# Engineering Acoustics/Basic Acoustics of the Marimba

## Introduction

!Marimba Band \"La Gloria Antigueña\", Antigua Guatemala,
1979

Like a xylophone, a marimba has octaves of wooden bars that are struck
with mallets to produce tones. Unlike the harsh sound of a xylophone, a
marimba produces a deep, rich tone. Marimbas are not uncommon and are
played in most high school bands.

Now, while all the trumpet and flute and clarinet players are busy
tuning up their instruments, the marimba player is back in the
percussion section with her feet up just relaxing. This is a bit
surprising, however, since the marimba is a melodic instrument that
needs to be in tune to sound good. So what gives? Why is the marimba
never tuned? How would you even go about tuning a marimba? To answer
these questions, the acoustics behind (or within) a marimba must be
understood.

## Components of Sound

What gives the marimba its unique sound? It can be boiled down to two
components: the bars and the resonators. Typically, the bars are made of
rosewood (or some synthetic version of wood). They are cut to size
depending on what note is desired, then the tuning is refined by shaving
wood from the underside of the bar.

### Example: Rosewood bar, middle C, 1 cm thick

+----------------------------------------------------------------------+
| The equation that relates the length of the bar with the desired     |
| frequency comes from the theory of modeling a bar that is free at    |
| both ends. This theory yields the following equation:                |
|                                                                      |
| $Length = \sqrt                                                      |
| {\frac{3.011^2\cdot \pi \cdot t \cdot v}{8 \cdot \sqrt{12}\cdot f}}$ |
|                                                                      |
| where t is the thickness of the bar, v is the speed of sound in the  |
| bar, and f is the frequency of the note. For rosewood, v = 5217 m/s. |
| For middle C, f=262 Hz. Therefore, to make a middle C key for a      |
| rosewood marimba, cut the bar to be:                                 |
|                                                                      |
| $Length = \sqrt{\frac{3.011^2\cdot \p                                |
| i \cdot .01 \cdot 5217}{8 \cdot \sqrt{12}\cdot 262}}= .45 m = 45 cm$ |
+----------------------------------------------------------------------+

The resonators are made from metal (usually aluminum) and their lengths
also differ depending on the desired note. It is important to know that
each resonator is open at the top but closed by a stopper at the bottom
end.

### Example: Aluminum resonator, middle C

+----------------------------------------------------------------------+
| The equation that relates the length of the resonator with the       |
| desired frequency comes from modeling the resonator as a pipe that   |
| is driven at one end and closed at the other end. A \"driven\" pipe  |
| is one that has a source of excitation (in this case, the vibrating  |
| key) at one end. This model yields the following:                    |
|                                                                      |
| $Length = \frac {c}{4\cdot f}$                                       |
|                                                                      |
| where c is the speed of sound in air and f is the frequency of the   |
| note. For air, c = 343 m/s. For middle C, f = 262 Hz. Therefore, to  |
| make a resonator for the middle C key, the resonator length should   |
| be:                                                                  |
|                                                                      |
| $Length = \frac {343}{4 \cdot 262} = .327m = 32.7 cm$                |
+----------------------------------------------------------------------+

### Resonator Shape

The shape of the resonator is an important factor in determining the
quality of sound that can be produced. The ideal shape is a sphere. This
is modeled by the Helmholtz resonator. (For more see Helmholtz
Resonator
page)
However, mounting big, round, beach ball-like resonators under the keys
is typically impractical. The worst choices for resonators are square or
oval tubes. These shapes amplify the non-harmonic pitches sometimes
referred to as "junk pitches". The round tube is typically chosen
because it does the best job (aside from the sphere) at amplifying the
desired harmonic and not much else.

As mentioned in the second example above, the resonator on a marimba can
be modeled by a closed pipe. This model can be used to predict what type
of sound (full and rich vs dull) the marimba will produce. Each pipe is
a \"quarter wave resonator\" that amplifies the sound waves produced by
of the bar. This means that in order to produce a full, rich sound, the
length of the resonator must exactly match one-quarter of the
wavelength. If the length is off, the marimba will produce a dull or
off-key sound for that note.

