1. INDOOR AND OUTDOOR
NOISE PROPAGATION,
PSYCHOACOUSTICS & NOISE
CRITERIA
Gayathri s Mohan

2. SOUND PROPAGATION OUTDOOR
⦿Sound waves emitted from a point source
propagate spherically - equally in all directions -
from the source.

3. ⦿Outdoors, sound waves travel in a
continuously extending spherical wavefront
from the source. For a point source that
emits a certain sound energy, this energy is
concentrated in a single point at the source.
At a distance from the source, the same
energy is distributed over a sphere. The
greater the distance from the source, the
larger the surface over which the energy is
dispersed. This may be illustrated by
studying a segment of the expanding
sphere.

4. ⦿The sound energy is dispersed over an imaginary
sphere with a surface that grows in proportion to
the square of the distance from a point source.

5. ⦿The surface of the sphere grows 4 times
with each doubling of the distance from the
source. The sound hence rapidly declines
with the distance from the source. Each
doubling of the distance from the point
source yields a 6 dB reduction of the sound
level.

6. SOUND PROPAGATION INDOOR
⦿Indoors, the sound wave hits building
construction surfaces before it is
significantly attenuated. The sound field
indoors is not spherical but depends on
the geometry and the acoustical
properties of these surfaces. The volume
of the room and the distances between the
sound source, the building construction
surfaces and the listening point are also
important.

7. ⦿The sound in a certain listening point in a
room is composed of the direct sound and the
reflected sound. The direct sound is the sound
that has not yet been reflected in a surface. The
sum of all reflected sound is called the
reverberent sound field. It consists of all
sound that has been reflected once, twice or
more in the building construction surfaces. The
sound reflected one time is called 1st
reflections, two times 2nd reflections etc.

8. Two-dimensional sound reflections in a room with only
reflecting surfaces.

9. ⦿The number of reflections affecting the
reverberent sound field depends on the
acoustic properties of the surfaces. If the
building surfaces were totally sound
reflecting, there would theoretically be an
infinite number of reflections.

10. Two-dimensional sound reflections in a room with one
absorbing surface.

11. ⦿ If the surfaces were perfectly absorptive,
there would be no reflections at all. In
reality, there is always a loss of energy
when a sound wave hits a wall. The air also
absorbs some of the sound wave's energy.
The sound absorption is frequency
dependent. High frequency sound is often
more easily absorbed than low frequency
sound.

12. PSYCHOACOUSTICS
⦿ Psychoacoustics is the scientific study
of sound perception, ie, the branch of science studying
the psychological and physiological responses
associated with sound including speech and music.
⦿ Hearing includes the mechanical wave propagation and
a sensory and perceptual event
⦿ When sound arrives at the ear, it is transformed to
neural action potential.
⦿ The ear has a nonlinear response to sounds of different
intensity levels; this nonlinear response is
called loudness

13. NOISE LIMITS
⦿The human ear can nominally hear sounds in the
range 20 Hz (0.02 kHz) to 20,000 Hz (20 kHz).
⦿The upper limit tends to decrease with age; most
adults are unable to hear above 16 kHz.
⦿Tones between 4 and 16 Hz can be perceived via the
body's sense of touch.
⦿The minimum threshold at which a sound can be
heard is frequency dependent. By measuring this
minimum intensity for testing tones of various
frequencies, a frequency dependent absolute threshold
of hearing (ATH) curve may be derived and can be
used to find the lower limits.

14. APPLIED PSYCHOACOUSTICS
Sound localization is the process of determining the
location of a sound source. The brain utilizes
subtle differences in intensity, spectral, and timing
causes to allow us to localize sound sources
Sound masking A weaker sound is masked if it is
made inaudible in the presence of a louder sound.

15. NOISE EVALUATION INDICES
AND BASIS FOR CRITERIA
⦿To properly evaluate noise exposure, both the type
and level of the noise must be characterized.
⦿The type of noise is characterized by its frequency
spectrum and its variation as a function of time.
The level is characterized by a particular type of
measurement which is dependent on the purpose of
the measurement

16. NOISE CRITERION
⦿The noise criteria (NC) is a single numerical index commonly
used to define design goals for the maximum allowable noise
in a given space.
⦿Noise Criterion - NC - were established in U.S. for rating
indoor noise, noise from air-conditioning equipment etc.
Noise Rating Curves – NR are commonly used for noise criteria
⦿Choosing an appropriate noise criteria is important when
specifying acceptable levels of noise.
⦿The NC criteria consist of a family of curves that define the
maximum allowable octave-band sound pressure level

17. NC CURVES
⦿The NC criteria consist of a family of curves that define the
maximum allowable octave-band sound pressure level
corresponding to a chosen NC design goal.
⦿The method consists of a set of criteria curves extending
from 63 to 8000 Hz, and a tangency rating procedure. The
criteria curves define the limits of octave band spectra that
must not be exceeded to meet occupant acceptance in
certain spaces.
⦿The NC rating can be obtained by plotting the octave band
levels for a given noise spectrum - the NC curves. The noise
spectrum is specified as having a NC rating same as the
lowest NC curve which is not exceeded by the spectrum.

18. NOISE CRITERIA DIAGRAM

19. NOISE CRITERIA LEVELS FOR ROOMS
Type of Space (and acoustical requirements)NC level
Conference rooms, churches, lecture halls, classrooms: NC 30-
35
Open-offices, schools, lobbies, public areas :NC 35-40
Large public offices: NC 40-45
Shops, garages, etc. (for just acceptable speech and telephone
communication): NC 50-60
For work spaces where speech or telephone communication is
not required,but where there must be no risk of hearing
damage: *NC 55-70

20. PSYCHOACOUSTIC MODEL
⦿The psychoacoustic model provides for high
quality lossy signal compression by describing
which parts of a given digital audio signal can be
removed (or aggressively compressed) safely —
that is, without significant losses in the
(consciously) perceived quality of the sound.
⦿compression is a feature of nearly all modern lossy
audio compression formats.

21. Thank you!!!.....