Learning Objectives Be able to explain how sound can be sampled and stored in digital form. Understand how sampling rate affects digital audio quality and file size.

The Sciencey Bit: Sound as a wave Sound comes from vibrations. These vibrations create sound waves which move through air* before reaching our ears. Sound waves need a material (liquid, solid or gas) but not a vaccum *or other stuff

Recording of Sound Waves 1928: Magnetic Tape 1877: Phonograph (Record Player) These are examples of analogue recording of sound waves. Can you guess when they were invented? (from props) Why was tape better than records? (small, portable)

Digital Sampling of a Sound Wave Computers (and CD players) are digital. They can only store binary numbers. 1982: Compact Disc 1 2001: Apple iPod

Digital Sampling of a Sound Wave Sound is stored in the computer by taking samples of the wave at regular intervals and storing them as binary numbers. The sample rate is the number of samples taken each second (measured in Hertz). Diagram is approximately 60 samples (so 60Hz if this is one second of audio)

Recreating the Sound Wave The computer converts the stored binary numbers back into a wave for playback, by joining the dots! What happens if there isn’t very much sample data?

Sample Rate and Sound Quality It is usual to take many thousand samples for each second of sound. Compare phone quality with CD quality. What is hold music like? Telephone calls are sampled at 8000 Hertz. CD-quality music is sampled at 44100 Hertz.

Sample Rate and Sound Quality Can you identify these people and songs from clips sampled at just 1000 Hertz?

Comparing Sample Rates: Audacity Task Change the Sample Rate here.

Comparing Sample Rates: Audacity Task 1. Record a short clip in Audacity 2. Export as WAV (note the file size) 3. Change the sample rate to 8000Hz 4. Export as WAV (note the file size) What do you expect the difference to be? WHY? What WAS the difference?