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Recently Australian speaker designer and manufacturer VAF Research introduced a new high-performance Public Address speaker to its range of highly regarded hifi and home theatre speakers. And they’ve been run off their feet ever since! We asked Philip Vafiadis, of VAF Research, to explain the philosophy behind the design of the I-201 Public Address Speakers. Defining The Ideal Public Address Loudspeaker L OUDSPEAKERS play an impor tant role in a public address sys tem (or for that matter any sound reproduction system) as the final link in the signal processing chain. They convert electrical energy from the power amplifier into acoustic energy in air that travels as sound waves to the listeners. Regardless of the quality of the preceding signal processing chain, if the loudspeakers are of poor quality or incorrectly connected or operated, the result will be poor quality sound. Typically, the performance of loudspeakers is orders of magnitude (ie, multiples to the power of 10) worse than what we would accept from other 6  Silicon Chip audio processing devices. At some time or other, we have all struggled to understand a public speaker in a church or school hall amplified with the typical “column” or horn loaded public address loudspeaker system. In fact, most people have concluded that high-quality vocal reproduction in a reverberant environment is difficult, if not impossible. So what exactly are the requirements for a public address loudspeaker? Well, it must reproduce acoustically the electrical input signal at an adequate level to be heard, without introducing distortion or colouration. The loudspeaker must accurately match the ‘timbre’ of the voice or instrument it is reproducing. The sound should be clear and intelligible for each listener, even though the listeners may be widely dispersed in three dimensions. If used inside an enclosed space, it must do this with the added encumbrance of the superimposed room acoustics. The loudspeaker should not be prone to feedback or howl-around, when used with open microphones. From a practical point of view, it should be small, light and visually unobtrusive. It should be physically constructed in such a way that it can be installed in optimal positions, both www.siliconchip.com.au acoustically and aesthetically. Finally, it must connect and function reliably. There are many methods of converting electrical energy into acoustical energy (including some esoteric ones) but the overwhelming majority of loudspeakers use electrodynamic transducers constructed with voice coils in permanent magnet fields driving a moving diaphragm. Electrodynamic transducers have so far proven to offer the best balance of performance and ruggedness at an affordable price. To achieve the performance ideals listed above, the following areas of loudspeaker system performance are important: ♦ Time Alignment and Source     Coincidence, ♦ Controlled Directivity, and ♦ Stored Energy. Time Alignment and Source Coincidence Currently available electrodynamic transducer technology dictates that a full bandwidth response requires two or more drivers to handle high and low frequency ranges. Therefore, the acoustic output of a loudspeaker system is the sum of the outputs of its individual drivers, which are in physically different locations on the baffle. Because of the displacement between drivers, the acoustic transmission path distance from the listener to each driver’s acoustic centre will be different for each listening position. Therefore, in-phase arrivals of two driver’s outputs can only occur exactly at one point in space for one listener where the path distances are equal (or offset as necessary). Typically, this point will be along the www.siliconchip.com.au loudspeaker’s main axis. Thus, a typical loudspeaker can only be exactly “time aligned” at one point in space. At all other locations it will be time misaligned! This misalignment causes ripple in the frequency response for about an octave either side of the crossover frequency. Unfortunately, this is typically where the ear is most sensitive, in the region from 1-4kHz! The individual driver outputs from a 2-way horn-loaded system at best can only sum properly in one plane and at worst along only one axis. Above and below the vertical axis, the path distances vary significantly between the horn and bass driver and correct reconstruction of the audio signal can not, and does not, occur. Even side to side, the effective path length through the horn can vary enough to cause ripple in off-axis frequency responses. In order to prevent the problem of different path lengths to the listener from each driver in a multi-way system, the drivers’ effective acoustic centres must be coincident. This can be achieved either by coaxial drivers, or by a mirror image array, with appropriate signal processing delays if necessary. Note that either arrangement of drivers in itself does not guarantee coincidence or time alignment! The high-frequency driver in a coaxial design has its terminating acoustic load modulated by the position of the bass driver’s cone, which in turn leads to high levels of intermodulation distortion. Unlike coaxial designs, the high-frequency driver in a mirror image array is decoupled from displacement modulation effects caused by the bass drivers and has low levels of intermodulation distortion. Controlled Directivity So called “high Q” or Controlled Directivity loudspeaker systems are all the