Psychoacoustic metrics for ITT equipment -
Part 1 (prominent discrete tones) and Part 2 (models based on human perception)
1st edition (December 2020)
This document describes two procedures for determining whether or not noise emissions contain prominent discrete tones: the tone-to-noise ratio method and the prominence ratio method.
Discrete tones occurring at any frequency within the one-third-octave bands having centre frequencies from 100 Hz to 10 000 Hz can be evaluated by the procedures in this document (i.e., discrete tones between 89,1 Hz and 11 220 Hz) inclusive, referred to the discrete tone frequency range of interest, hereafter).
All of the requirements of the test environment of ECMA-74, 8.3 apply. However, for the purposes of this document, corrections neither for background noise, K1, nor for test environment, K2 apply.
NOTE 1 Since some ITT equipment emit discrete tones in the 16 kHz octave band, the tone-to-noise ratio or the prominence ratio can be computed for these tones in accordance with the procedures in this document in an attempt to quantify their relative levels. However, the prominence criteria in either 11.6 or 12.7 cannot be applied, since there is no supporting psychoacoustical data on such high-frequency discrete tones.
Declaration of product noise emissions in accordance with ECMA-109 offers the option of stating whether there are prominent discrete tones in the noise emissions of a product, as determined by this document. Other standards, or other noise test codes relating to products besides ITT equipment, can also refer to this document for the declaration of prominent discrete tones. For the purposes of such declarations, either the tone-to-noise ratio method or the prominence ratio method may be used, unless otherwise specified in the standard or noise test code.
NOTE 2 The tone-to-noise ratio method can prove to be more accurate for multiple tones in adjacent critical bands, for example when strong harmonics exist. The prominence ratio method can be more effective for multiple tones within the same critical band and is more readily automated to handle such cases.
This standard describes a hearing model and psychoacoustic metrics dependent on the hearing model. The input to the hearing model are sound signals recorded using the procedures of ECMA-74 . The hearing model expresses specific loudness.  Psychoacoustic models use the specific loudness to express the strength of any tonalities or roughness in the sound generated by Information Technology and Telecommunications (ITT) equipment. While developed for ITT equipment, the psychoacoustic methods of this standard may be relevant to other applications like automobiles, consumer appliances, etc.
The tonality metric of this standard uses the auto-correlation function to describe causes of perceived tonality such as individual or multiple steady or time-varying discrete tones, individual or multiple spectrally elevated bands or slopes of noise, and combinations of these phenomena. A similar approach was published in 1998 to determine “pitch salience” .
The roughness metric presented in this standard uses a spectrum of the sound signal envelope, refined by a quadratic fit estimator, to describe roughness arising from sound signal envelope variations within a critical band at modulation rates between 20 and around 300 Hz. For steady sounds, roughness perception peaks at modulation rates of 70 Hz.
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