Room Acoustic Design Exercise

Room Acoustic Design Exercise

 

A) Guide to Acoustics

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1. Acoustic Primer

In order to fully understand
acoustics, it is important to know what sound actually is on a physical level. Sound
is quite simply a disturbance in a medium, which propagates as a pressure wave (Howard,
D.M. & Angus, J.A.S., 2017, pp. 2-10). To model how that wave actually
propagates through a medium, see Figure 1. This golf ball and spring model
shows that, when one golf ball is moved (representing the movement of a
molecule within a medium), the springs that connect them (representing the
intermolecular forces between the molecules within a medium) will pull the nearby
golf balls closer (compression) or will push them farther away (rarefaction).
This method of propagating by compression and rarefaction of molecules within a
medium is called a longitudinal wave. This type of wave is how most sound
travels, especially through air; however, sound can travel by other types of
wave including transverse, which is how sound often travels in musical
instruments such as strings and percussion. 

 

2. The Importance of Room Acoustics

The acoustical properties of a room can drastically affect
the quality of sounds within the room; therefore, it is important to know how
to design rooms that will have favorable acoustic properties and what measures
can be taken to improve the acoustics of a room. For example, bad acoustics in
a room in which someone is likely to be speaking in front of an audience can
make it difficult to understand the speaker due to a loss in sound clarity. Bad
acoustics in this type of environment also have the possibility of making sound
produced where the speaker would be standing within the room not as loud for
some locations in the audience as others. This is also a concern for people
designing concert halls, as it is important that the performance sound the same
regardless of where an audience member is seated within the room. The
reverberation within a room is also an important aspect of room acoustics, and
what is considered the desired reverberation time depends heavily on the
purpose of the room. Lecture halls, for example, need short reverberation
times, as too much reverberation can make it hard to understand the speaker. In
certain concert venues such as concert halls and churches, however, it is
desirable to have longer reverberation times, as it is pleasing to the ear for
the types of music that are likely to be performed in these venues. Room modes
can also negatively affect the way we perceive sounds within a room, as they
can drastically boost some frequencies and drastically cut others, leaving an
unwantedly skewed mix of frequencies.

 

3. Acoustic Treatment

Luckily, there are things we can do to improve the acoustics
of a room without building a completely new room. There are a variety of
methods and devices that can be used in a room to alter its acoustics qualities
to better suit the purpose of the room. For example, we may want to treat a lecture
hall so that it is less reverberant, making it easier to understand what he/she
is saying. Similarly, a room that is particularly modal can be treated with
devices that absorb a certain range of frequencies, in an attempt to flatten
the frequency response of the room. Additionally, a room can be treated to make
it quieter (as in a restaurant or factory) and can likewise be treated to keep
the noise from escaping the room or to keep outside noise from entering the
room.

 

4. Sound Absorbers and Diffusers

            One method
of altering the acoustic qualities of a room is by adding absorptive materials
in it, of which there are many different type. One such material is known as a
porous absorber, which absorbs sound energy through the friction between the
velocity component of the sound and the material’s surface (Howard, D.M. &
Angus, J.A.S., 2017, pp. 339-341). Because this friction increases with
frequency, the amount of sound absorbed by a porous absorber also increase with
frequency, as shown in Figure 2. One way to get more low-end absorption, or to
control what frequencies will be most affected by a porous absorber (if it is
being used to mitigate the effects of room modes), is to place the surface of
the material a certain distance away from a hard surface. Since the absorption
of a porous absorber depends on the velocity of the sound wave, the absorber
will work best when the sound wave is at its highest velocity (1/4 of a wavelength).
So, if the surface of the absorber is placed ¼ wavelength distance away from a hard surface, it will do a better job
of absorbing those frequencies which fit within that distance.

            Another type
of absorber is known as a resonant absorber and involves encapsulating an
absorptive material between a panel and a hard surface, such as a wall,
illustrated in Figure 3. The front panel vibrates causing friction in the
pressure component of the incident sound (Howard, D.M. & Angus, J.A.S., 2017,
pp. 341-345). Due to their resonant nature, these absorbers work best for low
frequencies, as shown by Figure 2. Because of this, resonant absorbers are a
great accompaniment to porous absorbers, which work best for higher
frequencies. Another type of resonant absorber is known a Helmholtz absorber.
This type of absorber is similar in construction to the panel absorber in
Figure 3; however, it uses a perforated panel to create resonance of a tube of
air. This alters the resonant frequencies of the absorber, as shown by Figure
2.

