Sound Power Measurement

The problem it solves

If one manufacturer measures in a hemi-anechoic room and another in an ordinary workshop, their sound pressure levels cannot be compared. A data sheet statement such as “sound power level ≤ 78 dB(A)” works as a common commitment because sound power is the total acoustic energy radiated per second, independent of measurement distance and environment. It is the quantity used in noise regulations, energy standards and purchase contracts.

Sound power cannot be measured directly; it is derived from sound pressure. Microphones are placed around the machine, the surface-averaged sound pressure level is measured, and the measurement surface correction is added while the influence of background noise and room reflections is subtracted. By hand this requires tables, area calculations and band-by-band corrections. The module automates the chain: enter the machine dimensions and measurement distance and the enveloping surface area is calculated; save the background noise with the machine stopped and it is subtracted band by band after start-up; choose an environment preset and K2 is calculated.

Functions

Screen layout

1. Result indicators: total sound power levels Lw(A) and Lw(Z) and the mean sound pressure level Lp(A) in large figures.
2. Total sound power history (upper right).
3. Octave chart of the measurement points (lower left): stepped 1/3-octave sound pressure spectra.
4. Machine sound power octave chart (lower right): the final Lw spectrum.
5. Measurement point table: total level of each point (Z and A) and 30-band details, including the background spectrum.

The left panel sets machine dimensions L1 × L2 × L3, measurement distance d, mounting condition, arrangement, environment preset and room dimensions, and background noise save and import.

Product noise ratings and data sheets

Sound power values stated for fans, pumps, compressors, motors, transformers and chillers are measured by the enveloping-surface method. Development, type approval and production sampling can use the module for Lw(A) data.

Tenders and contract acceptance

When a contract specifies “total sound power ≤ XX dB(A)”, buyer and supplier need the same method to reproduce the result. With the GB/T 3767 / GB/T 3768 correction chain built in, their data can be compared directly.

Energy labels and certification

Energy and noise labeling of appliances and office equipment uses sound power level. Third-party tests require traceable surface parameters and corrections; the module records the area, K1 and K2 calculation chain.

Ranking noise sources in a workshop

Workshop noise control starts with the question of which machine contributes most. Measuring Lw for each machine and comparing the bands identifies the main source and its frequency range, so investment goes where it is most effective.

Surveys of machines in service

Ageing and loose parts raise sound power. Regular surveys of Lw(A) compared with historical records can be an early sign of deterioration; the history curves form a record of each survey.

Training and demonstration

With the built-in example data, the whole sequence — enter dimensions, select condition, subtract background, obtain Lw — can be demonstrated, explaining the difference between sound pressure and sound power.

Enveloping-surface method

An enveloping measurement surface is imagined around the machine with evenly distributed points. The surface-averaged sound pressure level is calculated from the point levels and converted to sound power:

Lw = Lp,mean + 10 · lg(S / S0) − K1 − K2 [dB], S0 = 1 m²

S is the measurement surface area, K1 the background noise correction and K2 the environmental correction. The calculation runs band by band for every frame, giving the 1/3-octave sound power spectrum; the band energies combine into the total sound power level, unweighted and A-weighted.

Measurement surface and area

Reflecting planes taken up by the floor or walls are removed according to the mounting condition:

The box dimensions are the machine dimensions L1 × L2 × L3 extended by the measurement distance d (a = L1 + 2d, b = L2 + 2d, and the height extended by d or 2d depending on the condition). For the spherical arrangement the radius r is the measurement distance; if it is less than twice the characteristic source dimension (the machine diagonal D0), an engineering-method warning is shown.

Background noise correction K1 (per channel and band)

Background noise with the machine stopped raises the measured values and must be subtracted. With the machine stopped, the 1/3-octave spectra of all points are saved as a background file (.bgn, 8 channels × 30 bands). During measurement the level difference is calculated band by band:

Background noise differs between points and bands — for example, a neighboring fan may still be running after the machine stops and affect only some bands — so a single global correction would be wrong somewhere. When warning bands exist, the display states how many bands have ΔL < 6 dB.

Environmental correction K2 (absorption method, GB/T 3768 Annex A)

Room reflections return additional energy to the points and are subtracted according to the room absorption. Six presets are built in:

A = α · Sv K2 = 10 · lg(1 + 4S / A)

With a preset selected and the room length, width and height entered, the equivalent absorption area A is calculated from the inner surface area of the room Sv, and K2 updates with the measurement surface S. When A < 6S, “engineering method exceeded” is shown: the room is not suitable for the engineering method (GB/T 3767) with sufficient confidence, and the acoustic environment must be improved or the survey method used.

Spectral basis: 1/3 octave and A-weighting

The waveform of each point is windowed and transformed by FFT, and the energy in each of the 30 one-third-octave bands of IEC 61260 (20 Hz to 16 kHz) is summed to give the sound pressure spectrum. A-weighting follows the IEC 61672 class 1 transfer function band by band for Lw(A). All points are energy-averaged (Σ 10^(L/10) divided by the number of points), removing single-point directivity.

Display

Octave curves are drawn as steps from lower edge through center to upper edge. The total sound power history scrolls frame by frame (8192 points). The Lw spectrum and the point Lp spectra can be switched to check the pressure-to-power conversion.

Preparation

1. Measurement points: place eight microphones evenly on the enveloping surface of the selected arrangement (one channel per point); for the box arrangement at distance d from the machine surface (1 m recommended), for the spherical arrangement on the sphere of radius r (r ≥ 2D0 recommended).
2. Calibration: calibrate each channel with a sound calibrator.
3. Parameters: measure the machine dimensions L1 × L2 × L3 and note the mounting condition and approximate room dimensions.

Procedure

Measurement conditions

Floor mounting, box arrangement; the surface area is calculated from the machine dimensions and measurement distance. The background file saved with the machine stopped enables K1 band by band, and the “typical workshop” preset with the workshop dimensions gives K2.

• Results: Lw(A) = 113.68 dB, Lw(Z) = 113.83 dB, mean Lp(A) = 102.28 dB.
• Point table: channel 1 at 109.98 dB(Z) and channel 2 at 108.47 dB(Z); similar levels indicate a reasonable arrangement.
• The sound power octave chart shows energy spread over a wide band without an abnormal single peak.
• The sound power history rises smoothly and then levels off, showing that operation has reached steady state.

Read Lw(A) first against the data sheet value or limit, then the Lw spectrum for the main contributing bands, and finally check the correction warnings for data reliability. Sound pressure varies between about 102 and 110 dB with position, while the derived sound power level is a single value — which is why sound power is the identity of the machine.