What Material Makes the Most Resonant Soundboard?

Objective

The goal of this project is to investigate the effectiveness of various materials for use as musical soundboards. What materials provide the greatest amplification?

Introduction

Have you ever taken apart a music box to see the mechanism that makes the sound? The picture at right shows an example. The windup spring turns the cylinder. As the cylinder turns, the raised bumps contact the tines of the comb, causing them to vibrate. When the mechanism plays while suspended in the air, the sound it produces is very quiet. When the mechanism is placed on a hard surface, the sound produced is much louder. How does this happen? What materials provide the most acoustic "amplification?" This project will help you to answer questions like these. In order to do this project, you'll need to know how soundboards work, what sound is, and how the intensity (loudness) of a sound is measured. We'll provide a brief introduction to these topics below, but you should do more background research on your own.

The hard surface underneath the music box is acting as a soundboard. Stringed instruments such as guitars, cellos, pianos all have soundboards. The vibrating strings are not very loud on their own. In all of these instruments, the strings contact a bridge, which conveys the string vibration to the soundboard of the instrument. The soundboard has a much larger surface area than the string, so it can move air more efficiently, producing a louder sound.

What is sound? Sound is a wave, a pattern—simple or complex, depending on the sound—of changing air pressure. Sound is produced by vibrations of objects. The vibrations push and pull on air molecules. The pushes cause a local compression of the air (increase in pressure), and the pulls cause a local rarefaction of the air (decrease in pressure). Since the air molecules are already in constant motion, the compressions and rarefactions starting at the original source are rapidly transmitted through the air as an expanding wave. When you throw a stone into a still pond, you see a pattern of waves rippling out in circles on the surface of the water, centered about the place where the stone went in. Sound waves travel through the air in a similar manner, but in all three dimensions. If you could see them, the pattern of sound waves from the stone hitting the water would resemble an expanding hemisphere. The sound waves from the stone also travel much faster than the rippling water waves from the stone (you hear the sound long before the ripples reach you). The exact speed depends on the number of air molecules and their intrinsic (existing) motion, which are reflected in the air pressure and temperature. At sea level (one atmosphere of pressure) and room temperature (20°C), the speed of sound is about 344 m/s.

The human auditory system is sensitive to a wide range of sounds, both in terms of frequency (pitch) and intensity (loudness). Typically, a young person is able to hear frequencies ranging from 20 to 20,000 Hz (Hz is the abbreviation for Hertz, the name for units of cycles/sec). Humans can also detect sounds with intensities ranging over 13 orders of magnitude (powers of ten). In other words, the loudest sound a human can perceive is 10,000,000,000,000 times as loud as the softest sound that can be perceived.

When comparing sound intensities over such a wide range, it is inconvenient to keep lugging all of those zeros around, so units of decibels (dB) are commonly used instead. A decibel is defined as 10 × log(I ⁄ I_ref ), where I and I_ref  are the two intensities being compared.

So if I is 10 times louder than I_ref , that corresponds to an increase of:
10 × log(10 ⁄ 1) dB = 10 × 1 dB = 10 dB.

If I is 100 times louder than I_ref , that corresponds to an increase of:
10 × log(100 ⁄ 1) dB = 10 × 2 dB = 20 dB.

If I is 1000 times louder than I_ref , that corresponds to an increase of:
10 × log(1000 ⁄ 1) dB = 10 × 3 dB = 30 dB. And so on.

For each power of ten change in intensity, there is a decade change (±10) in terms of dB.

Our ability to detect changes in intensity (the "just noticeable difference" in loudness), is proportional to the original intensity of the sound. If you are in a very quiet room, you can hear a whisper. Another person whispering could also be heard: the added sound would be significant in relation to the existing sound level. On the other hand, if you're at a basketball game with a lot of people cheering, you're not going to be able to hear someone whispering two rows down, because now the added sound is insignificant in relation to the existing sound level. In other words, as sounds get louder, there needs to be a bigger change in intensity in order to detect it.

So you can see that decibels are used not simply for reasons of convenience, but also because when we express sound levels in decibels, we get the numbers that have significance in terms of human perception.

Decibels define a relative measure of sound intensity. In other words, it will tell you how much louder or softer one sound is than another. However, if we choose a fixed point for the reference intensity level, then we have an absolute measure of sound intensity. A reference level that is often used in human auditory science is Sound Pressure Level (SPL), the lower limit of human hearing, which is defined as 10^-12 W/m², and is given a value of 0 dB (SPL).

