Re: [Phys-l] sound demo/question

[alex brown <aesbrown77@yahoo.co.uk>]

only if the amplitudes of the waves are the same at the detector will you get completely destructive interference.  The equal amplitude aproximation is valid if the detector is far enough away so that the source seperation is small wrt to the distance
 
Alex 

--- On Thu, 22/4/10, Bob Sciamanda <treborsci@verizon.net> wrote:


From: Bob Sciamanda <treborsci@verizon.net>
Subject: Re: [Phys-l] sound demo/question
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Date: Thursday, 22 April, 2010, 21:23


Perhaps reverberations from furniture, walls, etc are confounding the 
expected wave patterns.  Try it outside in an open field.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

--------------------------------------------------
From: "Anthony Lapinski" <Anthony_Lapinski@pds.org>
Sent: Thursday, April 22, 2010 3:37 PM
To: <tap-l@lists.ncsu.edu>; <phys-l@carnot.physics.buffalo.edu>
Subject: [Phys-l] sound demo/question

> I am puzzled about something. I use two 256-Hz resonant tuning forks and
> an oscilloscope for some sound activities. When I hit both forks, the
> sound appears louder that just one oscillating. The mic picks this up, and
> the scope shows a higher (double) amplitude -- constructive interference..
> This always works. However, shouldn't there be places in between the forks
> that show destructive interference? And why doesn't the scope show this?
> No matter when I hit the forks and no matter where I position the forks,
> the waves always add up constructively. How can this be?
>
> Using the wave equation with v = 343 and f = 256, the wavelength is abut
> 1.34 m. So half of this would be 0.67 m for destructive interference to
> occur (crest meeting trough).
>
> Do I have to move the forks farther apart and not put the mic exactly in
> between the two? Or will the scope always show the amplitudes adding up?
> Or I am I misunderstanding something basic about this situation?
>
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