In a normal situation, the moment you press the brake pedal, the engine RPM (revolutions per minute) should drop. But the engine sound left on the black box of the City Hall Station crash vehicle was the exact opposite. The RPM was soaring in reverse. The engine sound was saying the driver's foot was on the accelerator pedal.
The voice of researcher Lee Jae-hyeong of the National Forensic Service was calm but firm. On July 1, 2024, the City Hall Station wrong-way tragedy took nine lives. The driver, a person surnamed Cha, 68, insisted to the end that the sudden unintended acceleration was caused by a vehicle defect, but the Supreme Court last month finalized a five-year prison sentence. Behind the judiciary's confidence in driver negligence was an engine sound analysis program developed by this researcher.
With this technology, the researcher proved the negligence of numerous offenders who tried to hide behind the shield of sudden unintended acceleration, and in recognition of that achievement, recently received the 11th Korea Public Official Award, the Presidential Commendation. In the past five years, there have been more than 400 crashes claiming sudden unintended acceleration.
Until now, readings of suspected sudden unintended acceleration crashes have relied entirely on the event data recorder (EDR) in the vehicle. But the distrust of bereaved families and the public always followed, asking how they could trust a device made by the manufacturer. In fact, in Korea, there has not been a single case in which the manufacturer's responsibility was recognized in a sudden unintended acceleration crash.
The researcher said he wanted to overcome the wall of distrust encountered in the field. The solution he found was sound, a dataset completely independent from the EDR. He built a system that analyzes the engine sound recorded on the black box to extract RPM values, cross-checks them with the EDR, and back-calculates the speed at the time of the crash. This technology is now used as the standard procedure for traffic forensics at the National Forensic Service.
He said, "The engine detonation sound recorded on a black box is mechanical vibration and cannot be manipulated," and noted, "When this research was published in the international journal Forensic Science International, I felt proud that our forensic work was recognized globally."
This program, unexpectedly, began with a news report of a drunk-driving crash 10 years ago. In 2015 in Gumi, North Gyeongsang, when a heavily intoxicated driver rear-ended a car in front, killing four people, the police asked the National Forensic Service for precise speed analysis.
The researcher said, "As I replayed the black box, the engine sound was unusually clear," and added, "I had an intuition that the answer was in that sound." He continued, "The frequency of the sound varies according to the number of explosions per second in the engine, and it is a principle that can be applied without exception to any internal combustion vehicle regardless of manufacturer."
Recently, the researcher has been studying new technology. As internal combustion cars disappear and the era of electric and hydrogen vehicles arrives, analysis using sound has hit a limit.
He said, "I tried every possible way to apply it to electric vehicles, but because they are so quiet, it is not easy to extract meaningful sound data," and added, "But forensic work must not stand still. If we cannot hear sound, it is the destiny of the National Forensic Service to find other evidence."