Star sprinter Oscar Pistorius is seen at the High Court in Pretoria on Monday, 30 June 2014 after spending 30 days under psychiatric observation to determine if he should be held criminally responsible for killing his girlfriend Reeva Steenkamp. Picture: Phill Magakoe/Independent Newspapers /Pool

Johannesburg - There’s a vehicle driving past the window outside the office. I’m trying to write this story and it’s distracting me.

The low purr of a truck, like a cat with a head cold.

I can’t see what’s going on down there, but I know they are ripping up Pretoria’s streets, so I don’t need to look. My ears can see the sound.

In my mind, it’s a truck loaded with concrete, under the hands of a construction worker. If I didn’t know about the construction, I would just be able to tell you – it’s a truck. Maybe delivering bread. Potentially a red Coke van.

If I worked in construction, I’d be able to deconstruct those sounds more carefully.

What brand of truck is delivering the concrete, what size tyres, what engine? Because, ultimately, the information we can extract from any situation starts with the knowledge we already have.

“Perception does sometimes vary, it all depends on your framework and your frame of reference,” said Dr Jackie de Wet from the criminology and forensic studies department at the University of KwaZulu-Natal.

He said that if you had been around firearms, for example, you wouldn’t only be able to say the sounds you heard were gunshots, but might be able to say what type of gun was used because you knew that weapon.

“There are a lot of variables that go into how you interpret different sounds,” he said.

The physical variables going into what was heard on the night of February 14 last year at Silverwoods Estate have been laid out in court.

The closed barriers, such as the toilet window, the weather, the height of the trees and even the length of the blades of grass.

But what about the psychological factors?

As defence witness sound expert Ivan Lin pointed out, the trauma of those sounds might change our perception of them.

De Wet said frames of reference were important in numerous ways in which we interacted with what was going on around us. He said some people would be better than others at perceiving tension in a conversation, for example.

De Wet said people who had grown up in a home in which they were exposed to a lot of tension would be more in tune to perceiving it than others.

“They would have a better appreciation of the context and what is happening within that context,” he said.

Beyond the physics of whether it was possible to determine if the screams were those of a woman or a man, it was completely natural for people to perceive the same situation differently.

Forensic expert Dr David Klatzow said they had conducted an experiment during which they fired off a gun unexpectedly in a full lecture theatre.

He said they had obtained a variety of answers when they interviewed students directly afterwards as to how many shots there were and how the shots had sounded. The students weren’t lying, he said, they just had different information at their disposal.

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The Star