The Body: Soundbox or Solid Plank

An acoustic guitar's body is not a decorative shell around the strings. It is the loudspeaker, and most of the work is done by the thin piece of wood facing you — the top, also called the soundboard, usually spruce a couple of millimetres thick.

The strings do not move much air by themselves. What they do is shake the bridge, which is glued to that top. The top has a large area, so when it moves, it pushes a serious amount of air. The soundhole lets the air inside the box breathe in and out, and the enclosed air itself resonates, reinforcing the low end.

This creates an engineering problem. Six steel strings pull on the bridge with something like 70 to 80 kilograms of tension, all of it trying to fold that thin top in half. The top has to be light enough to move and strong enough to survive. The answer is bracing: a skeleton of thin wooden struts glued to the underside.

How much does the top matter compared to the rest? Antonio de Torres, the Spanish luthier who set the shape of the modern classical guitar in the mid-1800s, settled the argument with a stunt: around 1862 he built a guitar with a fine spruce top and back and sides made of papier-mache. It worked. It sounded like a guitar. The top is where the sound lives.

Now the electric. By the 1930s, guitarists playing in dance bands had a straightforward problem — they could not be heard over the horns. The first fix was to hang a microphone-like pickup on a hollow archtop guitar. That worked until you turned it up, at which point the amplified sound came out of the speaker, shook the guitar's hollow top, got picked up again, and howled. Feedback.

The cure was to stop the body resonating at all. If the body is a solid plank, there is nothing for the speaker to set vibrating. You lose the acoustic volume entirely, which no longer matters, because the amplifier is doing that job.

So the family tree looks like this. Hollow archtops with carved tops and f-holes arrive with Orville Gibson around 1900 and mature into the Gibson L-5 in 1922. Flat-top steel-strings grow bigger and louder, giving the dreadnought, built by Martin from 1916. Solid-bodies arrive in the early 1950s — Fender's Broadcaster in 1950, renamed the Telecaster in 1951, the Gibson Les Paul in 1952, the Stratocaster in 1954. Semi-hollows split the difference from 1958.

None of these replaced the others. They all still sell, because they are answers to different questions.

Here is the same musical idea recorded both ways. Play a few bars over each and listen to what the body is doing — the acoustic's woody thump and fast decay against the electric's smoother, longer sustain.

An acoustic blues in E. Everything you hear from the guitars is wood moving air.

A slow electric blues, also in E. Same key, same form, an entirely different way of getting loud.