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Topic: Joe Thunder (Read 2537 times) |
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maryl
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As his name would suggest, Joe was not a meek, quiet fellow. In fact he had such difficulty keeping quiet, both his local and university library banned him. His roommate, being a music major and a sound engineer said he could solve his problem but Joe would need to trust him. He gave Joe his 75-watt ghetto blaster a cd, fake mustache and long blond wig. Joe sceptical but intrigued decided to give it a shot. Joe disguised as one of the professors entered the university library. He placed the CD player on the table in front of him; loaded the cd; cranked the volume full and braced himself as he turned it on. Surprisingly there was only a very very faint sshh. Joe then realized that he couldn't hear anything at all. He spoke normally, and then yelled but no one reacted. Awesome he thought as he began flailing his arms around and shouting as loud as he could. Unfortunately the librarian seeing him moving but hearing nothing assumed he was choking and preformed the Heimlich maneuver on him knocking his wig off; causing the librarian to perform the hind kick maneuver right out the door. Poor Joe.
But what had his roommate done? What was on that mysterious CD that made everything quiet?
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maryl
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Just to let you know, this is more of a scientific puzzle.
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towr
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Re: Joe Thunder
« Reply #2 on: May 19th, 2003, 1:57pm » |
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I'm wondering how the roommate got it on the CD.
Or was it a modified ghettoblaster, and did the CD only have a computer program?
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maryl
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on May 19th, 2003, 1:57pm, towr wrote:I'm wondering how the roommate got it on the CD.
Or was it a modified ghettoblaster, and did the CD only have a computer program? |
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It was recorded onto the CD.
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James Fingas
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Re: Joe Thunder
« Reply #4 on: May 20th, 2003, 11:05am » |
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A very loud noise that made everyone instantly deaf?
For noise-cancelling, there are limitations. Specifically, if you want to cancel sound at n locations, you need n speakers. Most ghetto-blasters only have two. Besides, the locations of the bystanders are not well known, so in addition to needing a computer program rather than a simple recording, you'd also need sensors to determine where everybody is.
You also need detailed information about what sound you're trying to cancel and its distribution. If the sound is picked up by a sensor, the sensor has to be between the source and the cancelling location, and you need a speaker between the sensor and the cancelling location. Otherwise, you're trying to cancel the noise after the hearer has already heard it.
So the answer can't be noise cancelling...
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maryl
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on May 20th, 2003, 11:05am, James Fingas wrote:A very loud noise that made everyone instantly deaf?
For noise-cancelling, there are limitations. Specifically, if you want to cancel sound at n locations, you need n speakers. Most ghetto-blasters only have two. Besides, the locations of the bystanders are not well known, so in addition to needing a computer program rather than a simple recording, you'd also need sensors to determine where everybody is.
You also need detailed information about what sound you're trying to cancel and its distribution. If the sound is picked up by a sensor, the sensor has to be between the source and the cancelling location, and you need a speaker between the sensor and the cancelling location. Otherwise, you're trying to cancel the noise after the hearer has already heard it.
So the answer can't be noise cancelling... |
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This is the answer I had, I guess you would disagree: Most airline pilots could tell you. Joe's roommate used the same technology that airline and fighter pilots have in their noise cancelling headsets. It works like this; the CD had a low frequency sound recorded on it. Something below 20 Hz which humans can't hear. As long as the volume of the low frequency is louder than your voice or other noise you hear almost nothing for the low sounds (which humans can't hear) drown out other sounds. I heard that when this was fist discovered that libraries actually used this concept to create quiet until they realized that even though humans couldn't hear the low sound if loud enough they would still damage the ears.
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aero_guy
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Re: Joe Thunder
« Reply #6 on: May 20th, 2003, 7:44pm » |
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There are several problems with this idea, several of which James has mentioned. Firstly, if this was the case, then why would pilots use it? They would be drowning out all sounds and effectively making themselves deaf. Active noise reduction requires not only n speakers to remove the sound in n locations, but a dynamically variable output that not only adapts to the frequency of the noise, but also to its phase. The timing of the CD must be perfect even if you know what frequency to play. Otherwise you could double the amplitude of the observed noise.
I didn't answer earlier as I figured you were baiting us into this answer.
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James Fingas
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Re: Joe Thunder
« Reply #7 on: Jun 10th, 2003, 10:31am » |
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Maryl,
That's very interesting. I've never heard about that technique before. It certainly isn't active noise cancellation like I was talking about.
It seems to me that this technique would require large amplitudes of sound waves to stop people's perception of noise. Most likely it works by taking advantage of the ear's non-linearity: in the high-pressure regions of the sound wave, the eardrum presses on the hammer and stirrup very strongly, damping out its sensitivity, and in the low-pressure regions, it pulls the hammer away from the stirrup, completely deadening the sound. Certainly sounds hard on your ear's hardware!
As collaborative evidence for this, consider being on a plane that's rapidly climbing. If you don't allow the air pressure to equalize in your inner ear, sounds definitely become quieter as your eardrum is pressed outwards. When you equalize the pressure, your hearing returns to its normal levels.
For the lazy experimenter, pinch your nose, close your mouth, and try to take a strong breath in. You should feel your ears being depressurized, and all sound will be muted. Returning sound to its normal levels is left as an exercise for the reader!
However, with the low-frequency sound, there will always be dead time when the air is at its usual pressure. Reducing the deadtime is possible, but requires a very large signal, so people will always be able to hear somewhat (just quieter).
You could probably also feel the sound waves, and it could make book shelves and/or window panes rattle (though I guess you couldn't hear them very well...).
The only major problem I can see is that a regular boombox is not likely up to the job. It wouldn't have a good enough bass, nor would it have enough power to move the air like that.
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