Most sound sources are better than most human hearing anyway

Definitions: (The higher, the better for all)

  • Bit rate is finished file size in kilobits per second and relates to audio quality.
  • Sample rate in kHz – the number of times per second the sound is sampled. 44.1kHz covers 5Hz -22.05kHz.
  • Bit depth – 8-bit (256 levels or 48dB), 16-bit (65,536 levels or 96dB), 24-bit (16,777,216 levels or 144dB), 32-bit or more is simply the granularity of data stored and relates to the dynamic range (dB). We perceive those lower bit depths as more noise. It is usually a simply hiss, but at low signal, levels can produce nasty effects. Those are usually dealt with by adding some dither noise.

Codec rates

MP3 uses a bit rate from 8-320kbps (typically 128kbps or ‘radio quality’) at a sample rate from 8-44.1kHz (generally 22kHz). It allows you to compress large music files to smaller sizes which are lossy (a smaller % sample of the original sound). For example, a typical MP3 produces a file size of 128 kilobits every second (they’re actually larger, thanks to metadata including album covers.)

AAC has a variable bit rate of 8-256kbps (typically 230kbps per channel) and a sample rate of 8-96kHz (almost always 44.1kHz). Widely used by Apple and can be less lossy. But it is only Bluetooth codec that makes uses of psychoacoustic modelling* to transmit data, so it’s a very processing-heavy codec.

CD sound requires 16-bit/44.1kHz (44,100 samples a second) sampling and 1,411kbps data rate. Some audiophiles comment that CD sampling only covers 80% of the original sound information.

DVD and Blu-ray audio are typically 24-bit/96 or 192kHz and cover almost all the original sound information.

By comparison, telephone quality is from 200Hz-3.2khz and uses 8 or 12-bit and a 64-96kbps data rate so you can see voice frequency range is quite limited.

Depending on your content type, the sound quality and frequency response vary. All tests should be at CD quality to be fair to the device.

You can read more and listen to different bit rate clips here.

* Psychoacoustic modelling determines which sound won’t be heard. For example, some sounds within a few milliseconds of louder sounds – even if they come first – won’t be heard. Models are used to determine those; then the encoder abandons them. Conceptually it’s the same for all lossy compression systems: MP3, AAC, WAV and so on. Just some do it better than others.

Then there is the device interface type

Most devices have 3.5mm (cable) audio, RCA, optical Toslink, HDMI, USB, Thunderbolt 3, Bluetooth or Wi-Fi interfaces.

3.5mm (or analogue) audio inputs may make their way in pure analogue format to the speakers, but there’s no guarantee of that. Many devices simply convert analogue to digital for processing, before converting back to analogue for output. In theory, pure analogue should be the best test of the speaker’s capability, but in practice, digital-to-analogue and analogue-to-digital conversion are so good that it’s largely indistinguishable from pure analogue, if competently performed.

A DAC takes the smooth analogy signal, samples it and plays back what it thinks the sound is. The higher the sample rate the more accurate the conversion.

But most music is digital and needs a chip to convert from digital to analogue (called a DAC or digital audio converter). These can vary enormously in quality and low and high filter capabilities.

Let’s remember that MP3 is 128-320kbps, and CD quality is 1,411kbps (1.411Mbps) (Source).

The difference is that MP3 – and other lossy compression systems – toss out the content that the psychoacoustic models judge to be inaudible. The higher the bitrate for a given codec, typically the better quality.

Bluetooth Codecs (typical or maximum rate with 44.1kHz quality)

  • Standard Bluetooth codec (Sub-band coding or SBC) is 127 (mono) to 328kbps (stereo)
  • aptX (mono/stereo) is 128/256/352kbps, aptX LL (low latency) is 352Kbps, aptX HD is 192/384/529Kbps and aptX Adaptive is 276-420kbps
  • Advanced Audio Coding (AAC) is 8-576kbps (stereo) but typically 256/320kbps over BT – good on iPhone (Apple AAC) but not so good on Android that uses the Fraunhofer AAC codec
  • LDAC is variable from 303/606/909kbps

Codecs also suffer latency (lag)

  • SBC: 150-250 ms (typically 175ms)
  • aptX: 130-180 ms (typically 166ms and aptX LL tries to keep this under 50ms)
  • AAC: 190-240 ms
  • LDAC: 160-210 ms
  • Wired <5-7ms

Interface speeds

  • Bluetooth 1/2/3/4/5 is 1/25/25/50Mbps, but the codec will slow it down
  • USB 1/2 is 12/480Mbps, and 3/4 is 5/40Gbps
  • Thunderbolt 3 is 20/40Mbps
  • Optical is 3.072Mbps
  • HDMI and DisplayPort are 36.863Mbps
  • Ethernet up to 1Gbps
  • Wi-Fi ranges from 50Mbps to as high as AX 11Gbps

The bottom line is that you need to test as a cabled device (if possible) to get the native sound signature and then preferably use a high-res BT codec like LDAC and USB 3.0 to get the range of sound signatures.

Now back to where we started – sound signatures

Sorry for the long tome but merely listening to a favourite music track does not cut it – it is plain wrong for all the reasons above.