Lecture program Psychoacoustics and instrumentation

1. Instrumentatio
n Used In The
Measurement
of Acoustic
Signals And
Auditory
Function
Dr. Ghulam Saqulain
M.B.B.S., D.L.O., F.C.P.S
Head of Department of ENT
Capital Hospital, Islamabad

2. NATURE OF SOUND
Describe the nature of a sound wave in
terms of a series of events happening to
air particles.
Describe acoustic signals in terms of their
amplitude, frequency, and phase.
Define the measure of sound level known
as the decibel

3. Nature of Sound
Sound is alternative waves of compression and
rarefaction, which travels through the medium
When an object surrounded by air particles vibrates,
the air particles adjacent to that object also vibrate.
Thus, when a sufficient force is applied to the air
particles by the moving object, the air particles will
be moved or displaced in the direction of the
applied force.

4. Once the applied force is removed, a
property of the air medium, known as its
elasticity, returns the displaced particle to
its resting state. The initial application of
force sets up a chain of events in the
surrounding air particles

5. Nature of Sound

6. Types of Vibrations
Free Vibration.
Once force is applied on an object ,it
keeps on vibrating ,e.g. drum in orchestra.
Maintained Vibrations.
When repeated force is applied on the
object and it keeps on vibrating ,e. g. vocal
cords and trumpets.

7. Frequency
Cycles per second or CPS is called frequency.
1CPS=1Hz
4CPS=4Hz

8. Period
Time for one cycle to occur is called
period.
F=1/P OR P=1/F
When period is long, frequency is small
b/c both are inversely proportional. Ear
perceives the changes in frequency and
periods as pitch.

9. Frequency

10. Loudness.
Amplitude is related to loudness in perception.
Intensity.
 It is derivative of amplitude and measurement in terms
of sound pressure.
Dyne/cm is a linear scale.
 Force per unit area is called sound pressure.
Dynes/cm2

11. amplitude

12.  In free vibration, as in the case of the pendulum
once the vibration was started by applying force, the
vibration continued without additional force being
required to sustain it. For free vibration, the form of
the vibration is always sinusoidal, and the frequency
of oscillation for a particular object is always the
same. This frequency is known as the object’s natural
or resonant frequency. The resonant frequency of an
object or a medium, such as an enclosed cavity of air,
is determined largely by the mass and stiffness of the
object or medium.

13. Resonance
 An object that vibrates maximally when a
force is applied, with the stimulating frequency
of the applied force corresponding to the
resonant frequency of the object, is said to be at
resonance.
Perhaps one of the most familiar examples of
acoustic resonance is the shattering of a
wineglass by the high-pitched voice of a soprano.

14. Mass and stiffness
 Generally
stiffness opposes low-frequency vibrations,
whereas mass opposes high-frequency vibrations.
The resonant frequency of an object or a
medium, such as an enclosed cavity of air,
is determined largely by the mass and
stiffness of the object or medium

15. Sound Intensity (I)
I = (Power transmitted by the Wave)/Area
Threshold of human hearing:
Io =1 x 10-12
Watts/m2
Threshold of Pain:
IP= 1 Watts/m2

16. Sound (Intensity) Level
A measure of our perception of the
loudness of the sound.
Unit: decibel(dB)

17. Sound (Intensity) Level
(decibel)
β= 10 log I , unit for β is dB (decibel)
I0
The decibel compares the sound intensity (I), to Io, the
threshold of human hearing.

18. SOUND PRESSURE LEVEL
– Our ears respond to extremely small pressure
fluctuations p
– Intensity of a sound wave is proportional to the
sound Pressure squared:

19. Sound Pressure.
Force being applied by sound wave against every cm
of air is called sound pressure.
If sound is concerned , we call it sound pressure . The
amount of sound wave is called its intensity. It is
measured in Dynes/cm.
Human ear can feel,
10 million:1 in Dyne
Finest Sound. 0.0002Dynes/cm
Loudest Sound. 2000Dynes/cm

20. SOUND PRESSURE and
INTENSITY
Sound pressure level is measured with a
sound level meter (SLM)
Sound intensity level is more difficult to
measure, and it requires more than one
microphone
In a free field, however, LI ≈ LP

22. FOUR ATTRIBUTES USED
TO DESCRIBE A SOUND:
Loudness
Pitch
Timbre
Duration

23. EACH OF THESE DEPENDS ON ONE OR
MORE PHYSICAL
PARAMETERS THAT CAN BE MEASURED:
Sound pressure
Frequency
Spectrum
Duration (measured)
Envelope
(Relating the SUBJECTIVE QUALITIES to the
PHYSICAL PARAMETERS that we can MEASURE
OBJECTIVELY. Is an important problem in

24. DEPENDENCE OF SUBJECTIVE
QUALITIES OF
SOUND ON PHYSICAL PARAMETERS

25. PLOT YOUR OWN
FREQUENCY RESPONSE
ASSIGNMENT: Plot your own frequency
response curves by using
www.phys.unsw.edu.au/~jw/hearing.htm
l

26. LOUDNESS OF COMPLEX
TONES
Loudness depends mainly on:
 SOUND PRESSURE. But it also depends
on
 FREQUENCY,
 SPECTRUM and
 DURATION

27. LOUDNESS
RECRUITMENT
UNUSUALLY RAPID GROWTH OF LOUDNESS
ABOVE A CERTAIN THRESHOLD
GENERALLY ASSOCIATED WITH HEARING
LOSS,BUT NORMAL LISTENERS EXPERIENCE IT
FOR TONES OF VERY HIGH OR VERY LOW
FREQUENCY