If you'd like to know what the German rock group Nektar sounds like, there is an easy, inexpensive way to find out. Wander over to the little triangle below Dupont Circle where 18th Street, M Street and Connecticut Avenue cross one another and stand out one the sidewalk tonight somewhere around the Cantine d'Italia.

The volume of sound pouring out of the L.A. Cafe, Washington's newest showcase of rock music (right next door to the Cantins) should be enough to satisfy all but the most decibelcrzed rock'fans; there are big glass windows right in back of the balcony-bandstand where performers in the L.A. Cafe do their thing, and the glass serves, apparently, was an efficient conductor of sound to the great outdoors.

It's more comfortable inside, of course (particulary in the weather we've been having), and there's plenty of sound left in there for the paying customers.

Inside or out, Nektar sounds something like another rock group named after a fluid - Cream, only a memory now, and a name to be spoken with an awed hush, in the voice, perhaps a catch in the throat.

During the three or four number I heard (I was one of the many standees in the standing-room crowd on opening night, and my ears felt all right buy my feet got sore), Nektar's four members showed a good technical grasp of the standard rock instruments (lead guitar, bass, keyboards and drums), and a fine sense of late '60s style, but there appeared to be no evidence of new musical ideas or a distinctive group personality. Their music was like their English - idiomatic, but evidently a result of careful study. If someone had told you they were a British rock group of 10 years ago, freeze-cried and freshly reconstituted, you would be tempted to believe it.

Of more enduring interest is the Cafe itself, a place designed as a showcase of rock music for moderate sized audiences and quite thoughs fully; cleverly designed. The basic shape (imposed, inevitably, by the oldish building it occupies) is the standard outline of a Washington townhouse - long and fairly narrow, not unlike the Childe Harold, though it seemed some what larger. A generous space at the front has been left open, rising through two stories with the side walls stripped to the bare brick. And about halfway up the front wall, a balcony has been constructed where the musicians perform.

The back part, where the audience sits or stands, was not quite finished on opening night but close enough to see what it will be like. It is divided into two levels, both open at the front and offering a reasonable view of the stage from most locations. In the upper level, you look down slightly on the musicians - a healthy viewpoint.

The most important element in a rock showcase is the sound system, and on this the L.A. Cafe scores extremely well. On the basis of its first night's performance, I believe it has the best system for rock I have heard in the Washington area.The volume was kept well below the pain threshold, the tonal balance was good (making allowances for the exaggerated bass and metallic treble one expects in live rock), and above all I found no traces of audible distortion - unintentional distortion, that is.

The Cafe's unusual spatial layout seems to offer patrons some options in terms of sound. The balance seems to be calculated to give the best effect on the lower level from the front to halfway back. On the upper level, it seems a bit louder (which many fans will like, of course), and vibration freaks can have the added kick of feeling the stronger bass notes vibrating in the upper level's floor. The softest sound (which I found quite pleasant) can he heard downstairs in back, standing at the bar, although at this location the sight lines don't let you see much of the musicians above the waist.