7_TSH Sound Amplification and Detection

Learning Outcomes for Sound Amplification and Detection

  • At the end of this lecture, learners will be able to:

    • Define sound intensity and its units.

    • Differentiate between intensity and sound intensity level.

    • State the decibel equation.

    • Explain decibel units and the decibel scale.

    • Discuss dynamic range and logarithmic response of the human ear.

    • Differentiate between dB sound intensity level and perceived loudness in phon.

    • Discuss thresholds: threshold of hearing and threshold of pain.

    • List amplification processes within the human ear and explain their basis.

    • Explain frequency detection within the cochlea.

Sound

  • Sound is a longitudinal pressure wave.

  • Requires a medium, typically air, but can also travel through fluids (like water/blood) and solids (like metal/bone).

    • Particles in the medium carry the sound energy by vibrating back and forth in the direction of the wave's travel, allowing the sound to propagate.

Frequency of Sound

  • Frequency is the number of wave cycles passing a fixed point per second

Frequency unit - Hertz (Hz)

  • Human hearing range: 20 Hz to 20,000 Hz.

Perceived Properties of Sound

  • Perception of sound is dependent on:

    • Intensity (amplitude)

    • Frequency (pitch)

Sound Intensity Levels

  • Sound intensity (I) defined as sound power per unit area (W/m²).

  • Intensity decreases with distance from the source following the inverse square law, meaning that as the distance from the sound source increases, the intensity of the sound diminishes proportionally to the square of that distance.

    • Doubling distance reduces intensity by a factor of 4.

Sound Intensity Level (SIL)

  • SIL is utilized due to the wide dynamic range of human hearing (12 orders of magnitude in intensity):

    • Formula: → see power point

SIL units = Decibels (dB)

Calculating Decibel from Sound Intensity

  • Example calculation:

    • For sound intensity = 500 mW/m²:

      • Simply substitute into the SIL equation to find the decibel level.

    • Formula given on exam sheet.

Behaviour of the Decibel Scale

  • Sound intensity from two sound levels:

    • If intensity (I1) = 250 mW/m² → SIL1 = 114 dB

    • If intensity (I2) = 500 mW/m² → SIL2 = 117 dB

  • Doubling intensity results in a 3 dB increase on a logarithmic scale.

Threshold of Audibility and Threshold of Pain

  • Threshold of hearing or threshold of audibility: lowest detectable sound intensity (0 dB at 1 kHz is considered just audible).

  • Threshold of pain (the loudest sounds we can tolerate) occurs at intensities of approximately 1-10 W/m².

Typical SIL of Common Noise

  • Examples of typical Sound Intensity Levels from everyday sounds.

Auditory Response

How the ear processes sounds, responding differently to various frequencies.

Loudness vs Intensity

  • Human ears vary in sensitivity to different frequencies; different sounds at equal intensity levels can be perceived differently on a loudness scale.

  • Units for perceived sound: phon.

Phon

  • The phon measures individual perception of loudness, referencing a standard frequency (1000 Hz).

  • Example: A sound that feels the same loudness as a 60 dB at 1000 Hz has a loudness of 60 phons.

Amplification within the Ear

Pinna - Outer ear

  • The pinna directs sound into the auditory canal, which enhances sound transmission to the tympanic membrane.

  • The area of the tympanic membrane is approximately half the area of the opening or the ear.

    • The acoustic energy is funnelled into a smaller area, increasing the sound pressure level and making the sound waves more effective at vibrating the ossicles in the middle ear.

    • Sound is amplified by a factor of 2

Anatomy of the Middle Ear

  • The middle ear contains air

Tympanic Membrane - A thin membrane that vibrates in response to sound waves, converting them into mechanical energy.

  • Sound pressure causes an inward movementof the tympanic membrane, which in turn moves the ossicles, a chain of three tiny bones known as the malleus, incus, and stapes, amplifying the sound before it is transmitted to the inner ear.

The ossicles -

  • Malleus

  • Incus

  • Stapes

These bones, ossicles, connect the tympanic membrane to the oval window, an opening into the inner ear

Sound Amplification by the Middle Ear

The ossicles act as a lever with a mechanical advantage of approx. 2

  • They provide a mechanical advantage of approximately 2, meaning they can increase the force of sound vibrations as they transmit these vibrations from the tympanic membrane (eardrum) to the oval window, which leads into the inner ear. This amplification helps to ensure that sound is effectively transferred into the inner ear for further processing, allowing for better hearing of faint sounds.

Tympanic membrane's area is approx. 20 times larger than the oval window.

Total Mechanical Amplification

  • 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.

Frequency Detection in the Inner Ear

Anatomy of the Cochlear

The inner ear contains a snail shaped structure known as the cochlea, filled with fluid, which is crucial for converting sound vibrations into neural signals. The cochlea is separated into different segments through several membranes:

  • Basilar membrane - a key structure within the cochlea that vibrates in response to sound waves, enabling the detection of different frequencies of sound.

  • Tectorial Membrane - a gelatinous structure that sits atop the hair cells in the cochlea, playing a vital role in the process of sound transduction by deflecting the hair cells as sound waves pass through the fluid, thereby initiating the neural response.

  • Reissner’s membrane - a membrane that separates the scala vestibuli from the scala media in the cochlea, crucial for maintaining the ionic composition of the fluid within these compartments, which is essential for proper functioning of the auditory system.

Place Theory

A theory that explains how different frequencies of sound are perceived based on the location of maximum vibration along the basilar membrane, with higher frequencies stimulating the base and lower frequencies stimulating the apex.

  • The basilar membrane exhibits different degrees of stiffness relating to different frequency responses.