A funny thing happened as I ambled out of the giant shed at the Tanglewood Music Festival where I had just heard the Boston Symphony, led by Leonard Bernstein, in a stunning performance of Mahler's Ninth. I ran into a friend whose seat was further back and more to left than mine. He enjoyed the music, he said, even though he had not been able to hear the last few minutes of the playing.

To understand this, you must know how the Mahler Ninth ends. As Michael Steinberg's program notes put it: "More and more, the music recedes . . . Grief gives way to peace, music and silence become one."

This effect was perceptible to a large part of the audience but obviously not to everyone. Playing a recorded version of the symphony, on a good system in a quiet room, can evoke it. It may take a just-so setting of volume and tone controls, but it's there to be savored. Also involved in this is the fact that the Tanglewood Shed, like so many outdoor concert settings, is literally open on three sides to the great outdoors. There are no side or rear walls to "define" the acoustic character of the place and there is relatively little "reflected sound" or reverberation, the kind of "room effects" that can reinforce the live sound and which indeed contribute to the sense of its being "live."

The closer one sits to the live-sound source -- i.e., the orchestra at one end of the Shed -- the less critical is the lack of "real boundaries." But farther back, or out on the lawn itself, a listener might wish for a time-delay ambient enhancer to make the live sound seem more live.

Someone seated very far over to the right or left might wish for a channel balance control to correct the lateral "spread" of the orchestra.

With some conductors and some compositions, one might feel the need of a graphic equalizer to help sort out some extra-thick ensemble textures, or permit a vocalist to be more readily heard against the orchestral accompaniment.

And anyone, seated anywhere for any performance, might yearn for a noise filter to help suppress the assorted coughs, whispers, seat squeaks, program rustling, occasional infant cries, and more than occasional starlings who compete with the highpitched woodwinds.

On the other hand -- and it's a very big one -- what kind of audio gadget could recreate the sheer excitement of an Andre Watts approaching the cadenza of the Beethoven Fourth Piano Concerto? Or simulate the presence of a Jessye Norman overpowering an audience with the Prelude and Love-death from "Tristan and Isolde"? Or tragedy that evolved magically between Bernstein and the BSO musicians as the Mahler piece progressed? Is there any audio device that lets you know when a great conductor has tears in his eyes at the end of a performance?