Hong Kong Regulations

NOISE CONTROL (AIR COMPRESSORS) REGULATIONS - SCHEDULE 2

TEST CONDITIONS FOR MEASURING AIRBORNE NOISE EMITTED BY AIR COMPRESSORS

[regulations 7 & 8]

1. OBJECT

1.1 The purpose of this test is to determine the noise emitted by
air compressors.

1.2 Both measured and calculated values obtained by this test constitute the
basic data for checking that the noise emission of air  compressors complies
with the prescribed standard.

2. SCOPE

2.1 This test is applicable to portable air compressors

(hereinafter referred to as "appliances"). The test specifies the procedures
for use in determining the sound power level of such appliances for the
purpose of testing as to conformity under the Noise Control (Air 
Compressors) Regulations.

3. DEFINITIONS

3.1 Sound pressure level LpA

The sound pressure level LpA is obtained by applying the weighting A to the
sound pressure level Lp.

The sound pressure level Lp, expressed in dB, of a noise is defined by-

L = 20 log10P/Po

Where- P is the effective sound pressure value measured at a particular point,
expressed in Pa; Po is the effective reference sound pressure, equal to
20x10-6 Pa. The value LpA of the A-weighted sound pressure level, expressed in
dB(A), is obtained by applying the weighting A to the measuring system.

3.2 Measuring surface

The measuring surface of area S (see paragraph 8.3) is a hypothetical surface
surrounding the sound source and on which the measuring points are arranged
(see paragraph 6.3).

3.3 Surface sound pressure level LpAm

The surface sound pressure level LpAm is the root mean square of the sound
pressures recorded on the measuring surface, calculated in accordance with the
method set out in paragraph 8.4.

3.4 Sound power level LWA

The sound power level LWA is obtained by applying the weighting A to the sound
power level LW. The sound power level LW, expressed in dB, of a sound source
is defined by-

LW = 10 log10W/Wo

Where- W is the total sound power generated by the sound source, expressed in
watts; Wo is the reference sound power, equal to 10-12W. The value LWA of the
A-weighted sound power level, expressed in dB, is obtained by applying the
weighting A to the measuring system.

3.5 Extraneous noise

Extraneous noise means the noise resulting from background noise and parasitic
noise.

3.5.1 Background noise

Background noise means any noise recorded at the measuring points which is not
generated by the appliance.

3.5.2 Parasitic noise

Parasitic noise means any noise at the measuring points which is generated by
the appliance but not directly radiated by it.

4. CRITERION TO BE USED FOR EXPRESSING RESULTS

4.1 Acoustic criterion for the environment

The acoustic criterion for the environment of the appliances is their sound
power level.

5. MEASURING INSTRUMENTS

5.1 General

The instruments shall be designed to measure the A-weighted level of the root
mean square of the sound pressure. The level of the root mean square value in
time for a measurement point is obtained either by direct reading of the
instrument, or by calculation in accordance with paragraph 11.

5.2 Measuring instruments

The following instruments may be used-

   (a)  a sound level meter meeting the requirements of International
        Electrotechnical Commission (IEC) publication 651:1979 (type 1). The
        meter shall be set at "slow" response;

   (b)  an integrating sound level meter effecting analog or digital
        integration of the squared signal over a given time interval meeting
        the requirements of IEC 804:1985 (type 1). The meter shall be set at
        "fast" response.

5.3 Microphone with cable

Use shall be made of a microphone with cable complying with IEC publication
651:1979, and calibrated for free field measurement.

5.4 Weighting network

Use shall be made of an A-weighting network meeting the requirements of IEC
publication 651:1979.

5.5 Inspection of the measuring apparatus

Before the tests, the acoustic properties of the entire apparatus

(measuring instruments including microphone and cable) shall be checked by
means of a calibrated sound source with an accuracy of at least 0.5 dB

(e.g. a pistonphone); the apparatus shall be checked again immediately after
each series of measurements.

