Acoustic Power Measurement by the Sound Intensity Method

When measuring acoustic power using the sound intensity method, it becomes possible to perform measurements even near machinery or in environments with steady background noise.
This method is very straightforward: the acoustic power is obtained by multiplying the average normal sound intensity over the measurement surface by the surface area.
The first step is to define this virtual measurement surface.

As long as there are no other sound sources or absorbing materials inside the defined measurement surface, its shape can be arbitrary.
The floor is assumed to reflect all sound perfectly and is therefore not included in the measurement surface.
Theoretically, the distance of the measurement surface from the sound source does not affect the results.

Two examples are shown below.

Rectangular Box

The shape and size of the measurement surface can be arbitrary.
A rectangular box is easy to define and, because of its flat surfaces, makes it simple to average the intensity over each plane.
The partial acoustic power from each surface is calculated individually and then summed.

Hemisphere

Using a hemispherical measurement surface minimizes the number of required measurement points.
In a free-field environment with an omnidirectional source, the intensity is uniform at all points on the hemisphere.

Spatial Averaging

Once the measurement surface is defined, the normal component of the intensity relative to the surface must be measured and spatially averaged.
The measurement surface can be defined physically using a grid, or simply by specifying distances from a reference point.

ISO 9614 consists of three parts, each defining a different measurement method:

• Part 1: Averaging at discrete points
• Part 2 and Part 3: Averaging through scanning across the measurement surface
• Part 3: Additional requirements apply for certain measurement environments

Averaging at Discrete Measurement Points (Discrete-Point Method)

In this method, the measurement surface is divided into small segments, and individual measurements are taken at each segment.
The measurement points are usually arranged in a grid pattern.
You can use rulers or tape measures, but creating a guide using string or wire is also effective.
By averaging the results and multiplying by the area, the total acoustic power for the entire measurement surface is obtained.

Neither method is perfect for every situation; in some cases, both methods can be valid.
The scanning method offers a closer approximation to continuous spatial integration and may provide more accurate results, but it requires moving the probe at a constant speed and covering the measurement surface evenly.

On the other hand, the discrete-point method offers excellent reproducibility.
When repeated measurements are required, both methods can be easily automated, improving precision.

Scanning the Measurement Surface (Scanning Method)

Using an appropriate averaging time, the probe is moved across the measurement surface much like painting the surface.
This produces a single value for the spatially averaged intensity.
Multiplying this value by the surface area gives the acoustic power from that surface.
The acoustic powers from all measurement surfaces are then summed.

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