Low-frequency applications for the

Acoustic Camera

Background on Acoustic Camera – Beamforming systems

Acoustic Camera resolution

Some new SW algorithms

Low frequency performance

Wind power plants applications

Conclusion

ACOUTRONIC

PAK and GFaI system measuring wind power plants

Background on Acoustic Camera systems

Developed in mid 90’s, commercial ~2000 GFaI

From 2004 more companies, 2010 approx 10 – 15 brands

2014: 3D microphone arrays & Laser scanning of rooms -> CAD-

drawing with all distances to each surface correct.

News - SW: PassBy, PsychoAcoustics, SoundPower, Acoustic Erase

ACOUTRONIC

Acoustic Camera project considerations

Frequency

- What frequency range are we interested in ? – HW sampling

Spatial resolution

- mm, cm, dm or meters ? – HW microphone array

Masking of sources

- Are we close to disturbing sources ? – SW algorithms

Dynamic Range

- Will the contrast in the photo be enough ? – SW algorithms

Extras

- Do I need 3D, Harshness, Orders (RPM) or extra sensor ? – SW & HW

ACOUTRONIC

Spatial Resolution vs. Sampling frequency

Array Plane Source Plane2 dot like white noise Sources

> 1m Distance = r

> 0.05 m apart from another = x

X = 0.05 m

1.0000000 m

1.0012492 m

0.0012492 m ~ 1.25 mm

With Beamforming the main information to find the location of the sound is the run time delay. The

spatial resolution is in direct relation to Sampling frequency in the time domain and the mic spacing.

To resolve two sources we need to find differences in the characteristic patterns in

in the run time delays independent of post signal processing. This is independent

from the resolution which is determined by the signal frequency

Geometrical

48 kHz => 7.2 mm => 12.02 cm

65 kHz => 5.3 mm => 10.31 cm

96 kHz => 3.6 mm => 8.49 cm

192kHz => 1.8 mm => 6.01 cm

22

min rrx

192000

344m

f

v

f

c

sampling

sound

minx – Uncertain circular area around a possible source (in time and space)

Uncertain area: λ/2=3cm

Frequency vs. Array size

Array Size Source PlaneExample of array sizes

Low frequency limits is in direct relation to the size of the microphone array.

To resolve two uncorrelated sources we need to detect differences in the characteristic patterns in the run time delays.

Frequency

63 Hz => 5.46 m

100 Hz => 3.44 m

1 kHz => 0.34 m

10 kHz => 0.03 m

mm

f

v

Hz

sound 4.3100

344

Frequency

63 Hz => 2.73 m

100 Hz => 1.72 m

ACOUTRONIC

Spatial Resolution vs. Frequency

spatial/frequency accuracy: source diameter of 46cm, 500Hz spatial/frequency accuracy: source diameter of 22cm, 1kHz

spatial/frequency accuracy: source diameter of 10cm, 2kHz

The acoustic pictures below displays the

same dot like white noise in a distance of

1m, with a 75cm Ring Array with 32 Mics.

All pictures display have dB-contrast 3dB.

spatial/frequency accuracy: source diameter of 2cm , 10kHz

Channel number vs. Sampling frequency

16 Chanl.

48 kHz

72 Chanl.

48 kHz

32 Chanl.

48 kHz

120 Chanl.

48 kHz

ACOUTRONIC

16 Chanl.

192 kHz

72 Chanl.

192 kHz

120 Chanl.

192 kHz

32 Chanl.

192 kHz

Channel number vs. Sampling frequency

ACOUTRONIC

- Acoustic Eraser – remove source

- High Dynamic Range HDR

Industrial Façade with leaks:

ACOUTRONIC

Source localization by Acoustic Camera – SW Tools

Left…: Source 66dBA, contrast 68-63(5)dBA

Middle: Erase dominating source, second source visible contrast 57-56 (1)dBA

Right..: HDR activated, contrast 68-48(20)dBA

By use of HDR we can se both dominating and other sources,

Artefacts can occur

Examples: With / Without HDR

ACOUTRONIC

Contrast: 50dB

Scale: 68–18 dB

Inbetween:-32dB

Contrast: 4dB

Scale: 68–64 dB

Between: - 2dB

Artifacts can occur

Source localization by Acoustic Camera – SW Tools

ACOUTRONIC

Low frequency performance

• Low frequency resolution is not so good depending on the size of array

Left: Source = Est. Ø=5-6m (1dB contrast) Right: Source Ø=2.2m (1dB contrast),

(Norsonic 848, Ø=1.6m, dist.= 2.5m) (GFaI Star array Ø=3.4m, dist.=5.4m)

Low frequency acoustic photos 1/3-octave160Hz in field and lab by two different systems:

HDR active with two

different settings:

Left..: SS=4, I=4, Ø=1.1m

Right: SS=8, I=8, Ø=0.8m

ACOUTRONIC

Applications – four different Wind Power Plants

• Wind Power plants is a possible application for acoustic camera, but it

is not always so obvious and easy…. especially for low frequencies.

