Unattended noise measurements: use of new technologies toautomatically qualify noise events for greater efficiency, precisionand time savings
Erik Aflalo
NTi Audio France304, RTE Nationale 6, 69380 Limonest, Francee.aflalo@nti-audio.com - +33 788 06 66 95
Thomas Hupp
NTi Audio AG
Im alten Riet 102, 9494 Schaan, Liechtensteint.hupp@nti-audio.com - +423 239 6075

ABSTRACT
For more than 40 years, the noise assessment in the environment has consisted of measuring andstoring the time history typically every second, then marking the events using post-processing
software or automatically or manually by coding. Technological advances now make it possible
to enrich the simple measurement of the time history thanks to additional data allowing a better
evaluation of the measured phenomena: audio recording and AI analysis of the type of noise,analysis of the predominant direction during a noise event. We propose to present an innovative
solution based on a 3D sound intensity probe allowing the determination of the azimuth and theelevation every time a noise event occurs. The results are expressed by frequency bands and Aweighted filter. The influence of the type of field (active and reactive intensity) will be developed.
Particular attention will be paid to the accuracy and the validity of the measurement as well as
the limits of the principle.
INTRODUCTION
1.
Determining the direction of noise can be done through various methods, depending on thecontext and resources available:– Human Hearing: This is the simplest method. Humans can often detect the direction of anoise by turning their head and listening for where the sound is loudest.
– Visual Cues: Sometimes, you can see the source of the noise. For example, if you see aconstruction site nearby and hear loud machinery, it's likely coming from that direction– Sound Intensity: by measuring sound intensity using a 3D probe, you can accurately
determine the intensity vector therefore the direction of the energy flow
– Time Delay of Arrival (TDOA) Techniques: By using multiple microphones spaced apart,you can measure the time it takes for sound to reach each microphone. By comparing
these time delays, you can estimate the direction of the noise source– Triangulation: If you have multiple listening points, you can use triangulation todetermine the direction of the noise. By comparing the level of the noise at different
locations, you can estimate its origin
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2024 provided permission is obtained from the author(s) and credit is given to the author(s) and these proceedings.

Proceedings of INTER-NOISE 2024

Beamforming: This is a signal processing technique that combines the signals frommultiple microphones to create a directional pattern. By analyzing the beamformed
signal, you can determine the direction of the noise source– Acoustic Cameras: These are devices equipped with microphone arrays that can visualize
sound sources in a particular space, effectively showing the direction from which noise iscoming.

–2.
Goal
The goal is to be able to enrich the sound pressure levels measurements with an additional
sensor to determine the sound direction of the predominant noise when an event occurs forunattended measurements in azimuth (0° - 360°) and elevation (-90° - +90°), function offrequency. The required bandwidth is from octave 125 Hz to octave 4 kHz.

3.
Challenges
The sensor must be simple and easy to install, low power and the size must be compatible withthe constraints of the sites to monitor. It has to be weather proof for unattended long-term
measurements outside.

4.
Which method has been selected and why?
Due to the physical encumbrance constraint, among the various methods, a 3D intensity probe
has been selected as the best compromise in terms of performance and size in a frequency
domain compatible with the required bandwidth.
Moreover, using sound intensity techniques, it is possible to quantify the quality of themeasurement by checking the dynamic capability of the probe during the measurement.
5.
Principle and design
8 microphones are installed at the tops of a cube of 2 cm side:

Figure 1: Microphones 3D probe setup.

The intensity vector is measured using 12 single axis intensity probes (2 microphones):
the result is obtained by the vector sum of the intensity over the 3 axes:

Proceedings of INTER-NOISE 2024���⃗ + ���⃗���⃗ ,𝐼𝐼⃗ = 𝐼𝐼𝑥𝑥
𝐼𝐼𝑦𝑦 + 𝐼𝐼𝐼𝐼
Where the mean intensity vector in each axis x, y and z is calculated:���⃗𝐼𝐼𝑥𝑥 =1∗ � �������⃗𝐼𝐼𝑥𝑥𝑥𝑥𝑥𝑥 , …
4(1)
(2)

Figure 2: 3D sound intensity probe principle.

Azimuth θ and Elevation φ are deducted using trigonometric formulas.

6.
Sound intensity in practice
Reference levels:
Sound pressure (𝑝𝑝0 = 20µ𝑃𝑃𝑃𝑃):

Sound intensity (𝑖𝑖0 = 1pW/m²):
Under free field conditions:
Acoustical analogy of Ohm’s law [1]:

𝐿𝐿𝐿𝐿 = 10 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (𝑝𝑝2 /𝑝𝑝0 2 ) ,(3)𝐿𝐿𝐿𝐿 = 10 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (𝑖𝑖 /𝑖𝑖0) ,(4)𝑢𝑢(𝑡𝑡) =Where 𝜌𝜌 ∗ 𝑐𝑐 = 415 𝑁𝑁𝑁𝑁𝑚𝑚−3 @20°𝐶𝐶 𝑎𝑎𝑎𝑎𝑎𝑎 1013 ℎ𝑃𝑃𝑃𝑃 :𝑝𝑝(𝑡𝑡)𝜌𝜌∗𝑐𝑐,(5)

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𝑝𝑝(𝑡𝑡)2𝑖𝑖(𝑡𝑡) = 𝑝𝑝(𝑡𝑡) ∗ 𝑢𝑢(𝑡𝑡) = 415 ,(5)𝑖𝑖(𝑡𝑡) ≈ (𝑝𝑝(𝑡𝑡)/20)2 ,(6)

𝐿𝐿𝐿𝐿 ≅ 𝐿𝐿𝐿𝐿,

Under free field conditions, pressure level and intensity level are almost identical.

