How it works: Dancing Lasers

How to observe the "acoustic activity" on the surface of sound and give an audio visual explanation of a stated law in physics. "The higher the frequency the shorter the wavelength"
 

Requirements.

This set-up is similar to method (1) discussed on another page. However the set-up  parameters are more refined.

First, the mirror used must be a surface coated mirror as opposed to the normal (silver back lined ) type. This is because the light has a clearer focus as no light is absorbed or reflected through a front glass piece as on a standard mirror.

So a front surfaced plane mirror is used, a plane mirror reflects light through  a 45 degree  angle. 

Video of slow motion laser

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How a speaker draws a piece of music

The properties of the laser should be known in advance. For better accuracy the diameter of the beam must be known as lasers have been known to "stray", so beam divergence should  be accounted for. The distance from the light source (laser) to the mirror must be accounted for and the distance from the reflected laser beam to the screen or wall should be known, as this can affect the focus and size of the projection. 

 

How a speaker draws a piece of music
Example
A sine square oscillator is used as an audio source.
This allows the user to input a specific frequency at a known volume.

The oscillator is connected to a set of loudspeakers via an amplifier, with the audio levels pre-set.

 

When a low frequency is generated by the signal generator (sine square oscillator ) a broad pattern emerges, as the mirror moves in and out to the diaphragm movement of the speaker, (it moves in step).

The dot from the laser is "stretched" across the screen and depending on the frequency used, it looks a bit like a quivering figure of eight. This shows that low frequencies have long wavelengths. The opposite happens at high frequencies, the laser-dot barely moves as higher frequencies have shorter wavelengths. 

Volume also plays a major part in this demonstration. As you're probably aware, when you increase the volume on your hi-fi, the speaker cone [diaphragm] moves in and out with more force. There is a relationship between the volume and the frequency.

This can be shown by using a higher frequency than previously used. Keep the volume level constant and turn up the frequency, as the frequency gets higher the pattern will start to shrink and the projection will slowly "turn" back into a laser-dot. Just below the point where the pattern resumes its dot like shape, leave the signal generator at this frequency and increase the volume. the pattern starts to get "stretched" again and a "similar" figure of eight appears.


Laser - Light Amplification by Stimulated Emission of Radiation.

A LASER is a device in which a subtitle material is pumped with energy. When this energy has doubled the material atoms release the absorbed energy by a small chain reaction in which the energy turns to light with its rays parallel to each other and in step. In other words an intense beam of coherent light.

HeNe Laser