Fundamentals of Sound Lecture

What is Sound?

Sound is both a physical phenomenon, and our

perception of it.

Orders of Sound

• There are three (or technically four) orders of sound/audio that we are concerned with:

1. Acoustics or Objective Sound (and Psycho-Acoustics or Subjective Sound)

2. Analog Audio 3. Digital Audio

Sound is Pressure

The sounds which we hear are based on sound waves.

These are longitudinal waves of molecules which result from regularly repeating changes in air pressure. A sound wave consists of micro-variations in air pressure above or below the ambient air pressure.

Sound Is Pressure

Amplitude

The distance between the positive and negative peaks is known as the amplitude.

Frequency

The speed at which one of these cycles is repeated every second is known as the sound wave’s frequency and is

expressed in Hz. If the air is vibrating very quickly then the frequency will be a high number such as 15,000 Hz. If the air is vibrating slowly then it will be a low number such as 40Hz.

Decades of Audio

WavelengthThe wave length is the distance through the

air that a sound wave travels to complete one of its repeating cycles.

Calculation of Frequency vs WavelengthThe frequency f specifies the number of cycles per second,

measured in hertz (Hz). Frequency and wavelength are related as follows:

• Wavelength (ft) = Speed of sound (ft/sec)/ Frequency (Hz)

which can also be written as:

• Frequency = Speed of sound/ Wavelength

The speed of sound in air is about 1,130 ft/sec at normal conditions.

For sound traveling in air our equation becomes:

Wavelength = 1130 / frequency (Hz)

Phase

Where might you find this short phase delay?

Harmonics

The second harmonic (or first overtone), twice the frequency of the fundamental, sounds an octave higher; 2:1 (octave)

The third harmonic, three times the frequency of the fundamental, sounds a perfect fifth above the second. 3:2 (perfect fifth)

The fourth harmonic vibrates at four times the frequency of the fundamental and sounds a perfect fourth above the third (2 octaves above the fundamental). 4:3 (perfect fourth)

Double the harmonic number means double the frequency (which sounds an octave higher).

Harmonics

Audio Definitions

• Sound waves are changes in pressure. • Amplitude is the amount that the pressure

changes.• Frequency is the speed that the pressure changes.• Phase is difference in pressure change between the

left and right ear.

• It is important that you understand these concepts since the purchase of equipment, the manipulation of sound and the recording of sound all depend on how the equipment responds to each aspect of a sound wave. We will be dealing with these three concepts for the rest of the semester.

Audio Basics ADSRAttack--how the sound starts. A sudden rise in amplitude is a fast attack. A slow increase in amplitude would be a slow attack. Sounds that make you jump usually have a very quick attack. Decay--After the initial rise in amplitude the perception of the sound usually decreases slightly.Sustain--How long does the sound stay at a steady amplitude (volume). The different lengths of sustain are the main differences between a sneeze and a shout. A sneeze has a very short sustain.Release--How does the sound die away? A sound that stops short or sounds "cut-off" would have a quick release. A sound that slowly fades away would be a slow release.

Transducers

mic-level signal (~2 millivolts) to Line level (~1.23 volts in pro gear)

A mic level is very weak. An XLR cable is a specific format of cable designed to carry mic level signals. In other words, XLR cables carry mic level signals and we need to use mic preamps to amplify a mic

level signal into a line level signal.