University of Parma, Italy

New measurement technique for 3D sound  characterization in theatres

A.Farina

, L. Tronchin

1

University of Parma, Italy

2

University of Bologna, Italy

Most of the acoustical measurements in theatres, concert halls and musical spaces are performed by using a single omnidirectional pressure microphone.

Introduction

Useful for reverberation time and other monophonic parameters

NO information about the direction of arrival of the sound

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Aim of the research: visual and dynamic display of the acoustical behaviour of the room under test (theatres, concert halls, etc.).

The dynamic vision of this behaviour could be useful for :

• Finding the origin of unwanted reflections (echoes)

• Evaluating the spectral content of those reflections

• Checking if the sound reinforcement loudspeakers are correctly located and aimed

Employing directive microphones and aiming them in

different directions it is possible to obtain a chart of the

behaviour of the sound in time and in frequency

Methods

• For mapping the direction of arrival of early

reflections, three methods have been successfully tested:

1. Good, old Ambisonics (1

order B-format) 2. A shotgun microphone over a turntable

3. A spherical microphone array (Eigenmike™)

Previous experience

• At UNIPR and UNIBO we have 10+ years of experience employing 1

-order Ambisonics microphones (Soundfield

, DPA-4, Tetramic, Brahma)

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Capturing Ambisonics signals

• A tetrahedrical microphone probe was developed by

Gerzon and Craven, originating the Soundfield microphone

Soundfield microphones

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• The Soundfield (TM) microphone provides 4 signals:

1 omnidirectional (pressure, W) and 3 figure-of-8 (velocity, X, Y, Z)

Ambisonics signals

Directivity of transducers

Soundfield ST-250 microphone

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Advanced IR capture and  rendering (      project)

•

In 2003 Waves launched a large research project, aimed to capturing a huge set of 3D impulse

responses in the most famous theatres of the world

•

The measurments did employ three diffrent

microphone systems, but here we are talking only about the Soundfield microphone, as in the

original Gerzon’s suggestion

•

More than 100 acoustical spaces were measured, including several historical sites, including the

Greek/Roman theatres of SIracusa and Taormina,

in SIcily

Measurement Setup

ƒ The measurement method incorporated all the known techniques:

o Binaural

o B-format (1

order Ambisonics)

o WFS (Wave Field Synthesis, circular array)

o ITU 5.1 surround (Williams MMA, OCT, INA, etc.) o Binaural Room Scanning

o M. Poletti high-order virtual microphones

ƒ Any multichannel auralization systems available in 2003 was supported

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Measurement Parameters

• Test Signal: pre-equalized sweep

Start Frequency 22 Hz End Frequency 22 kHz Sweep length 15 s Silence between sweeps 10 s Type of sweep LOG

z Deconvolution:

Test Signal – x(t)

Measured signal ‐ y(t)

™ The not-linear behaviour of the loudspeaker causes many harmonics to appear

Inverse Filter – z(t)

The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]

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Result of the deconvolution

The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders

2°1°

5° 3°

Transducers (sound source #1)

•

Equalized, omnidirectional sound source:

o Dodechaedron for mid-high frequencies o One-way Subwoofer (<120 Hz)

Dodech. LookLine D200

60.0 70.0 80.0 90.0 100.0 110.0 120.0

25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000

Frequency (Hz)

Sound Power Level (dB)

Unequalized Equalized

Lw,tot = 94.8 dB Lw,tot = 106.9 dB

Directivity of transducers

LookLine D-200 dodechaedron

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Directivity of transducers

LookLine D-300 dodechaedron

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Directivity of transducers

Omnisonic 1000 dodechaedron

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Transducers (sound source #2)

•

Genelec S30D reference studio monitor:

o Three-ways, active multi-amped, AES/EBU o Frequency range 37 Hz – 44 kHz (+/- 3 dB)

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Transducers (microphones)

• 3 types of microphones:

o Binaural dummy head (Neumann KU-100)

o 2 Cardioids in ORTF placement (Neumann K-140) o B-Format 4 channels (Soundfield ST-250)