Lastly, it might be assumed that sound absorbers can also be
used for sound isolation; i.e., keeping sound from escaping a room or keeping
outside sound from entering a room. While absorbers extract energy from a sound
wave, they typically do very little in terms of keeping sound from going
through them (Howard, D.M. & Angus, J.A.S., 2017, pp. 351).

            Unlike sound
absorbers, which absorb sound energy, diffusers are used to disperse the sound equally
through the space (Howard, D.M. & Angus, J.A.S., 2017, pp. 345). They
ensure that the reverberation time and absorption coefficient throughout the
space is constant (or as close to constant as possible). This is done using
uneven surfaces that will reflect the sound in multiple directions, such as the
surface shown in Figure 4.

 

5. Room Acoustics Guide

            There are some signs that can
indicate when certain acoustic treatments are likely to need to be used in a
room. For example, when a room is very rectangular, meaning it is a perfect
rectangular prism and all of its sides are parallel to the opposite side, this
typically causes room modes and flutter echoes, both of which can be cured by
adding sound absorbers. Flutter echoes are nasty repetitive reverberations that
occur when a sound reflects back and forth off of two parallel surfaces. Another
treatment that might be necessary in rectangular rooms is diffusion, which
helps to spread the sound energy evenly throughout the room and make the room
sound more spacious. Another sign of potentially necessary acoustic treatment is
if the room is built of reflective surfaces, such as marble of stone. This will
likely yield a very reverberant room, and sound absorbers might need to be added
to reign in some of the reverberation, depending on the context. If it is a cathedral
or large concert hall, it may be beneficial to have such long reverberation times,
as this is often a desirable effect in music; however, if the room is to be used
for lectures or speeches, it is likely that the reverberation time will need to
be shortened, so that the speaker can be more easily understood.

 

6. Further Reading

·        
Kindig, S. (2010) Room acoustics for home audio
[Internet]. Available from:
[Accessed 24 January 2018].

This source provides further general information about room acoustics
geared more towards home theater/stereo setups.

 

·        
Feinstein, S. (2016) The Importance of Room Acoustics
[Internet]. Available from:
[Accessed 24 January 2018].

This source provides simple, yet practical information about room
acoustics.

 

·        
Perry, T. (2016) Acoustic Treatment Setup 101: How to
Treat Your Room for High Fidelity Listening & Mixing [Internet]. Available
from:
[Accessed 24 January 2018].

This source provides practical information about room acoustic treatment
methods.

 

·        
E-Home Recording Studio (2017) Acoustic Treatment 101:
The Ultimate Guide for Home Studios [Internet]. Available from:
[Accessed 24
January 2018].

This source provides in-depth information about room acoustic treatment
and some useful products that are available.

 

B) Technical Report

1. Introduction

            If a room on campus was to be
converted into a small music venue, there are multiple aspects of that room’s
acoustic properties which should be taken into account, in order to get the
best listening experience in the room as possible. One good way to get a handle
on the acoustic characteristics of a room is to measure its impulse response. From
this measurement, we can calculate a variety of important properties and even
use it to convolve various anechoic recordings so that we can hear what
different sounds would sound like if they were played in that specific room.
This measurement will also tell us what sorts of acoustic treatments need to be
added to the room, in order to make it a better venue for small concerts.

 

2. Description of Room

            The room used for this report was a simple, rectangular teaching room. As
shown in Figure 5 and 6, the room has carpeted and wood floors, and it seems to
have sheetrock on the ceiling. The walls are also made of sheetrock; however,
there is a large chalkboard on the front wall, a whiteboard that takes up the
entire wall on the far side, and a window that takes up all of the back wall.
There are long black drapes in front of the back window and in front of the door
(on the right in Figure 5) which we had drawn for the taking of the impulse
response measurement. The room is 8.04m long, 5.9m wide, and 2.65m tall. We had
the speaker placed 66cm away from the front wall and the microphone placed 3.4
away from the speaker.            

