The document discusses various topics related to acoustics and noise measurement: - It describes how the audio frequency range is divided into standard octave bands and one-third octave bands. - It covers decibel additions, subtractions, and averaging used to calculate total sound pressure levels from multiple noise sources. - Key concepts around how sound is perceived by the human ear like loudness, equal loudness contours, frequency weighting, and threshold of hearing are summarized. - The effects of noise exposure like temporary and permanent hearing loss are outlined. - Common noise indices used to quantify environmental noise like LN, Leq, and Ldn are defined along with examples of how to calculate them

1. Octave band
• The audio frequency range is divided into 10
standard octave bands with center
frequencies 31.5, 63, 125,250,500,
1000,2000,4000,8000 and 16000 Hz
• Each center frequency is double the preceding
center frequency and each bandwidth doubles
the preceding one

2. • The upper and lower limits of frequency of
each band
• Because
• Bandwidth for an octave is

3. • Module I
• Introduction – Basic acous
and definitions – Plane wave
• Velocity of sound in invi
wavelength particle velocit
acoustic intensity – referenc
• CO1: Students should be
principles and terminologie

4. One-third octave band
• One-third octave bands are formed by subdivided each octave
band into three parts
• The successive centre frequencies increase in intervals by cube
root of 2
• Upper and lower frequencies are related to center frequencies
as
• Also
• Bandwidth is

6. Decibel additions, subtraction and
averaging
• Let Lp1, Lp2, Lp3, …Lpm are the sound pressure
levels
• We have
• Total sound pressure

7. subtraction
• If total sound pressure level Lpt and the sound
pressure level of back ground noise or any
other device is LpB, the sound pressure level of
source or another device is determined using

8. Averaging
• In certain cases SPL at any location is
measured several times and average SPL has
to be determined. Average SPL

9. • Find the total sound pressure level due to 96
dB, 87 dB and 90 dB
• Measurements indicated Lpt =93dB at a
specific location with a lathe in operation.
When the lathe is shut down the background
noise measures at 85 dB. What is the sound
level due to the lathe?
• Determine the average sound pressure level Lp
at a location for a series of measurements
taken at different times: 96,88,94,102 and 90
dB.

10. How sound is heard
• Sound wave enters the outer
ear and cause the ear drum to
vibrate
• These vibrations pass along the
middle ear via three bones
known as ossicles
• Ossicles amplify the vibrations
and transmit them to the
cochlea of inner ear
• Hair cells in the cochlea moves
in response to the vibrations
and send signals to the brain
through auditory nerves, which
helps to understand the sound

12. • The human ear does not possess uniform
sensitivity across the audio frequency range
• The frequency response of the ear is not
constant, but changes slowly with ageing
• Rapid reductions in sensitivity may occur if we
are exposed to damaging noise levels

13. Masking
• Speech which is perfectly audible in quiet
surroundings may become unintelligible as the
ambient noise level increases.
• Masking depends not only on the relative
pressure levels of two sounds, but also on their
frequency content.
• A pure tone is more effectively masked by a
second pure tone of similar frequency than by a
pure tone of quite different frequency.
• Low-frequency noises mask high-frequency
noises more than high-frequency noises mask
low frequencies.

14. Threshold of hearing and hearing area
The threshold of audibility of a
normally hearing person for
pure tones
Sensitivity of hearing is maximum
between 3000 and 4000 Hz
The sound pressure levels which will give
painful sensation
The region between the threshold of
hearing and the threshold of pain is
the ‘auditory area
Auditory area contains the frequencies and pressure levels of all pure tones which
our hearing can receive without causing any harm to our ear

15. Audiometry
• Audiometry is the measurement of hearing
sensitivity.
• The instrument used is called an audiometer.
• The subject wears headphones and listens to a
series of pure tones at pressure levels which can
be adjusted over a wide range.
• He is asked to indicate the threshold of
perception for each tone used, and the test is
repeated separately for each ear.
• Audiometry is done in a sound proof room in
order to avoid masking

16. Audiometer result

17. Loudness
• Loudness is subjective
• The loudness of an auditory stimulus is a psychological,
not physical attribute of the stimulus
• Loudness is the listener's subjective description of the
strength or volume of the stimulus
• It is the perceived volume of sound and perception
varies from person to person
• It is the subjective perception of changes in amplitude of
sound
• Unit of loudness is sone

18. • It is the perceived magnitude of sound as
estimated by listeners having normal hearing
based on the acoustic properties of the sound
and its manner of presentation to the listener.
• Loudness depends primarily on the sound
pressure level , it also depends on the
frequency, waveform, bandwidth and duration
of sound
• One sone is the loudness of sound whose
loudness level is 40 phon
• A sound that is twice as loud as another sound
has double the number of sone

19. • Loudness is a perceptional concept and
not a physical concept
• People are not equally sensitive to sounds
of all frequencies so perceived loudness of
a tone in fact depends on frequency as
well as intensity
• Two sounds can have the same physical
sound pressure levels but if they are of
different frequencies, they are often
perceived as having different loudness.

