Chemistry in Concert

The science of music has long been the domain of physicists. But don't let the eggheads fool you: There's plenty of chemistry in music.

Musicians know that it takes more than talent to make music that sounds good. The quality and performance of the instruments they play is important, too. To get a high-quality instrument, you need high-quality materials, and that's where the chemistry, particularly polymer chemistry, comes into the picture.

The first thing most polymer scientists talk about when discussing musical instruments--especially if they're talking about guitars and other stringed instruments--is wood. Wood, of course, is a composite of natural polymers. Cellulose, an organized, linear, high-molecular-mass polysaccharide, makes up roughly 50% of wood's structure. Hemicellulose, a less organized, branched polysaccharide with a lower molecular mass, also is found in wood in varying amounts. The components are held together by lignin, a complex branched heterogeneous aromatic polymer composed of phenylpropane units that are linked through aryl ether bonds.

Steven K. Pollack, a polymer scientist at the Naval Research Laboratory in Washington, D.C., started playing the acoustic guitar when he was in graduate school. He plays the instrument in the finger style that characterizes acoustic blues, he says, rather than the strumming of folk music.

Although the strings of a guitar initiate its sounds, Pollack explains, the body is not simply a passive component in the music-making process. As in all wooden instruments, "a lot of the sound comes from the body of the guitar itself," he says. The strings' vibrations couple to the top of the instrument, increasing the amount of air moved and making the guitar sound louder.

"The woods and other materials in the guitar can influence, both positively and negatively, the quality of the sound by dampening specific frequencies or enhancing them via resonance," Pollack notes. This resonant effect makes the materials used in the top of the guitar's body (the part of the guitar with the hole in it) critical, because that's where the acoustic guitar gets its "voice."

The type of wood used can also influence the quality of the sound. Wood has a porous structure, and the size of the pores varies depending upon the type of wood. Pollack says that this natural porous structure has a profound effect on the body's resonance because the pore size can amplify or dampen the sound. That's why finger players, like Pollack, prefer cedar for the tops of instruments, he says, whereas guitarists who strum the instrument tend to favor spruce.

In the 1950s, the first acoustic guitars made entirely of plastic were introduced. The instruments were less expensive than their wooden counterparts, but Pollack says they had a terrible acoustic tone. "It just didn't work" because the polymer absorbed all the wrong tones, he says. Plastic ukuleles, however, enjoyed a modest commercial success.

OTHER GUITAR MAKERS have incorporated synthetic materials into the acoustic guitar's body with more success. Some of the less expensive instruments have tops made from high-quality wood, but inexpensive composites make up the rest of the body. Graphite composite guitars also are gaining popularity because they're lightweight and tolerate a range of temperatures and humidities.

Unlike their acoustic cousins, most electric guitars have solid bodies. They produce sound via a magnetic pickup mounted under the guitar's strings. This electromagnet senses vibrations in the strings and transmits an electronic signal to an amplifier and a speaker. No cavity is needed to amplify the sound, which is why electric guitars can be shaped like a chevron or a rocket ship.

Even though the electric guitar's body doesn't produce the sound, the instrument's material still has an effect on the sound. "People think that because you have an electrical pickup, you can make an electric guitar out of a slab of anything, but that's not quite true," says Gary Wnek, a chemical engineering professor who started playing the guitar when he was 11. Wnek recently joined the faculty at Case Western Reserve University, Cleveland. He became a member of the Rock & Roll Hall of Fame & Museum there shortly after moving to the city--an opportunity that lets him see the guitars of many of his musical idols.

Wnek explains that in an electric guitar, the energy from the strings' vibration is transferred into the neck and body of the instrument. This energy causes the instrument to resonate and consequently can alter its tone as well as the length of time that the guitar sustains a note, although he says it's a subtle effect.

When thinking about polymer science and the electric guitar, Wnek starts at the same place as Pollack: with wood. The density and grain structure of wood can affect tonal structure, he argues. Lightweight woods like alder and poplar tend to have a warmer sound. Alder's grain isn't particularly striking, so Wnek says guitars made from this wood usually are painted. Maple, on the other hand, looks beautiful, but the wood is heavy and sounds abrasive. "In varying thicknesses, it can work miracles on a dull-sounding body as a laminate top," he notes. "Maple veneers are used often for their 'flamed' and 'quilted' appearance."

Wnek cautions that these are generalizations. Wood choice can be subjective when it comes to the electric guitar. "You can get 10 guitar players in a room, and they'll all have a different opinion," he remarks.

