Comprehensive Guide to Sound Intensity, Pressure, and Levels in Acoustics

Sound Intensity

Also known as Acoustic Intensity. The intensity of a sound wave is defined as the mean value of the acoustic energy which crosses a unit area perpendicular to the direction of propagation in unit time.

Intensity of Sound Wave

The amount of energy that is transported past a given area of the medium per unit of time.

Intensity

The energy/time/area quantity that represents the power, intensity is simply the power/area.

Typical units for expressing the intensity of a sound wave

Watts/meter².

Sound Intensity Level

Sound intensity is not the same physical quantity as sound pressure. Human hearing is sensitive to sound pressure which is related to sound intensity.

Inverse Square Law

States that a specific physical quantity is inversely proportional to the square of that physical quantity.

Sound intensity decrease with distance

As a sound wave carries energy through a two-dimensional or three-dimensional medium, the intensity of the sound wave decreases with increasing distance from the source.

Energy distribution of sound waves

The energy of the sound waves being distributed over a greater surface area.

Sound wave spreading pattern

The sound wave in a two-dimensional medium is spreading out in space over a circular pattern.

Intensity measurement

Since energy is conserved and the area through which the energy is transported is increasing, the intensity being a quantity that is measured on a per area basis must decrease.

Sound Pressure

The sound pressure of a spherical wavefront radiating from point sources decreases by 50% as the distance is doubled.

Reverberant Field

A steady state situation where the rate at which sound energy is being lost is exactly balanced by the rate at which the loudspeakers add energy.

Volume Level in Reverberant Field

In a steady state sound field, its volume level is always slightly greater than the volume level being pumped into the room by the speakers.

Free Field

A region where the acoustic waves can propagate free from obstructions that would otherwise interfere with the sound path.

Characteristics of Free Field

In a free field, there are no reflections; sound waves reach an observer directly from a sound emitting object.

Example of Free Field

The sound source is far enough away that it appears as a single point source, far in the distance.

Anechoic Chamber

A special facility constructed to approximate an acoustic free field by using materials to absorb sound waves before they can be reflected.

Diffuse Field

The opposite of a free field; diffusion with regard to sound is the scattering or reflections of sound waves.

Characteristics of Diffuse Field

A diffuse field describes an acoustic field where sound waves reach the observer from all directions.

Reflected Sound in Diffuse Field

The reflected sound is of similar magnitude to the direct sound when it reaches the observer, and as a result, does not appear to have a single source.

Microphone in a diffuse field

Measures the same magnitude regardless of orientation or location; the sound level is the same everywhere.

Near Field

The region of space close to the emitting source where there is typically a complex constructive and destructive interference pattern due to the waves being generated from an aperture of a set geometric size.

Far Field

The region where the source can be treated as a point source, and the wave front is deemed a plane wave, decaying at a rate of 6 dB per distance doubled from the source.

Evanescent Waves

Sound energy that circulates back and forth with the vibrating surface of the source, never escaping or propagating away.

Acoustic Far Field Start

Begins approximately at a distance of 1 wavelength away from the sound source and extends outward to infinity.

Sound Pressure Level Decay

In the far field, the sound pressure level decreases by 6 dB for each doubling of the distance from the source.

Acoustic Particle Velocity

In the far field, the sound pressure and acoustic particle velocity are in phase.

Acoustic Energy Behavior

Depending on how far away an observer is from a sound emitting object, the acoustic energy produced by the sound source will behave quite differently.

Sound Pressure Level

The sound pressure disturbance in the atmosphere measured using predefined conditions such as the location of the equipment, the environmental conditions, and the distance of the measurement from the measurement point.

Sound Power

The energy rate, or energy of sound per unit time (J/s or W), emitted by a source.

Sound Power Level

A measure of energy output of a sound source expressed in decibels.

Sound Pressure Unit

The unit for sound pressure is Pa or N/m².

Measurement Conditions

Conditions such as the location of the equipment, environmental conditions, and distance from the measurement point that affect sound pressure measurements.

Acoustic Environment

The specific location where sound energy is transferred and measured.

Sound Energy

The total airborne sound energy radiated by a sound source per unit time.

Sound Sources

Entities that emit sound energy into the environment.

Acoustic Wave Strength

The most commonly used indicator of acoustic wave strength correlating well with human perception of loudness.

Acoustic Measurement

The process of measuring sound pressure, sound power, and other acoustic properties.

Reference Sound Pressure

The pressure of sound related to a fixed reference.

Acoustic Energy Output

The measure of the acoustic energy emitted from a source of noise.

Sound Pressure Disturbance

The variation in atmospheric pressure caused by sound waves.

