THE MEASUREMENT OF THE SOUND POWER LEVEL OF TWO MICROCOMPUTERS IN LABORATORY CONDITIONS

The text hereunder is taken from a test report (Test ReporlTu99-1S I.e) of the Institute of Occupational Health by Mr. V. Hongisto and Mr. M. Lindgren, dated April 12^th 1999.

INSTITUTE OF OCCUPATIONAL HEALTH

To: Timo Koskinen

Dacco - Trading Oy

Juhana Herttuan puistokatu 23

FIN-20100 TURKU, Finland.

Test ReporlTu99-1S I.e

April 12, 1999

1. The purpose of the test

The sound power level of different microcomputers were determined in laboratory conditions using a precision standard method.

One composition (DTK Classic) was provided with a conventional central unit without any special noise reduction techniques.

The other composition (DTK Silent) was provided with a central unit, in which the fan noise was suppressed by means of a specially manufactured power source, (the Q Technology Ultra Quiet Power Supply). In addition, the noise caused by the internal hard disk was suppressed by placing it inside a capsule. (The SilentDrive™ from Silent Systems + Passive CPU cooler)

2. The test laboratory

The sound measurements were made in the laboratory of ventilation and acoustics (Lemminkäisenkatu 14-18 B Turku) in a reverberation room. The length, width and height were 6.9m, 4.5 m and 3.65 m, respectively. The reverberation room (size, shape and reverberation time) fulfils the requirements of ISO 3741 standard at frequency range 250-8000 Hz.

With this standard, results in the precision grade of accuracy can be obtained. The measurement uncertainty is below 2 dB at frequency range 250-4000 Hz and below 3 dB at 8000 Hz (octave bands). The uncertainty of the A-weighted sound power level is smaller than for individual frequency bands.

3. The measurement apparatus

Sound pressure level measurements. The acoustical measurements were made with a real time frequency analyzer (Bruel & Kjaer 2133). A condenser microphone was used (Bruel & Kjaer 4165) having the frequency response for a diffuse field flat in ± 0,1dB at the frequency range of 31-10000 Hz. The microphone was mounted on a rigid movable tripod. The whole measurement path including cables were calibrated at 1000 Hz frequency using a sound level calibrator (Bruel & Kjaer 4230). The sound level calibrator was traceably calibrated in June 1998.

Reverberation time measurements. The same measurement apparatus was used as in the sound level measurements. The test signal (pseudo-random pink noise) was generated by the analyzer. The noise was produced by a constant sound source (B&K 4224). The sound source was in the test room during the sound pressure level measurements.

The temperature in the test room was 22⁰C. The relative humidity was 20%. These measurements were made by Vaisala HMI 31. The barometric pressure was 1025 mbar.

4. Test specimen (devices to be tested)

Primarily, two different central units were to be compared. In addition, the display, the keyboard and the mouse were included in the test arrangement. The tested compositions are presented in Table 1. The devices were placed on the floor, approximately in the middle of the test room. The position of the central units were identical in sequential tests.

Table 1. The tested microcomputer compositions

Central unit

Peripheral device

1. DTK Classic

Display (MAG DJ 53O)

Keyboard (KeyTronic)

Mouse (Microsoft)

2. DTK Silent

Display (MAG DJ 53O)

Keyboard (KeyTronic)

Mouse (Microsoft)

All devices were switched on during the tests. The basic screen-saver (Windows 95 text form) was used to avoid fading of the display noise during the tests. There were no factors which could produce varying levels of noise and therefore the test was made in one single operating condition.

5. The test procedure

The reverberation time was determined as an average of 12 measurements. Two positions of the sound source were used and three positions of the microphone were used as recommended in the ISO 354 standard. Each measured decay signal comprised 180 sequential linearly weighted sound samples of length 30 ms. The reverberation time was analysed using the Schroeders backwards integration technique and 10 dB decay range.

The background noise levels L_PB and the sound levels caused by the equipment (and background noise together) L_p were measured at four fixed measurement points. The measurement period was 64 seconds. The microphone positions were more than 2m away from the tested device and 1.5m from the room boundaries.

The measurements were made in 1/3-octave bands at 50-10000 Hz. The results are given in 1/1-octave bands at 125-8000 Hz. The results fulfil the precision grade at and above 160 Hz third octave band.

In this laboratory of volume 113 m³, the lowest octave band fulfilling the precision grade is 250 Hz. The results will be presented also at the 125 Hz octave band but the uncertainty is not guaranteed by the ISO 3741 standard. At frequency bands 4000 - 8000 Hz there were also problems with the background noise level. The declared A-weighted sound power level is reliable because the background noise level was below 15 dB and the total sound power level will not be affected by these low noise levels.

