FUNDAMENTALS OF ARCHITECTURAL ACOUSTICS (This is additional information)
Architectural acoustics
Acoustics is the science of sound. It relates to recorded music, to speech and hearing,
to the behavior of sound in concert halls and buildings, and to noise in our environment.
It is the technology of designing spaces and systems that meet our auditory needs.
Architectural acoustics deals with sound in and around buildings of all kinds. Good
acoustical design ensures the efficient distribution of desirable sounds as well as the
exclusion of undesirable sound. All acoustical situations consist of three parts: (1)
source, (2) Path, and (3) Receiver.
Sound
Definition: An energy that is propagated by vibration in an elastic medium such as air,
water, most building materials, and earth.
Cycle, period, and frequency of sound: A full circuit by a particle of a medium
displaced by vibration is a cycle. Time required to complete one cycle is called
the period. Number of complete cycles per second is the frequency of
sound. Unit of frequency is Hertz (Hz).
Wavelength: The distance a sound wave travels during one cycle of vibration. Wavelength
= Velocity of sound/Frequency of sound.
Sound intensity: Sound travels freely in all directions (i.e. spherically). Sound
intensity is the strength of sound per unit area of a spherical surface.
The decibel scale: It is used to measure sound intensity. In decibel scale, (1) min.
intensity of perceptible sound is given a value of 0, (2) whole numbers are used, and (3)
an increase of every ten units equals a doubling of loudness. It is a logarithmic scale.
Inverse-square law: Sound intensity decreases at a rate inversely proportional to the
square of the distance from the sound source. The relationship can be expressed as:
I = W/4pr2
Where I = sound intensity in watts per square centimeter; W = sound power in
watts; r = distance from the sound source in centimeter.
If the distance is measured in feet, 4pr2
has to be multiplied by 930 (because 1 square foot equals 930 square
centimeter).
Sound propagation
Direct: Reaches the receiver directly from the source.
Reflection: Occurs when sound waves bounce off a surface at the same angle at which it
was incident on the surface.
Diffraction: It is the bending or flowing of a sound wave around an object or through an
opening.
Diffusion: Scattering or random distribution of sound from a surface.
Reverberation: Persistence of sound after source of sound has ceased. Results from
repeated reflections. Some reverberation is good (particularly for musical performances),
but not always desirable. Intelligibility and subjective quality of sound is rated by
reverberation time (RT).
Reverberation Time (RT): It is the time required for sound to decay 60 dB
after the source has stopped producing sound. Reverberation time = 0.05 * volume
of room/total absorption of sound. (Average ceiling height in spaces with upholstered
seats and absorptive rear walls is approximately related to mid-frequency reverberation
time. Ceiling height »20 * Mid-frequency
Reverberation Time in Seconds.)
Echo: Distinct repetition of original sound clearly heard above the general
reverberation. A reflected sound can be perceived as discrete echo if the reflected sound
wave is heard 0.05 second or later after it was heard as a direct sound.
Sound absorption
When sound energy strikes a surface, part of the energy is absorbed. Reverberation and
echoes may be controlled by effective use of sound absorption quality of a surface.
Acoustic absorption is defined in terms of an absorption coefficient. It is the ratio of
absorbed sound intensity by a material to the intensity of the sound source. Absorption coefficient = absorbed sound intensity/total intensity of sound source. Total absorption by a surface = surface area * absorption coefficient. Unit of
sound absorption is Sabin.
Ray diagram
Ray diagram is analogous to specular reflection of light. Analysis of ray diagrams can
be used to study the effect of room shape on the distribution of sound and to identify
surfaces that may produce echoes. A ray diagram shows both reflected and direct sound
paths. The difference between these two paths is called path difference (Path Difference =
Reflected Path - Direct Path). A path difference in excess of the distance that can be
traveled by a sound wave in 0.05 seconds indicates that the reflected sound can be
perceived as discrete echo.
Weighting networks
A-weighting network: Generally, the sensitivity of
human hearing is restricted to the frequency range of 20 Hz to 20,000 Hz. The human ear,
however, is most sensitive to sound in the 400 to 10,000 Hz frequency range. Above and
below this range, the ear becomes progressively less sensitive. To account for this
feature of human hearing, sound level meters incorporate a filtering of acoustic signals
according to frequency. This filtering is devised to correspond to the varying sensitivity
of the human ear to sound over the audible frequency range. This filtering is called A-weighting. Sound pressure
level values obtained using this weighting are referred to as A-weighted sound pressure
levels and are signified by the identifier dBA. Simply speaking, it may be defined as a frequency-response adjustment of a sound-level meter that makes
its reading conform, very roughly, to human response.
C-weighted network
: The C-weighted network provides unweighted microphone
sensitivity over the frequency range of maximum human sensitivity (over 1000 Hz).
STC is a single number rating of the air-borne transmission loss (TL) of a construction.
It measures the sound transmission loss (TL) of a construction at one-third octave band
frequencies.
For measurement, analysis, and specification of sound, the frequency range is divided
into sections or bands. One common standard division is into ten octave bands identified
by their mid-frequencies: 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, and 16000.
The STC of a given material is determined by comparing its measured TL values against a
standard STC contour using the following criteria:
The maximum deviation of the test curve below the standard contour at any
single test frequency shall not exceed 8 dB.
The sum of deviations below the standard contour at all frequencies of
the test curve shall not exceed 32 dB.
When the contour is adjusted to the highest value (in integral dB) that meets the above
requirements, the STC of the material would be the TL value corresponding to the
intersection of the standard STC contour and 500 Hz frequency ordinate.