Two acoustics experts told the House Assassinations Committee yesterday their tests showed 'beyond a reasonable doubt" that a second gunman fired at President Kennedy in Dallas 15 years ago from the area of the so-called grassy knoll.

They said the Warren Commission could have done the same work, and reached the same conclusion, back in 1964, without any great difficulty. Far from practicing any "modern electronic witchcraft" on a police recording of the sounds in Dealey Plaza when Kennedy was killed, the experts assured the committee that they used nothing more than "simple, basic physics and geometry," without the help of any newfangled gadgets.

"This is not an arcane science," one of the experts testified emphatically. "It is taught in high school and college-level physics... and I think [it] can be understood by anybody who has ever heard an echo."

With less than a week left before it goes out of business, the committee listened to the last-minute evidence with perplexed fascination. The members capped the day-long public hearing by moving into executive session for a potentially divisive series of votes on its findings in the president's murder. The findings are due next Wednesday.

A majority was expected to agree that someone in addition to Lee Harvey Oswald shot at the president just before he sustained a wound that literally exploded his head on Nov. 22, 1963. That, in turn, as Chairman Louis Stokes (D-Ohio) observed at the close of the hearing, "could point to a conspiracy in the assassination of President Kennedy."

The new findings involve a tiny segment of a police recording that started shortly before the assassination when a motorcycle patrolman left his microphone switch in the "On" position, deluging his transmitter channel with what seemed to be a lot of background noise.

The experts, Mark Weiss and Ernest Aschkenasy of Queens College in New York City, said they were sure of their findings even though they reached them only with trepidation. Weiss said he and his colleague were well aware of the "enormous impact" of their study. Stokes said it could "change the course of history."

With two gunmen firing at the presidential motorcade in Dealey Plaza within the same split second that day, Stokes observed in understated fashion, "one can assume an association" between the two assailants and, from that, "one can make a legal assumption of conspiracy."

Weiss nodded. One of the six experts who served on the courtappointed panel that found a series of evidently deliberate erasures in one of President Nixon's Watergate tape recordings, Weiss said during a break that the JFK tape study was by far the more onerous asignment.

"I can't tell you how many times we went through the agony of doing our calculations over and over again," he said during a break. In the Watergate tapes, he said, "there were six heroes" around to console each other and, even they kept calling each other and asking, "Are you sure?" This time, he pointed out, there were only two of them, himself and Aschkenasy, and, he emphasized, "we were nervous" when they realized what their study showed.

"Our initial reaction [to the police recording] was 'somebody's got to be kidding; these can't be gunshots,'" Weiss told the committee. But, he said, "the results of our analysis convinced us."

Aschkenasy agreed. "The numbers could not be refuted." he said, pointing out that he and Weiss used nothing more complicated than a hand calculator to do their computations, again and again.

The only other instruments they used, Weiss indicated, were pieces of string to measure distances on a 1963 survey map of Dealey Plaza, some thumbtacks to pinpoint locations, an oscilloscope to observe the waves and shapes of the sounds on the Dallas police recording, and another device to reproduce the waves graphically.

The two experts concentrated almost exclusively on a segment of the tape lasting only three-tenths of a second, which an earlier study had pinpointed as the possible impulse of a shot from the grassy knoll. That study, conducted by James Barger of the Cambridge, Mass., firm of Bolt, Beranak and Newman, had found a high degree of probability that three shots were fired at Kennedy, all from above and behind him, all from the direction of the Texas School Book Depository where Oswald was said to have set up his sniper's nest. To the committee's chargin, however, Barger calculated the odds on a fourth shot from the grassy knoll at an ambivalent 50-50.

Weiss and Aschkenasy said they were able to determine the location of Dallas policeman H. B. McLain's motorcycle, which inadvertently transmitted the sounds to a Dictabelt at police headquarters, with much more precision.

Their study indicated the motorcycle had just turned onto Elm Street, 120 feet in back of the presidential limousine, when the third shot rang out leaving a telltale signature or "fingerprint" on the police tape.Weiss said they found 10 echo patterns within the three-tenths of a second segment that precisely matched sounds emanating from the grassy knoll, traveling carefully measured distances to nearby buildings, and then bouncing off them to hit the motorcycle transmitter at the exact location predicted for it, give or take 18 inches.

He said they were similarly able to place the unknown gunman behind a picket fence at the top of the grassy knoll, in front of and to the right of the presidential limousine. Weiss said they were confident of that position, give or take five feet.

He and Aschkenasy added that they were sure, by 95 percent or better, that the sound they had traced was not simply a motorcycle backfiring, or a firecracker, but was indeed a bullet, probably a rifle bullet.

"If I were a betting man, I would say the odds are 20 to 1 [that this is not noise]," Weiss said. "What we're dealing with here is not noise, but in fact a bullet."

