Practical class on room acoustic for sound and recording

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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
ED Education in acoustics: Paper ICA2016-366
Practical class on room acoustic for sound and
recording degree program in Santa Fe, Argentina
Pablo Delmonte Zalazar(a), Paula Ayelén Gareis(b), Ernesto Accolti(c)
(a)
Instituto Superior de Música, Universidad Nacional del Litoral, Argentina,
[email protected]
(b)
Instituto Superior de Música, Universidad Nacional del Litoral, Argentina, [email protected]
(c)1
2
Instituto Superior de Música, Universidad Nacional del Litoral, Santa Fe; Instituto de Automática,
Universidad Nacional de San Juan, Comisión Nacional de Investigaciones Científicas y Técnicas,
[email protected]
Abstract
In this article the final work of students of the course in Room Acoustics for Sound and
Recording degree program of the Instituto Superior de Música from Universidad Nacional del
Litoral, Argentina, is described and results are shown. They designed, implemented and
aurally tested pseudo random sequence diffusers. One group of students designed a onedimensional quadratic residue diffuser, the second group an MLS and the third group a twodimensional quadratic residue diffuser. They tested aurally for themselves and then in a
practice shared with all the class in the recording studio facilities. The students experienced
how to build diffusors and how do they sound.
Keywords: Diffusor, Education, Acoustic Solutions.
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nd
22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
st
Acoustics for the 21 Century…
Practical class on room acoustic for sound and
recording degree program in Santa Fe, Argentina
1 Introduction
In this article we show the final work of students of the course in Room Acoustics for Sound
and Recording degree program of the Instituto Superior de Música from Universidad
Nacional del Litoral, Argentina. Three groups of the four groups of students that conform the
class decided to design, implement and test sound diffusers between a pool of suggested
topics.
One group of students designed a one-dimensional quadratic residue diffuser, the second
group an MLS and the third group a two-dimensional quadratic residue diffuser. The article is
not organized by group of students but following a fundamentals section, an implementation
section and finally the conclusions.
2 Fundamentals
The acoustical design of rooms, once its volume and dimensions fixed, consist in choosing
the more convenient materials to use as cladding, aiming to get optimal reverberation times
and other acoustic parameters.
Each one of the different types of materials and elements used, produce one or a
combination of the next effects on sound energy:
-
Sound absorption: due to the presence of absorbent materials in the room, resonant
elements, the audience and the seats.
Sound reflection: due to the presence of reflective materials used to generate useful
reflections to the audience area.
Sound diffusion: due to the presence of diffusers elements and irregular surfaces
used to disperse in multiple directions the incident sound energy.
The existence of sound diffusion in a room means that the energy of reverberant field will
arrive to the audience ears with similar probability from every direction. This will contribute to
the spatial impression and in turn to improve the acoustic quality of the room.
There are several types of diffusers and the correct choice depends on the room
characteristics and the use intended for the room. The students chose and developed the
following kind of diffusers:
QRD Diffusers (Quadratic residue diffuser)
! One-dimensional
! Two-dimensional
• MLS (Maximum length sequence)
Diffusers are arrays of elements that reflect the incident sound energy in several directions.
•
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
One of the principles that makes it possible is the interference of the portion of the sound
energy reflected in each element of the array.
Another phenomenon that contributes to reflect energy in several directions is diffraction. The
diffraction takes place in the boundaries of the elements of the array.
Diffraction happens for high frequencies, or short wavelengths compared with the
dimensions of the elements of the array. How diffraction contributes to diffusers can be
interpreted as the diffraction of the sound waves going through a hole or slot in a wall or
screen. If the slot is big compared to the wavelength of the sound, the wave fronts go
through the slot with almost no distortion. However, when the slot is smaller than the
wavelength, hemispheric wave fronts spread inside the shadow zone.
Figure 1. (α) A big slot compared to the wavelength allow the wave front go through with
almost no distortion shadow zone. (β) Wave fronts behaviour when the slot is smaller than the
wavelength.
