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.
Line array does reduce the effect of multipoint sources interfering with each other like the systems of twenty- five years ago, but it is still a long way from the superior results achievable with single point sources. A single point source sound system offers the highest possible definition and the highest possible dynamic range available today. Superior intelligibility is a by-product of this but is only guaranteed by maintaining high definition and high dynamics through the use of fast and accurate electronics and low distortion transducers. The frequency response graph below shows that when using a one axis multiple sound source like a line array (red), the audible suppression of higher frequencies starts as low as 2kHz. High frequency reduction is caused by mutual subtraction (cancellation) of signals from the individual components of multi point source. This reduction is typically -16dB @15kHz, so for a flat frequency response added equalisation is necessary. This reduction varies with position of the listener in space and air movement. Conversely when using one single point sound source solution the frequency response is not affected in the same way and is actually very flat (blue). Multipoint systems have the advantage of higher acoustic pressure than a single point sound source for frequencies up to 5kHz, but for superior quality and definition a single point source is the best solution.
Frequency response of the one axis multipoint sound source (red) and point source (blue)
VHD2.0 speaker Transfer function
Line Array Transfer function, (source: Live Sound International, May 2007 (Jeff Berryman)
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)
Illustration 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.
When using one point sound source, the listener in any location gets only a pure (not blurred) sound
VHD2.0 vertical polarplots at 1k, 1.6k and 4kHz
It is clear from the diagrams above that the issues of interference and random air movements will simply not affect a one point sound source, like KV2 Audio VHD and ES, regardless of their complexity or intensity. This will give a more equal coverage across the listening area with each individual listener enjoying a pure sound.