THE POLYMER CHEMISTRY of guitars goes well beyond wood and body composition. In acoustic guitars, varnishes can alter the mechanical properties of the instrument and, consequently, change its acoustics.

Two guitars "made of the same wood and made in the same way, but with different coatings, sound different," says David Bott, chief scientist of NanoMagnetics, in Bristol, England, and another guitarist who moonlights in polymer science.

"The coating acoustically couples with the wood. If you coat your guitar with a hard coating, it resonates at a high frequency. If you coat with a softer material, it resonates at lower frequency," he explains. "A good guitar maker understands all of this intuitively."

Varnishes and finishes are important to electric guitars as well, although Wnek admits that it can be more about aesthetics than sound. In image-conscious rock and roll, looking good can be as important as sounding good.

Finishes on electric guitars have to stand up to the rock and roll lifestyle, too. They have to take the abuse of the Keith Richardses of the world, who sweat on them, bang them against their belt buckles, and throw them around a bit. Even after that rough treatment, most rock stars expect their guitar's finish to look good. To achieve the right look and resilience, guitar makers have been turning to the specialty coatings and paints used in the automotive industry.

Wnek says polymer science also plays a big part in the instrument's construction materials. There are the acrylic knobs, Teflon or nylon nuts for holding the strings in place, and pickup housings. The glue that sometimes holds the neck to the body or attaches the frets to the fingerboard can also affect tone and note-sustaining time. "The bottom line is that the guitar construction does matter," he says.

The other major component of the guitar is string. Until the mid-20th century, guitar strings were usually made from another natural polymer: gut, usually pig gut. Stringmakers would cook the animals' long intestines, a process that would reduce the organs to long pieces of collagen and connective tissue. To achieve lower tones, instrument makers would make some of the strings heavier by winding fine wire around them. The process is still carried out today, although modern acoustic guitar strings are usually made from synthetic polymers.

Nylon, a polyamide, was first used in guitar strings in the 1950s, Pollack says. Polyacetal guitar strings have been introduced more recently. These robust polymers have a high tensile strength as well as a high impact strength, so they won't fracture or shatter even in the hands of the most enthusiastic strummer.

Polymeric strings also influence sound, Pollack says. Energy dissipates rapidly through the polymers via molecular motion. "That means you can play quick short notes," he explains. Compare that to the metal strings of the steel guitar. The energy can't dissipate in metal in the same way it can in a polymer, so the musician has to work to deaden the note.

"AS A SCIENTIST, you naturally think about the things around you," says T. C. B. McLeish, a polymer physics professor at the University of Leeds, in England. McLeish has been playing the French horn since he was 12, and he has often wondered how the sound of an instrument relates to the materials it's made of. For instance, why does the French horn sound so rich and round? Or, in an oboe--his daughter's instrument of choice--what makes a good reed versus a bad one?

Simplistically explained, brass and woodwind instruments produce vibrations, and consequently sound, via the movement of air through a constricted channel. In woodwinds such as the clarinet, that channel is a reed.

Like the wood in guitars, reeds are natural composites made of polymeric plant materials. Here again, cellulose forms bundles that are glued together by lignin. Compared to wood, reeds are flexible because of their higher cellulose-to-lignin ratio. That's not to say that they're without stiffness, though. Reeds have a fixed stiffness, and in creating sound, they resist pressure from the lips.

McLeish says that understanding the mechanical and viscoelastic properties of the reed material is important to understanding the sounds woodwinds make. It's a finely tuned system, he says. After about a week of steady use, moisture, from blowing through the instrument, and abrasion, from the musician's teeth, have usually degraded the reed's mechanical properties, and it has to be replaced.

For brass instruments, the musician's lips perform the same function that reeds do for woodwinds. The lips create a restricted channel for air. However, McLeish notes that there's one big difference when you compare the two: "Lips have to last a lifetime."

Lips can be soft, but they can also be very stiff. This active elasticity comes from the multitude of tiny muscles in the lips that are tensing and releasing, McLeish explains. Instinctively, a trumpet or horn player uses these muscles to change air flow and produce sound. A musician who plays the Australian didgeridoo illustrates this phenomenon in an extreme way, McLeish notes: He moves his entire mouth to play the instrument and achieve its remarkable range of sounds.

"The materials science and physical chemistry of the skin actually determine the frequency response that you hear," Bott adds. "The harder you blow, the shriller the sound becomes; the softer you blow, the warmer the sound becomes."

