The people of Hawaii have a wonderful tradition of being able to make music out of just about anything — a metal bolt or a needle lies on a guitar string and creates a new sound, a length of bamboo becomes a nose flute, a coconut shell becomes a drum.

So it’s no surprise that when you have a ready-made instrument such as a harmonica, people in Hawaii would use it to create an entire band.

What might be a surprise is that the cellphone-size instrument that stashes in your pants pocket has inspired not one, but two local harmonica groups: the Small World Harmonica Band in Honolulu and the Hawaii Reed 21 Harmonica Band in Mililani. In August they’ll perform together at Mission Memorial Hall Auditorium.

“This is just from general interest,” said Robert Omura, who founded the Small World Harmonica Band 20 years ago and remains its director. “I always played harmonica as a kid, and after I retired we started this group. None of these people are pure musicians. They all play because they enjoy music.”

His band comprises about 40 members, mostly retirees, who meet once a week to practice, said band member Albert Kaopuiki, 71, who is also president of the Hawaii Harmonica Society. The band occasionally performs at community events and senior citizen homes.

Many, if not most, of the members have little music training beyond grade school, but they can recognize a tune when they hear it. That’s pretty much all that’s needed to learn how to play the harmonica, which has been made easy thanks to Omura.

You’ve heard of paint-by-numbers? With harmonica it’s music-by-numbers.

Omura arranges the music in two-part harmonies, designating the different pitches by numbers that correspond to the holes in the instrument. If his arrangement calls for the No. 1 hole on the harmonica, then you blow. (The tricky thing is that when it says No. 2, you don’t blow, but instead inhale, or “draw” the air in the opposite direction, which creates a different pitch.)

The band’s repertoire reflects the tastes of its members. They play popular American tunes from years past such as “Red River Valley,” patriotic themes, Asian tunes such as “Sakura” and local songs like the “Hawaiian Wedding Song.”

The 84-year-old Omura, who used to “teach teachers how to teach” at the University of Hawaii at Manoa, has arrangements for more than 700 songs. “It’s a big repertoire and they’re all numbered like this,” he said.

It’s not just the size of the repertoire that impresses you with the dedication of these musicians. Many have spent a tidy sum to buy several instruments, which come in different sizes and are tuned to different pitches.

Omura said he started playing harmonica at age 8 on an instrument “that was about 50 cents.” Now his collection of instruments is worth about $2,000.

“Most of the people have eight or 10 harmonicas,” he said. “You can buy the small kind for less than $10, and then these go for $60 or $70 — each.”

His were designed by a Japanese company and manufactured in China, where the instrument made famous by American blues and rock artists actually has its roots.

“The first harmonica came from China. It was called the sheng,” Kaopuiki said. The instrument eventually made its way to Europe and then on to the Americas via trade, exploration and colonialism.

The Mililani group, Hawaii Reed 21, derives its name from the 21 holes in the standard harmonica. The group numbers about 20 musicians and started about 20 years ago at the Honbushin International Center, said band member May Tanaka. She has been with the group for about seven years and had no musical background when she started.

“I decided I’d do something after I retired,” said Tanaka, an energetic 89-year-old who also plays taiko. “We play anything — classical, semiclassical, Japanese, whatever.”

There’s nothing quite like a roomful of harmonicas playing. Reed instruments such as the harmonica tend to have a piercing sound, and harmonicas are usually double-reeded, with two reeds playing just slightly out of tune from one another. That creates a bright, warbling sound that can leave the eardrums ringing.

“When we play properly,” Kaopuiki said, “it’s music.”