The FBI yesterday disputed the scientific underpinnings for the House Assassinations Committee's conclusion that two gunmen were firing at President Kennedy when he was killed in Dallas in 1963.

In a 22-page report submitted to the Justice Department more than a year after the House committee completed its work, the FBI said the acoustical findings on which the committee relied were based on faulty premises and "must be considered invalid."

The now-defunct committee concluded after a $5.8 million investigation that Kennedy "was probably assassinated as the result of a conspiracy." The finding rested heavily on acoustical studies of sounds believed to have been accidentally picked up by a police transmitter in Dealey Plaza when the president was shot on Nov. 22, 1963.

Two highly regarded acoustics experts, Mark Weiss and Ernest Aschkenasy, told the committee their tests showed "beyond a reasonable doubt" that a second gunman fired at Kennedy from the area of the so-called grassy knoll while three other shots were coming towards him from another direction.

The experts said they were sure that the noises on the Dallas police department tape they studied were gunshots and that the echo patterns they left constituted a unique signature or "fingerprint" of Dealey Plaza.

The FBI's technical services division, however, charged that Weiss and Aschkenasy "neither proved that the impulses on the DPD [Dallas Police Department] recording were generated within Dealey Plaza nor that they were the sounds of gunshots."

As a result, the FBI study said, the House committee's finding that "scientific acoustical evidence establishes a high probability that two gunmen fired at President John F. Kennedy' is also invalid."

The FBI review leaned strongly on the bureau's investigation of a November 1979 shootout between members of the Ku Klux Klan, the Nazi Party and the Communist Workers Party in a residential area of Greensboro, N.C.

The bureau said that one of the shots in that confrontation, which was videotaped by local TV crews, produced impulses that virtually matched those on the Dallas police tape that were attributed to a shot from the grassy knoll.

This, the FBI said, showed that the sounds on the Dalls police tape, far from constituting a unique print of Dealey Plaza, could have come from any one of the many urban areas within range of the police recording system.

In fact, the FBI said, the Greensboro shot passed an even stricter test and thus "had a much higher degree of probability" of matching the impulse pattern on the Dallas police tape than the predicted echo pattern of a shot from the grassy knoll.

Aschkenasy had told the House committee that if he came across another tape with a matching sound pattern, he "would expect to find . . . a replica of Dealey Plaza at that location. That's the only way it can come out."

The FBI pointed out, however, that the residential neighborhood of Greensboro where the Klan shootout took place has narrow streets, low buildings and small fenced lots in contrast to Dealey Plaza's wide streets, tall buildings and small parks. It is "definitely not a replica of Dealey Plaza," the bureau said.

The FBI's analysts also disputed the notion that the sounds Weiss and Aschkenasy studied were actually gunshots rather than some other noises that could have produced echoes off surrounding buildings and vehicles. The bureau said its work in Greensboro showed that the simple sound of a stick hitting an object produced echo patterns and wave forms similar to those left by gunshot blasts.

In short, the FBI declared, "there is no proof provided by [the House committee's experts] that the four patterns on the DPD recording represent gunshot blasts and not some other sounds or electrical impulses produced internally by the DPD radio system."

Weiss and Aschkenasy withheld comment, saying that they had not been told of the FBI study and had not yet had a chance to examine it. "This is the first we've heard about it," Weiss said yesterday.

After citing what it called "numerous other problem areas and inconsistencies" in the acoustical work for the House committee, the FBI suggested that no further study was needed in light of the probable expense and the difficulty of reaching valid conclusions from such a poor quality tape.