Most analytical categories for sound treat it in the two dimensions of magnitude (which might be thought of as a "vertical" dimension) and time (the corresponding "horizontal" dimension). The term magnitude here can apply both to the scientist's desire to measure and quantify the physical "size" of the sound wave, and also to the listener's subjective evaluation of the loudness of the aural experience. The following section presents the concepts of magnitude in six sub-topics:

A) The physical parameters used by the acoustician to describe the magnitude of the sound wave;

B) The various so-called "level" measurement systems which quantify magnitude on a logarithmic scale;

C) The description and measurement of the subjective sense of loudness;

D) The thresholds of the auditory system's response to magnitude;

E) The description of how a sound's magnitude changes over time;

F) The electroacoustic treatment of the magnitude of a sound signal.

The level measurement systems mentioned in B) naturally lead into noise measurement systems, almost all of which are principally concerned with measuring the magnitude of noise, and community response to it. Given the specialized nature of this topic, it is dealt with separately in the document.

A) The physical parameters of magnitude: the scientist commonly uses four terms to describe the magnitude of a sound wave, each of which can be translated into an equivalent version of the other. "Amplitude" focusses on the size of the particle vibration, and "sound pressure" on the force which such vibration exerts on the surrounding medium. The other two terms, "intensity" and "power", place the emphasis on the more abstract notion of the "energy" of the wave, thereby relating it to other forms of energy transfer and exchange. 

Sound Pressure
Sound Intensity
Sound Power

B) Level measurements: the vast range of magnitudes of sound energy or pressure to which we are accustomed demands, first of all, that some conveniently usable scale be adopted whose smallest unit is meaningful in daily applications. Secondly, the logarithmic nature of significant increases in magnitude needs to be considered, and finally, the need to make relative, not absolute, comparisons between magnitudes has to be included. The solution is the so-called "level" measurement whose unit is the decibel, the most commonly used unit for describing acoustic magnitude. See also Appendix D.

Sound Pressure Level
Inverse-Square Law
Sound Level
Sound Level Meter
Ambient Noise Level
Peak Level
Intensity Level
Sound Power Level
Root Mean Square

C) Loudness: although people react to the magnitude of a sound quite subjectively, particularly when they are judging whether or not that magnitude is acceptable or appropriate to a given situation, it is still possible to quantify the auditory system's reaction to the magnitude of a stimulus (properly called "loudness") through theory and experiment. The problem is a classic one in psychophysics: to relate subjective sensation to the magnitude of the physical stimulus which causes the sensation. The problem is extremely complex, and work on it is not yet complete. See also Appendix F.

Loudness Level

D) Thresholds: the concept of "threshold" is central to all psychophysics. It specifies minimally perceptible changes, sometimes called "Just noticeable differences" of a stimulus, or alternatively, the magnitude of a stimulus that is just barely noticeable in the first place. It is significant with sound that this latter threshold constantly changes with environmental conditions. Hearing sensitivity is constantly changing, not only because our attentiveness changes, but also because our basic hearing changes with noise levels, as descrlbed by the term "threshold shift". Another use of the term "threshold" is to describe the onset of limiting subjective reactions, namely discomfort and pain.

Dynamic Range
Threshold of Hearing
Differential Threshold
Threshold Shift
Threshold of Pain

E) Time-dependence of magnitude: since all sounds have their own "lifetime", as it were, being born, sustaining their energy, and finally dying away, a set of terms has evolved to describe these temporal parts of a sound, most commonly and collectively called its "envelope". With constant or near-constant sounds, statistical methods need to be invoked to describe the time dependence.

Rise Time
Decay Time
Rate of Decay
Stationary State
Internal Dynamics
Stationary Sound
Impact Sound

F) Electroacoustic magnitude: the magnitude of an audio signal can be described in much the same way as that of an acoustic sound wave. The important difference is that in the electroacoustic case, the magnitude can and must be controlled, varied and manipulated. In some cases, the control is required because of the limitations of the medium itself, in other cases, because of design considerations in producing an acceptable product, whereas in others, outright manipulation of acoustic environments may be involved. Since acoustic power has always been linked with worldly power, the control of electroacoustic amplification has strong social significance.

VU Meter
Zero Level VU
Dynamic Range
Level Recorder