| Physics Of Sound | DIY Motor Construction & Invention |

Physics Of Sound:

1) The CD Electronic Organ

The sound we hear is due to vibration of air particles, which is in turn set to motion by other devices or musical instrument. The higher the frequency of vibration, the higher the pitch. To demonstrate this, a disc of alternating white and black sectors round the circle. There are more sectors in the outer rim. As the disc rotates, a light sensor is put near the disc to turn the alternating white and black sectors to vibration of the speaker connected to the sensor. As the light sensor goes from the inner circle to the outer circle, musical tones of increasing pitch can be heard. The tone due to the outermost circle is an octave higher than the tone due to the innermost circle. According to music standard, the frequency of vibration for reference A tone is 440 cycles per second (Hertz or Hz).

2) DIY Flute making and speed of sound

The speed of sound through air depends on temperature, humidity, pressure, carbon dioxide content, and other minor factors. But the biggest factor is the temperature. As a rough estimate,

Vs = 20 * sqrt(T+273)
Vs = speed of sound in m/s
T = temperature of air in degree Celsius

At 25 deg. C, Vs is about 345 m/s
Consequently, we can find the wavelength of sound in air as Vs / F
Where F = frequency of tone in cycles per second (Hz)

Now we get the frequencies of the various tones. There are many different tonality formulae but we shall use the piano's tuning here. First, the frequency of A is 440 Hz. The frequency ratio of each halftone is 2 to the power 1/12, which is about 1.06. The frequency of other tones is calculated accordingly. Details can be found in the following musical temperament page.

We can make a flute knowing that the length of a flute (measure from the mouth to centre of the first open hole, with compensations for both ends) to make a tone is half of its wavelength. That is Vs / F / 2. Due to end effects near the mouth and end of tube, the actual length needed is somewhat shorter. As a rough rule, subtract 3 times the diameter of the tube. The actual calculation is much more complicated and so the figures given below is just an approximation.

f/Hz L/mm
296.7 548
329.6 484
349.2 427
392.0 400
440.0 352
493.9 310
523.2 271

If we just want to make a working flute, we finish here. However, if we want to make a really accurate flute, we need to use a tuner to check the frequency of each note. First we bore smaller holes. Then we check the tones with our tuner. If it is flat, we bore the hole closer to the month and vice versa. Finally, use a big drill to expand all holes to same size. The final diameter of the flute holes is about 10 to 12mm. We can begin with a 6mm hole and expand the holes in the right direction under the guidance of a tuner.


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