Turntable

Binaural dummy head Cardioids (ORTF) Soundfield Microphone

Other hardware equipment

•

o Outline ET-1

z Computer and sound card:

– Signum Data Futureclient P-IV 1.8 GHz

– Aardvark Pro Q-10 (8 ch., 96 kHz, 24 bits)

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Measurement procedure

• A single measurement session play backs 36 times the test signal, and simultaneusly record the 8 microphonic channels

Theatres measured

N. Theatre N. sources/receivers

1 Uhara Hall, Kobe, Japan 2/2

2 Noh Drama Theater, Kobe, Japan 2/2

3 Kirishima Concert Hall, Kirishima, Japan 3/3 4 Greek Theater in Siracusa, Italy 2/1 5 Greek-Roman Theater in Taormina, Italy 3/2

6 Auditorium of Parma, Italy 3/3

7 Auditorium of Rome (Sala 700), Italy 3/2 8 Auditorium of Rome (Sala 1200), Italy 3/3 9 Auditorium of Rome (Sala 2700), Italy 3/5

10 Bergamo Cathedral, Italy 2/1

11 Teatro Valli, Reggio Emilia, Italy 5/1 12 Sydney Opera House, Opera Theatre 4/2 13 Sydney Opera House, Concert Hall 3/3 14 Sydney Opera House, The Studio 3/1

15 Tearo Regio, Parma, Italy 6/1

Reverberation Time T20

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

31.5 63 125 250 500 1000 2000 4000 8000 16000

Frequency (Hz)

T20 (s)

Uhara

Noh

Kirishima

Siracusa

Taormina

Audit. Parma

Roma-700

Roma-1200

Roma-2700

Bergamo Cathedral Valli-RE

SOH Concert Hall

SOH-Opera Theatre SOH-The Studio Regio Parma

Greek Theater in Siracusa

T 20 = 0.65 s

Roman Theater in Taormina

T 20 = 1.15 s

Current use of Ambisonics

• 1

order Ambisonics is still widely employed, as now it can be implemented employing very cheap

equipment (Tetramic, Brahma)

• Pulsive sound sources are usually preferred for a number of reasons

• Portable, battery operated recorders make it very easy to collect a large number of impulse responses

• A new digital method of processing the signals

provides much better polar response than those

available form the original Soundfield microphone

Current use of Ambisonics

• Balloons or Firecrackers as sound source

Firecracker Vs. Dodechaedron

• Comparison in Patras’ Odeion

Dodechaedron

Firecracker

Firecracker Vs. Dodechaedron

• Comparison in Patras’ Odeion

Dodechaedron Firecracker

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Other pulsive sources

• Balloons, starter pistol

Balloons

• Large ballons have more pronounced low frequencies

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Starter Pistol

The “clap machine”

• Good frequency response and repatibility

• Verification of the repeatibility

The “clap machine”

The “clap machine”

The “clap machine”

Current use of Ambisonics

• A portable digital recorder equipped with

tetrahedrical microphone probe: BRAHMA

Conversion from A‐format to B‐format

• A 4x4 filter matrix is employed in the X-volver free

plugin

ISO 3382 acoustical parameters

• Aurora plugin – processing the Odeion in Patra

ISO 3382 acoustical parameters

• Reverberation Time T30 – Odeion in Patras

ISO 3382 acoustical parameters

• Reverberation Time T30 – Audit. University of Patras

Spatial Analysis

• The direction of arrival can be found as follows:

• The Sound Intensity vector components are computed

• I

=w·x I

=w·y I

=w·z

• Also the total energy density is computed

• De=sqrt(w·w+x·x+y·y+z·z)

• They are averaged over 1ms time slices

• The ratio between active intensity and energy density is finally computed

• I

= sqrt(I

·I

+I

·I

+I

·I

) R=I

/De

• And azimuth and elevation of reflections are found:

