SPX (Speex Audio)
A speech-first audio format built for clear voice over slow and unreliable networks.
| Full name | Speex Audio |
| Extension | .spx |
| MIME type | audio/x-speex |
| Developer | Jean-Marc Valin / Xiph.Org Foundation |
| Released | 2002 |
| Type | Audio (speech codec) |
| Container | Ogg |
| Bitrate range | 2.15 kbps to 44 kbps |
What is a SPX file?
SPX is the file extension for audio encoded with the Speex codec and stored inside an Ogg container. Speex was designed specifically for human speech, not music, making it unusually efficient at compressing voice recordings. It was one of the first free and open-source speech codecs suited for internet voice communication.
Speex uses a technique called Code-Excited Linear Prediction (CELP) to model the human vocal tract and compress spoken audio at very low bitrates. Unlike general-purpose codecs such as MP3, it makes no attempt to handle music well — its entire design focuses on voice clarity. An SPX file pairs the Speex codec with the Ogg container format, the same container used by Vorbis and Opus audio. The format supports narrowband (8 kHz), wideband (16 kHz), and ultra-wideband (32 kHz) audio, so it can target anything from telephone-quality calls to higher-fidelity voice recordings.
History
Jean-Marc Valin started the Speex project on February 13, 2002, under the Xiph.Org Foundation, the nonprofit behind Ogg, Vorbis, and other open multimedia formats. Version 1.0 was announced on March 24, 2003, after roughly a year of development, and the codec was released under a BSD-style license to ensure it remained patent-free. Since 2012, the Speex developers themselves have considered it obsolete, recommending the newer Opus codec, which outperforms Speex across all use cases.
How it works
An SPX file is an Ogg bitstream where the logical bitstream contains Speex-encoded audio packets. The first Ogg page holds the Speex header, which declares the version, mode (narrowband, wideband, or ultra-wideband), sample rate, and channel count. Subsequent pages carry encoded audio frames, each frame typically representing 20 milliseconds of speech. The format supports variable bitrate (VBR) and discontinuous transmission (DTX), which stops sending packets during silence to save bandwidth.
What it is used for
- Archiving VoIP call recordings at small file sizes
- Storing voice memos and dictation where music quality is not needed
- Embedding voice audio in older VoIP applications that use Speex natively
- Converting legacy SPX recordings to modern formats like MP3 or Opus
How to open it
Most modern media players, including VLC, can play SPX files directly. If your player does not support it, converting the file to MP3, AAC, or Opus with an online converter will give you a file that plays anywhere.
Pros and cons
Strengths
- Very small file sizes for voice recordings at bitrates as low as 2.15 kbps
- Completely free and patent-unencumbered under a BSD-style license
- Supports multiple quality levels in a single bitstream via embedded coding
- Includes voice activity detection to skip silence and shrink files further
Trade-offs
- Designed only for speech — music sounds poor
- Considered obsolete since 2012; Opus is the recommended replacement
- Limited native support in modern browsers and mobile operating systems
- The .spx extension is not widely recognized outside VoIP and open-source software
Convert SPX files
Free, in your browser, no signup. Start at the SPX converter, or jump straight to a popular conversion below.
Curious how fast and how small? See our measured conversion benchmarks.
SPX FAQ
What is the difference between SPX and MP3?
MP3 is a general-purpose audio format that handles music and voice. SPX encodes only speech using a vocal-tract model, so it compresses voice far more efficiently than MP3 but sounds bad with music.
Is Speex still used in 2026?
Rarely. The Speex team declared it obsolete in 2012 and recommends Opus instead. You are most likely to encounter SPX files in old VoIP archives or legacy communication systems.
Can I convert an SPX file to MP3?
Yes. An online converter decodes the Speex audio and re-encodes it to MP3, which plays on any device. Quality will be equivalent to the original speech recording.
Why does my phone or browser refuse to play an SPX file?
Neither iOS, Android, nor major browsers ship with a built-in Speex decoder. You need a player like VLC or you need to convert the file to a widely supported format first.