The blank wall

''No Frisbee or ball playing,'' says the sign on one side of the barer-than-bare wall of the building. ''No loitering,'' another sign declares from the other side of the lifeless wall.

No walking, no laughing, no breathing, no socializing, no smiling, no strolling . . . no livingm, it might as well say, according to William H. Whyte.

Blank walls - the flat facades of much new architecture - minus windows, minus stores, minus ornament, minus murals, minus humanity, as the sociologist was saying last week, mar the streets of our cities.

The author of ''The Organization Man'' and the author of, most recently, ''The Social Life of Small Urban Spaces,'' was tracing the trend in a room filled with a modest but pointed exhibit in support of this thesis at the Municipal Art Society in New York (through April 8).

What others discern almost offhandedly, the man decribed as one of our best ''urban watchdogs,'' diagrams and deplores in bold print. His exhibition ''Blank Walls: The New Face of Downtown,'' does so again.

The show is the result of Mr. Whyte's travels in behalf of livability. Trying to make public plazas inviting to the public, he has crossed the country, and in doing so, found himself repeatedly face to face with this ''blank wall'' architecture.

''More and more what I was seeing were a lot of blank walls,'' Whyte told an audience at his exhibition.

Whether in Kansas City, Mo.; Richmond, Va.; Atlanta (''the spawning ground for the blank wall''); or Los Angeles, Whyte photographed these blank portraits for his show. If the show is intriguing, the man is more so. His face, long and dour, his accent equally deadpan, Whyte works through understatement and humor. In the end, he carries the impact of the barren walls of new buildings beyond mere aesthetics.

''These walls are meant to be blank, and they have a message,'' he says.

The message is security first. It is hostility to the city. It is flight from what the leadership in town after town labeled ''undesirables,'' he says. The blank walls are designed, says a label, to ''put down'' and intimidate.

Divided into categories, the clients for these monuments to fear and loathing are scored by the labels and slide lecture alike. Institutions and banks are the big blanksters, says the urban sociologist.

The convention center is another contributor. Even in balmy Dallas, the orientation is inside, renouncing street life. ''The day I took this picture there was not a soul to be seen outside,'' Whyte muses incredulously. ''I went inside and there were 3,000 doctors - 3,000, count 'em,'' he says with disbelief.

This inside-outside contrast - the draining of public for private life - characterizes what Whyte has been calling the double life of new cities. This architecture sequesters the visitor in cocktail pods overlooking fountains; excludes the residents with walls spiked with iron rods, and scares us all with television cameras spookily dogging our innocent steps.

Though the show confirms that this phenomenon exists ''from sea to shining sea,'' the engaging iconoclast who created it does not flail out at the architecture of these non-places indiscriminately. He can understand why an Investors Diversified Services mall and skywalk work in chilly Minneapolis in the dead center of downtown without dismissing the folly of Detroit's Renaissance Center a few blocks from downtown.

Whyte rarely blasts the architect per se; he is, in fact, rather gentle with designers who don't like the design but go ahead and do it. He will praise a Marcel Breuer building, despite its windowless state, or mention how another architect tries to relieve the tedium of the blank wall, aesthetically, with a bit of futile geometry.

Multiplication rather than polemics builds his case. See one tree in front of a blank wall, and you don't think twice about it. See one, two, three, four, five trees before the same barren facade in five cities and you laugh. And wonder.

Because Whyte loves life and street life, he loves New York. So what if Lexington Avenue is ''tacky,'' its architecture is alive - alive not only on the ground floor in zesty retail stores but on the floor above. Through his labors, the trend to blank out first- and second-floor windows was zoned out in New York.

Whyte acknowledges the difficulties of bumping one's way through a street filled with flowers, chairs, crowds. But the ''warm and gutty'' small-scale urban design of old-style cities stands in vivid contrast to the distressing and ''phony Disney World'' design of the enclosed mall, the fearsome megastructure, and the fortress city he has photographed.

Even the economics have begun to confirm his conclusions, the urbanologist says. Renaissance Center with its walled-off city is going bankrupt, and ''a lot of these places are not working at all,'' he says.

Nonetheless, though the trend ''reflects a certain contempt for surroundings, a certain contempt for nature,'' he fears it will endure. In the same way that we still suffer such post-World War II phenomena as the gutted inner city, the empty civic center, the closed anti-urban mega-structure, and the invasion of the highway, we haven't seen the last of blank walls.

''We are raising a whole generation of architects whose idea of a design is based almost entirely on a suburban shopping center,'' Whyte says, glumly.

''I'm sure,'' he concludes, ''we haven't reached the peak.''