2. The use of beam steering speakers in a Public Address system

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1 2. The use of beam steering speakers in a Public Address system According to Meyer Sound (2002) "Manipulating the magnitude and phase of every loudspeaker in an array of loudspeakers is commonly referred to as "beam steering". A clear understanding of beam steering is imperative for this investigation is to be a success, the previous quote by Meyer Sound is a only a window of information in regards to this area of science and the engineering behind today's modern line arrays. In order to prove that beam steering speakers can provide a more intelligible public address system at Kedleston Road's Atrium, it must first be established how line arrays work and how beam steering can be achieved. The line array Mitchell, J (2002) "A line array may form a complete loudspeaker or one or more bands thereof. In a line array, individual radiators are arranged in a straight line or an arc segment. It is also possible for a number of complete loudspeaker systems to be arranged as a line array. In the simplest form of the line array, each of the elements - usually a small cone transducer - is supplied an identical full range signal." In the simplest of terms a line array is a line of speakers which are working together to provide one wave front., but what is the benefit of using a line array over one large loudspeaker? The best answer is most likely, coverage! The theory behind a line array is to create a line source rather than a point source which would result with a cylindrical wave as opposed to the spherical wave defined by Howard and Angus (2006) as an application of the inverse square law. The benefit of forming a cylindrical wave is that sound will decrease by 3dB per doubling of distance as opposed to the 6dB decrease of the inverse square law. This is annotated by Pat Brown (2002) in his explanation of "the line source"; "Successful radiators have been constructed that radiate sound from a line rather than a point. The infinite line source emits a wave that is approximately cylindrical in shape. Since the diverging wave is not expanding in two dimensions, the level change with increasing distance is half that of the point source radiator. The sound level from such a device will decrease at 3dB per distance doubling rather than 6dB. It should be pointed out that these relationships are frequency and line length dependent, and what is being described here is the ideal case". Possibly the best practical example of an infinite line source can be found at gtaust.com (2012) " As an empirical example consider the observed behaviour of the noise pollution from a highway: People living near highways are the unfortunate victims of the highway noise only falling at only 3dB per doubling of distance from the highway. That's because the highway noise source precisely fits the model of an infinite line source, especially if it s in a valley and you live on the hill above."

2 Unfortunately creating an infinite line array or even a motorway sized line array is not realistic, this poses the question, can line arrays for cylindrical waves? Meyer Sound (2002) clearly state that they cannot; "A common misconception regarding line arrays is that they somehow magically enable sound waves to combine, forming a single cylindrical wave with special propagation characteristics. Under linear acoustic theory, however, this is impossible. "Cylindrical wave" is a marketing concept, not a verifiable acoustic reality." Although creating a cylindrical wave from a line array may be considered impossible in acoustic reality it is not impossible to create the characteristics of a cylindrical wave at certain frequencies. What these frequencies are is dependent on the size of the line array, a description of a finite line source is given by Pat Brown (2002) in his description of "the line source"; "If the line source is finite in length then there will be a phase differential between the sound radiated from different points on the source to a specific point in space. All of the points will be most "in phase" on a plane perpendicular from the end points of the array. As the point of observation moves away from the midpoint, phase interaction will produce lobes in the radiated energy pattern. The lobes can be suppressed by clever design, allowing the wave front to be confined to a very narrow vertical angle, yet with wide horizontal dispersion. Such a radiation pattern is ideal for some applications, such as broad, flat audience area that must be covered from ear height. Digital signal processing has produced well-behaved line arrays that can project sound to great distances." It can be concluded from this research that acoustically speaking a finite line source is not practically capable of producing a cylindrical wave, however, it can be deduced that digital signal processing or DSP can be used to suppress lobes and confine a wave to a narrow vertical angle. This process has to be a manipulation of phase and magnitude and according to Meyer Sound (2002); Such a process is referred to as beam steering and according to PAT BROWN (2002); The projection from a line array with this capability "is ideal for some applications, such as broad, flat audience area that must be covered from ear height "; a description which summarises the requirements of a Public Address system perfectly. Beam Steering with DSP According to Dr. Craig Richardson (2002); "Digital Signal Processing (DSP) is a technology and technique for analyzing and extracting information from signals, synthesizing signals, and manipulating signals. The acronym DSP is often used as both noun and an adjective. DSP also often stands for digital signal processor - the actual microprocessor/computer that is used to implement the system" From this information it can be determined that beam steering is made possible via specialist microprocessors (DSP); in order to achieve some understanding of how DSP's are configured within a line array to achieve beam steering, an explanation from a manufacturer of these specialist loudspeakers would provide the most reliable information. duran-audio.com(2012) "AXYS is a brand from Duran Audio, a company founded in 1991 by Gerrit Duran. From the outset AXYS was conceived as not only a superbly engineered range of professional audio products but as a series of innovative solutions to real-life acoustical problems - such as poor intelligibility in voice evacuation systems and unpleasant distortion in music reproduction."