McLeish finds it fascinating that, at a molecular level, the muscles in the lips look a lot like the cellulose and lignin structure in reeds. In reeds, the cellulose fibers are bundled together by lignin. In the lips, myofibril filaments are bundled together in each of the tiny muscles.

Nigel Clarke, an associate chemistry professor at England's University of Durham, says natural drumskins made from animal hides are structurally similar to synthetic polymeric drumskins. Clarke has been playing the drums since he was 18. He would have taken up the instrument when he was younger, but his mother wasn't so keen on the idea.

TO ACHIEVE the instrument's mechanical properties, the polymeric fibers in the drumskin need to be aligned. In animal drumskins, a triple helix of collagen forms the fibers. Drying the skins under tension puts these fibers in the proper alignment.

Likewise, synthetic polymers start out as films cast with no particular alignment. These films are then stretched during the manufacturing process to induce the appropriate alignment. This also introduces a degree of crystallinity, making the polymer a semicrystalline material.

"With a semicrystalline material, you get the best of both worlds: a high modulus and a high toughness," Clarke says. The material is rigid, but it can also absorb energy. If it were completely crystalline, it would be too brittle and would break fairly easily.

Drums are probably the oldest musical instruments, and Clarke says the form has changed little over the years. When drum makers introduced synthetic drumheads made from polyethylene terephthalate in the 1950s, however, they revolutionized the instruments. With a few exceptions, these PET drumheads quickly gained acceptance among drummers and replaced animal skins as the material of choice.

Clarke notes that this paradigm shift had more to do with pragmatism than with any particular musical benefits. "A synthetic drumhead is more durable and less susceptible to environmental changes," he explains. "Calfskin drumheads require frequent retuning due to their sensitivity to both temperature changes and humidity." Too much water breaks up the hydrogen-bonding network that holds the collagen helix together, basically turning the drumskin into gelatin.

Synthetic polymers work well in drums for a few reasons. They tend to be hydrophobic, so humidity isn't a problem. Polymers can also be manufactured so that they're highly regular. Drummers can rely on synthetic drumheads to sound the same. Natural drumskins, by comparison, don't have the same reliability, and variations in humidity mean they have to be constantly retuned.

"Polymers are very good at absorbing energy because of their long-chain molecules," Clarke adds. When you strike the drum, some of the energy gets transferred into molecular motion. In this way, the polymer dampens some of the high-frequency noise and gives the instrument a warmer sound. In a steel drum, the metal drumhead won't absorb energy in the same way, so high-frequency sounds are characteristic of that drum.

As with other instruments, polymers play an important role in holding the drum together. The adhesives that hold the drumskin to the drumhead's aluminum hoops need to be superstrong, but drum makers also want them to dry quickly to speed up manufacturing.

Adhesives also play a critical role in the musical instrument that's commonly used by most nonmusicians: the speaker. Bob Lituri, a polymer chemist and materials engineer with Bose, says there's a lot of sophisticated chemistry that goes into making those little boxes.

Part of Lituri's job is to keep your speakers from bursting into flames. "Most folks don't realize that the inside of a speaker is a pretty violent environment," he says, even though it would be highly unusual for a speaker to actually set itself on fire.

To produce sound, a speaker's components move back and forth, creating and propagating sound waves in the surrounding air. That motion generates a tremendous amount of heat. In fact, Lituri says, only 2% of the energy that goes into a speaker gets converted into sound. The other 98% is converted into heat that has to be absorbed and dissipated by the speaker's components without adversely affecting the sound quality.

THE LOUDER the speaker, the more heat it has to take. Audio-equipment makers used to make speakers louder by simply making them bigger. Heat wasn't a problem with these monsters because the larger components dissipate it easily. But Lituri says that the days of speakers that double as end tables are over.

"We tend to make our products very small, so they are invisible in the consumer's environment," he explains. "By doing that, we put tremendous restrictions on what materials we can use."

To make a sound, a smaller speaker has to work harder than a large one. Its components have to move farther than those of a large speaker working at the same volume. The increased movement makes the device hotter, but there's less material around to dissipate that heat. The smaller speaker's components, adhesives, and coatings have to be able to take the increased temperatures, Lituri says.

To accomplish that condition, Lituri says, he seeks out fairly exotic binders and adhesives that are lightweight and heat resistant beyond 150 °C. All the polymers he works with need to be processable and economical, he says.

Lituri, a guitar player of nearly 38 years, says he and his colleagues spend a lot of time thinking about chemistry so their customers don't have to. He says his main goal is to use chemistry to bring superior sound to people so they can kick back, relax, and just enjoy the music.

Now that sounds good.