rage today. Unfortunately, there are a number of false premises on which this trend has been based. First is the notion of matching the loudspeaker coverage pattern to audience area. In fact, if we reverse engineer the ideal coverage pattern for a loudspeaker in a typical application using a CAD simulation program, it can be shown that normally a conical or slightly elliptical (“squashed” conical) polar November 2001  7 speaker systems is that by definition they have severe discontinuities in their power responses. In other words, they fail to deliver the promise of clearer, more articulate sound. The bottom line is that a “low Q” loudspeaker system will sound more musical and need less equalisation than a “high Q” system. Feedback Stability Margin The I-201 from VAF Research is ideal for live voice or music and works equally well as a front-of-house, foldback, monitor, installed, arrayed or portable system. pattern is normally the best fit. A second false premise is the notion of minimising reflections off nearby walls for improved intelligibility. In fact, reflections in the first 20 milliseconds or so enhance intelligibility and reflections from walls much further away are typically attenuated enough by inverse square law so as not to be significant. A third false premise is the notion of minimising the reverberant energy field in the room, to maximise intelligibility and articulation. In fact, it is generally more significant that the reverberant sound field has even energy against frequency. In other words, articulate, intelligible speech is possible in a highly reverberant room despite the long reverberation time. 8  Silicon Chip For musicality and intelligibility generally, it is important that a loudspeaker system delivers a smooth power response. Power response is defined as the total acoustic power output in all directions against frequency, as opposed to frequency response, which only considers one point in space. Of course, the frequency response is important too but in all directions! Power response can be considered like an average of frequency responses taken in all directions. A smooth power response will mean that the decaying reverberant sound field in a room will more closely match the direct sound field. This is important to achieve a natural sound quality for both voice and music, and actually makes speech easier to understand. The problem with horn loaded loud- It is a commonly held belief that feedback in a sound system is a result of excessive gain at one frequency and can be controlled by reducing the gain at that frequency. And yes, those two statements are true – but there’s more to it than that. Feedback is always occurring in a public address system with an open microphone, because sound reproduced by the loudspeakers will be picked up by the microphone and reproduced by the loudspeakers and around the loop it goes again. Normally the total gain or amplification around the loop is less than unity, so the system remains stable and useable. Essentially, this is because the rate at which sound energy is being added is less than the rate at which it is naturally decaying. If, however, the gain around the loop exceeds unity, then the sound level will build up rapidly, causing the squealing known as feedback. Because of the imperfections of typical sound equipment, there will be a small number of frequencies with more gain around the loop than general and these frequencies will be the ones which “take off” when the system goes into instability. Perhaps surprisingly, these frequencies do not necessarily relate to peaks in the frequency response. The time for www.siliconchip.com.au SPECIFICATIONS Frequency Range ����������������45Hz to 25kHz Frequency Response �����������65Hz to 20kHz (±2.5dB) with grilles Power Handling �������������������For amplifiers to 200W RMS Impedance (nominal) ����������4W Sensitivity ���������������������������95dB (1W <at> 1m) Enclosure type ��������������������35-litre, vented Drivers ��������������������������������2 x 210mm fibreglass cone woofers with 37.5mm aluminium voice coil and synthetic high compliance surround; 1 x 25mm impedance-matched soft dome tweeter with aluminium voice coil. Tweeter protection provided. Crossover ����������������������������Displaced pole third order Crossover Components �������2% tolerance air-cored heavy-gauge inductors; close-tolerance metallised polypropylene capacitors. Crossover Frequency ����������2700Hz Connectors ��������������������������2 x Neutrik Speakon™ 4-pin connectors (in/out). Enclosure ����������������������������Arrayable (90° arc) and stackable, 18mm MDF with internal bracing. Finished in textured black. Black perforated steel front grille. Finish ����������������������������������Matte black. Black perforated steel front grille. Mounting ����������������������������Top hat suitable for 35mm poles. (Other options on request). Dimensions (mm) ���������������640 (h) x 490 (widest point) x 320 (deepest point) Weight ��������������������������������18 kg sound to travel from the loudspeaker to the microphone is typically 5-25 milliseconds, which is a lot of cycles at voice frequencies. In other words, there is plenty of time for the sound energy level to decay during the propagation around the loop – unless a resonant mechanism is at play. Resonances cause energy to linger at specific frequencies, making the system prone to feedback at those frequencies. Sources of resonance (or energy storage) in loudspeakers include, but