Convert source audio (WAV, high-bitrate MP3 from PDF CDs, etc.) to these settings before uploading. Keeps voice clarity while minimising storage cost.
tune Conversion settings
Parameter
Value
Why
Format
MP3
Works on all browsers + mobile/tablet
Sample Rate
22050 Hz
Nyquist 11 kHz preserves s / sh / f sibilants. 16 kHz cuts them → muffled.
Channels
Mono
IELTS is single-voice. Stereo doubles size for no gain.
Bitrate
64 Kbps
At 22 kHz mono, 64 kbps is transparent for speech. 48 kbps starts artefacting.
Sample Size
16 bit
Standard — don't change.
sd_card Expected file size (30-minute test)
Setting
Size
Notes
16 kHz / 80 kbps
~18 MB
Sibilants muffled · current default, not ideal
22 kHz / 64 kbps
~14 MB
Clear voice, lean size
44 kHz / 128 kbps
~29 MB
Overkill — wastes storage/bandwidth
WAV uncompressed
>100 MB
Exceeds 50 MB per-file cap
check_circle~14 MB per test · 100 tests ≈ 1.4 GB ≈ $0.04 / month on Firebase Storage.
warning Avoid these
Stereo — 2× size, zero quality gain for mono voice.
> 128 kbps — human ear can't tell the difference on speech through phone earbuds.
< 48 kbps — AM-radio artefacts, students lose consonants.
WAV / FLAC — uncompressed will blow past the 50 MB cap.
settings_applications AVS Audio Converter — how to apply
1. Click MP3 in the Formats toolbar
2. Set Sample Rate: 22050 Hz, Channels: Mono, Bitrate: 64 Kbps, Sample Size: 16 bit
3. Click Save next to Profile and name it NEXUS IELTS Listening — reuse next time
4. Convert Now! → upload the output .mp3 here
Import existing URL
Register a file that already lives somewhere (Firebase Hosting, CDN, another bucket). Library will index the URL so you can find it later — the file itself stays where it is.