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High Q Public A Loudsp This high quality column speaker will change your perception of Public Address (PA) sound. With a massive 200W By JOHN Public address systems are usually associated with poor quality sound. They often lack any bass below 100Hz and the upper frequency response rarely goes above 10kHz. As for the sound quality in the range from 100Hz to 10kHz, it is usually lacking clarity and is very peaky in its response. A peaky response, particularly in the mid frequency region, can cause acoustic feedback between microphone and loudspeakers. At best, PA speakers with a peaky response will have a tendency to ringing where the system is just on the verge of full        feedback or at worst, no reasonable sound level can be obtained before feedback occurs. Apart from flush-mount ceiling speakers, by far the most common loudspeaker type for indoor public address is the column or line source speaker. This consists of a vertical column of loudspeakers in a box and can be recognised by its long thin shape. These types of speakers have the advantage that the sound is dispersed in a horizontal plane so that levels are consistent throughout the entire Features Quality reproduction for voice Even sound distribution over the whole listening area Complete coverage of audible frequency range Smooth frequency response Good transient response Off-axis response up to 45 degrees High power rating and sensitivity 60  Silicon Chip listening area. Sound dispersion in the vertical plane is much reduced and this minimises reverb­eration from ceiling and floor reflections. A commercial pair of column loudspeakers typically cost around $750. Usually, they each contain four circular or oval shaped drivers all mounted on a loudspeaker box baffle. Commonly, the rear of the box is not sealed so that the dimensions can be made as small as possible. Because of the open back and the use of one driver type to cover the whole frequency range, the bass response is poor and the high frequency range is not covered well either. And while such speakers may be adequate (just) for speech they are usually plain awful with any sort of music program. The design presented here is intended to compete with commercial units in applications where size is not important and high quality sound is preferred. It is ideal for music and speech and the low frequency response makes Quality Address peakers continuous power rating, high efficiency and wide frequency response, it is ideal for music and voice in a large listening area. CLARKE it suitable for electric piano and organ. The prototypes have been installed in a small church. The 2-way loudspeaker system comprises two rows of loudspeakers, one with four 6.5" (165mm) woofers and another with four 1" (25mm) tweeters, all mounted in the one large box. We have specified Philips AD11600/T8 textile dome tweeters and these are relatively inexpensive compared to the more esoteric types with aluminium diaphragm and magnetic fluid damping. They provide a high level distortion-free sound and their typical resonant frequency is quite low at 1300Hz compared to many other tweeters of the same size. Its rated impedance is 8Ω and voice coil resistance is 6.3Ω. Measured Thiele-Small parameters are Qms 3.58; Qes 1.09; Qts 0.84; Re 6.2Ω; Le 1mH and fs 1132Hz. Bass drivers For the bass drivers we have spec- ified the Vifa P17WG-00-08 woofers which have a mineral filled polycone, high damping rubber surround, a smooth overall frequency response and an optimised off-axis response which enables operation up to beyond 4kHz. The resonant frequency is 37Hz which allows for good bass response in a suitable enclosure. Its Thiele-Small parameters are shown in Fig.6 which is a printout of the Bass Box 5.1 enclosure design. We opted for a bass reflex design so that the useable response could be extended to 30Hz; it is actually -10dB down at 30Hz. The -3dB and -6dB points are at 43Hz and 36Hz respectively. The bass reflex design also increases the power rating of the woofer below 100Hz due to the reduced cone excursion enabled by the use of the tuning port. Crossover network The crossover circuit is shown in Fig.1. It is a second order Linkwitz-Riley (Q = 0.5). It has the advantage that it does not cause any horizontal axis Specifications Power rating: Nominal Impedance: Sensitivity: Low Frequency Response: High Frequency Response: Box Size (external): Box capacity (internal) 200W RMS