• Below is four spectrograms from wind power plants:

Plant with modulation/tonal components: Y=0-400Hz Plant with high frequency: Y=0-12kHz

Plant with defects/birds: 0-10kHz Plant which is rather silent

ACOUTRONIC

Wind Power plants – High frequency disturbance

• Wind Power plants is a good application for acoustic camera, it is

easy to detect which blade is defective:

Spectrogram with one defective blade in high frequency region Y=10-11kHz

ACOUTRONIC

GearBox problem – Wind Power Plants

• Lets zoom in on the problematic areas: Filter out the birds

Plant with defects zoom: X=5.2 to

6.7s Y=0 to 1kHz

Plant with defects - original: X=32s, Y=0 to 20kHz

Acoustic photo of defect

Low frequency/ Tonal application: 90Hz tone is ”modulated”

Zoom shows the ”90Hz” noise to be varying in amplitude

Right: Zoom of 80-100Hz:

The average spectra for 6s segment shows…..: Fundamental at 30.1Hz

Tone 1: 90.2 Hz

Tone 2: 90.7 Hz

Tone 3: 91.4 Hz

Tone 4: 92.1 Hz

ACOUTRONIC

Amplitude modulation

Amplitude Modulation (AM)

In AM there is a carrier frequency and one(SSB)/two sidebands(DSB)

AM works by varying the amplitude of the carrier being sent. The average spectra for 6s segment shows: Fundamental at 30.1Hz

Tone 1: 90.2 Hz

Tone 2: 90.7 Hz

Tone 3: 91.4 Hz

Tone 4: 92.1 Hz

ACOUTRONIC

Low frequency application: 90Hz tone is ”modulated”

By analysing the envelop spectra we get modulation (F=91Hz): 91 / 432 = 0.211 Hz

Peak 1: 2x 0.211 = 0.44 Hz

Peak 2: 3x 0.211 = 0.63 Hz

Peak 3: 6x 0.211 = 1.25 Hz

Peak 4: 9x 0.211 = 1.90 Hz

ACOUTRONIC

Low frequency application: 90Hz tone is ”modulated”

Average over 7.2s between 89–111Hz: Tower is main source

ACOUTRONIC

Low frequency application: 90Hz tone is ”modulated”

Average over 7.2s but 91–111Hz: Tower and blade is source

ACOUTRONIC

Low frequency application: 90Hz tone is ”modulated”

Average but over 0.29s but 89–111Hz: Blade is source

ACOUTRONIC30 Hz tone looks like a straight line

The 30Hz tone

Extreme Zoom: FFT 28-32 Hz an Spectrogram 25-35Hz

Spectra from 6s average, frequency is 30.2Hz (0.1Hz resolution)

The variation is from 30.0Hz to 30.4Hz and 4 – 5 dB

The period time is approx 1.5s (0.67Hz)

ACOUTRONIC

Vib-measurements

have been done on

tower to confirm

presence of 30/90 Hz

vibrations in structure.

The Nacelle is radiating tonal noise between 200-230Hz

MP1 &MP4: The Nacelle is radiating tones between 200 – 230Hz

During 2min run-up the noise 200-230Hz starts after 48s

The tonal components is much more present in -90° direction

There are 2 different components: at 200Hz and varying 210-230Hz

The 200Hz tone comes from nacell (rotor resonance at 200Hz)

The varying 210-230Hz tone comes from nacelle

Overtone at +90Hz = 300-320Hz comes from nacelleACOUTRONIC

The Nacelle is radiating tonal noise between 200-230Hz

MP1: The Nacelle is radiating tonal noise between 206 – 218Hz

ACOUTRONIC

But from front view the Nacelle is not dominating

MP5: Spectrogram for 200-400Hz zoom: Marked area 207 -220Hz

MP5: Components at 200-230Hz are rather low in level

MP5: Acoustic Photo for 210-228Hz comes from blades

ACOUTRONIC

The Nacelle is radiating tonal noise between 200-320Hz

MP4: Spectrogram zoom between 200 – 350Hz MP4: The nacelle is radiating tonal noise between 207 – 227Hz

MP4: The nacelle is also radiating tones between 300 – 320 Hz (marked)

MP4: The nacelle do also have a clear tonal component around 290Hz

ACOUTRONIC

The Nacelle is radiating tonal noise between 200-230Hz

MP4: Acoustic Photo for 210-215Hz component

MP4: Acoustic Photo for 294-305Hz component (marked area)

ACOUTRONIC

Comparison of spectra from different views to achieve

directivity information for 90 Hz and 180Hz.

Comparison from 10°(370m): Left and -90° (316m): Right

Distance level comp:1dB

Spectrograms:

32s average

Acoustic Photos:

filtered 82–190Hz

(for creating spectras)

Spectras:

At acoustic photos

maximum point

= mid tower

10°: ”90Hz range” =15dBA -90°: ”90Hz range” =16dBA

10°: ”180Hz range” =13dBA -90°: ”180Hz range” =20dBA

10°:Variation: 90Hz/32s in 1.5s mov.ave: 13-27dBA -90°: 20-26dBA

10°:Variation:180Hz/32s in 1.5s mov ave: 17-32dBA -90°: 13-30dBA

Note: the tones and modulation vary a lot in frequency and level

Simultaneously measurements must be performed to achive directivityACOUTRONIC

Low frequency applications for the Acoustic Camera

Conclusion

The acoustic camera can be used for studying noise from Wind Power Plants

also for low frequency applications, but be aware of the limitations in

beamforming systems. We need an array of >3-4m diameter to achieve

enough resolution for the low frequency range around or below 100Hz.

By using high sampling frequency (200kS/s) & high resolution FFT’s, enables

possibilities to get better resolution

By using new algorithms like Acoustic Eraser and HDR makes it possible to

better localize and find masked sources.

By use of Acoustic Camera in WPP measurements it is possible to localize

sources of tonal components, modulation and directivity. The directivity is

strong and the variation large (10 – 15dB) and AM on top of that, meaning

there is a high risc for severe disturbance at residential areas.

ACOUTRONIC