• Az = atan2(I

,I

) El = asin(I

/I

)

Background image

• The Mercator projection is employed for creating a

rectangular image covering the whole surface of

the sphere

Image Composite Editor

• Creates a panoramic image ranging 360°horizontally

and up to 180°vertically

Reflection Mapping

• We can now plot a circle for every reflection, at the Azimuth and Elevation found, over a standard

Cartesian framework

• The radius of the circle is made proportional to the Sound Intensity level in dB

• The transparency of the circle is made proportional to the ratio R

• When R is low, the intensity is not indicating anymore a direction of arrival which can be perceived by the listeners

• When R is large (close to 1), the sound is strongly

polarized in one direction, which can be easily

perceived

Echo localization from B‐format IR

• Visual Basic program for displaying reflections

Sound  Intensity

Energy  Density

Echo localization from B‐format IR

• Odeion in Patras

Echo localization from B‐format IR

• Odeion in Patras

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Approach #2: a rotating shotgun  microphone

2008 - two similar opera houses chosen for the measurements:

Teatro Sociale

di Como Teatro Comunale di

Modena

Measurements equipment:

- Omnidirectional source

- Edirol FA-101 sound card and a laptop for the recording of the sine-sweep test signal

- Sennheiser ME66 directive microphone mounted on a

Outline rotating table

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- Azimuth: 18 steps (20°) - Elevation: 8 steps (22.5°)

- Impulse responses derived from sweeps

by using Adobe Audition 1.5 and Aurora

plug-ins

Some results

Teatro Comunale di Modena Source in orchestral pit

Point A

125 Hz 1000 Hz 4000 Hz

24 ms (direct sound)

The direct sound is rich of low frequencies for the diffraction of the pit

40 ms (first reflection)

The high frequencies arrive to the performer 16 ms after the direct sound

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Teatro Sociale di Como

S1 R

Source S1: the first reflection arrives from the back wall of the theatre after 40 ms

S2

4000 Hz

Source S2: direct sound at high frequencies is weaker than the first reflection coming after 56 ms from the proscenium arch

4000 Hz

Dynamic polar plot in vertical and horizontal plane.

Teatro Sociale di Como – 4000 Hz

Reflection from the proscenium arch

Approach #3: a spherical array

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How can we obtain lots of directive microphones with only one probe? Using an array of capsules!

2009: first prototype of a spherical array (32 capsules)

Expanded polyurethane (too much delicate for

handling!)

32 capsules for earing-aid

(poor quality)

The The EIGENMIKE EIGENMIKE ^{TM TM}

9 Array with 32 ½” capsules of excellent quality, frequency response up to 20 kHz 9 Preamplifiers and A/D converters inside the sphere, with ethernet interface

9 Processing on the PC thanks to a VST plugin (no GUI)

Cat5 cable

Traditional Spherical Harmonics approach

Spherical Harmonics (H.O.Ambisonics)

A fixed number of “intermediate” virtual microphones is computed (B-format), then the dynamically-positioned virtual microphones are obtained by linear combination of these intermediate signals. This limits both dynamic range and frequency range.

Virtual microphones

The signal processing

The idea Synthesis of 32 directive virtual microphones in the direction of the capsules employing a set of digital filters

M = 32 signals coming from the capsules

V = 32 signals yielding the desired virtual microphones

Bank of MxV FIR filters

y

(t) = x

(t) ∗ h

(t)

m=1

∑

Output signal of V mic.

Input signal from the m-capsule

Matrix of filters

Traditional design of the filters

The processing filters h_mv are usually computed following one of several, complex mathematical theories, based on the solution of the wave

equation (often under certain simplifications), and assuming that the microphones are ideal and identical

In some implementations, the signal of each microphone is processed through a digital filter for compensating its deviation, at the expense of heavier computational load

outputs of the microphone array are maximally close to the prescribed ideal

responses This method also inherently corrects

for transducer deviations and acoustical artefacts (shielding, diffractions, reflections, etc.)

