BUILDING SERVICES - IV

ACOUSTICS Introduction to Sound & Room Acoustics–BS-IV–

By:
Ar. Waqar Abid A Z
MODULE 1 (Introduction to Sound and Room Acoustics)

•Origin and nature of sound, its characteristics and measurement–

Amplitude, frequency, period, wavelength, velocity of sound, sound
pressure, sound intensity, decibel scale, sound and distance – inverse
Introduction to Sound: square law. human hearing, auditory range for humans (Frequency and
Intensity – threshold of audibility and pain), pitch (association with
frequency), tone, loudness (association with amplitude and intensity), Phon.

•Reflection - Nature of reflection from plane, convex and

concave surfaces, diffraction, Absorption, Echoes, focusing
of sound, dead spots, flutter echo. Room resonances,
Room Acoustics: Reverberation - reverberation time (RT) calculation using
Sabine’s and Eyring’s formulae. Effect of RT on speech and
music.
INTRODUCTION TO SOUND & ROOM ACOUSTICS 2
AR. WAQAR ABID
SOUND STATE ? ...... SOLID ? LIQUID ? GAS ? CHEMICAL ? LIGHT ? If you bang a drum, you make the tight skin vibrate at very
high speed (it's so fast that you can't usually see it), forcing
“ A wave motion in air or other elastic media (stimulus) or as that excitation of the the air all around it to vibrate as well. As the air moves, it
hearing mechanism that results in the perception of sound (sensation).” carries energy out from the drum in all directions.
Eventually, even the air inside your ears starts vibrating—
and that's when you begin to perceive the vibrating drum
Sound is a kind of energy created when something vibrates. When this vibration
as a sound.
reaches an ear, it is translated into what we recognize as a sound
physical process that produces sound energy to start with and
Subjective / Physiological Definition sends it shooting through the air
Objective / Physical Definition
psychological process that happens inside our ears and brains,
Sound is a form of energy. Sound is an auditory sensation produced which convert the incoming sound energy into sensations we
It travels in waves through elastic by stimulation of the organ of hearing, interpret as noises, speech, and music.
media and causes fluctuation of evoked by physical fluctuation of
pressure and particle displacement. pressure in media. physical process : the interest is
in the disturbance in air created
Study of sound in relation to people, both aspects of sound have to be treated by a loudspeaker, it is a problem
in physics
psychological process : If the
interest is how it sounds to a
person near the loudspeaker,

Sound vibration must travel

through matter. This is
typically air/water

INTRODUCTION TO SOUND & ROOM ACOUSTICS 3

SOUND AR. WAQAR ABID
A vacuum is an area without any air, like space. So sound cannot travel through space because there is no matter for the
vibrations to work in.

Can you hear sounds in space? Now you’ve probably heard that there’s no sound in
space
but technically that’s not true
Now yes, space is a virtual vacuum. However, sound does exist in the form of electromagnetic
vibrations that pulsate in similar wavelengths.

What NASA did was design special instruments that could record these electromagnetic vibrations,
and transferred them into sounds that our ears could hear

INTRODUCTION TO SOUND & ROOM ACOUSTICS 4

SOUND IN SPACE AR. WAQAR ABID
“Acoustics” is derived from the Greek wordἀκουστικός (akoustikos),
meaning “of or for hearing, ready to hear”
• It is a branch of physics that deals with study of all kinds of mechanical waves in
any medium

• It was Aristotle who first established the relation that anything that vibrates can
produce sound dates back to (384–322 BC)

• Architectural acoustics recognised as a science just over a century.

• The scientific knowhow to achieve a good sound within a space (building)

• Involves the study of speech intelligibility, speech privacy, music quality, noise
control and vibration reduction with in the built environment.

Acoustics is a branch of physics that study the sound, acoustics concerned with
the production, control, transmission, reception, and effects of sound.