6. Results

The background noise levels (L_pB), the sound levels caused by the tested devices (L_p) and the reverberation time (T) are presented in Table 2. All sound power levels are presented in reference I pW.

Table 2. The original data measured

Frequency (Hz)	T (s)	L_pB(dB)	DTK Classic L_p(dB)	DTK Silent L_p(dB)
125	1,86	31,0	36,8	33,4
250	3,41	17,0	46,3	27,7
500	4,61	10,4	40,5	25,1
1000	4,54	8,0	36,9	18,6
2000	2,64	9,8	28,8	15,2
4000	1,73	12,1	24,4	15,2
8000	0,77	13,1	15,6	13,2

7. Application of the results in workplaces

The number value of the sound power level is the same as the sound pressure level at a distance of 30 cm from the point-like sound source in a reverberant-free space. The sound pressure level can be calculated from the value of the sound power level when the distance to the sound source is known and the reverberation time of the room is small.

In practice, the distance between the microcomputer and the user varies a lot. The sound pressure level caused by the microcomputer in typically furnished office rooms can be calculated approximately by using Table 3.

The general rule is applied where the sound pressure level decreases by 6 dB when the distance doubles. This rule applies reasonably well at short distances in typical office rooms.

Table 3. The computational A-weighted sound pressure level of tested computers in different distances in a typical office room.

Distance (cm)	DTK Silent	DTK Classic
30	29,2	43,6
60	23,2	37,6
120	17,2	31,6

According to Finnish building regulations (Suomen Rakentamismääräyskokoelma osa C1 1985 Ministry of the Environment) the sound pressure level (L_A,_fast,max) caused by the fixed equipment of the building (ventilation, lifts, drain noise etc.) has to be below 35 dB(A).

Usually the sound pressure level is above 45 dB(A) due to the additional noise caused by other pieces of office equipment.

According to Table 3, the sound pressure level caused by DTK Silent is below 35 dB(A) already, at a distance of 30 cm. Therefore, the micro computer does not increase the noise level in an office workplace where the noise level is 35 dB(A) without the computer.

TURKU REGIONAL INSTITUTE OF OCCUPATIONAL HEALTH

Gustav Wickström Valtteri Hongisto

director research engineer

LABORATORY TEST REPORT

Client: Dacco Trading Oy Turku, Timo Koskinen

Task: Determination of sound power level ISO 3741:1988(E)

Laboratory: Laboratory of ventilation and acoustics

Reverberation room 2 (Volume 113 m³)

Personel: Mika Lindgren

Test date: March 17,1999

Test device: DTK Classic (PC), MAG DJ 530 (display), keyboard, mouse

Operation: Normal operation (central unit and display on)

Mounting: On the floor of the reverberation room

Results:

A-weighted sound power level (125 - 8000 Hz): L_W,A ** 43.6 dB (estimate)

A-weighted sound power level (250 - 4000 Hz): L_W,A 43.6 dB (ISO 3741)

Freq.	L_w
125*	42.1
250	48.3
500	40.7
1000	37.0
2000	31.1
4000	28.5
8000**	23.1

* The volume of the test laboratory should to be above 200 m³ at 125 Hz.

The uncertainty of the result is probably higher than 3 dB stated in ISO 3741

** The result is an overestimate at these frequencies.

The background noise level was too high. The background noise level has to be more than 6 dB below the noise level caused by the tested device.

LABORATORY TEST REPORT

Client: Dacco Trading Oy Turku, Timo Koskinen

Task. Determination of sound power level ISO 3741:1988(E)

Laboratory: Laboratory of ventilation and acoustics

Reverberation room 2 (Volume 113 m³)

Personel: Mika Lindgren

Test date: March 17,1999

Test device: DTK Silent (PC), MAG DJ 530 (display), keyboard, mouse

Operation: Normal operation (central unit and display on)

Mounting: On the floor of the reverberation room

Results:

A-weighted sound power level (125 - 8000 Hz): L_W,A ** 29.2 dB (estimate)

A-weighted sound power level (250 - 4000 Hz): L_W,A 26.4 dB (ISO 3741)

Freq.	L_w
125* **	38.8
250	29.7
500	25.3
1000	18.7
2000	17.5
4000**	19.3
8000**	20.8

* The volume of the test laboratory should to be above 200 m³ at 125 Hz.

The uncertainty of the result is probably higher than 3 dB stated in ISO 3741.

** The result is an overestimate at these frequencies.

The background noise level was too high. The background noise level has to be

more than 6 dB below the noise level caused by the tested device.