There are two main types of sound system designs that have been prominent in the market consisting of single point source or multiple point source concepts. Multi point source arose from the requirements for very high output power. The idea satisfied that criteria, but with the increasing number of sound sources came an overall reduction in the quality of the sound. The two big disadvantages of multipoint source systems were the suppression of the high frequency output and the physically time-shifted outputs from the individual speakers. Adding a number of time-shifted outputs from individual speakers together causes poor system impulse response.
The first types of multipoint sources were simply a large pile of cabinets, stacked together like building blocks and intended to array on all axis’. A major improvement in the next generation of systems was the introduction of multipoint, one-axis systems that provided better frequency response and increased definition than previous multi axes systems. Unfortunately, whilst a step forward, the frequency response and impulse responses were still not ideal and the coverage was often inconsistent. A typical representation of the one axis multipoint source sound system used commonly today is a line array system.
Single and Multi Point Source Impulse Response
To maintain a high-resolution audio signal, it is vital that the system is able to exhibit a short impulse response time. This will create a sound signal like the original. The figure below shows the comparable impulse responses of the one point source and one axis multipoint source (Line array). The Input pulse is 1V, pulse width 100 µs, period 10 msec. The damaged impulse response you see in the graph is the reason for the line array systems low resolution. Damaged impulse response is caused by mutual subtraction and addition of individual sources of multi point source system.
Impulse response of point source (orange) and multipoint one axis source system (blue)
Time-Shifts, Properties of Single Point source
llustration of the differences in distances to the listener from several Line array sources, each listener gets a blurred sound
Line array vertical polarplots at 1k, 1.6k and 4kHz
The figure above shows that the physical characteristics and dimensions of a multi-point system determine the time shifts between several sound sources. Transfer response and Pulse response will vary with the location of each individual listener. Time shifts for listener 1 are different to those for listener 2. Many manufacturers claim that time shifts are corrected using digital signal delays but this does not provide a solution because time shifts will infinitely vary with each new listener location. Even more critically, one factor over looked by the calculations and predictions of system engineers or line array prediction software is the RANDOM movement of the air in the listening area which causes huge changes in the transmission properties of multi-point systems regardless. This is the case when an audience arrives, after the system engineer has spent a whole day unnecessarily aligning the system to an empty but perfect theoretical environment – an environment that in the real world of random air movements and increasing temperatures will not exist.
Text and photos by courtesy of KV2 Audio. All trademarks, registered trademarks and copyrights are the property of their respective owners.