Sound sculptor Herbert Jercher has been building outdoor sound sculptures and playground instruments for more than 20 years. His work can be found in public parks and school playgrounds right across Australia. These durable instruments are now built under the name Herb's Acoustic Playground Instruments (HAPI).

These are not production-line models, each is a handcrafted original signed by the artist, and often designed especially for a site. To suit their outdoor locations, they are made to be virtually indestructible, yet produce a tone that is clearly (and first of all) musical. This makes a welcome change from "playground instruments" that lack proper tuning, or are mass produced as coarse noisemakers.

Designing any kind of amenity for a public space - let alone a musical instrument - can't be that easy. First of all, there are extremes of weather, and then vandal-proofing to consider. Add to this the many public safety considerations - and even local noise laws - and you begin to appreciate what it means to put a musical instrument out in the open for a community to enjoy.

All these things become evident when I visit Herb at his Sunshine workshop, and he takes me to his latest: a stainless steel tapper instrument that looks from a distance like a rocket ship out of a 1920s sci-fi movie. It's squat, silver body stands on four legs, and is ringed by thinner tubes of different lengths that could easily be some kind of booster rockets. I imagine it ready for a "Journey to the Moon", and pretty soon too.

"It's called a tadpole tapper," Herb tells me as we get closer.

A what?

"The client asked for something that sounded like a frog for their site theme and well, this one sort of sounds like a frog. Why don't you try to play it."

Then I notice the leather tappers over the tubes, a ring of tadpole faces smiling up at me! I begin to tap them with my hands like a bongo drum. Randomly at first, and with some force, then instinctively softer as the tapping starts to sound a musical scale, and then a melody.

"You can play a theme from Bartok's Concerto for Orchestra on it," I say after a while.

Herb nods. "That's because of the tritone in the scale."

Then he takes over and plays, using only his index fingers and thumbs, a pattern of notes in shifting harmonies, a texture of interlocking arpeggios. Occasionally the deep resonant sound of the instrument's body recalls an Indian tabla drum. I feel privileged to witness this short, private performance.

Then slowly it dawns on me, what I now call a HAPI contradiction. It's that the instrument, which is designed for a public place, creates instead a private space, and it does this by the nature and the quality of its sound. A private space to play, here within a public place.

It's a contradiction too, because it goes against what seems intuitive about musical instruments generally (and sound playgrounds in particular) - namely that the experience of the sound should also be public. There are other points of view of course, especially when you ask the neighboursI And so through a simple but careful design feature built right into the instrument, the needs of a civic space can be reconciled with the needs of a genuinely musical experience.

HAPI instruments don't openly announce their design features in these many respects. They are made from industrial materials and have the life expectancy of other outdoor playground equipment. Not bad for a musical instrument, which must also retain a quality that can only be appreciated from the experience. After all, the proof of the instrument is in the listening.

In the end Herb had convinced me on that other point too. In a strange sort of way, if you really, really listened, it did sound something like a frog... A field full of them in fact.