Diary of a Criminal Investigator-Chapter 280: The Case with Only Voices

"What's wrong? There isn't a case, is there?"

Yang Sen couldn't bear the thought of a case now. He finally had a chance to rest, and if a case came up, everything would be ruined.

Liu Guodong shook his head, "It's Old Yuan from the High-tech Zone Criminal Investigation Team."

"Yuan Shaokang?"

Liu Guodong nodded. Yuan Shaokang was the captain of the High-tech Zone Criminal Investigation Team and was quite familiar with the crime scene investigation team.

"What happened? Did something happen to him?"

"It's an old case," Liu Guodong looked at Lu Chuan, "He came to the bureau for a meeting this morning and heard about the Liu Bai case."

"He knows about Lu Chuan's work with identifying phone recordings and wants Lu Chuan to help analyze a case's recording."

Lu Chuan nodded, "No problem, just have them send over the recording."

"Alright, I'll give him a call back."

Soon, the High-tech Zone Criminal Investigation Team sent the recording over.

"This is a case from three years ago. It was quite high-profile back then."

Liu Guodong knew about this case, and so did Yang Sen and Old Bai.

Three years ago, a child trafficking ring emerged in Haizhou City.

In just three days, there were four reports of missing children.

At that time, the case drew significant attention, with Song Jiangguo serving as the overall commander and forming a special task force in the shortest time possible.

Of course, the final investigation results were positive; all four children were found.

However, finding the children did not mean the case was closed.

No one from the trafficking ring was captured.

"These people might be repeat offenders, their methods very skilled, and their ability to hide their identities quite good."

Yang Sen sighed, "We were involved back then too. The children were found in an abandoned warehouse."

"After the incident, the city paid great attention, and the entire city's transportation was locked down."

"The trafficking ring had no way to move the children, so seeing no other option, they fled, leaving the children behind."

"This recording was taken from surveillance video in the warehouse where the children were found."

"But the surveillance was actually broken; the camera wasn't working, it only recorded sound."

"The recording was analyzed by voice print experts from the neighboring province, but unlike fingerprints, there was no target to compare the voice against."

"So, the case remains unsolved to this day. The High-tech Zone was the main responsible party then, so they maintained jurisdiction over the case."

The three of them remembered the case well and as they talked, they clarified the situation.

"Listen to it first. If you find anything, it would be great."

None of them had high hopes.

In fact, Yuan Shaokang was just trying his luck.

After all, voice print analysis was hard to conduct back then as it needed to be taken to the neighboring province. Now, since Lu Chuan had this capability, trying it was better than not."

Using Old Bai's headphones, Lu Chuan played the recording.

The audio was long; it was a conversation.

And it wasn't just between one or two people, it was a conversation among four people.

"Let's just run; forget it this time."

This was a woman's voice, and judging by the intensity, she seemed middle-aged.

One can't accurately judge a person's age by their voice, but you can roughly estimate an age range. 𝗳𝚛𝗲𝕖𝚠𝚎𝚋𝗻𝗼𝕧𝗲𝐥.𝚌𝚘𝐦

"Why run? It took us great effort to get these 4 kids, and running now means getting nothing."

This sounded like a middle-aged man's voice.

"You're an idiot. Earn money to spend it alive. Everything's locked down outside; these kids can't be moved out. If we don't run and get caught, it's a dead end for us."

The first woman's voice spoke again.

"Let's vote, leave the kids, and those who want to go, raise your hands."

These were three voices, all male.

"I want to leave; the lockdown outside makes it too dangerous to take the kids."

This was the fourth voice, a woman's.

Then, the four voices continued to alternate, speaking back and forth for about ten minutes.

"Two men and two women, but they're suspected to all be middle-aged, around their thirties."

Seeing Lu Chuan remove the headphones, Liu Guodong discussed the situation from back then.

"How did it sound to you? Did you find anything?"

Lu Chuan shook his head slightly.

He'd only listened once; it wasn't possible to find something so quickly.

However, the age of one voice seemed different from what Liu Guodong mentioned.

"Master, you said they were all middle-aged?"

"Yes, that's what I thought from the recording—middle-aged, probably in their thirties or forties."

"We also brought in professional voice print experts from the neighboring province back then, and they concluded the same."

Lu Chuan shook his head, "Three voices seem fine, but one female voice..."

He was talking about the voice that said "I want to leave," the fourth female voice in the sequence—it sounded a bit off.

"Master, when will the spectrograph arrive?"

Before Labor Day, Qin Yong had mentioned purchasing the equipment, and it hadn't arrived yet.

"This week, Captain Qin mentioned in the morning meeting that the equipment is on its way and should arrive in three to five days."

"Alright, I've just picked up some issues by ear, but specifics will require spectrographic analysis to confirm."

"Okay, but what did you notice about that female voice?"

"That female voice seems like her vocal cords were damaged; she should be younger, but because of the damage, her voice is hoarse, making her sound older."

"But I'm not entirely sure; we'll have to analyze it with the equipment for a more accurate result."

Voiceprint analysis mainly examines the intensity and frequency of the sound.

The frequency of different people's voices is unique.

Like fingerprints, every person's voice has a unique identifier due to breath length and vocal cord development differences.

In this identifier, volume is the least important factor, while timbre and frequency are the most critical.

Timbre refers to the different qualities of voice—hoarse, sharp, deep, or magnetic.

In daily life, when people say some voices are similar, they're usually referring to timbre.

Frequency is the ultimate differentiator between different people's voices.

No two people's voice frequencies are identical, only similar, never the same.

A human's vocal frequency ranges from as low as 82 Hz to as high as 1200 Hz, and few normal people exceed this range.

In this range, the 800 Hz band is the primary frequency that affects timbre, and a fuller sound at this frequency makes for an appealing timbre.

A sound is composed of vibrations from different parts of the vocal cords simultaneously.

Thus, a sound's frequency will vary, and the differences in the composition of these frequencies produce a variety of voices.

By analyzing with a spectrograph, it's possible to discern these differences very intuitively.