Sound Measurement Techniques

Methods used to assess sound levels and properties in various environments.

Environmental Conditions

The surrounding factors that can influence sound measurements, including temperature, humidity, and background noise.

Acoustic Design

The process of designing spaces to optimize sound quality and control noise.

Decibels

A logarithmic unit used to measure sound intensity or power levels.

Sound Propagation

The movement of sound waves through different media.

Measurement Location

The specific point where sound measurements are taken.

Acoustic Properties

Characteristics of materials and environments that affect sound behavior.

Sound Disturbance

Any unwanted sound that disrupts the normal acoustic environment.

Sound Intensity Calculation

The process of integrating the acoustic intensity over a surrounding surface to derive sound power.

Power

The rate at which energy is transferred by the wave.

Intensity Equation

I = PA, where I is intensity, P is power, and A is area.

SI Unit for Intensity

W/m².

Pressure Variation (Δp)

The pressure amplitude in units of pascals (Pa) or N/m².

Density (ρ)

The density of the material in which the sound wave travels, in units of kg/m³.

Speed of Sound (vw)

The speed of sound in the medium, in units of m/s.

Sound Intensity Definition

The mean value of the acoustic energy which crosses a unit area perpendicular to the direction of propagation in unit time.

Average Rate of Sound Energy

The average rate of sound energy transmitted in a specified direction through a unit area normal to this direction.

Sound Intensity Level (β)

Defined as β(dB) = 10 log10(I/I0), where I0 = 10−12 W/m² is a reference intensity.

Logarithmic Perception of Sound

How our ears perceive sound can be more accurately described by the logarithm of the intensity rather than directly to the intensity.

Intensity Relationship

The intensity of a sound wave is related to its amplitude squared by the equation I = (Δp)² / (2ρvw).

Acoustic Energy

The energy of an oscillating element of air due to a traveling sound wave is proportional to its amplitude squared.

Sound Wave Production

The sound wave is produced by some vibration; the greater its pressure amplitude, the more the air is compressed.

Decibels (dB)

Sound intensity levels are expressed in decibels much more often than sound intensities in watts per meter squared.

Pressure Maxima and Minima

In greater-intensity sound, pressures are higher, allowing it to exert larger forces on objects it encounters.

Unit Area

The area over which the power is distributed in the context of sound intensity.

Mean Value of Acoustic Energy

The average value of acoustic energy crossing a unit area in unit time.

Sound Wave Amplitude

The amplitude of the oscillation is proportional to the pressure variation.

Intensity Variation

I varies as (Δp)².

Figure 1 Description

Graphs of the gauge pressures in two sound waves of different intensities.

I0

The lowest or threshold intensity of sound a person with normal hearing can perceive at a frequency of 1000 Hz.

Sound pressure level (SPL)

A logarithmic measurement of the effective pressure of a sound wave relative to a known reference value, typically measured in decibels (dB).

Loudness or intensity of sound

Quantified by SPL at a specific location, with a higher dB value indicating a louder sound.

0 dB

Represents the approximate threshold of human hearing.

Formula for sound level in decibels

1. Identify the intensity of the sound to be converted (I). 2. Divide the intensity (I) by the reference intensity (Io). 3. Calculate the base-10 logarithm of that ratio. 4. Multiply the result by 10.

Sound intensity level 0

1 × 10-12 W/m2; Threshold of hearing at 1000 Hz.

Sound intensity level 10

1 × 10-11 W/m2; Rustle of leaves.

Sound intensity level 20

1 × 10-10 W/m2; Whisper at 1 m distance.

Sound intensity level 30

1 × 10-9 W/m2; Quiet home.

Sound intensity level 40

1 × 10-8 W/m2; Average home.

Sound intensity level 50

1 × 10-7 W/m2; Average office, soft music.

Sound intensity level 60

1 × 10-6 W/m2; Normal conversation.

Sound intensity level 70

1 × 10-5 W/m2; Noisy office, busy traffic.

Sound intensity level 80

1 × 10-4 W/m2; Loud radio, classroom lecture.

Sound intensity level 90

1 × 10-3 W/m2; Inside a heavy truck; damage from prolonged exposure.

Sound intensity level 100

1 × 10-2 W/m2; Noisy factory, siren at 30 m; damage from 8 h per day exposure.

Sound intensity level 110

1 × 10-1 W/m2; Damage from 30 min per day exposure.

Sound intensity level 120

1 W/m2; Loud rock concert, pneumatic chipper at 2 m; threshold of pain.

Sound intensity level 140

1 × 102 W/m2; Jet airplane at 30 m; severe pain, damage in seconds.

Sound intensity level 160

1 × 104 W/m2; Bursting of eardrums.