continuous 8Ω 93dB SPL at 1 metre for 2.83V RMS input 3dB down at 42Hz, useable to 30Hz beyond 20kHz 460(w) x 750(h) x 370(d) mm 100 litres September 1996  61 Fig.1: Crossover circuitry for the column loudspeakers comprises 0.82mH inductors and 3.2µF capacitors (total) to form a high pass filter for the tweeters and a low pass stage for the woofers. The tweeters and woofers are connected in series-parallel to provide a system impedance of 8Ω. tilt at the crossover frequency due to different mounting centres between the woofer and tweeter. The Q value of the filter provides ideal damping and transient response, with a 3dB AUDIO PRECISION 100 drop in frequency response at the crossover point. Note that the phase of the tweeters is reversed from the woofers to provide the correct crossover blend IMPEDANCE (OHMS) vs FREQUENCY (Hz) between loudspeakers. This is not a mistake. The crossover frequency has been set at 3.1kHz so that both loudspeakers blend without any abrupt sound level changes and before any marked off axis rolloff by the woofer. This frequency is also more than one octave above tweeter resonance at around 1.3kHz. Each of the woofers is equalised with a shunt RC network consisting of a 12Ω 5W resistor and a 10µF non-polarised (NP) capacitor. This provides a more or less constant impedance load to the low pass filter network and ensures that its attenuation slope is close to the desired 12dB/octave beyond the crossover frequency. Signal to the tweeters is attenuated by a 6dB L-pad which compensates for their higher sensitivity compared to the woofers. The L-pad also helps to maintain an overall 8Ω impedance near the crossover frequency and also increases the power rating of the tweeters. An 8Ω system impedance is obtained by connecting the woofers and tweeters in a series-parallel arrangement. This configuration has two benefits. First, the overall power rating of the loudspeaker system compared to a single driver is increased by a factor of four. Second and more important, the series-parallel connection and mounting the four speakers on a common baffle increases the system efficiency by 6dB. This is equivalent to substituting a 200W amplifier for a 50W unit. The resulting overall efficiency of 93dB/1W/1m is very high for a wide range speaker system. Construction 10 1 10 100 1k Fig.7: measured impedance curve for the prototype column system. 62  Silicon Chip 10k 20k Fig.2 shows the dimensions of the prototype loudspeaker enclosure. The prototype was made from 18mm MDF (medium density fibreboard) and had internal cleats of 12 x 12mm quad at all corners. All cabinet joins were glued (with PVA glue) and screwed. All internal corners of the cabinet were then sealed with a fillet of PVA glue to make sure that it was airtight. The box can be finished with paint or a simulated wood grain material. There are two crossover network PC boards, one for the four tweeters and one for the four woofers. Both measure 120 x 93mm and their codes are 01310961 and 01310962. These Fig.2: dimensions of the prototype enclosure, made from 18mm MDF (medium density fibreboard with internal cleats of 12 x 12mm quad at all corners. The external dimensions will need to be increased if thicker material than 18mm MDF is used. The internal volume is 100 litres. September 1996  63 Fig.3: This overlay diagram shows how to wire up the crossovers. Make sure that the phasing is correct when wiring the loudspeakers. boards will be available from RCS Radio Pty Ltd. Phone (02) 9587 3491. Start assembling the crossover boards by inserting PC stakes at all the external wiring points. Then insert and solder in all the capacitors and resistors. The capacitors are all non-polarised (NP) types so there is 64  Silicon Chip no concern about polarity. Note that these non-polarised capacitors may also be labelled “BP” which stands for “bipolar”. The 0.82mH inductors are secured with a screw, nut and star washers or you can glue them in place. These inductors can be obtained from Jaycar Electronics stores (Cat LF-1320) or from Scan Audio Pty Ltd. Phone (03) 9429 9309. The tweeter crossover board should be wired up with 300mm lengths of hookup wire for both the tweeters and input terminals. Use red for positive (+) and black for negative. The board is Fig.4: manufacturer’s data for frequency response and impedance curves of the Vifa P17WG-00-08 woofer. The three curves for frequency response are for 0 degrees, 30 degrees and 60 degrees off axis. Fig.5: manufacturer’s