SOUND FLIPPER GUIDES
MP3 bitrate: CBR, VBR and actual file size
Understand MP3 bitrate modes, Sound Flipper's LAME settings and measured output size without assuming a bigger number guarantees quality.
By Singularity Shift Ltd · Updated · Preview guide
What bitrate measures
Bitrate describes how much encoded data is used per second. It is different from sample rate, which counts samples per second. A 44.1 kHz MP3 may be encoded at 128, 192 or 320 kbps; those figures answer different questions.
For a nominal constant bitrate, estimate audio payload as seconds multiplied by bits per second divided by eight. Three minutes at 320,000 bits per second is about 7,200,000 bytes before headers, tags and frame rounding. This is a calculation, not a promised output size.
CBR and VBR
Constant bitrate, or CBR, keeps a nominal stream bitrate. Variable bitrate, or VBR, lets the encoder choose different rates for different passages under its quality policy. A quiet interval and a complicated transient need not consume the same number of bits in VBR. The finished size is therefore more dependent on the audio.
Neither name alone identifies the encoder, algorithm quality or source quality. Comparing two files fairly requires those details as well as the bitrate mode. If a receiving service requires CBR, follow that requirement; if it accepts the original source, conversion may be unnecessary.
What this preview actually exports
Sound Flipper's qualified MP3 path uses LAME CBR with algorithm quality 0 and up to 320 kbps. The original sample rate is retained, so lower-rate sources have lower permitted MP3 bitrates. The resolved setting is shown before conversion. The preview does not currently offer a VBR MP3 control; the explanation here is not a claim that one is enabled.
For the retained 2.5-second, 16 kHz stereo speech fixture, the permitted CBR target is 160 kbps. The payload estimate is 50,000 bytes; the actual downloaded MP3 is 52,580 bytes. Headers, encoder delay/padding and whole MP3 frames explain why a simple bitrate calculation is not an exact file-size promise.
A higher bitrate cannot fix an old MP3
Re-encoding a 96 kbps MP3 at 320 kbps increases the data budget for the new encoding, but cannot reconstruct information absent from its input. Use the best source available, and export once for the destination. In the current conversion path, same-codec copying is a separate operation from encoding; check the displayed action rather than assuming a new filename means improved audio.
For encoder details, see LAME's own option documentation. Try WAV to MP3 for a lossless source, or read reducing file size for the other variables that matter.