Microphone Phased Arrays

(Acoustic Antennas)by V. Quaranta; presenter: A. Concilio

The Italian Aerospace Research Centre, CIRA

• 30 accidents per year (1959-2002):

– Casualties in 57%;

– Major damages and injuries in 41%;

– A/C loss in 51%.

• Major accidents take place in the ATZ (Aerodrome Traffic Zone).

• In the period 1980-2000 it results:

Motivations

• A/C accidents could be reduced by 80%, Flight Safety Foundationreports, by a better management of the TO and L phases.

• Over 50% National Agency for Flight Safety (ANSV) enquiries, regards accidents involving sport or VFR A/C.

• A/C traffic growth requires more and more safety tools and devices; many activities are devoted to improve RADAR systems.

•An alternative may be represented by the implementation ofMicrophones Phased Array as Monitoring Air Traffic Systems.

Objective

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� System addressed to improve air traffic management and

control in ATZ:

� Autonomous (small airports);

� Supporting other systsems (large airports).

� Acoustic Patrol (trajectory, SPL and identification);

� Military applications:

� Non-active system;

� Detecting “radar non-detactable” sources (stealth A/C);

� Low-profile surveillance of isolated buildings;

� …

Applications

Advantages

� Range (>100 km)

� Resolution (2 deg height, azimut)

� Indipendent on weather conditions

� Fast scanning period (≈ 6 sec.)

� No relevation time delay (light speed)

Limitations

� Cost (≈ 1M€)

� Constraints on site choice, devices nr. and transmission power

because of regulations on electromagnetic allowable exposure

� Security constraints on radar tracks

� Detectable device

� Non-detectability of “stealth” vehicles5

RADAR

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Advantages

� Low cost (≈100k€)

� No environmental impact (no emissions)

� Passive system, non detectable (military

applications, protection of sensitive obj.)

� Detectability of “stealth” vehicles

� Access to tracking data, real time

Limitations

� Low range (≈ 10 km)

� Resolution (≈ 100 m @ 5km with 512 mic.)

� Dependance on meteo conditions

� Time delay on the emissions analysis

Acoustic Detectors

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Acoustic detectors (aerophones) wereused:

� At first, 1880, to individuate ships in case of fog;

� From the end of the WW1 to the beginning of the WW2 to detect enemyaircraft

These systems became then obsolete, at the middle of the WW2, after the RADAR invention (RAdio Detection And Ranging).

Sergeant Jean Perrin (Nobel for Atomic Physics) with the first acoustic antenna, used by the French Army during WW1 (1914-1918) to detect enemy aicraft

Hystorical Briefs

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Hystorical Images

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� Development of an acoustic system to improve management of

the air traffic inside the ATZ

� The system is made of 2 rotating arrays with 512 mic to localise

A/C in the ATZ

� Partners: D’Appolonia, Rome University “La Sapienza”, ADR

Aeroporti di Roma, CIRA

� 36 months period: Jan. 2009 – Dec. 2011.

Rotating Arrays

GUARDIAN – A National Project

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� Innovative optoelectronic and acoustic sensing technologies for

large scale forest fire long term monitoring”

� Realised system is an acoustic antenna integrated with FO device

system, to detect fires in forest

� FO net aims at detecting several parameters (mainly temperature

and gas emissions)

� Partnership: D’Appolonia, CIRA, B&K, ADAI, Cyprus Univ.

� Period: 36 months: Sep. 2006 – Aug. 2009

A European Project

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� A system to manage air traffic

� Made of 4 monitoring (RTU), and

a central station (CPCU) linked

through a GPRS net

� Noise indexes measure & estimate

of A/C trajectory (simplified mod.)

� Sw products (code) for:

� optimisation of Nr & layout

� acoustic signature detection,

� a structured DB (meteo, noise

parameters, trajectories, …).

� Partnership: AirSupport, CIRA,

Labor, Aurotr., DC; TLC, CSCE.

� Period: 36 months; Mar .2003 –

Feb. 2006

MONSTER – A EU “Craft” Project

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Mic Phased Arrays: a set of n mic’s, somehow distributed.

Digital analysis allows achieving anequivalent mic characterised by:

• High S/N ratio

• High directivity

• Digital pointing

Wheel Array - Bruel & Kjaer

Directivity curves for a directive mic (left) anda mic array (right)

State-of-the-art

Acoustic CameraGFAI tech GmbHWind Tunnel Test

Fly over Test

Pass by Noise

Noise Test

Available products

Speaker localisation and virtualmicrophone

VideoconferenceHands free

communications.

Available products – Multimedia

• ShotSpotter – Acoustic triangulation (TDOA) to detect shots in urban areas• After few sec a warning is sent to police station, addressing placeand type of gun• Currently installed in 32 US cities• Results at now: 31% violent crimes; 60-80% shots; spari; 15 arrestsin a single evening.

Available products - Patrol

• ARDEC – Gunfire Detection System• Protect troops in Iraq & Afghanistan.• Detect, locate and identifies in few sec fire origin and allows a rapid reaction, increasing the possibility of surviving.• Fixed, car-borne and wearable system

Available products – Military patrol

• Intensimetry (multiple

sources in near-field)

• Acoustic olography

(multiple sources in near-

field)

• TDOA (single source in far-

field)

• Beamforming (multiple

sources in far-field)

Cube 32 Array, Acoustic Camera, GFAI

State-of-the-art: Acoustic characterisation

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Star Array - GFAI tech GmbH

State-of-the-art: Acoustic characterisation

• Signal from source arrives to the array mic with a delay that isfunction of the mic distance

• Detected signals are de-phased and summed

• A specific direction associated to a phases set, maximises the output

Sorgente

+

∆1

∆2

∆3

∆4

1/N

p(t)

t

∆

1

∆

2

∆

3

∆4

Array Time Delay

))((1

),(1

∑=

∆−=

N

i

irtp

NrtP

rr

),( rtPr

Beamforming

• To focus the array means to select proper delays• Array steering does not need physic movement• Once the noise event is measured, a digital steering

may be done, exploring all the scanning plane and identifying the source by the pressure peaks

Beamforming

Array parameters:

• Resolution (main lobe width

- 3dB)

• MSL (Maximum Sidelobe

Level)

• Range

• Frequency (Hz)

Design parameters:

• Antenna shape and

dimension

• Mic numbers

• Mic layoutAzimuth φ [0 - 360 deg]Elevation θ [0 (Nord) - 180 deg].

