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German IT-Equipment noise categorization concept: Measurements and final approach

Gregor Feneberg, Otto Martner

Mueller-BBM GmbH Robert-Koch-Straße 11

82152 Planegg / Germany [email protected]

[email protected]

Abstract An investigation project tendered by the German Federal Institute for Occupational Safety and Health (BAuA), with a planned duration of 1½ years, has been awarded to Müller-BBM. The existing concept of the quality classification of household appliances, where the energy efficiency categories A – F each represent a determined range of power consumption, is to be transferred to a system for the categorization or labeling of IT-equipment with regard to its noise emission. Currently it is almost impossible for a layperson to compare noise declarations provided by manufacturers even though there are standardized and very detailed specifications in ISO 9296 for the measurement and declaration of the noise emission of IT-equipment. One of the central issues of the project is the consolidation of existing procedures for noise emission measurements on IT-equipment with consideration of the impact of psychoacoustic parameters. A categorization proposal based on ISO 7779 will be presented. In order to check the neccessity of additional psychoacoustic metrics like tonality and transient loudness, several data projectors and other IT-equipment were measured according to the BAuA proposal. In addition, the annoyance of the operational noise was evaluated by a panel test. The results will be compared and contrasted with the current ISO / ECMA standard measurements. A final approach of the noise categorization concept will be presented.

Keywords: IT-equipment Noise Categorisation Concept

INTERNOISE 2010 │ JUNE 13-16 │ LISBON │ PORTUGAL

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1 Introduction

In order to check the neccessity of additional psychoacoustic metrics like tonality and transient loudness, 12 data projectors were measured according to the BAuA proposal. In addition, the annoyance of the operational noise was evaluated by a test panel. The results will be compared and contrasted with the current ISO / ECMA standard measurements [2] [3]. The project work started with some pre-tests on different kinds of IT-equipment like a page printer, a scanner, 2 keyboards and 2 notebooks. The main results showed a large range of additional sound characteristics regarding tonality and fluctuation. Indeed ISO 7779 offers an optional check on tonal and impulsive noise components after a subjective initial-test, but this is not mandatory. In addition the impulsive check method is out-of-date. After the first measurements, we made a first proposal to combine the sound power level, a slightly modified tonality measurement according to DIN 45 681 [5] and an impulsive measurement according to DIN 45 631/A1 [6]. The result was a so called “Overall Noise Level” as an addition of sound power level + tonal adjustment + impulsive adjustment and correlated quite good with the human perception. This investigation was started to verify the first results with a larger test field.

2 Investigation on 12 data projectors

For the investigation 12 different multi purpose data projectors were chosen in a wide price range from 500 € up to 3000 €. The projectors were encoded from P01 to P12. The physical resolutions of the projectors were between 1024 x 768, 1280 x 720 and 1920×1080 pixels. The data projectors were operated on a table in normal mode, not in the “eco” mode. The measurement and recording conditions were comparable to ISO 7779 and ISO 3745 (semi-anechoic room with an ambient floor noise below 5 dB(A)).

Picture. 1 - Devices under Test - Codes P01 – P12


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The sound power level measurements were made using five 1” microphones, the binaural sound recordings for the listener test were made with a dummy head system in a distance of 1 m to the projector. For each projector 30 seconds of the operational noise were recorded in 2-channel 16 Bit / 44,1 kHz resolution directly on computer.

Picture 2 - Sound power level measurement set up in a semi anechoic room condition

2.1 Listening Panel Test

The operational sounds of 12 projectors were presented to and evaluated by 19 subjects (average age 29,7 years; 32 % female) via freefield equalized electrostatic headphone. The subjects made a paired comparison with the full matrix without the self comparison and had to chose the more annoying sound. Each sample was compared to each other, all in all 66 evaluations per subject. The intraindividual variation of the subjects was determinded. The lowest consistence factor was 0,86 (average 0,97). The interindividual variation between the 19 subjects was 0,97, too. The result was the following ranking order of the projectors (1 = least annoyance, 12 = highest annoyance):


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Table 1 and 2 - Results of the panel test sorted by models and from the best to the worst model

2.2 Sound Power Measurements according to ISO 7779 and ISO 9296

After the measurement the sound power levels according to ISO 7779 were calculated and transformed to the declared A-weighted sound power levels according to ISO 9296 in bel (B). 1 B is 10 dB. The results are shown in the following table:

Table 3 - Results of the declared sound power level and the listening test ranking


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For the projectors with codes P03, P08 and P12 the ranking of the annoyance does not correlate with the declared sound power level. In order to get more data concerning the validity of the sound power level as a key metric for the determination of the sound the R² coefficient of determination between LwAd and listener test ranking was calculated with 0,9153.