2.1
MLS Diffuser
This is the more simple design but the frequency range for which the diffuser is optimal, is
only about an octave. Furthermore, this type of diffusers has less absorption at low
frequencies than PRD and QRD diffusers.
The design of this kind of diffusers is based in pseudorandom periodic sequences called
maximum length sequences (MLS). This sequences are binary.
The diffuser is created starting with a smooth and reflective surface, which is subdivided into
sections of equal width. Each section has assigned a value of the pseudorandom sequence,
according to the following procedure:
- If the value is 0, the section remains unchanged.
- If the value is 1, a slot is created in the space occupied by the stretch.
𝑊=
𝜆
2
𝜆
𝑑=
4
(1)
(2)
Where λ corresponds to the wavelength of the design frequency of the diffuser.
To design an MLS diffuser, we must initialize the registers at any state different from zero
vector. We initialised the vector at 1 0 1, and proceed as follows:
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
Figure 2. Procedure to obtain the seed
2.2
One-dimensional QRD
This design is based in the next formula:
hn =
c[n 2 mod(N )]
2N. fd
(3)
Where:
hn: slot height
c: sound speed
N: slots quantity in prime numbers
fd: design frequency
mod: modulo (remainder after integer division)
f max = fd ( N − 1)
(4)
1
d = λ f max
4
(5)
λ f max = maximum frequency wavelength
χ n = 2π [n 2 mod( N )]
(6)
hn =
χn
(2k)
(7)
Which:
fd =
ck
(2π )
(8)
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
Figure 3. One-dimensional QRD 3D view
2.3
Two-dimensional QRD
In this case the slots are replaced by wells and its dimension will affect the maximum
diffusion frequency.
𝑊=
𝐶
−𝑇
2𝑓!"#
(9)
Where:
W: width and length of the well
C: speed of sound (344 m/s)
𝑓!"# : maximum diffusion frequency
T: wells septum thick
𝑑!,!
(( ℎ ! + 𝑘 ! )𝑚𝑜𝑑 𝑁) 𝐶
=
⋅
𝑁
𝑓!
(10)
Where:
d: well depth
h and k: coordinate axis position. Values from 0 to N-1
N: number of wells in prime numbers
C: speed of sound (344 m/s)
𝑓! : design frequency
Figure 4. Two-dimensional QRD photograph
3 Implementation
In this section we show the implementation and tests performed by each group in the class.
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
3.1
MLS
This experience took place in a piano study classroom of the Instituto Superior de Música
from Universidad Nacional del Litoral.
Figure 5. ISM piano classroom
We set the design frequency at 1300 Hz because of space considerations.
Then we calculate the width and the depth of each section based on the wavelength at the
design frequency.
𝜆=
𝜆=
!"" !/!
!"## !"
𝑐
𝑓
(11)
= 0,26 m
The width is half a wavelength:
W= 0,26/2 = 0,13 m
The depth is also half a wavelength:
d= 0,26/2 = 0,065 m
The starting vector was: 1 0 1 and following the procedure shown in section 2.1 we got the
next sequence:
Figure 6. MLS sequence
In Fig. 7 we show a render of the designed diffuser.
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
Figure 7. MLS diffusor 3D view
3.2
One-dimension QRD
We choose a design frequency of 300 Hz to diffuse as much lows as possible without getting
the slots too deep but useful in the frequency range of human voice.
Based on equations (3-8), we made the next table:
Table 1 Obtained values from equations 3-8
N
7
7
7
7
7
7
7
n
1
2
3
4
5
6
7
n^2
1
4
9
16
25
36
49
n^2modN
1
4
2
2
4
1
0
hn
0,082
0,328
0,164
0,164
0,328
0,082
0,000
χn
6,283
25,133
12,566
12,566
25,133
6,283
0,000
C
344
344
344
344
344
344
344
fd
300
300
300
300
300
300
300
fmax
1800
Λ(fmax)
0,191
d
0,048
Thick
0,334
With the depths obtained we rendered the following isometric view:
Figure 8. One-dimensional QRD isometric view
To keep the symmetric design, we split in a half the first slot (cero depth) and distributed to
the extreme sides.