6. TEST CONDITIONS

6.1 Operation of the appliance during measurements

The appliance shall be brought to its steady-state temperature within the
range specified by the manufacturer. It shall be operated at its nominal speed
and its nominal pressure.

The nominal speed and pressure are those recommended by the manufacturer.

6.2 Measuring site

The measuring site must be flat and horizontal.

6.3 Measurement surface, measuring distance, location and number of measuring
points

6.3.1 Measuring surface and measuring distance

The measuring surface to be used for the test shall be a hemisphere. The
radius shall be-

-4 m, where the greatest dimension of the appliance to be tested is not more
than 1.5 m; -10 m, where the greatest dimension of the appliance to be tested
is more than 1.5 m but not more than 4 m; -16 m, where the greatest dimension
of the appliance to be tested is more than 4 m.

6.3.2 Location and number of measuring points

For the measurements there shall be 6 measuring points on a hemisphere of
radius r, i.e. points 1, 2, 3, 4, 5 and 6 arranged as illustrated in figures 1
and 2, with variations in the value of z of points as shown in Table 1.

For testing of the appliance, the geometric centre of the appliance shall be
positioned vertically above the centre of the hemisphere.

The x-axis of the set of coordinates, in relation to which the positions of
the measuring points are fixed, shall be parallel to the main axis of the
appliance.

The 6 measuring points shall have the following coordinates (Table 1) as
measured from the centre of the hemisphere-

TABLE 1

X Y Z 1 r 0.0 1.50 m 2 0.0 r 1.50 m 3 -r 0.0 1.50 m 4 0.0 -r 1.50 m 5 -0.27 r
0.65 r 0.71 r + 1.5 m 6 0.27 r -0.65 r 0.71 r + 1.5 m

r = radius of hemisphere

7. MEASUREMENTS

7.1 Measurement of extraneous noise

Only the background noise shall be taken into account for the purpose of
corrections. Background noise shall be recorded at the measuring points

(see paragraph 6.3.2) with the appliance switched off, i.e. no sound emission
(see paragraph 7.2).

7.2 Measurement of the sound pressure level LpA

To measure the sound pressure level LpA, use shall be made of an instrument as
defined in paragraph 5.2. If a sound level meter is used, 5 readings shall be
taken at each measuring point. The mean value of these 5 readings shall be
calculated in accordance with paragraph 11. If an integrating sound level
meter is used, the integrating time shall be 15 seconds.

8. USE OF RESULTS

8.1 Calculation of root mean square values

8.1.1 Root mean square value at a measuring point

The values obtained from the measurements referred to in paragraph 7.2 are
root mean square values in time.

8.1.2 Root mean square value on the measuring surface

The level corresponding to the root mean square value in space of the sound
pressure levels for all the measuring points shall be calculated on the basis
of the values established in accordance with paragraph 8.1.1.

8.2 Calculation of average extraneous noise level

The average extraneous noise level over the measuring surface is obtained by
applying the method described in paragraph 8.1.2 to the extraneous noise
levels established at the different measuring points.

8.3 Calculation of the area S of the measuring surface

The area S, expressed in m2, of the measuring surface is determined as
follows- S = 2πR2 The area of the measuring surface may be calculated
approximately. It should be noted that an error of ±20% in this area gives a
deviation of ±1 dB in the value of the term-

10 log10 S/So (surface level)

8.4 Calculation of the surface sound pressure level LpAm

The surface sound pressure level is the level calculated in accordance with
paragraph 8.1.2 and then corrected as described in paragraph 8.6.

8.5 Calculation of the sound power level LWA

The sound power level LWA of the sound source is calculated by means of the
following equation-

LWA = LpAm + 10 log10 S/So

Where- LWA = sound power level of the source tested, expressed in dB (see
paragraph 3.4); LpAm = the surface sound pressure level, expressed in dB, as
defined in paragraph 3.3; S = the area of the measuring surface in m2,
calculated in accordance with paragraph 8.3; So = reference area of 1 m2.