            Given its physical characteristics, it
is likely that this is a pretty dry room. It has very few reflective materials
in it, the only major one being the whiteboard on the far wall, and the carpet
and drapes are going to absorb a lot of the sound energy, leaving very little
reverb in the room. We can estimate the reverberation time of the room (T60)
using the equation,

We can
calculate the volume of the room (V) and its surface area (S) using the room
dimensions listed above. As for the absorption coefficient (?), we’ll have to approximate a bit. The sheetrock walls and
ceilings probably have absorption coefficients of 0.1 or less, the drapes
probably have an absorption coefficient of 0.3 or more, and the carpeted floor
probably has an absorption coefficient of 0.3. So, the average absorption
coefficient of the entire room could be approximated as being about 0.2.
Plugging this all back into the equation above,

Mode
#

Frequency
(Hz)

Mode
Type

1

21.33

Axial

2

29.07

Axial

3

36.05

Tangential

4

42.66

Axial

5

51.62

Tangential

6

58.14

Axial

7

61.93

Tangential

8

63.99

Axial

9

64.72

Axial

10

68.14

Tangential

Additionally, since it is so rectangular, it is likely that
this room is quite modal. There are three different types of room modes, axial,
tangential, and oblique. Axial modes occur across one dimension of the room;
tangential modes occur across two dimensions of the room; and oblique modes
occur across all three dimensions. The first ten room modes are given in Figure
6 below.

Figure
6
– First 10 Room Modes

 

            Another potential issue with this
room is sound insulation, mostly through the back window. While we were in the
room in fact, I noticed how easily outside noises entered the space. If this
room is going to be converted into a small music venue, this would probably
need to be addressed, especially to keep any loud concerts from spilling over
into neighboring rooms.

 

3. Acoustic Properties of Room

Center of Frequency Band (Hz)

C50 (dB)

D50 (%)

63

0.452105

52.6038

125

2.3548

63.2302

250

5.03625

76.11755

500

7.93015

86.107

1000

10.8335

92.37455

2000

12.4777

94.6442

4000

15.20115

97.07275

8000

18.63175

98.64825

             Overall, this room is quite dry, as can be
heard in its impulse response; it has a reverberation time that is excellent
for a teaching room, but perhaps a bit short for a small music venue. One way
to get a better, more objective understanding of such a subjective acoustic
quality of a room is to calculate its clarity (C50) and definition (D50), shown
in Figure 7. Clarity is a ratio of the early sound energy to the late sound
energy, measured in dB, and is typically used as a measure of how easy it is to
understand speech within a room. Definition is a ratio of the early sound
energy to the total sound energy, measured as a percentage, and is

Figure
7 – Calculated Clarity and
Definition of Room

typically
used as a measure of how clear a piece of music played within the room will
sound to the listener (PureBits, 2004). Both the clarity and definition of this
room are very good; they do decrease as frequency decreases, leaving less
clarity at lower frequencies, but this is to be expected.

            Even though this room already has
very good clarity and definition values, there are still acoustic treatments
that could be done. First of all, to combat the room modes that are present in
most rectangular rooms like this one, resonant absorbers, tuned to trap particularly
prominent modes, should be added. This will even out the frequency response of
the room; however, too many absorbers can make the room too dry. Fewer than ten
acoustic panels to be installed along the walls and/or ceiling of the room
would cost approximately £400. Secondly, since the whiteboard along
the far wall will most likely want to be removed anyways, as this room is no
longer going to be a teaching room, it could be a good idea to add a diffuser
in its place for under £300. A diffuser will not absorb any
sound energy and will not reduce any reverberation in the room, but it will
spread the sound out around the room, ensuring that sound in the room sounds
the same no matter where in the room the listener is. Lastly, something should
be done to better insulate the large window across the back of the room. One
option would be to take out the window entirely and replace it with a solid
wall. While this would better insulate the room, it would be more expensive.
One alternative is to replace the existing windows with better soundproof
windows, although this is likely to be about as costly. Another option is to
add mass to the windows, which will keep sound from passing through them. This
can be done using a sheet of vinyl insulation for around £50. Additionally, keeping the existing drapes will help with
insulating the windows.

 

4. Conclusion

                        While this room has some beneficial acoustic qualities
(such as a low reverberation time, good clarity, and good definition), there
are still some problems with it that need to be addressed before it can be
converted into a small music venue. As with many rectangular rooms like this
one, it is quite modal. Having prominent room modes in a room makes it
difficult to get an accurate balance over the frequency spectrum; however, we
can treat the room with resonant absorbers to reduce the sound energy at any
unwanted frequencies. Additionally, there are a few different treatments that
can be done to better insulate the room to keep unwanted sound out and to keep
the sound of the performance in. Lastly, a diffuser will help spread the sound
of the performance evenly around the room, ensuring each listener receives the
same acoustic experience.

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