Defense attorneys further chipped away at the case against accused killer Wayne B. Williams today, calling an accoustics expert who attacked the credibility of the police officer who set off the alarm that ultimately led to Williams' arrest.

The testimony of the expert, Mark Oviatt, a Marietta, Ga., sound engineer, was intended to cast doubts on the police contention that Williams crept slowly across the Jackson Parkway Bridge last May to dump the body of a victim in the Chattahoochee River.

Prosecutors say the young Atlanta police recruit, Robert Campbell, heard a splash in the river moments before Williams was stopped for questioning. The splash, they contend, was the sound of Nathaniel Cater's body hitting the water.

Patrolman Campbell, on a predawn stakeout beneath the Jackson Parkway Bridge on May 22, testified earlier that he never heard Williams' car make a noise as it drove across the bridge expansion joint above him. Police contend the joint never rattled because Williams was driving too slowly, to avoid detection.

Tests by police investigators showed that the cable made a "popping" sound loud enough for someone on the river bank to hear only when a car was driven across the bridge faster than 10 miles per hour.

But Oviatt testified today, in a special session called to speed up the trial, that the expansion joint made an audible sound when his investigators drove the Williams' station wagon across the bridge at 3 to 4 miles per hour.

His elaborate sound tests were an attempt by defense lawyers to refute police tests and bolster their contention that the former recruit was asleep on duty at the time.

Earlier, Judge Clarence Cooper allowed the prosecution's sound tests introduced as evidence, but rejected a defense attempt to introduce similar tests because conditions on the bridge last May 22 differed too greatly for the defense test results to be relevant.

But defense lawyers won on their second attempt today.

The bridge incident is a strategic chapter in the prosecution's circumstantial case against Williams, 23, because it was the first time he came to the attention of police. Everything in their files builds from the moment Williams drove across the bridge and Campbell heard the splash.

Two days later, Cater's body washed up downstream near the spot where Jimmy Ray Payne, 21, was found a month earlier.

Williams is accused in their murders, two in a string of 28 slayings of young blacks that terrorized this city for two years. But prosecutors have also been allowed to introduce evidence allegedly linking him to 10 other killings to show a "pattern or bent of mind."

The sound engineer's tests are part of an effort to disprove that the splash Campbell heard was Cater's body. An Israeli army pathologist testified earlier for the defense that the body was too decomposed to have been in the water only two days.

As the jury of eight blacks and four whites begins the eighth week of the roller-coaster murder trial, it must weigh such experts against eyewitness testimony placing Cater with the defendant. One witness said he saw them holding hands outside a movie theater six hours before Campbell heard the splash.

Oviatt testified today that a "normal human being" would be able to hear the expansion joint popping if a car drove across the bridge at 3 to 4 miles per hour. He set up ultra-sensitive sound meters at the spot Campbell was staked out. But at that low speed, the noise would not be loud enough to rouse a person sleeping, he added.

"If a person were asleep, wouldn't a loud splash in the river wake him up?" asked assistant Fulton County District Attorney Jack Mallard on cross-examination.

Before he could answer, Mallard fired off another question, trying to salvage the credibility of a key prosecution witness. "If he were asleep, it would take a horse or something bigger to wake him up, wouldn't it?"