The study of acoustics has been fundamental to many developments in the arts,
science, technology, music, biology, etc

“A science that deals with the production, control, transmission, reception, and
effects of sound.”
INTRODUCTION TO SOUND & ROOM ACOUSTICS 5
ARCHITECTURAL ACOUSTICS AR. WAQAR ABID
 Study sound as a wave

Categorized as a line diagram , Reflection/

Refraction / Absorption of the sound is
translated as a Ray . More important for
architects

In this domain , we study the various

frequencies of pressure( level of sound
intensity) .To design for a certain intensity of
sound in a room, this statistical data gives a
scale for various materials that could be used

INTRODUCTION TO SOUND & ROOM ACOUSTICS 6

ARCHITECTURAL ACOUSTICS AR. WAQAR ABID
 INTRODUCTION TO SOUND & ROOM ACOUSTICS 7
SOUND AR. WAQAR ABID
 Longitudinal Wave
In a Longitudinal Wave the particle
displacement is parallel to the • Motion of a String
direction of wave propagation • Water Ripples
• S-Wave
• Electromagnetic Wave

Transverse Wave
In a Transverse Wave the particle
displacement is perpendicular to the
direction of wave propagation
• Motion of a String
• Water Ripples
• S-Wave
• Electromagnetic Wave
INTRODUCTION TO SOUND & ROOM ACOUSTICS 8
TYPES OF WAVE MOTION AR. WAQAR ABID
 Particle motion in AIR
Linear medium

in other medium The back and forth motion of the particle is shown above.
If a stone is dropped on a calm water
surface, concentric waves travel out
from the point of impact, and the
water particles trace circular orbits (for
deep water, at least)

The tiny elements of the string move

transversely, or at right angles to the
direction of travel of the waves along
the string.

For sound travelling in a gaseous

medium such as air, the particles move
in the direction the sound is travelling.
These are called longitudinal waves,

INTRODUCTION TO SOUND & ROOM ACOUSTICS 9

Characteristics of Sound AR. WAQAR ABID
Amplitude
The maximum extent of a vibration or oscillation in a propagating wave
motion, measured from the position of equilibrium is called Amplitude

Peak - Amplitude
The extent of a vibration between the Crest to the position of equilibrium
in a propagating wave motion is called Peak-Amplitude [1]
Peak – to – Peak - Amplitude
The extent of a vibration between the Crest to Trough in a propagating
wave motion is called Peak-to-Peak Amplitude [2]
Root-mean-Square Amplitude
The square root of the squared average values of the waveform is called
Root-mean-Square Amplitude [3]
In the case of the sine wave, the RMS value is 0.707 times the peak value
Sine wave Language
The sine wave is a specific kind of alternating signal and is
described by its own set of specific terms. Viewed on an
oscilloscope, the easiest value to read is the peak-to-peak
value (of voltage, current, sound pressure, or whatever the
sine wave represents), the meaning of which is obvious as
shown in the figures below. If the wave is symmetrical, the
peak-to-peak value is twice the peak value. INTRODUCTION TO SOUND & ROOM ACOUSTICS 10
Characteristics of Sound AR. WAQAR ABID
Complex Sound Waves
The variation in pressure caused by speech, music or noise as shown below.
Most sounds are complex, consisting of a variety of pressures which vary
with time. The threshold of hearing for humans is one millionth of normal
atmospheric pressure.

Time Period
The time needed for one complete cycle of vibration to pass a given
point is called Time Period.

Frequency
Frequency describes the number of complete wave cycle that
pass a fixed point in unit time.
Usually frequency is measured in cycles per second (CPS ) or
hertz unit, named in honour of the 19th-century German
physicist Heinrich Rudolf Hertz
INTRODUCTION TO SOUND & ROOM ACOUSTICS 11
Characteristics of Sound AR. WAQAR ABID
 INTRODUCTION TO SOUND & ROOM ACOUSTICS 12
Characteristics of Sound AR. WAQAR ABID
 Wave length

• The distance between a successive crests or troughs, or the

distance of a complete cycle of a wave propagation of the
sound wave in the direction of wave motion is called
Wavelength