20. Loudness level
• Sensitivity of human hearing is frequency dependent
• Response of the ear depends on frequency as well as
pressure amplitude
• Equal loudness contours(Fetcher-Munson curves)
• Subject is presented with a 1kHz tone, which is the
reference tone, and a second sound
• The subject is asked to adjust the level of sound until it
sounds same as that of the reference sound
• Contours are drawn in this manner.

21. Equal loudness contours(

22. Hearing loss
• Excessively loud noise can damage the structure of the ear,
which results in temporary or permanent loss of hearing.
• Mechanical damage of hair cells, seriously impairs the normal
function of the ear
• Sudden exposure to very intense sound damages hair cells
• Regular exposure to excessive noise results in the formation of
harmful molecules in the inner ear as a result of stress caused
by noise-induced reductions in blood flow in the cochlea.
• The harmful molecules build up toxic waste products known as
free oxygen radicals which injure essential structures in the
cochlea, causing cell damage and cell death, resulting in noise
induced hearing loss
• This type of hearing damage is often accompanied by
permanent tinnitus or “ringing in the ears”.

23. – Trauma
High-intensity sounds like explosions or jet
exhausts can rupture the eardrum.
Damage of Ossicles, sensory hairs on the basilar
membrane or cochlea also lead to this
permanent damage
Occurs suddenly and is irreversible
Sudden exposure to noise of SPL around 150 dB
may lead to this

24. – Chronic hearing loss
The gradual loss of hearing caused by persistent exposure to
high noise levels, for those people working in factories and
workshops.
Limited time exposure of loud noise results in temporary
threshold shift (TTS)
For a person with normal hearing, the threshold of hearing is
20micropascal or 0dB at 1 KHz
The hearing threshold can rise by up to 20 dB at 4 kHz after
exposure to loud noise.
If the exposure is short, loss is temporary else permanent loss
develops over a period of time
It is irreversible also
Greatest reduction in hearing occurs at 4kHz

25. Frequency weighting-necessity
• Human perception of loudness depends on the
frequency of sound
• Perception of human ear is high in the mid
frequency rather than low or high frequencies
eventhough all are of equal energy
• A noise whose energy is concentrated mostly in
the mid frequency of the audio spectrum is
perceived as louder than noise of equal energy
concentrated in low or high frequency region

26. • This effect is more pronounced for soft sounds
than loud sounds
• Thus loudness control is provided on some
audio amplifiers, which provides greater
amplification to the high and low frequency
contents

27. • In order to account for the difference in the
actual sound pressure level and the perceived
sound pressure level, frequency weighting is
done.
• ‘A’ weighting is exclusively used in
measurements that involve human response
to noise
• It is reported in dB(A) or dBA

28. • For 1kHz frequency band actual and perceived
sound pressure levels has no difference
• ‘A’ weighting is introduced for sound levels
below 55dB (low sound), ‘B’ weighting for
levels between 55dB and 85 dB (moderate
sound) and ‘C’ weighting for levels exceeding
85 dB(high sound)

29. Frequency weighting curves

30. Conversion table of sound levels in
1/3 rd octave band

32. Performance indices for environment
noise
• For quantifying noise exposure performance
indices are using
• These indices are single number criteria used for
assessing environment noise
• Three indices based on A-weighted
measurements are
– LN (gives the levels exceeded N% of the measurement
time)
– Leq –the equivalent continuous sound pressure level in
dB(A)
– Ldn- the day-night average sound level in dB(A)

33. • LN criteria are specified in terms of sound
levels that exceeded 10%, 50%, and 90% of
the time
• Level corresponding to percentage of total
observation is determined using the
probability of exceeding each range of decibel
levels
• Sound levels that are exceeded a particular
percentage of time is estimated.

34. A histogram showing probability of exceedance for plant noise

35. Equivalent sound level Leq
• It is the sound energy averaged over a given
period of time T
• It is the rms or mean level of the time varying
noise

36. Day-night equivalent sound pressure
level Ldn
• It is a modification of Leq
• Mainly used for evaluating community noise
problems
• A night time a penalty of 10dB is imposed on
measurements between 10 pm and 7 am

37. • Find Leq in the case where Ln = 90.5, 95, 103,
88 and 98 dB(A) are obtained as the
respective average levels for five short, equal
time intervals.
• Find Ldn for the situation where the day time
equivalent sound level is 82 dB(A) and the
night time equivalent sound level is 76 dB(A)

38. Noise and number index(NNI)
• NNI was used to assess aircraft noise disturbance
near major airports in UK since 1963
• It was formerly used in the United Kingdom as a
measure for evaluating the long term average
sound pressure levels from aircraft near airports
• It was used to measure the subjective noiseness
of aircraft
• It was devised by the Wilson Committee of noise
in Britain in 1963

39. • It was the degree of annoyance
• It was prepared by relating people’s reactions
to aircraft noise, at various positions near
Heathrow airport using measured noise levels
and periodicity of flights in those positions

40. • It uses the PNdB (peak nose dB) as a basis and
takes into account the number of aircraft per
day (or night) as the key annoyance factor
• NNI=L+15(log10N)-80
• where L is the logarithmic average of aircraft
noise level heard and N is the number of
aircraft heard during the day between 0700
and 1900 hours local time on an average
summer day