1 Fundamentals Of Sound


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1 Fundamentals Of Sound

  1. 1. Fundamentals of Sound Introduction In this experiment I learnt more about what sound is, what it looks like and how it is measured. I discovered that sound is vibrations in the air; they go through the air as sound waves. By using the signal generator and an oscilloscope I could see these sound waves and learn how to measure them. We learnt about amplitude, cycles, periods and frequency of sound waves as well as phasing and noise and how this can affect the sound we hear. Experiment Sine Waves The sine wave can be used to make any sound. It is a pure tone so the waveform is very basic as you can see below: http://library.thinkquest.org/06aug/02101/images/Waveforms.png A sine wave can be created using a signal generator. By connecting the signal generator to the mixing desk I could listen to the sound of a sine wave. By splitting the signal from the signal generator, one side to the mixing desk and one side to the oscilloscope, I was able to see a graphic display of the wave form. Adjusting the Volts/Div dial and the Time/Div dial allowed me to create a clear looking sine wave. Signal generator Oscilloscope http://www.indiamart.com/crownelectronicsystem/pcat- http://www.testequipmentconnection.com/images/prod gifs/products-small/digital-function-generator.jpg ucts/BK_PRECISION_5105B.JPG Cycles A cycle is the section of the waveform that repeats itself. The waveform pictured below has 5 cycles. 1 cycle http://ee.stlcc.info/131/oscope.gif Amplitude The amplitude is how loud the signal is. When I turned up the volume of the sine wave the waveform on the oscilloscope increased in height. Therefore the taller the waveform is, the bigger the amplitude of the wave and the louder the sound.
  2. 2. The display screen on the oscilloscope has grid markings on, which are called divisions. Once positioning the waveform in the middle of the display screen I was able to count the divisions to find out measurements of the sine wave. To find out the amplitude of the waveform I counted the divisions from the centre line to the point where the waveform is at its peak. Peak Amplitude Centre Line Trough http://ee.stlcc.info/131/oscope.gif My waveform came to 1.1 divisions. This number then has to be multiplied by the value that appears on the Volts/Div dial. This was 5 so the amplitude of my waveform was 5.5 volts. Frequency Frequency is similar to the pitch of sound. It is a measurement of how high or low a sound is. To measure the frequency of the sine wave I have created, I first needed to measure the period of the sound. The period is the time it takes to do one cycle (from the centre point of the wave past the peak and trough and back to the next centre point) Period Centre Line http://ee.stlcc.info/131/oscope.gif To measure the period I had to count the divisions again, but this time horizontally between two adjacent centre points. The period of the sine wave was 5 divisions. To get the measurement this had to be multiplied by the reading on the Time/Div dial. This was 1 millisecond so the period is 5 milliseconds. The frequency of a wave is the number of cycles in a second. The frequency can be worked out by doing 1 divided by the period in seconds. This expression is often written as ƒ = 1/T (T being the period). The period of my sine wave was 5 milliseconds so in seconds this would be 0.005 seconds. 1 / 0.005 = 200. This number is measure in hertz so the frequency of the sine wave i created was 200Hz. This is correct as the frequency is displayed on the signal generator. Phase By activating the dual button on the oscilloscope I was able to have two sign waves displayed. This allowed me to experiment with phase. Phasing is when: “Two travelling waves which exist in the same medium will interfere with each other. If their amplitudes add, the interference is said to be constructive interference, and destructive interference if they are "out of phase" and subtract.” [1] Once I had the two waves in phase I could move them out of phase by changing the EQ of each sine wave via the mixing desk. The difference between two waveforms is known as phase shift and this is measured in degrees. 0 ° are when the two waves are in phase, 180° is when they are out of phase and 360° is when they are back in phase again. Phase cannot be heard but it is important not to have waves phasing as when adding two waveforms it can cause problems.
  3. 3. By generating a sine wave in Audacity it became clear what happens when two sine waves are added together. These two sign waves are 180° out of When the two waves are combined phase. they cancel each other out. Due to this the two frequencies go in These two sign waves are both at and out of phase repeatedly when different frequencies. combined Noise By turning off the signal generator I created a jagged looking waveform like below: www.neighbournoise.com/images/oscilloscope.jpg This is called noise but the graphic on the screen wasn’t just noise. There was a waveform underneath with regular cycles just like the sine waves. After measuring the frequency of this wave in the same way I had done previously I found the wave was at the frequency of 50Hz. “Mains electricity is an A.C. supply. In the UK it has a frequency of 50 cycles per second (50 hertz)” [2] After research on the internet I found that this wave was actually the frequency of mains electricity and although we could not hear it, it was still appearing on the oscilloscope. Conclusion Learning about the fundamentals of sound is very important if I wish to work in the music industry. Being a sound engineer or producer it is important to know about how sound works when recording sound in the studio. Mix downs in the studio could be of poor quality if you don’t know what sound is and how it is affected. An example could be phasing. If parts of your recordings are out of phase destructive interference may occur and the mix will be ruined.
  4. 4. References 1. Hyper physics. Interference of Sound [Online] Available at: http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/interf.html [Accessed 30th November 2008]. 2. Antonine Education. What is mains electricity and how can it be used safely? (Topic 7). [Online] Available at: http://www.antonine-education.co.uk/physics_gcse/Unit_2/Topic_7/topic__7.htm [Accessed 30th November 2008]. Rossing, Moore & Wheeler (2002) The Science of sound third edition, San Francisco, Addison Wesley, p.46 Sciencescope, Frequency of a wave [Online] Available at: http://www.sciencescope.co.uk/frequency_of_a_wave.htm [Accessed 30th November 2008] Hyper text book, Periodic Waves [Online] Available at: http://hypertextbook.com/physics/waves/periodic/ [Accessed 30th November 2008]