Sound in the real world is a smooth analogue wave. A computer can only store numbers, so it measures the wave thousands of times a second and saves each measurement in binary. Change how often it measures and how precisely, then listen to the difference.
Each sample measures the height (amplitude) of the wave at one moment. It's rounded to the nearest available level, and the level number is stored in binary using the bit depth. These are the samples in the view above.
| Sample | Time | Exact amplitude | Nearest level | Stored in binary |
|---|
File size = sample rate × bit depth × length in seconds (× 2 for stereo). That gives the size in bits. Divide by 8 for bytes.
| Quality | Sample rate | Bit depth | Channels | Size |
|---|
Here 1 KB = 1,000 bytes and 1 MB = 1,000,000 bytes. Formats like MP3 use compression to make files much smaller than this.
Computers can only store numbers, so sound has to be converted from a smooth wave into a list of binary values. These are the key ideas for GCSE Computer Science.
Sound is an analogue wave. A microphone turns it into an electrical signal, and an analogue-to-digital converter (ADC) measures the height (amplitude) of that signal at regular intervals. Each measurement is a sample, stored as a binary number.
The sample rate is how many samples are taken each second, measured in hertz (Hz). CD audio uses 44,100 Hz. A higher sample rate captures more detail, but every extra sample must be stored, so the file gets bigger.
Bit depth (also called sample resolution) is the number of bits used to store each sample. More bits means more levels: 8 bits gives 2⁸ = 256 levels and 16 bits gives 65,536. Each sample is rounded to the nearest level, so more levels means a more accurate recording.
File size in bits = sample rate × bit depth × length in seconds. Multiply by 2 for stereo, then divide by 8 to get bytes.
Sample rate is how often the sound is measured. Bit depth is how precisely each measurement is stored. Raising either one improves quality and increases file size.
Detail is lost, and high-pitched sounds can come out at the wrong pitch. The sample rate needs to be at least double the highest frequency you want to record.
MP3 uses compression to remove data that people are unlikely to notice, so files are much smaller than the uncompressed sizes worked out here.