Part 1: Getting the best out of your auditorium audio
When trying to get the best out of your auditorium, one of the key components to consider is how it sounds. The function of a PA system is to deliver sound to your audience at the highest quality possible. Achieving satisfaction on this front requires consideration of three key criteria:
• Audio level, in relation to purpose
• Low distortion, low noise and a flat frequency response
• Adequate clarity in relation to purpose
The right audio level for the venue or application is easily achievable in recognising how much amplifier power you require, while PA noise should only be audible if your system has a fault. By contrast, frequency response and clarity have historically caused problems. PA loudspeakers tend to have frequency responses which can cause unwanted distortion when not treated properly, while concert scenarios can prove to be a minefield for achieving perfect clarity.
With that in mind, we’ll be taking a look at line arrays, how they work, and how they are adapted to auditoriums. Although these loudspeaker systems have historically been used in live music, they are becoming more prevalent in corporate and commercial public address.
Line arrays are loudspeaker systems made up of multiple discrete loudspeaker cabinets mounted in a line, designed as such so as to produce an iso-phasic wavefront, or a flat wavefront that has constant phase over its entire surface. They can be oriented in any direction however their main application is vertically, in Public Address. To understand how they work, let’s first take a look at three different types of sound source:
– Point Source – A source that is, in practise, dimensionally smaller than the wavelength of the emitted sound. Point Sources emit sound omni-directionally, and as the wavefront pulsates outwards covering a larger area, sound pressure decreases by 6dB for every doubling of distance from the source (known as the inverse square law).
– Plane Source – Sound energy produced from a plane source is very tightly directional, and often constrained to travel within an enclosed medium, meaning that that no sound pressure level drop occurs.
– Line Source – Larger than the wavelength that it is emitting, a line source is known for being more directional, and the larger the source, the more tightly directional the wavelength becomes. As with a point source, an SPL drop is seen as distance from source is doubled, however this is only 3dB.
When considering a seated audience in an auditorium or conference hall, sound engineers need to ensure that as much sound as possible is directed at the audience. Imagine standing on a stage and looking at your audience – spread widely from left to right, but narrowly from front to rear – the loud speaker system needs to focus so as not to cause reflections off of walls and ceilings which will arrive at audience ears out of phase with the source. In simple terms, line arrays combine Point Source and Line Source theory to direct sound energy at the audience. Of course there are a number of other factors involved in getting the wanted sound reproduction out of the system, but we’ll come on to that later.
So, with the spread of the audience as it is, what needs to be achieved is a wavelength with near-zero dispersion vertically, but wide enough dispersion horizontally to cover the width and depth of the audience. Line arrays set up vertically are essentially a solution to providing sound that is tightly focused in the vertical dimension but spreads in the horizontal. This is where the theory can start to affect the practicality of system design however. To achieve near-zero dispersion, a line source needs to be around four times the size of the wavelength. To give an example, let’s take the lower end of the human hearing spectrum – 20Hz – and work out how big the source would need to be to provide a focused sound wave. To do this, we must take the linear formula for calculating wavelength (λ) relative to speed of sound (c), and frequency (f):
The wavelength of a sound wave at a frequency of 20Hz is approximately 16.5 metres, meaning that a line array would need to be in the region of 66 metres in height to produce near-zero dispersion of frequencies that low. In a real world environment this is simply impractical due to height (and health and safety) constraints. At the expense of losing some of the very low end in the audible frequency spectrum, more realistic line source sizes can be calculated. Take a reasonably low frequency of 170Hz for example, with a wavelength of 1.95m, and it will require a line source of about eight metres in height.
We’ve made a realistic decision on how tall we want our line arrays to be, but now we need to consider how the array is going to tackle the frequency spectrum. Each individual loudspeaker cabinet in an array will consist of LF (Low Frequency), MF (Mid Frequency), and HF (High Frequency) drive units to cover the full range, in this case down to a low end of 170Hz, but so as to ensure the system works as if it were a genuine line source, the individual drive units must couple together so as to constructively interfere with each other. For this to happen the drive units must be separated by less than half a wavelength. For low to mid frequencies up to about 450Hz this is easy – for 450Hz the cabinets would need to be 38cm (15”) or less – however it gets a lot more difficult the further up the frequency spectrum we go. If each loudspeaker cabinet contains an LF drive unit of 15”, there will be more than half a wavelength between the individual MF and HF drive units respectively and in practice that means the line array system will only act as a line source for low to mid frequencies.
With that in mind, how do the MF and HF drive units produce the required wavefront with near-zero dispersion in the vertical dimension?
There are a number of solutions that can be employed, so as to ensure the HF attains directional characteristics similar to those of the LF and MF, but we’re just going to take a look at a couple of practical and favoured techniques. The first is the use ribbon drivers, which consist of a thin metal-film ribbon suspended in a magnetic field. As electrical signal is applied to the ribbon, it vibrates to create sound, and yields very good HF response. The downside however is that they have lower output levels than other solutions, and adjacent units will be more than half a wavelength apart and will not couple well.
An alternative option is wave guides (horns) coupled to compression drive units, which have a very narrow vertical and very wide horizontal dispersion. These horns achieve directionality by reflecting sound into a specified coverage pattern which should closely match the LF and MF directional characteristics of the array, however they do not have linear phase signal at the mouth of the horn. A popular method to accomplish a linear phase signal here is the use of variable density foam which acts as a kind of acoustic lense, slowing the speed of sound through the more dense foam medium towards the centre of the horn.
We have now have a line array that produces an iso-phasic wavefront across low, mid, and high frequencies, but now let’s look at the other factors involved in ensuring that the whole audience receive direct, consistent audio from the loudspeaker system – remember delivering sound to an entire audience at pretty much the same level and clarity, front to back.
The first solution that might come to mind is what is known as Intensity Shading. This involves reducing the output level of the lower section of the array. The front rows of the audience are much closer to the loudspeaker cabinets than they are to the upper cabinets, so reducing the level of the lower cabinets will lower SPL (Sound Pressure Level) delivered to the front of the audience while the rear of the audience still receives the louder signal. It should be noted however, that the front rows will still hear the sound from the upper cabinets clearly because they are louder. The two wavefronts with different SPLs will combine and there will be a discontinuity at the juncture of the two that will be heard as a phase difference.
The sensible alternative to Intensity Shading is Divergence Shading, whereby a line array is curved to point directly at the different parts of the audience. Remembering that curving the array up at the ceiling will cause unwanted reflections and reduction in clarity of audio, the top of the array should retain its flat positioning pointing at the rear of the audience while the bottom is curved to point at the front of the audience and in the process create a ‘J’ shape. Angling the lower cabinets apart means that the sound they produce has to cover a wider angle, therefore reducing the intensity at the listening position.
There we have it, a (detailed) introduction to line arrays. Naturally, technology is rapidly moving on, and there are various software aided solutions out there that can help engineers configure their systems to best suit the acoustics of the room or auditorium they are working with. Next time we’ll be looking at Martin Audio’s Multi-Cellular Line Array system specifically and how it is attempting to move line array technology on in terms of clarity, flexibility, and quality.