3 The Duran Audio website also explains that some loudspeaker manufacturers were "a little sceptical" about the innovative technology that they were using, but that in 1994 the world's first sound system using "Digital Directivity Control" (DDC) to minimise sound reflections was installed by them at Schipol Airport in Amsterdam, Holland. A technical report entitled "Intellivox DDC2.0", duran-audio.com(2012), details how DDC is made possible through the use of DSP in a line array from Duran Audio's "Intellivox" range, this contributes a substantial amount of information towards clarifying how beam steering can be achieved via DSP and why it is so ideal for Public Address systems. The report states that "DDC is a multi-channel loudspeaker array technology where the single loudspeaker elements are positioned in space according to a patented algorithm" The adjacent diagram, taken from Intellivox DDC2.0, illustrates the audio path inside an Intellivox array. The "DDC PROCESSING" element is the line arrays DSP and it is here where individual audio signals are manipulated according to specific requirements before following its assigned audio path through individual amplification. According to Duran Audio (2012) this means "each loudspeaker or group of loudspeakers can have their own unique set of filters. The intention of this technology enables users to electronically manipulate the vertical dispersion of an Intellivox array". The report goes on to state that "The beam steering capability of the Intellivox means that you can maintain even coverage across the listening plane whilst steering the beam away from surfaces that may cause unwanted reflections. This results in a very high direct to reverberant sound ratio which is essential for achieving acceptable levels of speech intelligibility within reverberant spaces". The image of the waveform (Intellivox DDC2.0) illustrates the directivity properties that the Intellivox range is capable of. A description of this kind of waveform was quoted previously in this investigation from Pat Brown (2002); "very narrow vertical angle, yet with wide horizontal dispersion." It was that description which suggested a successful application of this technology could be a Public Address system.

4 The capacity to manage phase and amplitude of individual signal paths within a line array can yield the desired waveform necessary for optimized speech intelligibility, this optimisation is characterised best by "even coverage across the listening plane whilst steering the beam away from surfaces that may cause unwanted reflections." However there are other requirements if this type of sound reinforcement system is going to offer the versatility required to be effective in all environments. This means that the requirements for every installation of the PA system will differ according to the environments acoustical properties and these differences will need to be addressed before an installation can take place. in regards to the Duran Audio Intellivox range mentioned previously, the requirements of each installation are calculated and used as part of their "Digital Directivity Synthesis" (DDS) described in the technical report "Digital Directivity Synthesis (DDS)" duran-audio.com(2012) duran-audio.com(2012); "The electronic aiming available with DDC products, which can be thought of as aiming and focusing a light, allows users to aim the sound onto the audience area; from the acoustic centre of the array." By looking at the image below, Digital Directivity Synthesis (DDS), it is possible to determine the factors which are considered for Digital Directivity Synthesis, which is used to achieve Digital Directivity Control. duran-audio.com(2012)"dds is based on a unique, specially adapted "constrained weighted least-squares" optimisation algorithm". The algorithms themselves are unique to Duran Audio, which makes pin pointing exactly how DDC is made possible, impossible. The diagram illustrates the data which is consolidated as part of the synthesis and this synthesis provides a unique set of filters. By using DSP each filter is assigned to its relevant audio channel, this means the phase of every loudspeaker in the array can be manipulated. Having isolated amplification for each audio channel as well as phase manipulation means the aggregate of the signal reaching each loudspeaker is specific to its requirements. This is beam steering!

5 Summery Thanks to the research which has been anthologized in this section of the Investigation it has been possible to determine how a line array is constructed and how specialist technologies such as DSP make beam steering possible. It is established that creating a line source capable of providing a complete cylindrical wave is not an acoustic reality but that modern technologies and innovative thinking has created a process for creating waveforms with similar properties to cylindrical waves. The unique Digital Directivity Synthesis provides the potential to create specific loudspeaker filters within an array and thus provide their Intellivox range with the versatility required to be successful in almost any environment. Considering these factors, it is hard not to determine that an Intellivox PA system has the potential to be successful at the Kedleston Road atrium. In order to make an installation successful some acoustical evaluations will need to be made at the Kedleston Road atrium in order to provide data for Digital Directivity Syntheses which in turn provides Digital Directivity Control. The benefits of DDS & DDC are summarised below by duran-audio.com(2012) DDS; Flexible array set up Pre-defined direct SPL distribution over (complex shaped) audience planes while minimum energy projection at hall boundaries constant spectral balance for all listening positions Optimum direct-to-reverberant energy ratio Both far field and near field control DDC; The sound is digitally aimed at the listener There is less sound reflected from walls and ceilings therefore you hear less reflections it is highly efficient at distributing the available power from the loudspeaker The SPL of the loudspeaker is approximately the same if you are close or if you are 60 m away Thanks to DDS, DDC is made possible which can ultimately be described as beam steering, the benefits outlined above suggest this method of sound reinforcement will provide a clear and direct sound, this is precisely what an ideal PA system should provide.

6 References from chapter 2 Duran Audio. (2012). Intellivox DDS (Digital Directivity Synthesis). Available: Last accessed 17/02/2012. Duran Audio. (2012). Intellivox DDC2.0 (DDC EXPLAINED). Available: Last accessed 12/02/2012. Duran Audio. (2012). About Us. Available: Last accessed 20/02/2012. Pat Brown. (2002). Fundamentals of Audio and Acoustics. In: Glen, M. Ballou Handbook for sound engineers. 3rd ed. USA: Focul Press Meyer Sound. (2002). DSP beam steering with modern line arrays. Technical report.. (.),.. Meyer Sound. (2002). Can Line Arrays Form Cylindrical Waves? A Line Array Theory Q&A. Available: Last accessed 30/01/2012. Mitchell, J. (2002). Loudspeakers. In: Glen, M. Ballou Handbook for sound engineers. 3rd ed. USA: Focul Press David M. Howard & Jamie Angus. (2006). Introduction to sound. In:. Acoustics and Psychoacoustics, third edition. 3rd ed. Great Britain : Elsevier ltd. 28. DR. Craig Richardson. (2002). DSP Technology. In: Glen, M. Ballou Handbook for sound engineers. 3rd ed. USA: Focul Press