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Sound is generated by movement of air particles caused by an object vibrating. The moving air particles generate a longitudinal wave (you might have studied this in Physics) with areas of high pressure (compression) and low pressure (rarefaction). These changes in sound pressure are detected by structures in the ear which convert them into nerve signals which are interpreted by the brain as 'sound'.
Task 1.1 Ruler music Where we use a vibrating ruler to generate analogue sound
You would normally get told off for this, but I'm asking you to get some rulers and have a go at playing a song! Work in groups using 5 rulers each and try to create a 'keyboard' to play the first part of 'Baa, Baa, Black Sheep'. Answer the following questions in your notebooks (in full sentences of course).
Sound waves are described in terms of their frequency or pitch which is measured in cycles per second or hertz. The higher the frequency, the higher the pitch of the note we hear. We also sense the loudness of sound - the height or amplitude of the wave. The higher the amplitude, the louder the sound. Look carefully at the following pictures ...
Real sound do not look as simple as this when represented as a wave. A very small portion of a real sound might look like this.
Where we use a sound editor to 'inspect' a digital sound wave
Download and open the drumbeats.mp3 file with Audacity sound editor. Play the file, zoom in on the file and inspect the structure of the sound wave. Can you find loud and quiet parts? Can you find parts with high and low pitch? Write about what you have found out in your notebooks, on paper or using a word processed document. Add an image of part of the sound wave to illustrate your work.
To store an analogue sound wave on a computer, it needs to be converted to digital data (think binary). The structure of the analogue wave is stored by measuring the height of the sound wave at regular intervals and storing it's height as a binary number.
Task 3.1 Sampling exercise Where we perform manual sampling of an analogue sound
Either get a copy of the Sound Sampling Exercise worksheet from your teacher or print yourself a copy. This is quite complex stuff, so your teacher will need to show you what to do. Pay attention!
When you have done all the exercises you can, answer the following questions.
Task 3.2 Videos Where you watch two videos and make some excellent notes
Get your headphones on, watch the videos and take some notes in your notebooks / on paper.
What is Sample Rate? : iZotope Pro Audio Essentials (3:29)
Music Technology 101 : Sampling Rate and Bit Depth Explained (9:23) Task 3.3 Inspect sound files Where we listen and evaluate the quality of sounds
The videos discussed the effect of altering the sampling rate and sampling resolution (bit depth) of a sound file. Get some headphones on and then double click each sound file in turn and 'listen' to the quality of the sound. Inspect the properties of each file in turn and make a note of it's file size and the bit rate (kbps). We'll learn about bit rate shortly. Write down your findings in your notebooks. You may wish to use a table (hint). Task 3.4 Complete the sentences Where we learn how sample rate affects file size and quality
Complete the following sentences but choosing the correct word from the pair. When you write your answer, make sure you underline the word you have chosen so it's easy for me to check!
As you should have seen, the size of sound file is related to the both the sampling rate and the sampling resolution. To calculate the file size of a sound file, use the following (super easy) formula ...
Wow - how many terms are there in that formula? Task 4.1 Uncompressed file size Where we learn to calculate the uncompressed file size of a sound file Calculate the approximate, uncompressed file size in the most appropriate unit, for the following sound files (notice the capital 'B' which stands for 'bytes', not 'bits'). Be careful - your answer to the initial calculation will be in bits.
Since songs tend to vary in length, often bitrate is used to compare the quality of sound files, especially in streaming music services.
Bit rate is given by the following, slightly simpler, formula ... Notice that there is no metadata included in this calculation because the file is streamed and does not have the same format as a physical file. The bitrate is normally quoted in kilobits per second (kbps). Notice the lower case 'b' which stands for 'bits'. To convert bits per second (bps) to kilobits per second (kbps), simply divide the answer by 1024. Task 5.1 Calculating bitrate Where we learn to calculate the bitrate of a sound file Calculate the bit rate in kbps for the following sound files.
Click to load key word list to help you make your own flash cards
Tone generator MIDI Extension MIDI stands for 'Musical Instrument Digital Interface' and is a way of creating pure digital sounds using a keyboard or other compatible musical instrument. Download and install a copy of MidiEditor, download the 'chopsticks.mid' file from the lesson resources and do a little experimentation. If you hear no sound from MidiEditor, make sure you choose 'Midi > Settings' and then select 'Microsoft GS Wavetable Synth' from the 'Midi I/O' section. |