8.6 Corrections to be made to measurements

The average sound pressure level over the measurement surface, calculated in
accordance with paragraph 8.1, shall be corrected, if necessary, to take
account of the extraneous noise determined in accordance with paragraph 8.2.
The correction K1 in dB, which shall be subtracted from the average sound
pressure level over the measuring surface is given in the table below.

Difference in dB between the sound pressure level calculated when the
appliance is operating and the sound pressure level due to extraneous noise
alone
Correction K1 in dB less than 6 6 7 8 9 10 more than 10 no valid measurements
1.0
1.0
1.0
0.5
0.5
no correction

To allow for tolerances, 5 dB(A) shall be deducted from the A-weighted sound
power LWA determined for the appliance and the net result shall then be
compared with the relevant standard.

9. DATA TO BE RECORDED

A model report is provided in the Appendix.

10. DATA TO BE INCLUDED IN THE REPORT LAID DOWN IN PARAGRAPH 9

The data shall be reported in accordance with paragraph 9. The report shall
state clearly that the sound power levels were obtained in accordance with the
test conditions. It shall specify that these sound power levels are given in
A-weighted dB, reference 10-12 W.

11. METHOD OF CALCULATING THE AVERAGE LEVEL CORRESPONDING TO THE ROOT MEAN
SQUARE VALUE OF THE VARIOUS SOUND PRESSURE LEVELS

The root mean square value of the various sound pressure levels resulting from
a series of measurements made at different points on the measuring surface
(root mean square in space) can be determined by the following formula-

LpAm = LpAo + 10 log10 1/n i = n
Σ gi
i = 1

Where- LpA = the sound pressure level of the i-th measurement; LpAo = an
auxiliary sound pressure level to simplify the calculation (for example, the
smallest of the LpAi values); gi = the auxiliary variable for the i-th
measurement- gi = 100.1 (LpAi-LpAo).

APPENDIX

MODEL REPORT

TEST REPORT FOR AIR COMPRESSORS

1. Test Article

Manufacturer
...........................................................................
.................................... Model
..................................................... Serial No.
.................................................. Description
............................................. Mass
......................................................... Type: Compressed
air, hydraulic, electric, petrol(1)

2. The Compressor

Length ............................................. m Width
.............................................. m Height
............................................. m Mass
............................................... kg

3. Operating Conditions(1)

3.1 Operation of the drive motor

Make and type
...........................................................................
.................................. Energy used: petrol, diesel, electricity,
gas(1) Nominal speed
...........................................................................
........................... rpm Motor power
...........................................................................
.............................. kW

3.2 Operation of the compressor

Make and type
...........................................................................
..................................

Nominal speed at full load
...........................................................................
............ rpm Nominal delivery pressure
...........................................................................
............ kPa Nominal air flow at the above speed and pressure
...........................................................
...........................................................................
............................................... m3/min

(1) Delete as appropriate.

4. Test conditions

Barometric pressure .......................................... Ambient
temperature ........................... Composition and dimensions of
reflecting surface
............................................................
...........................................................................
.......................................................... Remarks
...........................................................................
............................................

5. Equipment

Microphone ............................................... Serial No.
............................................... Sound-level meter
...................................... Serial No.
............................................... Calibrating equipment
................................. Serial No.
............................................... Miscellaneous (e.g. wind
shield or recorder)
................................................................. Serial No.
...............................................

6. Test Set up

Diagram showing microphone locations and, if applicable, the direction of the
exhaust and the position of any large objects situated less than 25 m from the
appliance under test.

7. Acoustic data

-area S in m 2 of measuring surface and value of 10 log 10 S/So;
-sound-pressure levels measured at measuring points; -average sound-pressure
level over measuring surface; -any corrections in decibels; -surface
sound-pressure level LpAm; -sound-power level; -date and time of measurements.
Extraneous noise Background noise Parasitic noise