruments, a guitar, a saxophone,
a clarinet, a violin, a turkey saz and a balinese gender (metallophone), we
have performed a microrhythmic analysis of the initial transients. Microrhythm
here means the time period of neighboring amplitude peaks in the sound. These
peaks could be understood as an impulse train reaching the ear. If this train
would have a characteristic rhythm for each musical instrument, the ear could
distinguish the sounds very fast without the help of the frequency components
of the sound.
And indeed, different instruments have different occurences of microrhythmic
relations with their standard deviation shown as vertical lines through the
points. The figures below show three example plots of occurence of relations
vs. relations. For example, if the time between one pair of amplitude peaks
is twice as long as the time between the next pair of amplitude peaks, then
the ratio is 1/2; if three times the length, then 1/3. The vertical axis shows
how often such patterns occur.
The mean occurences of microrhythmic relations from 144 tones of a classical
guitar are shown in this plot. It results in a beautiful figure showing preferred
harmonic relations.
This is not true for the quasi-steady state of the guitar tone. Here more irregular relations occur.
Another example is the violin. Here we have more values on the upper half of the plot and the whole picture tends to more irregular relations.
