Acoustics and Lighting Lecture Notes

Flashcards covering the fundamental concepts of acoustics, including sound properties, behaviors, room design for speech and music, and sound insulation principles.

Acoustics

A branch of science that deals with the control and propagation of sound.

Sound

A physical wave originating from a vibrating body, defined as a series of pressure vibrations spread in an elastic medium (air or building materials).

Rarefaction

The instantaneous, local reduction in density of a gas resulting from the passage of a sound wave; also known as the elongation of sound.

Frequencies

The number of cycles per second (cps) or vibrations per second (vps) of sound waves.

Hearing Range

The range of sound frequencies a healthy young person can hear, typically from $20$ to $20,000$ Hz.

Velocity of Sound in Air ($20ºF$)

$1125\,ft/sec$; velocity increases $1\,ft/sec$ for every $10ºF$ rise in temperature.

Molecular Absorption / Masking of Sound

A phenomenon where a sound is covered up or absorbed by another sound, usually of lower frequency than the sound it masks.

Soporific Effect

The effect brought about by a low-steady sound.

Wavelength

The distance between two crests of a sound wave; calculated as $WL = \frac{\text{distance traveled by sound wave in one second}}{\text{frequency of the wave}}$.

Diffraction of Sound

One characteristic where sound, like light, can bend around an obstruction; low frequency sounds are diffracted more easily than high frequency sounds.

Standing Wave

A wave created when the wavelength of a sound is exactly the same as one of the room’s dimensions.

Direct Sound

The original sound from the source that is delivered directly to the hearer.

Reflected Sound

The sound that reaches the hearer from all directions of a room after striking surfaces.

Velocity of Propagation (Standard)

At sea level, sound velocity is $344\,m/sec$ or $1130\,fps$.

Velocity in Steel

$4900\,m/sec$ or $16,000\,fps$.

Velocity in Glass

$5000\,m/sec$ or $16,400\,fps$.

Echo

A reflected sound that reaches an observer delayed by more than about $0.058$ seconds relative to the direct sound.

Reverberation

A series of echoes or successive reflections of sounds in a room from the same source.

Resonance

Occurs when a body vibrates in harmony with another vibrating body; happens if the resonator has a dimension which is a multiple of the wavelength of the sound source.

Sound Absorption

The phenomenon where sound waves strike an absorbent surface and are transformed into heat energy through friction on the pores.

Sabin

A unit used to measure the sound absorption efficiency of a material, where $1$ sabin implies perfect absorption and $0$ means no absorption.

Decibel (db)

A measurement of the intensity of sound; $40\,db$ is the limit for comfortable hearing, while $100\text{--}120\,db$ is painful.

Reverberation Time (RT)

The time in seconds that a reflected sound takes to diminish by $60\,db$ after the original sound has stopped.

Reverberation Time Formula

$RT = \frac{0.05V}{a_1s_1}$, where $V$ is the volume of the room in cubic feet, $s_1$ is the area of the absorbent surface, and $a_1$ is the coefficient of absorption.

Transient Sound

The direct sound which stops if the originating body stops vibrating.

Diffusion

The state where reflections are so large that the sound level becomes uniform throughout the room, eliminating acoustical shadows.

Near Field

A sound field generally within one wavelength of the lowest frequency produced by the source.

Free Field

A sound field existing between the near and reverberant fields where intensity varies inversely with the square of the distance.

Inverse Square Law

States that the intensity of sound is inversely proportional to the square of the distance from the source.

Ordered Sound

Sound in which the instantaneous pressure follows a regular pattern, such as music.

Disordered Sound

Sound in which the peak of acoustic power occurs more or less at random, lacking periodic qualities.

Response Approach (Music Therapy)

Focuses on the therapeutic power of music heard from an external source and the body/mind's reaction to it.

Expression Approach (Music Therapy)

Identifies music’s power as coming from its creation as well as its reception.

Directional Nature of Speech

In plan, seats should be positioned within an angle of $140º$ relative to the speaker, as sound intensity decays rapidly outside this range.

Clouds

Pieces of cloth hung on the ceiling to absorb sound, typically not less than $2\,ft$ in depth.

Definition (Music Quality)

Refers to hearing each type of musical instrument clearly so they are readily distinguished from each other.

Balance (Music Quality)

The correct loudness ratio between the various sections of an orchestra.

Blend (Music Quality)

The hearing of a body of instruments as a homogeneous source rather than a collection of individual sources.

First Reflections (Music)

Reflections that blend into direct sound and give body to music, arriving no later than $0.35$ second after the direct sound.

Omnidirectional Microphone

A microphone equally sensitive to sounds arriving from any direction.

Directional Microphone

A microphone that receives sound from one direction; better for speaking purposes as it reduces feedback.

Time Delayed Sound (Sound Systems)

The mechanical interval (usually $0.035$ of a second) used so that the natural sound is heard before the loudspeaker sound to maintain source location realism.

High-level Sound System

Consists of a single loudspeaker with a high level of sound, applicable for rooms with good acoustics.

Low-level Sound System

Consists of several loudspeakers at strategic locations operating at low volume, useful for rooms with bad acoustics like cathedrals.

Sound Insulation

The process or method of making a room invulnerable against the entry of noise.

Noise Control

The process of elimination or limiting the propagation of noise.

Airborne Sound Insulation

Insulation against noise originating in the air, such as voices.

Impact Sound Insulation

Insulation against noise caused by physical impacts, such as footsteps or falling objects.

Resiliency Principle

A structural principle where the building is designed to go with the vibration to prevent damage, as seen in Frank Lloyd Wright’s Imperial Hotel.