RCSI TSH.7 Sound Amplification and Detection PDF

Thermoregulation, Special Senses, Endocrine System, Reproductive System TSH.7 Sound Amplification and Detection INGMAR SCHOEN DAT E : 3 0 t h J a n u a r y 2 0 2 4 Learning outcomes At the end of this lecture, the learner will be able to Define sound intensity and its units. Differentiate be...

Thermoregulation, Special Senses, Endocrine System, Reproductive System TSH.7 Sound Amplification and Detection INGMAR SCHOEN DAT E : 3 0 t h J a n u a r y 2 0 2 4 Learning outcomes At the end of this lecture, the learner will be able to Define sound intensity and its units. Differentiate between Intensity & Sound Intensity Level. State the decibel equation. Explain decibel units and the decibel scale. Discuss the dynamic range and logarithmic response of the human ear. Differentiate between dB Sound Intensity Level and perceived loudness in phon. Discuss threshold of hearing and threshold of pain. List the amplification processes within the human ear and explain their basis. Explain frequency detection within the cochlea. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 2 Sound T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n Sound (recap) Sound is a longitudinal pressure wave. Requires a medium to travel. Particles in the medium move to carry the sound. Generally, the medium is air. Other media are fluids such as water/blood or solids like metal/bone. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 4 Frequency of Sound Different sounds can have different frequency. The number of waves that pass a fixed point in 1 second is called frequency. Frequency is measured in Hertz (Hz). 1 Hz = 1 s-1 – SI base unit Humans can detect frequencies ranging from 20 Hz to 20,000 Hz. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 5 Perceived Properties of Sound What we perceive is dependent on both the Intensity (amplitude) Frequency (pitch) T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 6 Sound Intensity Levels T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n Sound Intensity Intensity is defined as sound power per unit area. Unit W/m2 Note: Sound will decrease in intensity according to distance from the source following an inverse square law. i.e. if we double the distance from the sound source, the intensity decreases by a factor of 4. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 8 Sound Intensity Level The wide dynamic range of human hearing (12 orders of magnitude in sound intensity) makes the unit of W/m2 difficult to use. Instead, we use the Sound Intensity Level (SIL) which has units of decibel (dB) and is based on a logarithmic scale T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 9 Calculating decibel from sound intensity Example: If we are given a sound intensity of 500 mW/m2 We substitute into the equation Formula given on your exam sheet T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 10 Behaviour of the decibel scale I1 = 250 mW/m2 SIL1 = 114 dBSIL I2 = 500 mW/m2 SIL2 = 117 dBSIL Note: if we double the intensity, the SIL does not double but it increases by 3 dB (log scale) T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 11 Threshold of Audibility and Threshold of Pain The lowest intensity of sound that the ear can detect is called the threshold of hearing or threshold of audibility. At a frequency of 1 kHz, this intensity, , would be just audible = 10-12 W/m2 SIL = 0 dBSIL All other sound intensities are compared with this to give sound intensity levels (SIL). The loudest sounds we are able to tolerate have intensities of about 1-10 W/m2. This is known as the threshold of pain. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 12 Typical SIL of Common Noise T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 13  Pause point Do you have questions on Sound Intensity Levels? 14 Auditory Response T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n Loudness vs Intensity The ear is more sensitive to some frequencies than others. 100 Hz and 1 kHz sounds of equal intensity (1 W/m2) will have different perceived loudness. We need a different unit for perceived sound: > phon T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 16 Phon The phon is used to indicate an individual’s perception of loudness. 1000 Hz is chosen as a standard frequency, then equal loudness curves can be referenced to the decibel level at 1000 Hz. If a given sound is perceived to be as loud as a 60 dB sound at 1000 Hz, then it is said to have a loudness of 60 phons. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 17  Sound intensity level (dB) In this example a 50 dBSIL sound at 100 Hz would equate to 40 phons, i.e. it would be perceived with the same loudness as a 40 dBSIL sound at 1000 Hz. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n At 1 kHz phon = dBSIL (by definition) 18  range of highest sensitivity (3-4 kHz) ~120 phon Please see “notes” section for further detail. ~0 phon T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 19  Pause point Do you have questions on the auditory response? 20 Sound Amplification within the Ear T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n Outer Ear Pinna may help direct sounds into the auditory canal – more on this in a future lecture. Auditory canal “funnels” and helps to enhance the amount of sound that reaches the tympanic membrane. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 22 Auditory Canal Area of the tympanic membrane is approx. half the area of the opening of the ear canal (Area ∝ r2). The acoustic energy is funnelled into a smaller area. >> Sound is amplified by a factor of 2. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 23 Anatomy of the Middle Ear (recap) The middle ear contains air. Tympanic membrane: sound pressure causes an inward movement. Three bones (ossicles) Malleus Incus Stapes connect the tympanic membrane to the oval window, an opening into the inner ear. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 24 Sound Amplification by the Middle Ear The ossicles act as a lever with a mechanical advantage of approx. 2 >> Mechanical amplification by a factor of 2 Area of tympanic membrane is approx. 20 times that of the oval window at the base of the stirrup bone >> Mechanical amplification by a factor of 20 The ossicles transform low force, high amplitude vibrations to low amplitude, high force vibrations at the oval window (see notes section). T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 25 Sound Amplification within the Ear The total mechanical amplification of the sound pressure in the 3000 Hz range is about 2 × 20 × 2 = 80 times. Because the intensity is proportional to pressure squared, the intensity at the oval window is amplified by a factor of about 6400. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 26  Pause point Do you have questions on sound amplification within the ear? 27 Frequency Detection in the Inner Ear T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n Anatomy of the Cochlear (recap) The inner ear contains a snail-shaped structure called the cochlea, filled with fluid. The cochlea is separated into different segments through several membranes: Basilar membrane Tectorial membrane Reissner’s membrane T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 29 Place Theory The cochlea analyses sound frequencies by means of the basilar membrane, which exhibits different degrees of stiffness, or resonance, along its length: High frequencies at the base, low frequencies at the apex. This is known as place theory. T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 30 T S H. 7 S o u n d A m p l i fi c a t i o n a n d D e t e c t i o n 31  Pause point Do you have questions on frequency detection in the inner ear? 32 Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA N T INGMAR SCHOEN EMAIL: [email protected] 33