Standard ISO 3747 Acoustics – Determination of sound power levels – survey method using a reference sound source

This standard describes a method based on relative measurements using a so-called calibrated sound power source.. The standard permits reflecting surfaces in the vicinity of the measurement object.

Before becoming too specific about the contents of the standard, we begin by getting acquainted with some background. For a calibrated sound power source, there should be a calibration protocol that gives the sound power in third octave and octave bands, and with A-weighting included. Two types of calibrated sound power sources exist.

Firstly, there are the electroacoustic sources that consist of a generator/amplifier part and a loudspeaker part. Secondly, there are fan-type sound sources that contain a radial fan driven by a powerful electric motor, as in Figure 3-21. The overdimensioned motor supplies a constant rotational velocity which, in turn, generates a stable sound spectrum.

An important requirement for such reference sources is that, in addition to a constant sound power, they shoud also have a low directivity, i.e., they should radiate about the same in all directions.

The standard does not make any demands on the measurement environment. That means that the sound waves recorded by the measurement microphone can be either direct sound from the source, or reflected sound; see Figure 3-23.

Figure 3-23 The standard for sound power determination ISO 3747 specifies no requirements for the measurement environment, which means that both direct and reflected sound can reach the microphone positions.

According to the preceding section, the sound power level is a measure of the radiated sound power from a machine, and essentially independent of the acoustic environment, whereas the sound pressure level depends on both the distance and the direction to the machine, and the environment in which it is placed.

For a certain acoustic environment, i.e., a specific room, and a specific distance to the sound source the machine’s sound power level LW can be related to the resulting sound pressure level Lp at a point, using the so-called room correction K [dB], in the formula

K L

L_p = _W + . (3-5)

The method for the sound power determination with a calibrated sound power source is based on determining the room correction K using the known sound power of that source.

Microphone Source

Direct sound

Reflected sound

The sound pressure level from the measurement object is measured first, after which it is replaced by the calibrated reference source, and the sound pressure level measured again at the same microphone positions. The sound power level LW of the measurement object can then be determined from (3-5), as

(

)

p

W L L L

L = + − , (3-6)

where L_p is the sound pressure level from the measurement object,

Wr

L is the sound power level of the reference source,

pr

L is the sound pressure level of the reference source.

Usually, the sound pressure level is measured at a number of points on a measurement surface around the measurement object. Then, the spatial average of the sound pressure levels L_p is used in (3-6), as

) 1 10

log(

10

1

∑

=

⋅

= ^N

n L

p ^pn

L N [dB], (3-7)

where Lpn is the sound pressure level at point n.

Example 3-2

At five measurement points, the sound pressure levels 79, 82, 83, 79 and 81 dB are measured. Determine the spatial average.

Solution

(3-7) gives

(

)

5 log 1

10 ^{7.9 8.2 8.3 7.9 8.1}⎥⎦⎤= ≈

⎢⎣⎡ + + + +

⋅

p =

L .

ISO 3747 places no demands on the measurement environment, and is intended for measurement objects that cannot be moved to a measurement environment that gives a higher degree of precision. The overview given below is considerably shortened, and assumes a measurement object placed freely on a reflecting floor.

Measurement object: Machines that cannot be moved, and their parts and components.

Running condition is to be specified.

Sound Character: Stationary. Broad band, narrow band, discrete tones and combinations of these.

Quantities

determined: A-weighted sound power level.

Frequency region: Octave bands with center frequencies from 125 Hz up to 8000 Hz.

Background levels: The background level for each octave band is to be at least 3 dB

measurement object or reference source are operated. The correction for the background level is to be made if it is not at least 10 dB lower; see example 1-6.

Placement of

reference source: The calibrated reference source can either be placed at the ordinary position of the measurement object, or at one or more points around the measurement object, depending on its size.

Microphone positions: To find the microphone positions, a hypothetical parallelepiped reference surface, surrounding the measurement object and reference source positions, is first defined. The microphone positions are then placed on another, typically parallelepiped, surface, with its sides parallel the reference surface, and at a distance of 1 m from it. For measurement objects with horizont-al dimensions of less than a meter, the microphones are placed in the middle of the five measurement surfaces, see Figure 3-24.

1m

1m 1m

Microphone position Measurement

surface

Reference surface Reflecting floor

3.3.2 Standard ISO 2631-1 1997 Vibration and shock – Guidance for evaluating the effects of whole-body vibrations on man. Introduction

Machines and means of transport of all kinds subject man to vibrations that affect comfort, working ability, and, in the worst case, health. Despite the inherent difficulties due to the involved situations, and to some extent the lack of reliable knowledge, an international standard has nevertheless been agreed upon. The standard primarily seeks to simplify the assessment and comparison of measurement results concerned with whole-body vibrations.

The standard is concerned with vibrations transferred to the whole body from the surface one stands, sits, or lays upon. It consists, presently, of two parts:

Part 1 ISO 2631-1 General requirements

Figure 3-24 The microphone positions on a measurement surface are at distances of 1 m from the reference surface. The reference surface surrounds the measurement object and the reference source.

Pat 2 ISO 2631-2 Vibration and Shock - Measurement and guideline values for the assessment of comfort in buildings.

The standard includes directions on how to apply the results, and how and what should be measured. In the following two sections, the main features of parts 1 and 2 are briefly described.