Sound wave : Frequency & Wave length

wavelength (ft) x frequency (Hz) = speed of sound λ X f = 1130

INTRODUCTION TO SOUND & ROOM ACOUSTICS 13

Characteristics of Sound AR. WAQAR ABID
Variation of Frequency , Amplitude and Wave length
Displacement vs Directivity
diagram

Nomogram of Frequency & Wave Length

Sound Wave Propagation in Air

This compression and Rarefaction synchronises
the movement of sound in the air

Tuning fork
INTRODUCTION TO SOUND & ROOM ACOUSTICS 14
Characteristics of Sound AR. WAQAR ABID
 Speed of Sound
Velocity of Sound in Air (c) is depend upon:
• Atmospheric Pressure (p)
• Density of Air (ρ) rho
• Temperature of Air (ta)

The speed of sound at room temperature

Air : 340 m/s

Water : 1,440 m/s

Steel : 5,000 m/s

Glass : 4,500 m/s

Helium : 1,005 m/s

INTRODUCTION TO SOUND & ROOM ACOUSTICS 15

Characteristics of Sound AR. WAQAR ABID
Sound Pressure

Sound travels in an elastic medium such as air, water , earth etc. an elastic medium returns to its normal
state once a force is removed.

Pressure is a force per unit area. Sound energy progresses rapidly, producing extremely small changes
in atmospheric pressure, and travel great distances.

“The fluctuation in the atmospheric pressure caused by the vibration of air particles due to a
sound wave is called Sound Pressure.”

Pitch
“ The attribute of an auditory sensation which enables us to order sounds on a scale
extending from low to high is called pitch”

It is a quality that makes it possible to judge sounds as "higher" and "lower" in the sense
associated with musical melodies
• Pitch is the subjective response of human hearing to frequency.
• It is the subjective physiological equivalent of frequency.
• The pitch depends primarily upon the frequency of the sound stimulus
• Low frequencies generally are considered "bloomy"
• High frequencies generally are considered "screechy" or "hissy"

INTRODUCTION TO SOUND & ROOM ACOUSTICS 16

Characteristics of Sound AR. WAQAR ABID
Tone A sound sensation having pitch is called tone.

Pure tone (or simple tone) – Single Frequency is a sound sensation of a single
frequency characterized, therefore, by its singleness of pitch, e.g., that produced by
striking a tuning fork, vibration of string of a guitar

Complex Tone (More than one frequency) is a sound sensation characterized by

more than one pitch, e.g., that produced on musical instruments., Speech , Road side
noise
• Whether or not a person hears a tone as simple or complex depends on ability, experience and listening
attitude.

Sound Intensity Sound intensity also known as acoustic intensity is defined as the Energy carried by the
sound waves per unit area.
• The unit of sound intensity is watt per square meter (W/m2).

• The rate at which sound energy passes through a unit area held perpendicular to the direction
of propagation of sound waves is called intensity of sound

Sound intensity, denoted I, is defined by

Sound Intensity (I) = Sound Pressure (p) X Particle Velocity (v)
Both I and v are vectors, which means that both have a direction as well as a magnitude. The
direction of sound intensity is the average direction in which energy is flowing.
The average sound intensity during a time T is given by,
(I) = 1/T
INTRODUCTION TO SOUND & ROOM ACOUSTICS 17
Characteristics of Sound AR. WAQAR ABID
Loudness
• Loudness is the characteristic of a sound that is primarily a psycho-physiological correlate
of physical strength (amplitude).
• It is defined as "that attribute of auditory sensation in terms of
which sounds can be ordered on a scale extending from quiet to
loud".
It also shows that humans with normal hearing are most sensitive to sounds around 2– 4
kHz.

Phon Unit of measurement of loudness

Human hearing and Speech

• Humans are generally capable of hearing sounds between 20 Hz and 20 kHz.
A is more A is less
L is more L is less
• Sounds with frequencies above the range of human hearing are called ultrasound.

• Sounds with frequencies below the range of human hearing are called infrasound.

