Previously, we have discussed constant voltage based PA systems and where we would use them. Now it’s time to look into their low impedance counterparts. Most consumer audio systems are low impedance in operation to provide higher audio quality and power output requirements, and these concepts scale up to larger Professional level PA systems.
Most, if not all, large installed or touring systems operate in low impedance due to the power outputs required, and specifying such a system requires us to know the clients goals in relation to total sound output level. From here we can gauge the total power of the system required, as well as the style and specification of the speakers and amps to be used.
In this article we will discuss the different formats and technical specifications of low impedance speakers, to assist in selecting the appropriate products for a project. First we will look at “Point and shoot” style box speakers.
“Point and shoot” speaker boxes usually incorporate “drivers” (the mechanical units responsible for re-creating sound) that cover most of the audible frequency range, and as such can be used on their own in a smaller system with appropriate results. Some units only cover low frequencies down to a certain point and are used in tandem with a subwoofer to complete the frequency range and increase the overall power output of the system. A subwoofer in turn only produces frequencies in the lower range.
Another feature of this speaker style is the range of ‘dispersion angles’ available. This specification indicates the nominal angle at which the high frequencies emanate from the speaker and are usually specified in both horizontal and vertical angles. These frequencies allow a speaker to be selected based on how far or wide it is expected to throw sound.
For example, Houses of Worship often feature column style speakers with a wide horizontal (100 Degrees plus) and narrow vertical (~ 10 degrees) dispersion allowing for visually discreet installation and wide coverage with minimal speaker boxes. We will touch more on column speakers in a later article.
Alternatively, speakers mounted up high are often specified with narrow vertical and horizontal dispersion, to ensure the sound energy reaches its target efficiently over a greater distance. Narrow dispersion speakers can also be implemented in place of ‘delay’ speakers (speakers mounted further back from the front of house PA to ensure coverage over the length of a venue) in basic systems or where delay stacks cannot be positioned appropriately.
The last major specification of a box speaker is the material with which the box is constructed, which changes the overall ‘tone’ of the box, and is usually proportional to the purchase price of the speaker. Entry level, economical and most IP rated speakers are made of ABS plastic, making them light and robust but compromises the quality of the ‘cabinet resonance’ (The sound the box makes in sympathy with the active drivers within). Mid-range to high quality, speaker boxes are generally wood enclosures ranging from Multilayer ply to more exotic timbers. These materials work in conjunction with the speaker drivers to deliver warmer or clearer sound for professional applications at a greater financial cost and considerable increase in weight over the plastic enclosures. The weight is particularly relevant when mounting permanently installed speakers, or moving speakers around in portable systems.