The tape also contained strong indications that it was a rifle bullet since the recording showed a so-called "N," or shock, wave traveling faster than the speed of sound and hitting the motorcycle transmitter milliseconds before the arrival of the noise of the muzzle blast itself.

This supersonic phenomenon is characteristic of rifle bullets, but the committee's chief deputy counsel, Gary Cornwell, informed the members that there were handguns available in 1963 that also shot bullets at supersonic speeds.

Since the basic findings of the study were made public more than a week ago by Rep. Harold Sawyer (R-Mich.), there have been contentions that the recording could not have been made in Dealey Plaza, but Aschkenasy made short shrift of such suggestions.

Asked about the chime of a carillon bell that can be discerned on the tape following the shooting, Weiss agreed that no such chime could be heard in Dealey Plaza, but he said the sound could well have been picked up when some other police motorcyclist in another location in Dallas "tried to get on the channel."

"During the five minutes (the approximate duration of the police Dictabelt and the tape made from it)," Weiss added, "you can in fact hear other transmitters trying to come on. You do hear other voices coming on... low but intelligible." In fact, he said, after the shots have been fired, "You can hear more people coming in with comments that somebody's got his button stuck."

Rep. Christopher Dodd (D-Conn.) wondered if the recorded sounds could possibly have come from a motorcycle in another location.

Aschkenasy's rejoinder was explicit and sweeping.

"If someone were to tell me that the motorcycle was not in Dealey Plaza," he told Dodd, "I would go there -- and I would expect to find a replica of Dealey Plaza in that location."

Working from the acoustical findings, committee chief counsel G.Robert Blakey said the panel reviewed photographic coverage of the motorcade until it found the motorcycle in question and identified the rider as Officer H. B. McLain. (The film showing McLain evidently was one turned up just last month by outside critics.)

Called as a witness yesterday afternoon, McLain, a 26-year veteran who is now a Dallas police accident investigator, said he couldn't remember having a stuck microphone that day, but he said that happened on his old Harley-Davidson so often "that I'm scared to say." He confirmed his position in the motorcade as being to the left of the line of cars and behind Vice President Johnson's limousine.

The Weiss-Aschkenasy study did not cover the three shots from behind, but Barger, who also testified, said he was confident those had taken place. He also endorsed the new findings and agreed the probability of a shot from the grassy knoll was 95 percent or better.

The combined studies thus show the first shot coming from the book depository, a second shot coming from the depository just 1.6 seconds later; a third from the grassy knoll 5.9 seconds after that, and the final shot from the book depository a half-second after that.

Committee members were evidently impressed, although several, such as Dodd and Rep. Sam Devine (R-Ohio), were plainly reluctant to accept the implications. Nonetheless, Rep. Floyd Fithian (D-Ind.) said of Weiss and Aschkenasy after the hearing:

"These guys were unshakable. We hit them with every question but we couldn't shake them. Until some mathematician comes along and proves otherwise, I believe them."

The prolonged session ended with a hurried summary by Blakey who said that that other scientific work done for the committee, including medical, ballistics and trajectory tests, indicated strongly that the shot from the grassy knoll missed the presidential limousine as did one from the book depository, probably the first to be fired from there.

Blakey took the position that the second shot from the book depository hit both Kennedy and Texas Gov. John Connally and that the fourth shot, again from the depository, struck Kennedy in the head, killing him.

Even so, the evidence of a second gunman, whether he fired accurately or not, ironically leaves the House committee in much the same position as the Warren Commission was years ago, scrambling to patch up its findings and meet a deadline. Stokes appeared to close the door on a renewal of any congressional inquiry and suggested that any loose ends would be bequeathed to the Justice Department.

Regarding its inquiry into the murder of the Rev. Dr. Martin Luther King Jr. in 1968, Stokes said the committee had developed "evidence of the outlines of a likely conspiracy" there.

He was evidently alluding to the committee staff's contentions that James Earl Ray was in part encouraged to commit the murder by reports of a $50,000 bounty on King offered by two now-deceased St. Louis men. The committee has also sought to develop evidence that Ray's two brothers, John and Jerry, may have helped him and known of his plans. But the testimony has been far from conclusive.

Stokes said he regretted having to leave behind the "loose ends" in both investigations but observed, as he had before, that "life itself has loose ends." In any case, he said one clear lesson could be drawn now from the committee's work and that is "we did not give these men (Kennedy and King) the type of investigations in death which were commensurate with the dignity of their lives. We can and we must promise ourselves that this history will never again be repeated in this nation."

The committee then began wrestling with its official conclusions in a secret session that lasted well into the night. Members refused to comment on what transpired, reportedly passing a resolution to say nothing until a scheduled appearance by Stokes Sunday on CBS television's interview show, "Face the Nation." A summary of the finds, however, may be released before then.