Destructive interfrence in music concerts (basic question)

[beldiniz]

Hi there. I am sorry I am not a technician, but a physicist and a science writer. I have to review some basic-level educational videos and in one of them the teacher makes a statement that I need to confirm with expert opinions. In very large music concerts, he says, sometimes there is a point in the middle of the public where the sound is accidentally cancelled out due to interference effects. You hear perfectly in a certain place but if you give, say, one step backwards you would hear almost no sound. I have learned that there is some destructive interference near neighbor loudspeakers, but I am afraid the previous statement is an exaggeration. I would like a confirmation of the (non-)existence of this effect in the public area of and its cause. I have tried the Internet and other virtual social networks, unsuccessfully.

If you think I should not be posting this here, please feel free to tell me and I will look for another source and be sorry for the inconvenience. I just had no more options to decide about the question above.

Roberto Belisário
Campinas - SP
Brazil

 

[GregLocock]

At one frequency, and its harmonics, there may be a dead spot. However only the most boring of music would only have one note and harmonics for any significant amount of time.

Different notes would have dead spots in different places.

So yes it is hopelessly exaggerated.




Cheers

Greg Locock


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[beldiniz]

Thank you, Greg. I hadn't thought of "dead spots" as a keyword for Internet research; it helped me to find a clear answer. I found numerous references in the net about dead spots - points in an auditorium or in a concert hall where the sound volume and quality are sensibly decreased because of destructive interference (the fact that music is a mess of waves does not prevent destructive interference; it just makes it less pronounced). Probably the teacher has heard about that but, in order to provide an example that could be recalled by his students' own experience, he talked about large rock concerts.

 

[GregLocock]

The teacher could very usefully set up 2 large speakers on a fottball pitch and demonstrate the effect with a large crowd of volunteers.

That is a lesson you'd never forget. Video the results as you ask people to stand in the dead (or loud) spots for each frequency.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rob768]

"At one frequency, and its harmonics, there may be a dead spot. However only the most boring of music would only have one note and harmonics for any significant amount of time."

I can imagine some electronically produced bass-tone to be present at a constant frequency, having a dead spot at the same location (which would be where i was standing, if forced to be present