Most of my instruments come in common western equal temperament tunings. All of these (excluding some Wrenchophones, Chimasaurs, Whale Drums, and experimental instruments) are at concert pitch : A=440hz. Over time, the wood instruments will travel some out of concert pitch, but stay in tune with themselves. Depending on the instrument, they are available in either Pentatonic, Diatonic, or Chromatic. For those unfamiliar with these terms see them described below.
There is also a whole world of other tuning systems and scales out there. I've experimented with some tunings from other cultures, and eagerly anticipate doing more. For those with an itch to break from Western European patterns of scale, I'm happy to help.
The Pentatonic scale is probably the worlds most universal scale. It's found in the folk traditions of Russia, India, Africa, Asia, Europe, and pre and post Colombian America.
The pentatonic I use is a five note scale in which all the notes are harmonious with each other. It's most easily found by playing just the black keys on a piano. You could pick out any other notes on the keyboard, keeping the same intervals of the black keys, and you can get a variety of pentatonic scales. Another way to imagine it is: do re mi - so la - do, with the "fa" and "ti" left out.
Because of the lack of dissonance, the sound is very sweet to the ear. Anyone of any musical ability can improvise to their hearts content and it all sounds harmonious.
The limitation comes in when you want dissonance in your music or you want to play most modern written music. Then you'd want to go to a diatonic scale or some combination (see "Orff" below).
The diatonic is your basic western scale of seven notes: do re mi fa so la ti do (for a major scale). Most modern Western music is written diatonically. On a piano, starting with middle C, play seven notes on the white keys and that's the tuning of most of my diatonic instruments. Of course, I can custom tune to any scale of your choosing. If you want to have a variety of scales available, then you'd either need a bunch of instruments or a chromatic one.
A chromatic instrument has available all twelve notes in Western music. Therefore you can pick out any combination of notes for whatever scale you choose to play in. Most of my chromatic instruments are two octaves, starting and ending with C. They usually come in a marimba like setup. The whole notes (white keys) are on one base close to the player. The sharps and flats (black keys) are on a slightly higher base which sets next to the first, with the keys slightly overlapping for easier reach.
The Orff System:
German composer Carl Orff developed a system of music education called Orff-Schulewerk. The Orff system was designed to emphasize the development of creativity and improvisation in children, largely through use of the pentatonic scale. The Orff system starts the young musician on the pentatonic scale to encourage improvisation. Once that's mastered, most Orff instruments have spaces to add keys to then make a diatonic scale. This system is fairly popular in schools throughout the US and Germany.
I'm currently contemplating introducing a line of Orff instruments; but in the meantime I'm happy to custom make or adapt my current instruments to the Orff system for anyone interested.
Now for the more ethereal. Beat frequencies happen when you have two notes tuned slightly apart played simultaneously. You hear the two notes plus feel a third which is tuned to the difference between the frequencies: the beat frequency. The pitch of the beat frequency is far lower than we can consciously hear, but you can hear a kind of wavering "wa,wa,wa". This frequency can be tuned to match the frequencies of our brain in various states of consciousness. Through entrainment (the "craving" of similar frequencies to beat as one), we can actually lead our brain into different states of consciousness.
This is basically how "mind machines" and subliminal tapes work: By setting up a series of beat frequencies to bring your mind to a receptive state of conscience. As I don't necessarily trust giving over my brain to a tape or machine, I use the same principals in user-controlled musical instruments. I usually take a pentatonic tuning and add various "beat" notes next to them (ie. Fountain, River Drum, and Miriam's Aquarion on Experimental... page, Aquabeat on Glass Instruments page). I usually tune the beat frequency to lead one into a Theta mind state. Theta is the most intuitive state of mind between asleep and awake. It's the only time the two halves of the brain synchronize, and your mind is literally expanded. It's where most forms of meditation intend to take you.
My musical instruments using beat frequencies are intended to help expand the mind and help break old patterns of thought. All can be custom made to suit your needs.