TH 1551
Sound for the Theatre

Sound Terminology

Sound Waves
Molecular movement (air) at 1,130 ft. per second in all directions
(at sea level)

Sound Waves - Air Compression

Sound Wavees

Phasing
Diagram of sound pressure movement

Frequency - measured in Hertz
Number of peaks (cycles) in a given distance (time)

Amplitude
Intensity of frequency, height of sine wave = Volume
Change in height of peaks (number of cycles per second remains the same)
Volume measured in decibels (dB).
More dB = more Volume.
0 dB is arbitrarily set at the threshold of hearing

Contrasting Sound Wave Graphics


Inverse Square Law
As distance from the source increases, volume drops at the square of the distance from source.
For a sound to have the same apparent volume at twice the distance, it must be amplified four times as much. At a distance three times as far away, the sound must be amplified nine times as much.
Inverse Square Law Diagram

Harmonics
Equal division of Cycles

Middle C = 256 Hz
1 octave above middle C = 1st Harmonic: 512 Hz (256 x 2)
2 octave above middle C = 2nd Harmonic: 1024 Hz (512 x 2)
In and out of Phase
Multiple sound sources in combination
Phasing Diagram

Doppler Effect
Sound approaching appears to increase in pitch
Sound waves are compressed creating the illusion of more cycles per second.

Doppler Effect

The Human Ear

Human Ear

Inner Ear
Cross section of Cochlea

Discernment
Ears are located on the sides of human heads

Horizontal discernment: 15 degrees
Vertical discernment: 45 degrees
Dynamic Range
20 - 20,000 Hertz

Envelope
Shape of sound as defined by how it begins, changes internally over time, and how it ends.

Sound Envelope Examples


Reverberation
RT60: Time of decay 60 dB (1 millionth of original sound)
Reveerberation Illustration

Directionality of various pitches (higher frequencies are more directional)