are not limited to, misaligned crossovers, cabinet panels, dust caps, diaphragm break-up modes, and cavities including those in the horns of horn-loaded systems. Unless well damped, these resonances will be the feedback break points in the system. Eliminating or at least severely attenuating system resonances improves the feedback stability margin to near the theoretical maximum. In practice, this provides more than enough gain before feedback for most applications, even in difficult acoustic environments such as churches, provided good quality microphones are used. Horn-loaded loudspeakers are at a distinct disadvantage in feedback stability margin because of the number of resonant modes supported in a typical horn. Often the assumed gain-before-feedback advantage of controlled directivity is more than traded off because of this. www.siliconchip.com.au Kits without compromise VAF Introducing the VAF Research I-201 Recognising that loudspeakers are by far the weakest link in the public address audio chain today, VAF Research has used CAD (Computer Aided Design) and CAM (Computer Aided Manufacture) techniques to develop a new high-performance public address loudspeaker system, the I-201. It is intended for use as a general purpose PA loudspeaker in small to medium-sized performance spaces. It is ideal for churches, hotels, restaurants, function centres and other live performance spaces. The I-201 can be used for live voice or music, foldback monitoring or recorded program reproduction. Multi-purpose cabinet design The unique cabinet geometry of the VAF Research I-201 lends itself to almost all applications. The ideal angle for a foldback wedge, the I-201 also sits up closer to a side-wall than a traditional trapezoidal design, minimising aesthetic and sight line problems in critical applications. Two I-201s will provide for 180° coverage from a side-wall position and four I-201s form a perfect circle for all-round coverage in a gymnasium, for example. Where additional reach is required, I-201s can be stacked vertically and/or “Sound quality to die for” Rolling Stone Magazine “..A new benchmark in every criteria” Best Buys Home Theatre Speaker Kits without compromise from $312 pr to $8,863 pr FreeCall 1800 818882 www.vaf.com.au vaf<at>vaf.com.au November 2001  9 I-201 HIGHLIGHTS ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Affordable, multi-purpose arrayable, stackable cabinet design. Works equally well as Front-of-House, Foldback, Monitor, Installed, Arrayed or Portable system. Full-bandwidth design for music reproduction with reduced need for subwoofers. Breakthrough low stored energy design for high feedback-stability margin in reverberant environments. High internal acoustic damping, MDF cabinet construction – not a plastic box! Powder-coated, anti-resonant, dent resistant, curved metal grille. True acoustic time-aligned point source – no electronic processing required. Internal crossover with high-frequency driver protection. Flush, concealed carry handle positioned on centre of gravity for easy carrying. Recessed metal terminal plate with dual linked Speakon™ style connectors. Flush 38mm stand-mount adapter in base of cabinet. 25mm dome high-power tweeter with acoustic dispersion control. Dual 200mm fibreglass long-throw precision bass drivers. the angle between pairs reduced from 90° to as little as 0°. Full bandwidth audiophile quality Unlike other small public address loudspeakers, the VAF Research I-201’s have been engineered to provide strong usable bass for full-range music reproduction. What’s more, the high frequency response extends well past 20kHz in a typical PA loudspeaker fashion! True point source time alignment The I-201’s Vertical Mirror Image Array with time-aligned drivers behaves as a true point source at all frequencies in all directions. The effective acoustical centre of the two bass drivers always remains at the same point as the effective acoustical centre of the tweeter for waves radiated in any di- 10  Silicon Chip rection. Acoustical reconstruction is accurate regardless of polar direction, giving the I-201 the smoothest total power response possible. Feedback The I-201’s are an ideal choice for a very reverberant hall, in spite of their “low Q” design. Combined with reasonable condenser microphones, there will be plenty of loop gain available, even for a quiet talker standing behind a lectern. There are three secrets to this success. First, I-201’s are a low stored energy system, much lower than typical PA designs. Second, typically only one cabinet a side is needed for coverage, which eliminates phasing between boxes and the consequent side lobes. And third, the polar response of the I-201 is very smooth, thanks to unique diffraction control incorporated into the baffle. Pricing The new VAF I-201 is available only direct from VAF Research, in the following forms: ♦ Fully assembled and tested: $1100 each. ♦ Ready to assemble kits where the cabinets are fully built and finished leaving only the final assembly: $940 each. ♦ Ready to assemble kits without cabinets but including cabinet plans: $599 each. Insured freight to most of Australia $30 each. If you buy the I-201 as a kit including cabinets, rather than fully assembled, you will get the same high level of performance as the factory finished version. VAF research can be contacted on FreeCall 1800 818882 or vaf<at>vaf.com.au More information on the new I-201 or other VAF models can be found at www.vaf.com.au SC www.siliconchip.com.au