The Sounds Produced by the Vibraphone and the Marimba

The Sounds Produced by the Vibraphone
and the Marimba
Nicholas Dettman and Dana Kurtz
Professor Steve Errede
Physics 199 POM
Fall 2004
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Many different kinds of percussion instruments exist, which are played and
produce sound in a variety of ways. For our project, we investigated the different sounds
that can be produced by two different mallet percussion instruments, the marimba and the
vibraphone. The modern marimba is similar to the xylophone, with wooden, pitched bars
that are struck with a mallet. The marimba has shallower, wider bars than a xylophone,
and the bars are usually made out of rosewood. Each marimba bar is suspended above a
tubular metal resonator, giving the marimba a full, resonant tone. Marimbas usually have
a four octave range, from C3 to C7.
The first vibraphone was constructed in the United States in 1907. Originally,
vibraphone bars were made of bronze or steel. Now, they are typically made of a very
hard light-metal alloy. Like the bars of a marimba, the bars of a vibraphone are
suspended over metal resonator tubes. The vibraphone, however, has a small electric
motor which controls rotating metal disks located at the top of each resonator tube. When
the motor is turned on, the rotation of the disks causes the air columns in the tubes to
pulsate, resulting in a tremolo effect. Usually, a vibraphone has a three octave range,
from F to F3. A pedal presses a felt lined rail against the rows of bars, allowing the bars
to be damped or ring freely. Like the marimba, vibraphone bars are struck with a mallet.
Marimba and vibraphone bars can also be bowed, usually with an orchestral bass bow.
We decided to keep a constant C4 note in all of the sound files, so that we could
observe the difference of three variants: instrument, type of mallet, and location. For the
instrument, we used a Deagan 3-octave vibraphone and a Musser 4 1/3-octave rosewood
marimba. For the mallets, we decided on six varying styles of mallets, ranging from hard
rubber to soft yarn, in addition to the bow. The location refers to the location we struck
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the bar. The maximum resonance is achieved when the exact center of the bar is struck,
directly above the resonator tube. The minimum resonance is achieved when the node of
the bar, the area just above where the strings suspend the bar, is struck. Here is a visual
representation of the playing surface:
The first thing we wanted to look at was how the bow would affect the sound
waves produced. The harmonics that were most prevalent in both instruments were the
fundamental, fourth, seventh, and tenth; or every third harmonic. Those are the only
harmonics we looked at for the bow analysis.
There were two main aspects of the sound wave that we looked at: amplitude and
frequency. The amplitudes of the harmonics we looked at are similar, yet slightly
different between the two instruments. In both instruments, the most prevalent harmonic
was the fundamental, with the least prevalent being the seventh harmonic. With the
vibes, the tenth harmonic was more prevalent than the fourth harmonic, whereas with the
marimba, it’s vice versa. What really sticks out is how much fluctuation happened in the
amplitudes of these harmonics with respect to time. Here’s what the vibes looks like:
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The graph of the amplitudes of the harmonics on marimba looks even more hectic than
this does. Also, all the harmonics were louder on the vibes.
Analysis of the frequencies produced some pretty crazy results as well. We found
that at the higher harmonics, the frequency becomes very unstable. We observed this in
all of our analyses of these mallet instruments, but found it most noticeable when we used
the bow. Looking at just the two instruments, the vibes appear to have more stable upper
harmonics than the marimba does. It was very noticeable in the sound file itself also,
because if you listened to it you could hear the note bend a little flat towards the end of
the recording, which is right where the fourth and seventh harmonics start to waver, as
you can see here:
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The frequencies the vibes produced were much more stable than that, with only the tenth
harmonic showing any kind of flux. We found that there’s a lot of instability in the tone
that’s produced when a marimba or vibraphone is played with a bow. Luckily, it’s not
the quality of tone that we enjoy, but the uniqueness of the sound that it produces, which
is likely in part of its instability.
Next, we decided to look at the harmonics produced by the different locations of
the bar, the center and the node. For this analysis, we used a pair of Malletech NR13B
medium-hard rubber mallets. We obtained some very interesting results about the
amplitudes and frequencies produced by striking the node of the bar versus hitting the
center of the bar, as well as playing either a marimba or a vibraphone.
We also found some intriguing results when we looked at which harmonics were
most prevalent in each of the four scenarios. We found that the first and fourth
harmonics were very apparent in all but one situation: when the marimba was played on
the node. One of the harmonics that stuck out didn’t appear to actually be an integer
harmonic, but rather, a harmonic halfway in between the fifth and the sixth harmonics.
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Also, when the vibes were struck on the node, one of the harmonics that really stuck out
was the 17 th harmonic. Both of these harmonics aren’t as prevalent in any analysis we
did of recordings in which the center of the bar was played.
Just looking at the vibraphone, it was interesting to see that striking the node took
away the presence of the first harmonic. The left graph is with the center of the bar, and
the right graph is the node of the bar. The first two columns are the same harmonics: first
and fourth, and look at the relationship between them:
It’s clearly visible that the second harmonic is the most prevalent when the node of the
bar is struck instead of the center of the bar.
When the harmonics of the two instruments are analyzed between the different
mallet choices with the location constant, there are some general trends that can be seen.
The harmonics that are most prevalent when the vibes are played are the first, fourth,
eighth, tenth, and thirteenth. With the marimba, the eighth and thirteenth harmonic were
never as apparent as they were in the vibes sound. The most prevalent harmonics for the
marimba only included the first, fourth, and tenth.
One interesting observation from looking at the different types of mallets that
were used, is that when the hard rubber mallets are compared with the two-tone yarn
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mallets (two-tone is when the outside is really soft and the inside is really hard, so
depending on how hard you strike the instrument you can either get a soft, more mellow
sound or a harder, more striking sound), the two-tone mallets excited more of the second
harmonic than the hard rubber did for the vibes, but had the opposite affect when played
on the marimba. Also, on the vibes, the two-tone mallets had a much more stable
thirteenth harmonic than the hard rubber, as is shown below:
The graph on the left shows the two-tone mallet, while the graph on the right shows the
hard rubber mallet. The purple line shows the thirteenth harmonic.
The same observation can be made when looking at the frequencies of these same
two mallets on the vibes. When the hard rubber mallet is played, the thirteenth harmonic
fluctuates much more than it does when the vibes are played with the two-tone mallet:
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Another interesting thing to note is that the tenth harmonic (the light blue line) has
greater fluctuation when played with the two-tone mallets. This kind of instability
distinction from the harmonics is not seen in the analysis of the marimba recordings.
Bibliography
Blades, James. Percussion Instruments and their History. London: Faber and Faber,
1975. 407-410.
Brindle, Reginald. Contemporary Percussion. London: Oxford UP, 1970. 41-47.
Holland, James. Percussion. London: Macdonald and Jane's, 1978. 163-165.
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