The ED is reportedly in possession of an alleged audio recording of a conversation that Bhadra had, which it  claims is a crucial piece of evidence in the school jobs scam case. It has been thus been trying for over six months to obtain this sample.

Here is all you need to know about the use of voice samples in investigations.

How is a voice sample taken?

An investigating agency generally moves court, seeking permission to collect a person’s voice sample in connection with a case. Upon receiving permission, there are specific requirements for collecting the sample.

An official from Central Forensic Science Laboratory (CFSL), requesting anonymity, had told The Indian Express in 2023: “A voice sample is generally taken in an echo-proof room for a controlled and noise-free environment, and a voice recorder is used … the person is asked to say a specific clue word from a statement already part of the evidence.”

The official added that certain technical parameters are kept in mind while recording a person’s voice sample. “A spectral analysis of the audio speech is undertaken where the pitch, energy and frequency of the voice are kept as the base to study and match the original audio sample,” he said.

Forensic officials use international phonetic alphabets while recording a voice sample and ask the subject to pronounce only a small part of the original statement so that both vowels and consonants in the spoken bit can be alternatively analysed.

“There are two methods of comparison, you either get an anonymous voice sample and check it with a suspect list of five persons, or you know who the speaker is and corroborate both the voices,” said another official.

An official from Forensic Science Lab, Rohini, said the semi-automatic spectrographic method of voice sampling is used in Indian forensic labs while some countries use the automatic method where a likelihood ratio of the voice samples is developed, which increases accuracy.

The spectrographic method for speaker recognition makes use of an instrument that converts the speech signal into a visual display.

While the results of the voice sample either turn out to be positive or negative and the final report is submitted by the forensic lab to the investigating agency, officials said that inaccuracies mainly arise when the person’s voice is altered due to the effect of medicines, or if the person is suffering from a cold.

How does this help the police’s case in court?

Senior Delhi Police officers had said that a matching voice helps confirm the evidence already collected by the investigating agency.

Retired Delhi Police officer Maxwell Pereira said in 2023: “Depending on the evidence, an investigating officer directs the forensic expert to extract a particular sample which will help their case in court. The credibility of the sample depends on the technique used by the expert and how the court analyses it for proving a person’s culpability.”

The US Federal Bureau of Investigation (FBI) had first used the technique of voice identification analysis in the 1950s, but the procedure gained legitimacy in a 1962 study by Lawrence Kersta.

Later, a research paper titled ‘Forensic Speech and Audio Analysis, Forensic Linguistics – A Review: 2001- 2004’, by APA Broeders, Chief Scientist, Netherlands Forensic Institute, said the parallel with the fingerprint that the voiceprint evoked, “was shown to be utterly misleading”.

The study, however, states that, “Unlike fingerprints, or friction ridge patterns, spectrographic representations of speech are not invariant over time but highly variable within speakers, reflecting the inherent within-speaker variability that is characteristic of speech”.

What are some cases in India where voice samples have been used?

A special NDPS court had in February 2023 allowed a plea moved by the Narcotics Control Bureau (NCB), seeking collection of voice samples among 33 accused in a drugs case it was investigating after the death of actor Sushant Singh Rajput. NCB had claimed that it needed to verify certain voice calls that had emerged among the accused.

In December 2022, the voice sample of Aftab Poonawala, accused of killing his live-in-partner and chopping her body into several pieces, was taken at CFSL after a local court allowed a police plea in light of an audio recording surfacing of the couple purportedly fighting.

In April 2023, Congress leader Jagdish Tytler submitted his voice samples to the CBI in connection with his alleged role in three murders in Delhi’s Pul Bangash area by a mob during the 1984 anti-Sikh riots.

This is an edited version of an explainer published last April.