What makes the sound when you hear an acoustic guitar (with no amplifier)? Did you say the strings? That's right -- but the strings need help to do the job.

Guitar strings produce sound through vibrations. The strings by themselves, however, cannot be heard well without some sort of amplification. An electric guitar uses magnetic pickups, but acoustic guitars rely on wood.

When the strings vibrate, it makes the bridge of the guitar vibrate. These vibrations are passed along to the wood that makes the top of the guitar. This causes the air inside the guitar to vibrate, and finally the sound is projected out of the sound hole.

Different types of woods can be used for the top and back of a guitar. Each type imparts its own tonal qualities to the sound. One of the most sought after woods for acoustic guitar tops is the Sitka spruce.

The Sitka spruce is found in the temperate rain forests of the Pacific Northwest. At more than 260 feet, it is third tallest conifer (cone-bearing tree) in the world. Its wood makes a good guitar top because it can bend very well without breaking or becoming brittle.

Sitka spruce is also used in pianos, harps and violins, and has historically been used for airplane fuselages, propellers, sail masts, barrels and paper.

Because of the varied uses and high demand, much of the native western spruce forest has been cut, but the Sitka spruce is fast-growing and is planted in many parts of the world where the climate and soil are favorable, such as Ireland and Norway.

To learn more about the science of sound and music, the different materials used in making guitars and the different sounds they produce, visit "GUITAR: The Instrument That Rocked the World," the traveling exhibit of The National Guitar Museum, at Carnegie Science Center through Sept. 30.

First Published: September 13, 2012, 4:00 a.m.