2.3 Tonality Measurements according to DIN 45 681

For the determination of tonal components in sounds several standards and methods exists. A procedure to assess the influence of single tones is given in the standard DIN 45 681. Basically it describes the method to calculate the so-called tonal adjustment for the assessment of noise immissions in dB between 0 dB (no tonal components) and 6 dB (one or more strong tones), which will be added to the noise rating level. The procedure in DIN 45 681 gives the possibility of a more detailed assessment of the influence of single tones. The level difference between the prominent tonal components in a sound and the masked threshold (as defined in the standard) is used within the tests on data projectors to predict the subjectively assessed tonality of that sound. The main result of the calculation according to DIN 45 681 is the level-difference L : dB vaLLL GT (1)

with

TL tone level in dB; GL masking noise in dB; va masking index in dB,

va has values between –2 dB at low frequencies and –6 dB at higher frequencies. Some results of the application of the calculation procedure according to DIN 45 681 are shown in the form of spectrogram representation in [1]. In the following table 4 the results of the projector’s level differences regarding the pitch strength are shown:

Table 4 - L Level differences results and tonal adjustment table from DIN 45 681


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According to former investigations of IT devices with high tonality like flatbet scanners, the measured values of L were much higher than 12 dB, in some cases twice as high. Due to this fact we propose an adapted tonal adjustment scale for the transformation in Table 4, right picture. The new proposed scale is “Difference L” x 2, for example 18 < L <= 24 dB = 5 dB. The modified KT values are shown in the following table 5:

Table 5 - Results of the level differences with modified tone adjustments in the right column.


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3 Combination of Sound Power, Tonality and Impulsiveness

3.1 Overall Noise Level Proposal

Former investigations with several IT products like a page printer, a scanner, 2 keyboards and 2 notebooks showed the influence of additional psychoacoustic parameters like transient loudness (impulsivness) and the tonality on the overall sound impression. For the impulsiveness the instationary loudness according to DIN 45631/A1 [6] turned out as a good metrics for the impulsiveness. The proposed metric is based on the N50/N10 percentile loudness ratio. This sum of the sound power level in dB(A), a slightly modified tonality measurement according to DIN 45 681 [5] and an impulsive measurement according to DIN 45 631/A1 is called “Overall Noise Level” (ONL) and correlates quite good with the human perception. Due to the fact that the sound power level is a room independent metric important for planners and acoustical experts, the idea was to combine this main metric with the most important psychoacoustic metrics to a new introduced value ONL in dB. The impulsive part was not included in the evaluation described here, because in the current test of the operational sounds of projectors the transient components were measured, indeed, but gave no reason for a further impulse adjustment. In the current test in case of the three data projectors, code P03, P08 and P12 the ranking of the annoyance listening test does not correlate with the declared sound power level. Here the results of the level differences according to DIN 45 681 are the explanation for this finding. This is shown in the following Table 6:

Table 6 - Results of the level difference explains the relatively poor correlation of listening test and sound power level exclusively


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3.2 Result and coefficient of determination

In the following table 7 the addition of the sound power level in dB according to ISO 7779 and the modified tonal adjustment according to DIN 45 681 called Overall Noise Level (ONL) is shown.

Table 7 - Overall Noise Level as an addition of sound power level and tonal adjustment The ONL shows a further improved correlation to the subjective listening test ranking compared to the sound power level (see Diagram 1 and Table 3). The R² coefficient of determination increases from 0, 9153 to 0,9529. In case of data projector P12, which has the most annoying operational noise, an additional sound impression can be observed. The sound character is quite rough. This additional psychoacoustic parameter was not evaluated, but in the former investigations these parameters gave some valuable extra information [2]. The data projector’s roughness was 0,69 asper, compared to the other data projectors more than double as rough.


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4 Conclusions

The results show that the sound power level offers a good but not perfect correlation to the human perception even within the same IT product group. If there was an additional measurement of the tonality and an appropriate combination of sound power level and tone adjustment, the correlation to the listener’s impression would get even better. In the current discussion regarding a product noise rating and labeling system additional psychoacoustic metrics should be used in order to avoid misjudgements.

References

[1] G. Feneberg; O. Martner; C. Zerbs, Different requirements on describing quantities for Sound Quality, Euronoise 2009.

[2] Widmann, U.; Feneberg, G.: Anwendung psychoakustischer Berechnungsverfahren zur vergleichenden Bewertung der Betriebsgeräusche von Konsumgütern. In: Fortschritte der Akustik – DAGA '97 (Kiel 1997) Bad Honnef : DPG-GmbH, 1997, S. 361-362

[3] ECMA-74 Measurement of Airborne Noise Emitted by Information Technology and Telecommunications Equipment, 10th edition December 2008

[4] ISO 9296 Acoustics – Declared noise emission values of computer and business equipment 1988-04-15

[5] DIN 45 681 Acoustics – Determination of tonal components of noise and determination of a tone ajustment for the assessment of noise immissions - March 2005

[6] DIN 45 631/A1 Calculation of loudness level and loudness from the sound spectrum – Zwicker method – Amendment 1: Calculation of the loudness of time-variant sound – January 2008

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