Waves and Sound - Module 2-4

Vocabulary flashcards covering key terms from the lecture notes on transverse/longitudinal waves and sound propagation.

Transverse wave

A wave in which the medium's displacement is perpendicular to the direction of propagation.

Longitudinal wave

A wave in which the medium's displacement is parallel to the direction of propagation.

Oscillation direction

The direction in which particles move during the oscillation of a wave.

Propagation direction

The direction in which the wave travels through the medium.

Wavelength (λ)

The distance between consecutive crests (or troughs) of a wave. SI Unit is m (metre)

Crest

The highest point of a transverse wave.

Trough

The lowest point of a transverse wave.

Amplitude (SI Unit: metre)

The magnitude of the maximum displacement from the rest position (or equilibrium) of the wave.

Speed of sound

The rate at which sound waves propagate, depending on the medium's rigidity, compressibility, and density.

Medium

The substance through which a wave travels (gas, liquid, or solid).

Inverse square law

In a free field, sound intensity decreases with the square of distance (I ∝ 1/d^2).

Free field

Condition of sound that occurs outdoor in an open field with no sound reflections.

Reverberant field

Indoors sound energy drops off free field conditions only near the source (usually <5ft for small rooms) because room surfaces reflect sound.

Sound intensity

Power per unit area carried by a sound wave (W/m^2).

Sound intensity level

SIL in dB determined as the power ratio: SIL = 10 log10(I/I0).

Sound pressure level

Decibel measure of sound pressure: SPL = 20 log10(P/P0).

Reference intensity

I0 = 1×10^-12 W/m^2, the threshold of human hearing.

Reference pressure

P0, the reference sound pressure (commonly about 20 μPa in air).

Decibel

A logarithmic unit used to express ratios of sound intensity or pressure.

Source-Path-Receiver model

Model describing sound propagation with Source Level (SL), Transmission Loss (TL), Ambient Noise Level (NL), and Receiver metrics.

Source Level

Sound level at the source before transmission losses.

Transmission Loss

Loss of sound energy as it travels through the environment.

Ambient Noise Level

Background noise present along the sound path.

Received Level

Sound level at the receiver after transmission losses and ambient noise.

Signal-to-Noise Ratio

Difference between Received Level and Ambient Noise Level; measure of audibility.

Detection Threshold

Minimum signal level detectable by the receiver.

Wavefront

An imaginary line that connects all the crests or points in phase on a wave.

Frequency (f)

The number of crests (peaks) or troughs that pass a point per second; the number of complete waves generated per second. (SI Unit: Hz hertz)

Velocity (m)

The distance a wave travels per second (SI Unit: m/s)

v = fλ

Equation linking velocity, frequency, and wavelength.

Line source vs. point source

Line sources spread energy cylindrically; point sources spread energy spherically.

6 dB distance rule

For a point source in free field, sound level drops by about 6 dB with each doubling of distance.

3 dB distance rule

For a line source in free field, sound level drops by about 3 dB with each doubling of distance.

Ultrasound

Sound with frequency above the human hearing range (>20 kHz).

Compression

A region of higher pressure in a longitudinal wave (part of compressions/rarefactions).

The Speed of Sound

varies greatly depending upon the medium it is traveling through.

The Speed of Sound

The greater the density of a medium, the slower the speed of sound.

Sound propagation

One of the more popular model used to describe the propagation of sound through a medium is the source, path, receiver model.

RL = SL - TL (received level = source leve; - transmission loss)

A simple definition of sound propagation is:

Sound Intensity (Free field propagation)

A useful quantity for describing the loudness of sounds is called sound intensity. In general, the intensity of a wave is the power per unit area carried by the wave. Power is the rate at which energy is transferred by the wave.

I=P/A

In equation form, intensity I is :

Free Field

Sound level drops 6dB for every doubling of distance for sound traveling spherically.

The decibel scale

You may have noticed that when people talk about the loudness of a sound, they describe it in units of decibels rather than watts per meter squared.

While sound intensity (in W/m2) is the SI unit, the sound intensity level in decibels (dB) is more relevant for how humans perceive sounds.

Inverse Square Law (free field condition)

The sound intensity is inversely proportional to the square of its distance traveled (assume spherical spreading). A simple definition of this principle can be expressed by:

I=W/4πd²

Sound intensity level (SIL) and sound pressure level (SPL)

Are both measured in decibels (dB).

Sound Intensity Level (SIL)

also called acoustic intensity level, is the absolute logarithmic unit of the sound intensity relative to a reference value of 10⁻¹² W/m² —human hearing threshold in the air.

The speed of sound

the speed of sound in air is low, air is compressible