Psychoacoustic metrics for ITT equipment Part 1 (prominent discrete tones)
ECMA-418-1 specifies in detail methods for determining and reporting the presence of prominent discrete tone in airborne noise emissions of information technology and telecommunications equipment.
The technical contents of the 1st edition of ECMA-418-1 are based on, and mostly identical to ECMA-74, Annex D.
ECMA-418-1 has the following annexes:
- Annex A (normative) Tone-to-noise ratio calculation based on mean-square sound pressure data
- Annex B (normative) Prominence ratio calculation based on mean-square sound pressure data
Tones must be audible
Discrete tones should only be classified as prominent if they are, in fact, audible in the noise emissions of the equipment under test. For the purposes of the screening test, it is assumed that the level of the noise being measured is well above the threshold of hearing.
Discrete tones or tonal components that can be present in the noise emissions are occasionally not audible due to masking by the noise itself or for some other reason (e.g. the tones can be harmonics of a lower fundamental tone and not individually audible).
Therefore, an initial aural examination of the noise emitted from the equipment under test shall be made at the specified microphone position, with the following cases applied.
a) If one or more discrete tones are audible, then the measurement procedures of this document for either the tone-to-noise ratio or prominence ratio, or both, shall be carried out for each audible tone.
b) If no discrete tones are audible in the noise emissions, and there is a high degree of confidence in this conclusion, the procedures of this document may not be carried out and a statement such as “no audible discrete tones” or “no prominent discrete tones” may be included in the test report.
c) If there is doubt as to whether a discrete tone is audible in the noise emissions (e.g. if the test engineer has a hearing loss or is not a trained or experienced listener), then other, more objective evidence should be sought. For this purpose, a preliminary FFT analysis shall be taken of the noise emissions at the specified microphone position(s).
If the spectrum indicates the presence of potentially audible discrete tones or tonal components (i.e., if the spectrum shows one or more sharp spikes), then the measurement procedures of this document for either the tone-to-noise ratio or prominence ratio, or both, shall be carried out for each potentially audible tone.
NOTE The aural examination in cases a) and b) can be bypassed, and the preliminary FFT analysis of case c) used directly as this screening test for the audibility of discrete tones.
Any discrete tone that is determined to be prominent in accordance with the either the tone-to-noise ratio method or the prominence ratio method shall also meet the audibility requirements.
This module calculates the Prominence ratio (Annex B) from an arbitrary audio recording. You can specifiy the area of interest graphically.
Akulap supports 2 modes:
1) interactive: monitor the level of the middle, upper and lower critical band, while moving the mouse over the FFT spectrum
2) display the prominence ratio vs. frequency