Friday, 23 November 2012

Isolated EVPs mystery

Shaken sound spectrumPlease play this audio clip: sound1. You should set your audio software to repeat, or continuous play, to try to catch the words, if any. If you have any problems playing the sound, see 'Advice about playing the clips' below.

Then try playing this audio clip: sound2.

Something that has puzzled me for some time is why the most convincing EVP recordings all seem to be single words or short phrases with little or no accompanying noise. In such examples words appear suddenly, from a silent (or low noise) background, just like normal human speech. In my experiments to reproduce EVP using formant noise (as featured in the EVP gallery) it wasn't like that. On my original recordings there were several minutes of noise produced by whatever natural sound source I was using. I would then go through the recordings and pick the bits that appeared to sound like voices. Once isolated, these cropped samples sounded quite voice-like with words discernible by many people. But it wasn't the same as those dramatic recordings where words just comes out of silence and then vanish, like normal human speech. Though my method demonstrated that ambient noises could produce voice-like sounds from which words could be heard, it was not a convincing reproduction of the best 'isolated' EVPs. If someone heard longer versions of my recordings, they might well have recognized the source of the noise and not noticed any 'words' at all!

So, on to the sound samples. If you haven't played them already this will come as a spoiler! I'll tell you what I hear in sound1. Against a somewhat noisy background, I hear a voice suddenly appear and distinctly say 'shaken, something I need frequently'. You may hear different words or maybe none at all. Anyway, to me it sounds just like some of those more convincing examples of EVP where a voice appears from silence, says some words and stops again, just like a normal human voice.

In sound2, I hear a rustling noise. It has the same rhythm as sound1 and appears to be a different version of the same clip. However, there is no obvious voice this time. However, to me at least, there is still a hint of the same words but they are difficult to hear against the rustling noise. If I hadn't heard sound1 first, I might easily not have picked up the idea of a voice in this sample at all!

So, what's going on? Sound2 is the original recording. It is of a piece of paper being rustled in someone's hand not far from the recorder. It is not that obvious what it is but you might guess from the sound alone. Sound1 was produced by filtering out all the frequencies above 2300 Hz. It then acts like formant noise, producing 'words' by combining the simultaneous frequency peaks around 150 Hz and in the 1500 to 2300 Hz range (see sound spectrum graph above). If you remove the higher frequency peaks, above 1500 Hz, the voice vanishes altogether because the formant noise effect no longer works. Here is a final sample sound3 where just those frequencies have been filtered out. You shouldn't hear any words now!

So what did I conclude from this experiment? Firstly, the more impressive short isolated EVPs can, indeed, be produced by bursts of suitable ambient noise (rustling paper in this case). Secondly, the very fact that the noise is short may make it unrecognizable and more likely to be heard as a voice rather than what it really is. Not all such short bursts of noise will sound voice-like. I had to reject quite a few to get this one. The ones that don't sound voice-like may be dismissed as noise by people reviewing the recording!

Incidentally, it is not at all unusual for people to filter out certain frequencies in EVP recordings in order to 'remove noise'. In reality, such filtering can enhance the formant noise effect, making it more likely to be heard as words. Of course, you may not have heard any words at all in any of these recordings. All I can say to that is that I heard some and they sounded just like recordings I've heard of isolated EVPs elsewhere, so I've reproduced the effect to my own satisfaction. Other people will have to do their own experiments!

Advice about playing the clips: The files are in WAV format. You may need to associate WAV files with a default sound application on your computer for the links to work. You shouldn't need to use headphones to listen to these clips (indeed it may be inadvisable as some are loud!). Ordinary computer loudspeakers should work fine though you may need to have the volume set high for some of the clips.

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