In this project, you'll use a sound level meter, calibrated in dB (SPL), to measure the sound produced by a music box mechanism. You'll investigate how well different materials function as a soundboard for the mechanism. What material do you think will produce the most sound?

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• decibels,
• resonance,
• soundboard wood,
• soundboard materials,
• density.

Bibliography

• Here are three references on decibels:
  
  • Gotfrit, M. et al., 1995. "Decibels," Zen Audio Project, School for the Contemporary Arts, Simon Fraser University [accessed April 21, 2006] http://www.sfu.ca/sca/Manuals/ZAAPf/d/decibel.html.
  • Elsea, P., 1996. "Decibels," Technical Essays, UCSC Electronic Music Studios [accessed April 21, 2006] http://arts.ucsc.edu/EMS/Music/tech_background/TE-06/teces_06.html.
  • Bertrand, R., 2002. "Decibels," Online Radio and Electronics Course, Radio & Electronics School [accessed April 21, 2006] http://www.radioelectronicschool.net/files/readings/reading22.pdf

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• a music box mechanism (search online for 'music box movement'; the 18-note versions are the least expensive),
• sound level meter (e.g., Radio Shack #33-2055),
• same-size samples of various materials to use as soundboards,
• four pieces of foam packing material for soundboard supports,
• a quiet room.

Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms and concepts above.
2. It is important to make your sound level measurements under the same conditions. For example, all measurements should be made in the same room, with the same relative placement of soundboard and sound level meter. The material samples should all be the same size. The only variable that should change is the soundboard material.
3. It is also important to make the sound level measurements in a quiet room. You want to be sure that you are measuring the sound of the music box, not some other background sound.
4. Place the soundboard material to be tested on top of the four pieces of foam packing material (one piece for each corner). The purpose of the foam is to isolate the soundboard from its surroundings. This way, the sound level meter will measure sound produced by the soundboard alone, and not any additional vibrations that might be produced if the soundboard were in direct contact with, for example, a wooden tabletop.
5. Wind the music box mechanism and place it on the soundboard.
6. Hold the sound level meter at a fixed distance from the soundboard and take at least 5 separate dB readings.
7. Calculate the average dB reading.
8. Repeat steps 4–7 for each of the soundboard materials.
9. Measure the density of each of your soundboard materials, recalling that density is mass/volume.
10. What is the sound level of the music box mechanism alone, without any soundboard? Subtract this value from the average sound level measured for each soundboard. The result gives you the amount by which the soundboard increased the sound level.
11. Make a graph of increased sound level vs. material density. Is there any consistent relationship? Why or why not?

Variations

• Use a magnifying glass to examine a cross-sectional sample of each of your materials. Measure the size of the air pockets (if any) in the material. Is there any relationship between air cell size and sound level produced by the material?
• What other properties (in addition to density and air pocket size) can you think of that might influence a material's effectiveness as a soundboard? Think of ways to measure those properties and see if you can find a relationship with the sound level produced by the material.
• Predict how the measured sound level would change as you vary the area of the soundboard. Design an experiment to test your hypothesis.
• Predict how the measured sound level would change as you rotate the soundboard with respect to the detector. Design an experiment to test your hypothesis. For a more advanced project, consider the effect of reflected sound and try to minimize it in your experimental design.
• Advanced: Soundboard Frequency Analysis. Of course, amplification is only part of the story. Human auditory perception ranges over a wide range of frequencies, from 20 to 20,000 Hz. Do the soundboard materials you tested amplify all audible frequencies equally? You can use a spectrum analyzer to measure the amount of sound energy that is produced in each frequency band. Stand-alone spectrum analyzers are expensive, so if you have access to one, by all means use it (and consider yourself lucky!) A less expensive alternative is to record the sound digitally, and do the spectral analysis with software. The digital sampling process itself will color the spectral frequency of the recorded sound. You will have to do background research on digitization of signals so that you understand this phenomenon and can take it into account when analyzing your results.

• For more science project ideas in this area of science, see Materials Science Project Ideas

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

The idea for this project came from:
Vig, R.J., 2005. "What Material Makes a More Resonant Acoustic Instrument Soundboard?" California State Science Fair Project Abstract [accessesd April 21, 2006] http://www.usc.edu/CSSF/History/2005/Projects/J0223.pdf.

Last edit date: 2006-06-11 10:30:00