When symphony orchestras play, who makes up the deficit?

Big business's investment in the arts has not exactly been insignificant. But long-term commitment has been harder to come by.

One outstanding example was begun in 1979 when AT&T announced the ambitious program, ''Bell System American Orchestras on Tour,'' to the tune of $12 million. It was at first limited to the big five orchestras, but it has been expanded -- and the list for 1982 consists of 30 American orchestras, including the big five (Boston, Cleveland, Chicago, New York, Philadelphia) and the best of the rest.

I would not normally devote this column to one company's efforts on behalf of the arts, but in this era of waning government support, the AT&T program looms large. As you read, the fate of the program rests in the balance: Under the agreement with the federal anti-trust folks, AT&T is, in effect, divesting itself of control over the Bell System. Since it was AT&T that conceived of this project, there is a tangible danger that the soon-to-be-more-independent operating companies will not make room for it financially or organizationally.

This unprecedented program arrived just at the right time for maximum impact. Orchestras are dealing with costs that are escalating at a furious rate. Their revenues go only so far, and the cost of a ticket can go only so high. (Nowadays , it is not uncommon for an orchestra to charge $25 for a special event, $17.50 for a normal evening at the symphony.)

An orchestra that records needs to tour to sell those recordings. Other orchestras need to tour to establish their place in their own community - the prestige of travel and major-city reviews draws in crowds and grants. But costs being what they are, touring is a financially draining experience. With AT&T in the picture, that cost has often been offset. In other words, the difference between ticket receipts and costs has been made up.

Kenneth Haas, general manager of the Cleveland Orchestra, says: ''When we play Eugene, Oregon, people pay $8 or $9. The program helps us reach out, and reach an audience.'' Ernest Fleischmann, Los Angeles Philharmonic executive director, puts it in different words: ''We are able to play in places that could not afford to have us.''

Mr. Haas reaped the most recent benefit of the program, when the Cleveland came to New York at the end of May to bid farewell to exiting music director Lorin Maazel in a series of concerts concluding with a large (and costly) Verdi ''Requiem.'' In fact, it was the single largest project within the program this season, and involved some $200,000.

But the program is not content with just doling out cash or footing some hefty bills. The program sees to it that a visiting orchestra has some impact on the community.

''It's wonderfully well done,'' remarks Mr. Haas, who adds that ''the Bell locals have been taught how to get the most out of us.'' Master classes featuring first-class players are given in the music schools. Ensembles formed from within the orchestra give noontime concerts in malls and atrium-plazas around the city.

Of course, a certain amount of formal socializing is necessary on tours. Since the local Bell System affiliate has helped foot the bill, they chair these receptions. Mr. Haas, for instance, marvels that speakers at the receptions have all the proper facts.

Edward Block, AT&T's vice-president, public relations, explains the system this way: ''By touring the orchestras, you get them into other communities, and hopefully you get people who would not otherwise have had the opportunity to see , say, the Cleveland, or the Boston, or Los Angeles.''

What is perhaps most remarkable about this program is its overall elasticity. It does not dig itself into a rigid format now that it includes the smaller-league orchestras. It has proved that quality is the goal, not just window-dressing glamour. The variety of the program is seen in the Cleveland tour, the regional touring of large orchestras, and even the exclusively intrastate touring of the North Carolina Symphony.

Mr. Haas, who was in on the planning stages of the entire program, notes that the AT&T program has inspired other corporations to be more visible in their support of the the arts. Citibank has agreed to underwrite many of the New York Philharmonic's upcoming tours. As Mr. Haas so succinctly puts it: ''It has changed our method of operation. If it does not continue, we will have to find other ways, (to fund tours).'' He says this with a look of discouragement. Mr. Fleischmann goes a step further: ''I am terrified that because of the change either the program will be killed, or so watered down as to be rendered ineffective.''

No one is trying to pretend the program is pure altruism. Mr. Block says that ''any way we look at it, it's a good business proposition. There is a higher percentage of the kind of people we want to reach in the arts. If it's okay to take a customer to lunch or to a ball game, then why not to the symphony? You think in terms of what it costs you to reach whatever kind of person you're trying to reach, and clearly our support of orchestras is a very attractive business proposition. Symphonies have remarkably appealing demographics,'' he continues.

''The median age of the people who attend symphony performances is 40, which is a touch under the national average. The median income is something like $21, 000, which is 50 percent over the national median. Seven out of 10 are college graduates, three out of 10 have advanced degrees. And if you talk about numbers of people who pay their own good hard money to go to an event, symphonies beat professional sports in almost every major market. There is a higher percentage of the kind of people we want to reach in the arts. And it is the best of all worlds. First off, you're doing something important - support of the arts -- and secondly, you're doing something the communities clearly favor.

''The business network and the arts network haven't yet really come together. We've got to somehow bridge that gap.''.

''American Orchestras on Tour'' must not be allowed to founder. This program has been an unprecedented benefit for anyone who loves orchestral music. It has enabled all of us to hear the great and the up-and-coming orchestras -- either with regularity, or perhaps for the first time, depending on the city or region in question. It seems to be sparking other corporations into action. It has an unlimited growth potential, and in this financially precarious age, anything that encourages the arts to flourish must be encouraged. A letter to your local Bell would not be out of order. For when the support comes from the people local companies serve, it carries tremendous impact.