Lecture 2 a instrumentation used in the measurement of acoustic signals and auditory function
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1. The document discusses instrumentation used to measure acoustic signals and auditory function. It describes the nature of sound waves and defines characteristics like amplitude, frequency, period, and decibels. 2. Resonance is explained as occurring when a force is applied at an object's natural frequency, causing maximal vibration. The resonant frequency of an object depends on its mass and stiffness. 3. Sound intensity and pressure are defined, with thresholds for human hearing and pain. The decibel is introduced as a unit used to measure sound levels perceived by humans.
Lecture 2 a instrumentation used in the measurement of acoustic signals and auditory function
- 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
- 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