An audio amplifier's heating, and the energy available at its outputs, depend heavily on the signal it is driven with. A full-power sine wave heats an amplifier far more than music ever would. Driving it with silence hardly loads it at all. Safety standards therefore define a standard test signal that represents real programme material, so that temperature-rise, energy source classification and abnormal operation tests are realistic and repeatable.
Why Pink Noise
White noise has equal energy per hertz, so most of its energy lies at high frequencies. Pink noise has equal energy per octave, which is much closer to the spectrum of music and speech. IEC 62368-1 Annex E, like IEC 60065 before it, specifies pink noise band-limited by a defined filter, giving a standardised, music-like load. A 1 kHz sine wave is also specified for some determinations, such as setting the non-clipped output power.
How the Signal Is Used
In outline, the amplifier is first characterised with the sine signal to establish its maximum non-clipped output power into its rated load. For normal operating conditions it is then driven with the filtered pink noise at one-eighth of that non-clipped output power, the same condition IEC 60065 used, while temperatures are measured. In voltage terms, one-eighth power is about 35% of the non-clipped output voltage. Multi-channel equipment is driven on all channels. The exact conditions depend on the equipment type and the edition of the standard, so always work from the clause that applies to the product.
What the Signal Source Needs
- Pink noise filtered to the Annex E spectrum, with known RMS level and crest factor.
- Low-distortion sine signals, including 1 kHz.
- Enough output level to drive line, microphone and other input sensitivities.
- Several synchronised channels for stereo and surround equipment.
The LISUN LS8280 is a two-channel, PC-controlled generator built for this. It produces pink noise to IEC 62368-1 Annex E.2.1 and IEC 60065, sine signals to Annex E.2.2, and swept sine, bursts, white and narrowband noise. Its channels run independently or synchronised at 0° or 90°. The supplier specifies a frequency range of 1 Hz–70 kHz within ±0.2 dB, with three output ranges up to 5 V for sine and 1.7 V RMS for filtered pink noise.
Frequently Asked Questions
Can I use pink noise from an audio test CD? Only if its spectrum and level match the Annex E definition and you can show that they do. A dedicated, characterised source makes laboratory traceability simpler.
Does IEC 60065 still apply? IEC 62368-1 has replaced IEC 60065 and IEC 60950-1 for audio/video and ICT equipment in most markets, but the test signal concept carried over largely unchanged.