Electroacoustics

This section turns to devices that are supposed to make sound: amplifiers, loudspeakers and microphones, and the broadcast programme they end up carrying.

Electroacoustics covers the IEC 60268-3 distortion set (THD, THD+N and SINAD, intermodulation and DIM) and the H1/H2 frequency-response estimators with coherence. The two device families have their own type-test pages: Loudspeaker Characterisation (IEC 60268-5) with the radiating-piston model behind a loudspeaker’s directivity, and Microphone Characterisation (IEC 60268-4) with the polar patterns and the inherent-noise conventions, both ending in a one-page report laid out the way an accredited laboratory lays one out, with the rated-characteristics table beside the response, polar and noise panels drawn to the IEC 60263 scale conventions. What makes such a sheet a test report rather than a design study is the measured input data and the declared standard, and both of those you supply. The loudspeaker page then puts the characterised device back in a room with an open microphone, where the question stops being a datasheet number and becomes whether the loop is stable: Long’s gain-before-feedback criterion, the correction for the number of open microphones and the 10 dB margin an equalised system is designed to are computed there. If you are designing or diagnosing a sound-reinforcement system rather than testing a loudspeaker, that is the section to go to.

The three device pages divide by what is under test, and therefore by what the bench looks like. An amplifier works into its rated load impedance from its rated supply and source impedance, and IEC 60268-3’s standard measuring conditions (clause 3.1.3) then drop the source e.m.f. 10 dB below the rated value, so a distortion figure is meaningless without saying which of the two it was taken at. A loudspeaker is measured on its reference axis at a stated distance in a free or half-space free field, with its mounting declared. A microphone is measured against a calibrated reference. The two type-test pages start from measured curves rather than telling you how to acquire them, so the conditions under which those curves were taken travel with every number in the resulting fiche and belong in it. Swept-sine distortion and phase utilities extends the bench with the one-sweep alternative: the Farina / Novak harmonic separation that turns a single exponential sweep into the full set of harmonic frequency responses and a THD measured as a function of the excitation frequency, plus the minimum-phase, group-delay and excess-phase utilities that dissect what the measured response’s phase is made of.

Programme loudness covers the signal the devices carry: the ITU-R BS.1770-5 loudness of a broadcast or streaming programme in LUFS, the EBU R 128 normalisation to -23 LUFS, the EBU Mode momentary/short-term/integrated meters, the loudness range and the oversampled true-peak level in dBTP.

If you are here to bench-test audio gear, go straight to Electroacoustics; if you are here to level a programme, go to Programme loudness. To measure what a machine emits rather than what a transducer reproduces, the Sound power and intensity pages are the place to start.

Pages in this section

• Electroacoustics: distortion and frequency response: the IEC 60268-3 distortion metrics and frequency-response estimation with coherence.
• Loudspeaker Characterisation (IEC 60268-5): the sensitivity conventions, the radiating piston and the IEC 60268-5 characteristics fiche.
• Microphone Characterisation (IEC 60268-4): the sensitivity references, directional patterns and inherent noise of the IEC 60268-4 fiche.
• Swept-sine distortion and phase utilities: harmonic separation and THD(f) from one exponential sweep (Farina / Novak synchronized swept-sine), and minimum phase, group delay and excess phase from a measured response.
• Broadcast: the loudness problem solved with a measurement rather than a compressor, one gated number per programme and the range that says how much it moves.
• Programme loudness and true peak: the ITU-R BS.1770-5 programme loudness and true-peak level with the EBU R 128 normalisation practice, EBU Mode metering and loudness range.

What this section does not cover

• The type-test pages reduce and report what a laboratory measured; they do not acquire it. The free-field response, the polar cuts, the noise spectrum and the distortion-against-level sweep come in as data, and no procedure here tells you how to run the anechoic room or the substitution measurement that produced them. Two implemented editions are pinned rather than current: the distortion metrics follow AES17-2015 and not the 2020 revision, and the microphone report follows IEC 60268-4:2014 and not the 2018 one. Thiele-Small parameter extraction from an impedance curve is not implemented, and the electrical and mechanical power-handling ratings of IEC 60268-5 clauses 17 and 18 are stated by the manufacturer rather than computed. The feedback criterion is level bookkeeping, not an acoustic model: it consumes two direct-field levels you supply, does not compute them from a coverage pattern, and predicts neither the ring frequency nor the effect of an equaliser or frequency shifter. And IEC 60268-16, the speech transmission index, is not part of this section at all — it is in Speech.