3.2.1 Test & measurement
The test was made in moderately reverberant closed room. It consisted in placing a
measurement microphone 1 meter from a reflecting surface (See Fig. 9), then excite the
room with an impulsive sound source (snare drum). The audio was recorded for a later
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
spectral analysis. Then we repeated the experience placing the diffuser in front of that
reflecting surface (See Fig. 10).
Figure 9. Elements distribution for the experience without diffuser
Figure 10. Elements distribution for the experience with diffuser
We repeated the experience in the class using the recording studio facilities. The students of
the class agreed in that the sound of the snare, when recorded using the diffuser, was
perceived slightly more defined and with a less coloration.
3.3
Two-dimensional QRD
In this case the design frequency chosen was 700 Hz with N=11. The construction material
available were 45x45 mm wood strips. The highest diffused frequency is constraint by the
dimensions of the section of the strips.
Using equation (9), we calculated fmax:
𝑓!"# = 3882 ℎ𝑧
Then using the equation (10) we obtained the block positions with their corresponding height:
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
Table 2 Block positions with their corresponding height
To keep a symmetric scheme we moved the design to maintain that symmetry in both axis.
The final design is shown in Fig. 11. and Fig. 12. and a photograph of the diffuser
constructed by the students is shown in Fig. 4.
Figure 11. Two-dimensional QRD diffusor modelled with QR Dude free software
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
Figure 12. Two-dimensional QRD diffusor modelled with QR Dude free software (other)
3.3.1 Test & measurement
The measurement was made in a shoebox room (2,3mx5mx2, 5m) and consisted in an
impulse generated by a speaker analysed by RTA microphone (75cm from the rear wall). As
well as the one-dimensional case, the experience was repeated with the diffuser for further
comparison. The impulse was a snare drum sample.
Figure 13. Elements distribution for the experience
Figure 14. Elements distribution for the experience (photograph)
Once again the experience was repeated in the recording studio facilities with the class. The
perceived difference was a slightly, but greater than the case of the one-dimensional QRD,
better definition of the sound and a richer spectral timbre.
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22 International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
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Acoustics for the 21 Century…
4 Conclusions
Each group of students designed one kind of diffuser. A one-dimensional and a twodimensional quadratic residue diffusers examples were also constructed. They tested aurally
for themselves and then repeated the experience in a practice class in the recording studio
facilities. The students experienced how to build diffusors and how do they improve sound of
a recorded material. The perceived result was an improve on the definition and armonic
richness of the recorded signals.
Acknowledgments
We wish to thank our friends and colleagues Federico Galiano, Demian Gonzalez Petrich,
Pablo Guillen and Andres Gordillo Pizarro for their contribution and collaboration.
References
[1] Antoni Carrión Isbert. “Diseño acústico de espacios arquitectónicos”. Primera edición
1998.
[2] Cox, T.J., and D’Antonio. “Acoustic absorbers and diffusers: theory, design, and
application”.
[3] Wikipedia, Online enciclopedia: http://es.wikipedia.org/wiki/Difusor
[4] Bongiovanni Pablo, Cascino Marcelo, Sanso Marco. “Análisis y diseño de difusores
acústicos”. Universidad Tecnológica Nacional 2011.
[5] García Céspedes María de los Ángeles, Hernández Almazán Marisela. Tesis “Diseño
de un difusor acústico”. Instituto Politécnico Nacional México 2010.
[6] http://www.subwoofer-builder.com/qrdude.htm
[7] Miyara, F. (1999) “Control de Ruido” 1ra edición, Rosario, Argentina.
[8] Miyara, F. (2003) “Acústica y Sistemas de sonido” 4ta edición, Rosario, Argentina
[9] Huizar,
J.
(2012)
“Acústica
en
los
Recintos”
https://
m2db.files.wordpress.com/2012/11/acustica-en-los-recintos-jesus-huizar.pdf
[10]
Burred Sendino, J. (1999) “La Acústica del Piano”, Conservatorio Profesional
de Música Arturo Soria, Madrid. Versión revisada en Septiembre de 2004
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