Sound: What the heck is it?
Properties, Waves, Cycles, Frequency, Period, Velocity, & Wavelength
prepared for participants of AudioSoup @ CFRC 31 Jan & 01 Feb 2004

Sound: A physical definition could describe sound as a wave motion propagated in an elastic medium, travelling in both transverse and longitudinal directions, producing an auditory sensation, by the change of pressure at the ear.

Some other thoughts about sound might touch on its evernascence (always being born) and evanescence (vanishing away), the differences between visual and oral/aural culture; and how power, territory, property, ownership, and dominance could be expressed in each.

With sound and in hearing, one stands at the centre of the universe, and gauges each phenomenon in relation to oneself - subjectification

With light and seeing, one stands a some distance from the object perceived - objectification.

Visually based words - focus, point, parallel, bottom line, &c.
Aural based words - resonance, tenor, tone, &c

For all the visual emphasis in our culture, we still highly prize the meanings embodied in words depicting the oral/aural such as "hearing" and "audit" to pursue truths.

Wave: A disturbance travelling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself. A sine wave, is the simplest form of this motion. The image (Figure 01) below shows the vertical movement of a point travelling around a circle over time transposed to a graph where the y axis (the vertical) represents the vertical movement and the x axis (the horizontal) represents the passage of time.


Figure 01: a sine wave -

Cycle: A complete vibration. For a sine wave (see Figure 02), it is the complete event starting with a rise from zero energy to a maximum amplitude, its return to zero, the rise to a maximum in the opposite direction, and then its return to zero.


Figure 02


Frequency: The number of cycles of vibration in a given unit of time. The number of cycles in a second is one Hertz (Hz), after the German physicist. Therefore 1Hz equals one cycle per second.

Period: the time it takes to complete one cycle. (see Figure 03)


Figure 03

Wavelength: The distance sound travels during one period, regardless of frequency.

Speed of Sound: at 21C (70F), 344 meters per second, 1,129 ft per second, 1,233kph, 770mph. At freezing, the numbers are 331 m/s or 1087 ft/s. The Speed of sound in water is 1480 m/s or 4856 ft/s. More than 3,000 miles per hour.

Wavelength, Frequency & Speed of Sound
Wavelength x Frequency = Speed of Sound, or,
Wavelength = Speed of Sound / Frequency, and
Frequency = Speed of Sound / Wavelength

As frequency increases (becomes higher), the wavelength becomes longer.
As frequency decrease (becomes lower), the wavelength becomes shorter.

 

A chart of a few selected frequencies and their correlative wavelengths
 Frequency
    in Hertz
Wavelength
in feet and inches or metres and centimetres
20Hz
56.5ft
17.22m
50Hz
22.6ft
6.89m
100Hz
11.3ft
3.44m
400Hz
2.83ft
0.86m
1,000Hz
1.13ft
0.34m
5,000Hz
2.71in
6.89cm
10,000Hz
1.36in
3.44cm
20,000Hz
0.68in
1.72cm

Audible frequency range: 15Hz to 20,000Hz
Carrier frequency of CKWS Kingston: 960,000Hz
Carrier frequency of CFRC Queen's 101,900,000Hz


Heinrich Rudolph Hertz (1857-1894)

 

Adding Waves
Let's start with a 100Hz sine wave


Then let's add a 200Hz and a 400Hz sine wave.

What we get is a combination of 100Hz, 200Hz & 400Hz with the resultant wave form shown as the highlighted line

If we were to run this wave through a spectrum analyser we might see something like this.

Related Links or Links of potential interest

Pitch and Frequency
http://www.radiosite.ca/training/articles/pitch_n_freq.html

Understanding Comb Filtering http://www.radiosite.ca/training/articles/UnderstandingCombFiltering.html

Wiring: Balanced to Balanced Explained
http://www.radiosite.ca/images/wiring_balanced_to_balanced_lpb.gif

Wiring: Unbalanced to Balanced Explained
http://www.radiosite.ca/images/wiring_unbalanced_to_balanced_lpb.gif

How to make a patch cable to use a professional microphone with a minidisc
http://www.trentu.ca/trentradio/audiosoup/as_cfrc040131/mic2minidisc.htm

Principles of Digital Audio is an excellent primer on digital audio and other computer audio related topics, from the Center for Electronic and Computer Music (CECM) at Indiana University. There's also a link here for all CECM documents.

Restoring old Records from someone who loves old LPs and want's to share restoration and preservation experience.

The Project Studio Handbook sponsored by StudioForums.com is a great stating point for all matters audio.
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