Design of a spherical array of microphones for room acoustics … · 2014. 6. 2. · Laboratory of...

Design of a spherical array of microphones for room acoustics applications.

ABAV AG 28 may 2014 1

Intelsig Research group Laboratory of Acoustics

H. Feron (*) and J.J. Embrechts (Intelsig group, Laboratory of Acoustics, University of Liege, Belgium)

(*) Now at Deltatec s.a. (Ans-Liège)

ABAV AG 28 may 2014 2

Intelsig Research group Laboratory of Acoustics

The room impulse response RIR

(between an omnidirectional source and an omnidirectional receptor)

time

sou

nd

pre

ssu

re

P S

P S

ABAV AG 28 may 2014 3

Intelsig Research group Laboratory of Acoustics

The directional (or spatial) room impulse response DRIR

(between an omnidirectional source and a directional receptor)

time

sou

nd

pre

ssu

re

ABAV AG 28 may 2014 4

Intelsig Research group Laboratory of Acoustics

Directional room impulse responses DRIR can help to:

- Evaluate « spatial » room acoustics parameters, such as the lateral energy fraction LEF, the left/right ratio LRR or the reverberation time in a particular direction,

- Detect possible (flutter) echoes in a particular direction,

- Create 3D auralization of acoustic spaces.

ABAV AG 28 may 2014 5

Intelsig Research group Laboratory of Acoustics

How to measure Directional room impulse responses DRIR ?

- With a microphone antenna (microphone array), which can provide beamforming and beamsteering.

Beam pattern of array output

ABAV AG 28 may 2014 6

Intelsig Research group Laboratory of Acoustics

Which kind of antenna for that application ?

(form, size, number of microphones,…)

- Master project of Hermine Feron (2013),

- Spherical array, 10cm radius,

- Rigid sphere,

- 16 microphones,

- Nearly uniform distribution

on the sphere,

- Two souncards (8 inputs each),

- Recordings in *wav format,

- DSP in Matlab.

ABAV AG 28 may 2014 7

Intelsig Research group Laboratory of Acoustics

How does it work ?

(some hopefully few mathematics)

- The sound field P(k,r,W) existing around the antenna can be described by a series of spherical harmonics functions.

W = (f,d).

- k=2pf/c (the wave number).

Spherical coordinates

ABAV AG 28 may 2014 8

Intelsig Research group Laboratory of Acoustics

How does it work ? (some hopefully few mathematics)

- If you know the spherical coefficients Pnm(k,r), then you obtain the sound field in any direction W.

- « n » is limited to the order « N » of the array => the directional lobe of sensitivity has a non-zero extent.

N=1 N=2 N=3

How does it work ? (some hopefully few mathematics)

- The spherical coefficients Pnm(k,r) are determined by combining the 16 pressure signals measured in the directions Wj (j=1,16).

ABAV AG 28 may 2014 9

Intelsig Research group Laboratory of Acoustics

Array output:

= =

W=WN

n

nml

n

nm

nmlPWY

0

* )(),(),( P(,Wj)

16 microphones signals

Pnm()

(N+1)² Spherical coefficients Several algorithms…

Weights: they depend on the steering (look-up) direction Wl.

ABAV AG 28 may 2014 10

Intelsig Research group Laboratory of Acoustics

Measurement of Directional room impulse responses DRIR.

Sound source: log sine sweep signal

(250-4000 Hz).

Deconvolution of the 16 recorded sweeps leads to 16 RIRs (not yet DRIRs !)

ABAV AG 28 may 2014 11

Intelsig Research group Laboratory of Acoustics

Example 1: One reflecting panel in the anechoic room.

ABAV AG 28 may 2014 12

Intelsig Research group Laboratory of Acoustics

Example 1: One reflecting panel in the anechoic room.

Time evolution of array output in the horizontal plane: Algorithm ‘Delay and Sum’ (DAS)

Spherical coordinates

ᵠ [°]

Tim

e [m

s]

ABAV AG 28 may 2014 13

Intelsig Research group Laboratory of Acoustics

Example 1: One reflecting panel in the anechoic room.

Space distribution of array output in a specified time window (direct contribution):

Algorithm ‘Minimum-variance distortionless response’ (mvdr) Spherical coordinates

ᵟ [°

]

ᵠ [°]

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Intelsig Research group Laboratory of Acoustics

Example 2: DRIRs in a shoebox reverberant room.

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Intelsig Research group Laboratory of Acoustics

Example 2: DRIRs in a shoebox reverberant room.

Direct

Floor

Ceiling

f [°]

ᵟ [°]

Tim

e [m

s]

Tim

e [m

s]

14 to 16 ms

10,7 to 12,7 ms

17 to 18,2 ms

ABAV AG 28 may 2014 16

Intelsig Research group Laboratory of Acoustics

Example 3: DRIRs in a long corridor, closed by reflecting doors.

0 200 400 600 800-200

-150

-100

-50

0

X: 529

Y: -46.95

temps [ms]

Réponse impulsionnelle omnidirectionnelle

Flutter echoes

Omnidirectional RIR

Time [ms]

ABAV AG 28 may 2014 17

Intelsig Research group Laboratory of Acoustics

Example 3: DRIRs in a long corridor, closed by reflecting doors.

0 50 100 150 200 250 300 350 400 450 5000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

X: 417.6

Y: 0.02576

temps [ms]

Réponse impulsionnelle directionnelle dans la direction dl = 0° et f

l =180°

1

2

3

1

180° 0°

ABAV AG 28 may 2014 18

Intelsig Research group Laboratory of Acoustics

Example 3: DRIRs in a long corridor, closed by reflecting doors.

0 50 100 150 200 250 300 350 400 450 5000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

X: 417.6

Y: 0.02576

temps [ms]

Réponse impulsionnelle directionnelle dans la direction dl = 0° et f

l =180°

1

2

3

2

180° 0°

180° 0°

ABAV AG 28 may 2014 19

Intelsig Research group Laboratory of Acoustics

Example 3: DRIRs in a long corridor, closed by reflecting doors.

0 50 100 150 200 250 300 350 400 450 5000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

X: 417.6

Y: 0.02576

temps [ms]

Réponse impulsionnelle directionnelle dans la direction dl = 0° et f

l =180°

1

2

3

3

ABAV AG 28 may 2014 20

Intelsig Research group Laboratory of Acoustics

Example 4: DRIRs in an auditorium.

ABAV AG 28 may 2014 21

Intelsig Research group Laboratory of Acoustics

Example 4: DRIRs in an auditorium.

Direct sound

Floor and tables

Ceiling

Blackboard wall

ABAV AG 28 may 2014 22

Intelsig Research group Laboratory of Acoustics

Example 4: DRIRs in an auditorium.

Effect of absorbing material on the blackboard wall.

DRIR between 15 and 25ms

ABAV AG 28 may 2014 23

Intelsig Research group Laboratory of Acoustics

Future works … - Improving the selectivity of the antenna:

- Illustration of the dynamic evolution of the

reflections (at least the first-order ones).

- Automatic computation of the room acoustics’ spatial parameters.

- Tests in great volumes (theatres, industrial halls).

ᵠ [°]

Tim

e [m

s]