## Why would the marimba need tuning?

In the theoretical world where it is always 72 degrees with low
humidity, a marimba would not need tuning. But, since weather can be a
factor (especially for the marching band) marimbas do not always perform
the same way. Hot and cold weather can wreak havoc on all kinds of
percussion instruments, and the marimba is no exception. On hot days,
the marimba tends to be sharp and for cold days it tends to be flat.
This is the exact opposite of what happens to string instruments. Why?
The tone of a string instrument depends mainly on the tension in the
string, which decreases as the string expands with heat. The decrease in
tension leads to a flat note. Marimbas on the other hand produce sound
by moving air through the resonators. The speed at which this air is
moved is the speed of sound, which varies proportionately with
temperature! So, as the temperature increases, so does the speed of
sound. From the equation given in example 2 from above, you can see that
an increase in the speed of sound (c) means a longer pipe is needed to
resonate the same note. If the length of the resonator is not increased,
the note will sound sharp. Now, the heat can also cause the wooden bars
to expand, but the effect of this expansion is insignificant compared to
the effect of the change in the speed of sound.

## Tuning Myths

It is a common myth among percussionists that the marimba can be tuned
by simply moving the resonators up or down (while the bars remain in the
same position.) The thought behind this is that by moving the resonators
down, for example, you are in effect lengthening them. While this may
sound like sound reasoning, it actually does not hold true in practice.
Judging by how the marimba is constructed (cutting bars and resonators
to specific lengths), it seems that there are really two options to
consider when looking to tune a marimba: shave some wood off the
underside of the bars, or change the length of the resonator. For
obvious reasons, shaving wood off the keys every time the weather
changes is not a practical solution. Therefore, the only option left is
to change the length of the resonator.

`   As mentioned above, each resonator is plugged by a stopper at the bottom end.  So, by simply shoving the stopper farther up the pipe, you can shorten the resonator and sharpen the note.  Conversely, pushing the stopper down the pipe can flatten the note.  Most marimbas do not come with tunable resonators, so this process can be a little challenging.  (Broomsticks and hammers are common tools of the trade.)`

### Example: Middle C Resonator lengthened by 1 cm

+----------------------------------------------------------------------+
| For ideal conditions, the length of the middle C (262 Hz) resonator  |
| should be 32.7 cm as shown in example 2. Therefore, the change in    |
| frequency for this resonator due to a change in length is given by:  |
|                                                                      |
| $\Delta Frequency = 262 Hz - \frac {c}{4\cdot (.327 + \Delta L)}$    |
|                                                                      |
| If the length is increased by 1 cm, the change in frequency will be: |
|                                                                      |
| $\D                                                                  |
| elta Frequency = \frac {343}{4\cdot (.327 + .01)} - 262 Hz = 7.5 Hz$ |
+----------------------------------------------------------------------+

The acoustics behind the tuning a marimba go back to the design that
each resonator is to be ¼ of the total wavelength of the desired note.
When marimbas get out of tune, this length is no longer exactly equal to
¼ the wavelength due to the lengthening or shortening of the resonator
as described above. Because the length has changed, resonance is no
longer achieved, and the tone can become muffled or off-key.

## Conclusions

Some marimba builders are now changing their designs to include tunable
resonators. There are in fact several marimba companies that have had
tunable resonators for decades. However, only a few offer full range
tuning. Since any leak in the end-seal will cause major loss of volume
and richness of the tone, this is proving to be a very difficult task.
At least now, though, armed with the acoustic background of their
instruments, percussionists everywhere will now have something to do
when the conductor says, "tune up!"

## Links and References

1.  <http://www.gppercussion.com/html/resonators.html>
2.  <http://www.mostlymarimba.com/>
3.  <http://www.craftymusicteachers.com/bassmarimba/>

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