Glossary Terms For P.A.
Here’s a glossary covering some of the most relevant terms used in this guide.
A signal routing path, for example on a mixing console, which allows for the summing of selected input channels to be sent, and returned, to various locations other than the main output, at a gain specified by the operator.
Diffraction around objects: Effectively the bending of the wavefront as sound travels around an object. Due to their longer wavelengths, lower frequencies are more likely to be successful in diffracting around objects than higher frequencies. In order to travel around an object, the wavelength (of a particular frequency) must be longer than the longest axis of the object.
Diffraction relating to openings between objects: The diagram below helps to visualise this part of the acoustic phenomena, and pertains to sound at the throat of a loudspeaker’s horn. A smaller throat opening (on the right) causes sound to diffract (that is to bend and spread out), more so than the wider throat opening (on the left).
(Stark, Diffraction Principle, 2004, p.161)
Dispersion & Pattern Control
Technically speaking, there is a difference between the terms dispersion and pattern control. According to Stark, (2004, p. 154), dispersion describes the spreading out of sound energy, whereas pattern control is the ability to maintain that dispersion over a range of frequencies, and in both the horizontal and vertical planes of sound propagation.
These are a series of quickly repeating reflections which occur between hard parallel surfaces. Here are some audio examples;
(Link with kind permission of Matt Mcglynn, 2011)
Abbreviation for Front Of House. In the context of the guide, this refers to the main P.A. system which faces the audience.
(Davis & Jones, A One Third Octave Graphic Equaliser, 1989 p.251)
Graphic equalisers area ‘band pass/rejection filters’ (Davis & Jones, 1989) characterised by;
- Being able to adjust the gain of 8 or more (typically 32) bands of frequency at the same time.
- Having linear slide controls.
Professionally speaking, according to McCarthy (2008), their primary role is in the rejection of feedback.
Horizontal and Vertical Planes Of Sound Propagation From Loudspeakers
The difference between horizontal and vertical planes of sound propagation from a loudspeaker can be seen below.
(d&b Audiotechnik GmbH, 2015)
A method for displaying the pattern control over dispersion of loudspeakers, in a chart. They are produced by plotting -6dB attenuation points for a range of frequencies, as the measurement microphone is moved away from the centre axis. (The centre axis is explained below). Lines are drawn between plotted points in order to join areas of equal pressure. Isobars give an indication of how flat, or otherwise, the frequency response of a loudspeaker is off-axis.
(A venues) Noise Floor
In the context of this guide, the term noise floor is used to describe the ambient noise that exists at a reasonably continual level in a venue. This does not include the homogenous reverberant sound field; but rather is most largely contributed to by people talking.
As the direct sound from the P.A. system comes within less than 10dB of the venues noise floor, the clarity of sound reinforcement is severely challenged.
This refers to the angle between which the dispersion of a loudspeaker varies by no more than 6dB. The centre point is on-axis, (see the diagram under the On Axis definition title below) and the measurements are made by moving off to the sides (or above) until the level has dropped by 6db. This is done at specific frequencies, but for brevity manufacturers usually average these, and this is what we would refer to as nominal. When a nominal dispersion is given, e.g. 90° x 45°, it is typically specified in first horizontal then vertical planes.
Different frequencies in sound have different wavelengths: (the audible bandwidth of frequencies to humans ranges from 17m to 1.72cm). If a sound source has a diameter that is greater than the wavelength (frequency) which it is creating, then it will maintain directional control. This is affectively because a driver becomes big enough that sound propagating from different areas of its own cone / diaphragm, begin to interact with each other in such a way which causes cancellation (attenuation) off to the sides. As a general rule, if you were to measure across the diameter of a driver (or the mouth of a horn), you’d discover the frequency down to which that sound source maintains pattern control: (or some degree of beaming on-axis). Frequencies that are lower than this become more omnidirectional.
Occurs when the loudspeaker’s driver and as a result air molecules, return to their rested state creating an area of lower pressure. It is the inverse of compression (in an energy transfer sense).
A term used to describe a wave’s average strength. RMS allows for assessment of amplitude: if + and – excursions were simply averaged, the net result would be zero (Stark, 2014 p. 60). RMS is ‘the equivalent AC voltage to that found in a DC circuit (McCarthy, 2008 Loc 947), where the RMS value of a pure sine wave is 70.7% of the peak value.
A measure of the time taken for the reverberant sound field in a room to decay by 60dB once the initial excitation has ceased (Rational Acoustics, LLC, 2015). Although more reliable indicators of speech intelligibility exist, it used within this guide during discussions on clarity, in order to offer general context to rooms.