No theory is assumed: the set of h_mv filters is derived directly from a set of impulse response measurements, designed according to a least- squares principle.

Novel approach

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Virtual microphones synthesized for this research:

4^th ORDER CARDIOIDS

Matlab script

•Inputs:

9 2048 samples of each IR

9 The number of virtual microphones 9 Directivity of each virtual microphone 9 Azimuth and elevation of each virtual

microphone

IRs Matrix inversion

Output: FIR filters matrix

✕ =

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The virtual microphones

The probe is not calibrated with an absolute level: every measurement has its own level normalization and colour scale.

BUT

Growing the distance between source and receiver the reflections become more relevant in comparison with the direct sound.

1 3 2

24

24 virtual microphones for horizontal polar

1

2 32 3

32 virtual microphones for 3D map

Explanation

Whilst Sherical Harmonics are the “spatial” equivalent of the Fourier analysis of a wavefront,

Our “virtual microphone” approach is the equivalent of representing a waveform with a sequence of pulses

(PCM, pulse code modulation)

1

2 32 3

32 virtual microphones for 3D map

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Image coverage from the 32 virtual microphones

In reality, 32 “spatial samples” are not really a lot: some interpolation is required for getting sensible colour maps

1 2

3

4 5

6

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11 12

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14

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21 22

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27 28

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32 17

19 21

25

The post processing software 1

step

Xvolver is employed for generating the virtual microphones

MATLAB script

Mesh map of the levels on a 360°x180° picture of the room

Dynamic plots

2

step

Matlab script for creating a video animation

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The new session of measurements Two different kind of musical space:

Sala dei Concerti

(Casa della Musica - Parma) Teatro “La Scala”

(Milano)

Equipment:

•Eigenmike

probe as receiver

•Laptop as recorder

•“Lookline” Dodecahedron as omni-direcitonal source

•Sine sweep as test signal

Teatro “La Scala”: 4 kHz – root 2 – 8000 samples

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The direct sound

500 Hz

1000 Hz Radiation of the

wooden stage

The loudspeaker was here

The source is close to the receiver

The direct sound masks on the map the first reflection on the floor

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The first visible reflection comes from the column behind the source, followed by a reflection on the floor and by a reflection coming from the opposite direction of the first one.

The reflections

4 ms

57 ms

The sound bounces

between the columns of the proscenium arch…

… and then arrives the ceiling reflection.

“Sala dei Concerti”: 4 kHz – root2 – 8000 samples

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The direct sound

500 Hz - the sound in this freq. band anticipates the direct sound. Is the effect of the radiation of the wooden structure of the stage

500 Hz

4000 Hz

21 ms

Strong lateral reflection (90°)

coming from a plane even surface.

On the opposite side that reflection is not

present because of an absorbent curtain.

After this reflection the sound appears quite diffuse in the

room apart from a little bounce between the lateral walls.

97 ms

Strong reflection from the back of the room.

The effect is audible

also from the stage and causes problems with high repeated notes to the performers

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Conclusions

We presented a new approach to the impulse response measurements. It permits the dynamic view of the impulse responses plotted on a panoramic picture (360°x180°) by using 32 high directive virtual microphones.

Positive aspects

Easy visualization of undesired reflections

Easy way for locating their arrival direction

Easier way for correcting them

This is a first step in the research, a lot of work has to be done:

• Calibration of the probe

• Creation of a plug-in (VST or Audacity-based)

• IP-camera with parabolic mirror for taking the panoramic pictures

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Future Work

Hardware for 360° video

• A 2 Mp hires Logitech webcam is mounted under a parabolic mirror, inside a Perspex tube

• The video stream from the Logitech webcam is processed with a realtime video-

unwrapping software, written by Adriano Farina in “Processing”, a Java-based programming language and environment

• It is possible to record the

unwrapped video stream to a

standard MOV file

Video unwrapping

Unwrapped image Original image

Thank you for the attention

angelo.farina@unipr.it

University of Parma