Sound
Unit 5 Summary - part 3
Sounds:
formed by movements in the air
Sound waves:
movements that make sounds.
Frequency:
is how quickly the waves follow each other.
High frequency is high in pitch because the frequency waves are narrow and close together.
Amplitude/intencity/volume/decible:
is how tall the waves are on the graph.
High amplitude sounds are very loud.
Do you know that sound waves and frequency
can be seen and can move objects?
Analogue vs. Digital
Analogue/analog value:
is original/natural sound, it has continual smooth change, the sound doesn't jump from one value to another.
Digital recording:
Computer stores sound in binary, so it converts analogue to digital form of 1s & 0s, and the signals jump from one value to another.
Microphone:
to capture sound and turn a sound into a wave of electricity.
Storing Frequency in a Computer
Digital sound jumps from one value to another, so to have a similar quality to its original analogue, sampling must be taken very often.
Sampling:
is amplitude changes per second. The more often the sample is taken, the better the sound quality.
Hertz:
measurement of sampling rate, abbreviated to Hz.
1 Hz = 1 sample per second
44100 Hz = 44 thousands of samples per second.
High sampling rate:
-
has more digital values
-
more detailed sound is recorded
-
high-frequency sounds are recorded
-
sounds are similar to the original analogue
-
larger file size.
Low sampling rate:
-
has fewer digital values
-
less detailed sound is recorded
-
only lower frequency sounds are recorded
-
doesn't sound similar to the original analogue
-
smaller file size.
Storing Amplitude in a computer
Remember binary lesson?
In 8 bit binary, there are these columns:
If you put 1 in each column, it results in 255.
There's also 00000000 binary number, so altogether, 8 bits can have 256 possible values.
Bit depth/sample resolution:
Computer stores amplitude into bit depth/sample resolution.
So bit depth/sample resolution is a sound values range available.
High quality recording uses 2 bytes (16 bits) to store the amplitude.
Low quality recording uses 1 byte (8 bits)
High bit depth:
-
uses 2 bytes (16 bits) to store the amplitude.
-
2 bytes can make more than 65000 different values.
-
The numbers can be a close match to its original analogue sound.
-
The recording is high quality.
-
the file size is bigger.
Low bit depth:
-
uses 1 byte (8 bits) to store the amplitude.
-
1 byte can make up to 256 different values.
-
The amplitude is rounded up or down to the nearest number, so it doesn't really match to its original analogue sound.
-
Only a narrow range of amplitudes is recorded, so there's a loss of sound quality, that makes the quality low.
-
the file size is smaller.
Extensions:
- Monophonic:
1 channel
- Stereophonic:
2 channels
- Quadrophonic:
4 channels
Stereo and quadrophonic sound is more realistic than monophonic sound.
That's because we hear natural sound coming from different directions around us.
Stereo and quadrophonic sounds reflect this.
The more channels a sound has, the bigger the file size.
Bit rate:
a number of bits stored in a second.
Bit rate depends on:
- sample rate (sound wave changes per second).
- bit depth (sound values range available).
- number of channels (monophonic,stereophonic, quadrophonic, etc).
- duration of the audio.
Multiply all of them to get the bit rate.
CD-quality standard:
has 44100 Hz, 16 bits.
A CD audio has a higher quality than an MP3 file.
Let's calculate a file size of a CD audio.
Example:
Sample rate: 44100 Hz
Sample resolution/bit depth: 16 bits per sample.
Channel: 1
Duration: 60 seconds (1 minute).
44100*16*1 = 705,600 bits per second.
Let's multiply it by the duration of the audio:
705600*60 = 42,336,000 bits per second.
or
42,336,000/8 = 5,292,000 bytes.
or
42,336,000/8000 = 5,292 kB
or
42,336,000/8000000 = 5.292 MB.
Oxford AQA IGCSE 2020
06.1.
Describe the steps that are involved in sampling sound so that it can be stored digitally on a computer. [3 marks]
__________________________________________________________________________________
Answer:
-
Microphone converts sound/soundwave into analogue/ electrical signal;
-
Analogue/electrical signal sampled/ measured at fixed/regular time intervals;
-
Amplitude of signal/wave measured;
-
Then coded into a fixed number of bits/binary.
06.2.
A recording is made of a sound that lasts for 5 seconds using an 8-bit sample resolution and a sample rate of 10 000 Hz Calculate the amount of memory, in bytes, that will be needed to store the entire recording.
Show your working. [2 marks]
__________________________________________________________________________________
Final answer: (bytes) ___________________
Answer:
The formula is sample rate * bit depth * duration.
The final answer is in bytes, so it's useless to multiply 10000 by 8 bits, because you're going to divide it again by 8 to get bytes (10000*8/8).
Multiply directly 10000 by duration:
10000*5 = 50000
06.3.
The same sound is sampled again for 5 seconds using an 8-bit sample resolution, but this time at a sample rate of 20 000 Hz
Describe the exact effect on the amount of memory that is needed to store the entire recording if the sample rate is increased from 10 000 Hz to 20 000 Hz [1 mark]
__________________________________________________________________________________
(It doesn't ask you to calculate, and it only gives 1 mark.)
Alternative answers:
-
Doubles the amount of memory/size, or increases it to 100000 (bytes);
-
Doubles from the answer given in part 6.2.
06.4.
Describe the effect of increasing the sample rate on the quality of the recording. [1 mark]
__________________________________________________________________________________
Answer:
-
Better sound quality.
-
played back sound more closely resembles its original source.
Oxford AQA IGCSE Mock Paper
11.1.
Explain why sound must be sampled before it can be processed by a computer system. [2 marks]
__________________________________________________________________________________
11.2.
An audio signal has been sampled and recorded as a sound file on a computer.
The signal lasted for 30 seconds and was recorded with a sample rate of 8000 Hz and a sample resolution of 16 bits.
Calculate the size of the sampled audio signal in bytes. [2 marks]
__________________________________________________________________________________
11.3.
The signal could have been recorded with a sample rate of 4000 Hz instead of 8000 Hz.
Describe one advantage and one disadvantage of using a sample rate of 4000 Hz instead of 8000 Hz. [2 marks]
Advantage:
__________________________________________________________________________________
Disadvantage:
__________________________________________________________________________________
