← ACARA Digital Technologies Years 9–10

Audio bitrate (kbps)

The number of bits used for each second of audio or video, measured in kbps. A lower bitrate gives a smaller file but lower quality, e.g. 128 kbps is smaller than 320 kbps.

Part of: ACARA Digital Technologies Years 9–10 — Data Compression — Module 32

Understanding Audio bitrate (kbps) in ACARA Digital Technologies Years 9–10

What it does: Each second of audio uses a set number of bits, so total size is bits per second times the number of seconds. Multiplying the bitrate (128 kbps) by the length (10 seconds) gives the file size in kilobits. More bits per second keeps more detail in the sound, but it also makes the file bigger. A lower bitrate saves space, which is helpful when storage is limited. Quality is lower than 320 kbps.

Common mistakes: File size depends on bitrate multiplied by length, so the line should use * (128 * seconds), not +. 128 kbps is less than 320 kbps, so the lower bitrate is the one that gives the smaller file. The test should be low < high.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 155–225) and word search (Modules 116–225).

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Learning
Duo
What does a bitrate of 128 kbps tell you about an audio file?
128 thousand bits are used for each second of audioThe file is 128 kilobytes in total
Matches
A 320 kbps song is smaller than a 128 kbps song
↓
This is backwards. A higher bitrate uses more bits per second, so the 320 kbps file is larger, not smaller.
Applying
Meanings
Term
bitrate
Meaning
The number of bits used for each second of audio or video, measured in kbps. A lower bitrate gives a smaller file but lower quality, e.g. 128 kbps is smaller than 320 kbps.
Practice
lowBitrate ← 128
Testing
Gaps
= 128
Check Answer
Spots
1seconds = 200
2size_kbits = 128 + seconds

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Quick Questions

1. What does a bitrate of 128 kbps tell you about an audio file?
A. The file is 128 kilobytes in total
B. 128 thousand bits are used for each second of audio ✓
Bitrate is measured per second of audio. kbps means kilobits per second, not the total file size.
2. Which audio file will usually be smaller if the songs are the same length?
A. The 128 kbps file ✓
B. The 320 kbps file
A lower bitrate uses fewer bits each second, so the file is smaller.
3. Which statement about a 320 kbps file compared with a 128 kbps file is correct?
A. It usually has higher quality but a larger file size ✓
B. It usually has lower quality but a larger file size
More bits per second keeps more detail in the sound, but it also makes the file bigger.
4. Which formula estimates file size from bitrate?
A. file size = bitrate + length in seconds
B. file size = bitrate x length in seconds ✓
Bits used per second multiplied by the number of seconds gives the total bits in the file.
5. A song is 100 seconds long at 128 kbps. How many kilobits is the file?
A. 12,800 kilobits ✓
B. 228 kilobits
128 x 100 = 12,800 kilobits. You multiply bitrate by length, you do not add them.

Match the Pairs

bitrate = 128 kbps → A lower bitrate: about 128,000 bits per second, giving a smaller file with lower audio quality
bitrate = 320 kbps → A higher bitrate: about 320,000 bits per second, giving better audio quality but a larger file
File size in bits is the bitrate multiplied by the length of the audio in seconds → size_bits = bitrate * seconds
A 10 second clip at 128 kbps uses 1,280 kilobits (128 x 10) → 128 * 10 = 1280
A 320 kbps song is smaller than a 128 kbps song → This is backwards. A higher bitrate uses more bits per second, so the 320 kbps file is larger, not smaller.
size = bitrate + seconds → File size is found by multiplying, not adding. The formula should be size = bitrate * seconds.

Key Term

bitrate
The number of bits used for each second of audio or video, measured in kbps. A lower bitrate gives a smaller file but lower quality, e.g. 128 kbps is smaller than 320 kbps.

Fill in the Blanks

1. bitrate = 128 print(bitrate, "kbps")
Hint: Type the variable name for the number of bits used for each second of audio.
2. size = bitrate * seconds
Hint: Type the operator that multiplies the bitrate by the audio length to give the file size.
3. print(128 < 320)
Hint: Type the comparison operator that is true when the first bitrate is lower than the second.
4. low = 128 high = 320 print(high > low)
Hint: Type the variable name for the bigger bitrate, which is used in the last line.
5. bitrate = 128 seconds = 180 size = bitrate * seconds
Hint: Type the variable that holds the audio length, defined on the line above.

Spot the Error

Question 1
A seconds = 200
B size_kbits = 128 + seconds
C print(size_kbits)
Line B — File size depends on bitrate multiplied by length, so the line should use * (128 * seconds), not +.
Question 2
A low = 128
B high = 320
C if low > high:
D print('smaller file')
Line C — 128 kbps is less than 320 kbps, so the lower bitrate is the one that gives the smaller file. The test should be low < high.
Question 3
A kbps = 320
B seconds = 60
C kbits = kbps * seconds
D kbytes = kbits * 8
E print(kbytes)
Line D — There are 8 bits in a byte, so converting kilobits to kilobytes means dividing by 8, not multiplying: kbits / 8.
Question 4
A song_secs = 180
B kbits = 192 * song_sec
C print(kbits)
Line B — The variable is called song_secs. Writing song_sec uses a name that was never created, so the program fails.
Question 5
A bitrate = 128
B seconds = 240
C kbits = bitrate * seconds
D mbytes = kbits / 1000
E print(mbytes)
Line D — Bitrate gives kilobits. They must be divided by 8 to get kilobytes before dividing by 1000 for megabytes: kbits / 8 / 1000.

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