• Typical sounds produced by human speech have frequencies on the order of 100 - 1,000 Hz
F is less F is more
P is less P is more • The peak sensitivity of human hearing is around 4000 Hz.

INTRODUCTION TO SOUND & ROOM ACOUSTICS 18

Characteristics of Sound AR. WAQAR ABID
Decibel Scale
L1 = 10 log l/l0
where, L1= Sound Intensity Level (in dB)
l = Sound Intensity (in W/ m2)
l0= Reference Sound Intensity 10-12 (in
W/m2)

INTRODUCTION TO SOUND & ROOM ACOUSTICS 19

Characteristics of Sound AR. WAQAR ABID
Threshold of Audibility and pain
• The level above which sound intensity, at
any specified frequency, must rise in
order to be detected by the average
human ear is Threshold of Audibility.

• This threshold value decreases with increasing

frequency from a value of about 1microwatt m-2 at
50Hz to slightly less than 10- 6microwatts m-2 at about
2000Hz, and then increases with increasing frequency
to about 104microwatt m-2 near 30000Hz.

As the sound intensity rises above the threshold of audibility at any frequency,
continued increase will finally raise the intensity to a second type of limit, the
threshold of pain (threshold of discomfort, threshold of feeling).

This threshold is rather insensitive to frequency, being found at about 106microwatts m-2 for nearly all
frequencies in the audible range. It represents the limit above which appreciable increase in sound intensity will
lead to sensible pain in the average human ear. Thresholds of hearing for male (M) and female (W) subjects
between the ages of 20 and 60

INTRODUCTION TO SOUND & ROOM ACOUSTICS 20

Characteristics of Sound AR. WAQAR ABID
 Sound can be described by two things
Wavelength is more Frequency is less (bloomy)

Amplitude is less Pitch is

Sound can be described by two things
low Less Vibrations Tone ?
Wavelength is less Frequency is more (screechy/hissy)
Loudness is less (relates to
Sound intensity) SPL (Sound Amplitude is more Pitch is high
pressure Level)
Larger Vibrations Tone ?
Loudness is more (relates
to Sound intensity)
(depends on energy of the
wave)

INTRODUCTION TO SOUND & ROOM ACOUSTICS 21

Characteristics of Sound AR. WAQAR ABID
Sound shadow • A phenomenon caused by the absorption or obstruction of a sound wave by an object in its
path.

• The effect produced is perceived as a reduction in loudness depending on the observer's position with
respect to the sound source and obstructing object and is greatest when the three are aligned.

Source -------- Obstructing Object -------- Observer/ Reciever

• Sound masking is the addition of natural or artificial sound (such as white noise or pink
Sound masking noise) into an environment to cover up unwanted sound by using auditory masking.

• This is in contrast to the technique of active noise control.

• Sound masking reduces or eliminates awareness of pre-existing sounds in a given area and can make a work
environment more comfortable, while creating speech privacy so workers can better concentrate and be
more productive.

• Sound masking can also be used in the outdoors to restore a more natural ambient environment.

• Covering a distracting sound with a more soothing or less intrusive sound.

INTRODUCTION TO SOUND & ROOM ACOUSTICS 22

Characteristics of Sound AR. WAQAR ABID
Sound and Distance - Inverse square law
• Sound waves from a point source outdoors with no obstruction (called free-field
conditions) are virtually spherical and expand outward from the source.

• In Physics an inverse square law is - any physical law stating that

a specified physical quantity or intensity is inversely proportion to
square of the distance from source of physical quantity.

• Power is a basic quantity of energy flow.

•The intensity from a point source outdoors at a distance d away
is the sound power of the source divided by the total spherical
area 4 d2 of the sound wave at the distance of interest. This
relationship can be expressed as

INTRODUCTION TO SOUND & ROOM ACOUSTICS 21

Characteristics of Sound AR. WAQAR ABID
T h a n k y o u
Contoso Ltd.
INTRODUCTION TO SOUND & ROOM ACOUSTICS
AR. WAQAR ABID