Study Notes for SLPA 3705 - Speech Science

The Nature of Sound

SLPA 3705 - Speech Science

Overview

• This document encompasses the principles of sound, including its properties, definitions, relationships with air, and characteristics that pertain to speech science.

Key Concepts of Sound

Air as a Medium

• Air: A gas consisting of billions of molecules in constant random motion.

  • Brownian Motion: Described by Robert Brown, it refers to the random motion of molecules in a gas due to thermal energy.

  • Ambient Pressure (Pam): The constant pressure produced by the collisions of air molecules.

Properties of Air

• Pressure: A force acting perpendicularly on a surface, varies based on location and area.

• Volume: The physical space occupied by matter.

• Density: Mass per unit volume; e.g., water (1 g/mL) vs. lead (11.3 g/mL).

  • Boyle’s Law: Relates pressure and volume—inverse relationship if density remains constant. If density changes, pressure changes (direct relationship).

Boyle's Law and Respiration

• Inspiration and Expiration:

  • During inspiration, diaphragm contracts downward; pressure decreases.

  • During expiration, diaphragm relaxes upward; pressure increases.

Sound as a Disturbance

• Definition: Sound is a disturbance in a medium by a source—essentially vibrations of air molecules or changes in air pressure.

• Medium: Can be gas, liquid, or solid; air serves as a perfect medium for sound.

• Sound Wave Generation:

  • Sound waves consist of tiny movements of air molecules around their resting positions due to disturbances from sound sources.

  • These waves travel longitudinally in all directions, like ripples in a pond.

Wave Dynamics and Properties

• Slinky Analogy:

  • Representation of air molecules—when pushed/pulled, it mimics the act of sound propagation, causing condensation and rarefaction in the air molecules.

  • As sound travels, it results in waves of compression and rarefaction, altering air’s volume, density, and pressure.

Forces in Sound Generation

• Key Forces at Work: When a force displaces air molecules, three forces are involved:

  • Elasticity: Restores displaced objects back to position.

  • Inertia: Maintains the motion of displaced molecules.

  • Friction: Opposes motion and dissipates energy until the sound ceases.

Changes in Air Pressure by Sound

• Displacement Effects:

  • Molecules exhibit compressive actions (collide closely) and rarefaction (overshoot resting point).

  • Changes in air pressure correlate with these movements:

  • Compressed phase: Increased density, positive pressure.

  • Rarefied phase: Decreased density, lower pressure.

The Hearing Process

• Sound Wave Transmission:

  1. Sound propagates by changing ambient pressure.

  2. Air pressure manipulation pushes tympanic membrane inwards (compression) and allows it to move outwards (rarefaction).

  3. Tympanic membrane vibration sets the ossicles in motion within the middle ear.

  4. Vibration stimulates fluid movement in inner ear, triggering hair cells (nerve cells) to generate nerve impulses.

  5. Brain interprets these signals as sound.

Dimensions and Characteristics of Sound

Frequency

• Cycle: A complete set of compression and rarefaction.

• Frequency (F): Number of cycles per second; measured in Hertz (Hz).

  • Example: A wave with a frequency of 2 Hz completes 2 cycles in one second.

Period

• Definition: Time taken for one cycle to occur; measured in seconds (T).

  • For a 2 Hz wave, the period is $T = rac{1}{f} = rac{1}{2} = 0.5$ seconds.

Wavelength

• Definition (λ): The distance of one complete cycle; measured in meters (m) or centimeters (cm).

• Propagation Velocity (C): Average speed of sound in air, approx. $330 ext{ m/s}$.

• Wavelength Calculation: $ext{Wavelength} (λ) = rac{C}{F}$, e.g. for 250 Hz, $λ = rac{330 ext{ m/s}}{250 ext{ Hz}} <br>ightarrow 1.32 ext{ m}$.

Frequency/Pitch Relationship

• The higher the frequency, the higher the pitch perceived by humans, within a range of 20 Hz to 20,000 Hz, with speech sounds typically between 100 and 8000 Hz.

• Pure Tone vs. Complex Sound:**

  • A pure tone comprises a single frequency.

  • A complex sound contains multiple frequencies, resulting in harmonic structures.

  • Fundamental Frequency (F0): Lowest frequency of a complex sound with harmonic frequencies (whole number multiples of F0).

Intensity and Loudness

• Amplitude: Distance molecules are displaced from resting position; correlates with pressure.

• Intensity: Power needed to create molecular movement; measured in watts (W).

  • Intensity and Amplitude Relationship: $ext{Intensity} = ext{Amplitude}^2$. E.g. if amplitude increases by 3, intensity increases by $3^2 = 9$.

• Decibel Scale (dB): Measures loudness with logarithmic adjustments.

  • 1 dB is 1/10 bel; not linear—each increase on the scale indicates a 10x increase in intensity.

Decibel Reference Sounds

• Standard Reference Sound (SRS): Softest sound heard, defined as 0 dB at a pressure of 0.0002 dynes/cm2.

• Sound Pressure Level (SPL) vs. Intensity Level (IL):

  • SPL measures amplitude. For instance, $30 ext{ dB SPL} = 1000 ext{ times greater than 0.0002 dynes/cm²}$.

• Decibel Calculation:

  • For intensity: $N(dB) = 10 imes ext{log}<em>{10} rac{I</em>1}{I_0}$.

  • For amplitude: $N(dB) = 20 imes ext{log}<em>{10} rac{P</em>1}{P_0}$.

Fletcher-Munson Curve

• Graph illustrating perception of sound across different frequencies in terms of loudness at various dB levels.

Audiogram Characteristics

• dB Hearing Level (dB HL): An adjusted scale used in audiology to reflect individual hearing, equalizing sounds at various frequencies to present a more accurate measure of hearing capability.

• Speech Banana: The area on an audiogram representing where phonemes/speech sound segments fall, critical for understanding speech perception in various auditory conditions.