Physics of Sound and Air Pressure

Speech and Hearing Sciences Notes

• Sound has no weight, substance, or mass.
• Sound is a set of movements, vibrations, disturbances, or perturbations.
• Sound does not travel itself, but is continuously transferred.
• Sound is invisible.
• Sound is (almost always) very complex.
• Sound is the propagation of a pressure wave in space and time through a medium.

Sound System

• Source: Anything that vibrates.
• Medium: Anything with molecules.
• What about sounds in a vacuum such as space?
• Receiver: Anything that can detect vibrations.

Metaphysics of Sound

• An important aspect of sound is detecting vibrations of a source that pass through a medium.
• For people, brains are important detectors.
• Acoustics: Branch of physics dealing with the generation, transmission, and modification of sound waves.

Speed of Sound

• Propagation velocity depends on the properties of the medium.
• Elasticity: Rigidity, deformation characteristics, absorption of energy; higher elasticity results in faster speed.
• Hooke's Law: Stress is negatively proportional to strain, or stress ∝ -strain. Young's modulus is the ratio of stress to strain, measured in gigapascals (GPa). For example, steel is 200 GPa, rubber is .01 GPa, human tendon is 7 GPa, and bone is 14 GPa.

Air Pressure Behavior

• Made up of many molecules of many chemicals.
• These molecules move: constantly in random patterns (Brownian Motion), or drunkard's walk; at high speeds, into collisions with each other and whatever is in the way.
• These collisions produce pressure.
• Brownian motion: Random movement of small particles.

Air pressure

• Pressure: A force that acts perpendicularly on a surface.

Measurement Units of Air Pressure

• Pressure is calculated by: force (newton, dyne, kilopound, pound), and the surface area (square foot, m2, acre, square inch, mi2, cm2).
• PSI (pounds per square inch)
• N/m2 (Newtons per square meter)=Pascals (1/1,000,000 Pa = µPa)
• Dynes/cm2 (dynes per square centimeter) = microbar
• mmHg (millimeters of mercury)
• Dynes per square centimeter (d/cm2).

International System of Units (SI)

• Distance, mass, and time units (meters, kilograms, seconds).
• Newtons (N)
• Pascals (Pa). Micropascals (µPa)

Higher altitudes = lower pressure.

Physical Acoustics

• Movement of air from high pressure to low pressure.
• Flow measures are in volume/time units, like liters per second (l/s), liters per minute (l/min), milliliters per second (ml/s), and milliliters per minute (ml/min).
• Rate of flow = volume velocity or speed occurring in a particular direction of air flow.
• Driving pressure: The difference in pressure causes air to flow from higher to lower pressure. This is important for generating speech.
• Laminar flow: Smooth, parallel flow of air molecules at the same speed.
• Turbulent flow: Random flow of air molecules, results in swirls and eddies that cause noises.

Volume and Density

• Volume: Amount of space occupied in 3D.
• Density: Amount of mass per unit volume.

Inverse Relationship Between Pressure and Volume

• Inverse relationship: Volume increases, pressure decreases; volume decreases, pressure increases.
• Proportional relationship: Both volume and pressure increases or decrease (when temperature is constant).
• Boyle's Law: As volume decreases, the pressure of air increases, keeping constant air temperature.

Changes in Pressure

• Ambient pressure (Pam) = relatively constant pressure around us.
• For sound to be generated, constant Pam must be disturbed by a force.
• Example: striking a tuning fork creates a disturbance resulting in sound.

Mass-Spring Model

• Molecules behave like masses connected by springs.
• No energy when spring hangs itself. Energy input when the spring is pushed. Energy loss through friction.

Sound: Changes in Air Pressure

• Vibration spreads in time.
• Compression: Increased air density (increased pressure)
• Rarefaction: Decreased air density (lower pressure).

Elasticity and Inertia

• Movement of air molecules eventually comes to a resting position.
• Elasticity: The restoring force, refers to an object springing back to its original shape after being stretched. This is pertinent to all materials (solids, liquids, and gases) and their ability to return to their resting position after displacement.
• Inertia: Refers to the tendency of a body to remain at rest or in motion with a constant velocity unless acted upon by a force, and causes the molecules to overshoot their resting positions and oscillate back and forth due to elasticity. This back and forth allows sound to continue.

Other Wave Characteristics

• Damping: Reduction of amplitude over time, due to frictional resistance in the air.
• Coupled oscillators: Happens when two masses are coupled, causing them to be synchronized, like in biological cycles.
• Frequency (f):Cycles per second (Hertz [Hz]); the pitch; how many times a wave repeats itself every second.
• Period (t): Time for one cycle in a wave. The reciprocal of frequency, t = 1/f.
• Wavelength (λ): Distance between two wave fronts. Calculated by dividing the speed of sound by the frequency (λ = c/f).

Complex waves

• Periodic complex waves: Regular, repeating waveforms with a fundamental frequency and harmonics
• Aperiodic complex waves: Random, do not repeat, and have no fundamental frequency nor harmonics.

Complex waves contain more than one frequency.

Explore the concepts of sound and air pressure in this quiz. Understand how air moves, the laws governing volume and pressure, as well as the properties of sound waves. Perfect for students looking to deepen their knowledge in the physics of acoustics.