Resolution MSL

-3 dB

Array directivity pattern

Array design parameters

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(a) 65-ch. cross-array(b) 64-ch. grid array (c) 66-ch. random array (d) 66-ch. arch.spiral array(e) 66-ch. wheel array(d) 66-ch. half-wheel array

MSL minimisation

Gfai Star -36ArrayGfai Sphere -32 Array

Array Shape Vs. MSL

3D-Geometry array pattern

• Optimal phase linearity

• High sensitivity (≈50 mV/Pa)

• Independent on environment (rain, humidity, T, EMI,…)

• Low cost: development and manufacture of a PMA system

(parallel microphone array) with 32 FBG mics: 160 kUSD

32 by 16 sonar frame design 4600mm diameter

FBG Mic

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� Original code for A/C acoustic signature detection.

� Set-up of a noise DB for A/C in TO and L @ Napoli A/P

� Experimental system validation

Acoustic signature

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Critical issues:

� Characteristic parameters identification

� Reduction of the parameters into vectors of suitable length

� Classification method

Acoustic signature, 2

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Available techniques (unsteady time signal):

Characteristic parameters extraction:

�Short Time Fourier Transform

�Biologic ear models

�Wavelet analysis

Classificators:

� k-nearest neighbor

�Neural networks

�Fuzzy Logic

Acoustic signature, 3

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Flow-chart

� Wavelet

Decomposition

� Features

extraction through a

statistic analysis

� Classification

� Training

� Identification

Fase di Training Fase di Classificazione

Acoustic signature, 4

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Graphic Interface

• Noise characterisation of an L A/C; Dept. of Helicopters and Aeroacustics @ NLR;

• Higher resolution (13 m array) shorter range (243 mic’s)

Ongoing activities @ NLR

• Test facility with more than 600 ground-based microphonesarranged in a custom spiral pattern spread over the end ofthe runway in a 250-foot-wide by 300-foot-long area

• Acquisition of the noise of a 777-300ER as it flew overheadand immediately retrieved and processed the data to get anacoustical image of the airplane.

Ongoing activities @ Boeing

• Gasoline cars show a flatspectrum, with a meancontent at 70 dB (standard distance).

• Sport cars are characterisedby a low band spectrum(100-200 Hz), often attainedby a proper silencerdimensioning. Medium tohigh freq spectrum is insteadsimilar to the classical cars.

Car noise

General vehicles show a

high spectral content at low frequency, with the major

exception of A/C.

Car noise

Figure 1. Spectrogram of the SPL from the passage

of a diesel Intercity 125 (V ≅≅≅≅ 160 km/h).

Figure 2. Spectrogram of the SPL from the passage of an

electric Intercity 225 (V ≅≅≅≅ 200 km/h).

Figure 4. Spectrogram of the SPL from the passage of a

diesel freight train (V ≅≅≅≅ 85 km/h).Figure 3 Spectrogram of the SPL from the passage of a

electric multiple unit (V ≅≅≅≅ 105 km/h).

Railway noise

Sources of Railway Noise• Traction Noise – For diesel engines, exhaust noise, engine and transmission vibrations. Electric power

units are much quieter, though noise is emitted from the traction motor and extra cooling fans. Pantograph noise is significant at high speeds.

• Rail/Wheel Noise – Rail and wheel are set into vibration. This produces external and internal noise. Main sources are poorly aligned track joints and the roughness of the wheels and the track.

• Auxiliary Equipment Noise – compressors, ventilation and brake systems.• Aerodynamic Noise – Produced by passage of the train through the air. Its contribution to the total noise

level increases with speed.• In addition, one can recognise flange squeal, bridge noise and ground vibrations. • For diesel locomotives with block braking systems traction and rail/wheel noise is about the same above

100 km/h. With disk braked stock the traction noise dominates up to the maximum speed. For diesel multiple units traction noise does not dominate at the top of the speed range unless disk brake are in use. For electrical units rail/wheel noise dominates above 30 km/h.

Spectrograms for four major train types, diesel Intercity 125, electric Intercity 225, electric multiple unit and a diesel freight train, are provided in Figures 1-4, respectively.

• Figure 1 shows two substantial peaks, which occur when the diesel engines on either end of the train pass the receiver. The engine noise sources are predominant and their spectrum extends between 100 and 6500 Hz. Fans and coolers also produce high levels of noise in diesel trains. Most of the noise from the carriages is rail/wheel interaction noise and aerodynamic noise. Figure 1 shows the spectrum of the horn which is blown at t ≅ 5 sec.

• In the case of an electric Intercity 225 train (see Figure 2) the noise spectrum is relatively stable during the train pass-by. Traction, rail/wheel and aerodynamic source of noise are the prime contributors. Traction and rail/wheel noise sources are also predominant mechanism for noise generation in electric multiple units (see Figure 3).

• In the case of the freight train broad band noise (100-6500 Hz) is emitted during the pass-by of the locomotive (see Figure 4, at t ≅ 9 sec). For this train the noise spectrum emitted during the passage of its carriages is related to rail/wheel vibration and is relatively stable.

Railway noise