(7)

Under reverberant conditions the sound intensity is zero as the energy flow is coming randomly
from all directions. Hence there is no net flow of acoustic energy at any point.

7.
Measurements
Various measurements have been performed in an anechoic and a reverberant chamber toqualify the accuracy and determine the limits of the transducer:
A pink noise is used and the results are obtained per octave from 125 Hz to 4 kHz and A
weighted. The 3D probe is mounted on a turntable and the angle resolution is 1°.
7.1. Anechoic chamber

Figure 3: Measurement setup in the anechoic chamber.

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Figure 4: ∆𝑝𝑝𝑖𝑖 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 − 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖.

7.2. Measurement results azimuth

Figure 5: Azimuth error [°]The azimuth error graph shows:– For octave 125 Hz a maximal error < 45° on the smoothed curve– for octaves from 250 z to 2kHz a maximal error < 10°– for octave 4 kHz a maximal error < 15°. The periodic variation can be explained by themechanical construction (the tube in the middle of the cube used for the metrologicalmicrophone on top has a masking effect)– for A weighted a maximal error < 5°.

7.3. Measurement results elevation

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Figure 6: Elevation error [°]The elevation error graph shows:– For octave 125 Hz a maximal error < 30° on the smoothed curve– for octaves from 250 z to 2kHz a maximal error < 20°– for octave 4 kHz a maximal error < 30°– for A weighted a maximal error < 15°

When assessing outdoor environmental noise, the field conditions are close to free field if all
precautions are taken to avoid reflections.
These results show that under free field conditions the 3D sound intensity probe “NL1 - Noise
Locator” allow the determination of the azimuth and elevation of a sound source with asufficient accuracy in most cases.
7.4. reverberant chamber

Figure 7: measurement setup in the reverberant chamber

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The purpose of the measurement in a reverberant chamber is to determine the residual
intensity measured by the 3D probe. The difference between pressure and residual intensity
characterizes the dynamic capability of the probe and is called ∆𝑝𝑝𝑖𝑖0 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 −𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖.

Figure 8: Pressure – residual intensity index

Figure 9: Azimuth error [°]In a reactive (reverberant) sound field, the difference between pressure and intensity ismaximal; the consequence being the measurement of azimuth and elevation are not relevant.

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8.
Validation of the measurement
The validity of the measurement depends on the type of sound field and the complexity andamount of noise sources. When assessing noise in the environment, the sensor is placed outside
usually in a free field (avoiding reflections nearby).
When a single noise event occurs, it is a predominant noise source emerging from thebackground noise. In this case the measurement in an anechoic chamber is reproduced (∆𝑝𝑝𝑖𝑖
close to zero) and the measurement is valid.
If several noise sources without predominance appear at the same time the pressure will behigher than the intensity. In this case (∆𝑝𝑝𝑖𝑖 not close to zero) and the measurement is not valid.
∆𝑝𝑝𝑖𝑖 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 − 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 gives a perfect indication of the reactivity of the sound field.
When it is close to zero, it means that free field conditions are fulfilled and the measurement isvalid. On the opposite, if ∆𝑝𝑝𝑖𝑖 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 − 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 is close to ∆𝑝𝑝𝑖𝑖0 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 −𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖, the measurement is not valid.

9.
CONCLUSIONS
A 3D sound intensity probe for assessing noise in the environment is a useful tool to determine
the direction of a predominant noise source. Despite its small size (cube of 2 cm size), theresolution in frequency starts at 125 Hz with a sufficient accuracy.
When the sound field is complex and the sound sources multiple, the measurement will not give
relevant results. Which is predictable as there is no specific direction result to expect!
However, under outdoor free field conditions, if a predominant noise source emerge from thebackground noise the 3D intensity probe “NL1 – Noise Locator” will measure correctly its
azimuth and elevation.
The validity of the measurement is verified by comparing pressure and intensity levels;
therefore, the implementation of a specific condition on ∆𝑝𝑝𝑖𝑖 : 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 − 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 foreach octave band calculated during the noise events will validate or invalidate the direction
determined by the sensor.
ACKNOWLEDGEMENTS
We gratefully acknowledge the R&D teams of the companies NTi Audio AG and CAE Software
und Systems GmbH for their deep implication in the design of the probe as well as the massive
dataset collected for this survey.
REFERENCES
1. Gade S. Bruel and Kjær Technical Review N°3 and N° 4, 1982; N°4, 1985
2. Roland J. “What are the Limitations of intensity techniques in a hemi-diffuse field”, 1982,
Internoise Proceedings pp.715-718
3. Rasmussen, G & Brock M. “Acoustic Intensity Measurement Probe” Acoustic Intensity –Senlis 1981 pp.81-84