data for frequency response of the Philips AD11600/T8, measured on axis at a distance of 1m with 1W input power. Note the increased sensitivity compared to the woofer. Shown on the same graph is the impedance. Note the rise in value to the resonance at around 1.2kHz mounted centrally on the tweeter side of the box using self tapping screws. Tie the pairs of red and black leads for each tweeter together with a knot to make sure that they will be connected up correctly later. Similarly, wire up the woofer crossover with 300mm wire lengths for the centre two speakers and 500mm lengths for the outside woofers. Attach this board centrally on the woofer side of the box and tie each pair of the red and black leads together. Attach the terminals for the speaker on the rear of the box and solder the crossover input wires to it. Line the box with Innerbond on all sides except for September 1996  65 PARTS LIST    (for one loudspeaker box) 4 Vifa P17WG-00-08 woofers (Scan Audio Pty Ltd) 4 Philips AD11600/T8 or AD11610/T8 tweeters (Jaycar Electronics or Dick Smith Electronics) 1 woofer crossover PC board coded 01310961, 120 x 93mm 1 tweeter crossover PC board coded 01310962, 120 x 93mm 1 1m length x 910mm Innerbond 2 66mm I.D. ports (76mm long) (Jaycar Cat CX-2682) 2 0.82mH speaker crossover inductors (Jaycar or Scan Audio) 4 10µF 100VW NP electrolytic capacitors 2 2.2µF 100V metallised polyester capacitors 2 1.0µF 100V metallised polyester capacitors 4 12Ω 5W resistors 4 10Ω 5W resistors 4 3.9Ω 5W resistors 20 PC stakes 1 10m length of heavy duty red hookup wire 1 10m length of heavy duty black hookup wire Fig.8: full size artwork for the two crossover network PC boards. Hardware 1 12m length of 12 x 12mm quad section wood 2 1200 x 900 x 18mm MDF panel 1 speaker grille kit (Jaycar Cat CF-2750) or wood frame and four speaker grille clips 1 piece of speaker grille cloth 800 x 500mm 1 loudspeaker terminal posts 1 2m length of speaker sealant 16 self tapping screws to mount woofer 24 self tapping screws to mount tweeter and ports 8 self tapping screws to mount crossovers 2 self tapping screws to mount speaker terminal 66  Silicon Chip the baffle. Pass the speaker wires from the woofer cross­over through the material. Glue the Innerbond to the panel surfaces with PVA to keep the material away from the port holes. Once the glue has dried, the loudspeakers can be connected to the wiring and secured in position. Correct phasing for the loudspeakers is important and is normally indicated on the loudspeaker terminals with a red dot or with a (+) sign on the magnet label. The convention is that a positive voltage applied to the plus terminal will cause the cone to move outward. We used speaker sealant around edge of the woofer mounting holes to ensure that the box is sealed properly. The tweeters have an integral sealing washer. It is a misconception to think that the box need not be sealed properly because it has port holes anyway. To work properly, the ports rely on an airtight box. Any leaks will affect the low frequency response of the loudspeakers, cause extraneous noises and reduce efficiency. Now cut the 66mm I.D. ports to 76mm in length and secure each one to the baffle with four screws. The grille can be constructed using a wooden frame with the cloth secured with tacks or staples. It can be attached with grille clips. Alternatively, you could use a grille kit from Jaycar Electronics. This comprises Fig.6: Predicted low plastic strip mouldings frequency performance for the sides which of the woofer using the BassBox 5.1 CAD are attached to corner software. pieces. The cloth is held using the supplied double sided tape and the whole assembly is secured to the loudspeaker baffle with grille clips. Positioning Used in a hall, column loudspeakers are best mounted one on each side of the hall, forward of the stage area. This positioning will reduce the possibility of acoustic feedback between microphones and loudspeaker. The loudspeakers should be angled downward so that they each point to the centre of the audience area. Some adjustment of the position may be necessary for best results. Alternatives Although the AD11600/T8 has been specified, you can also use the AD11610/T8. The only real difference between these tweeters is that the specified unit has a textile dome while the second version has a polycarbonate dome. An alternative woofer is the more expensive P17WJ-00-08. They are available from Jaycar Electronics or Scan Audio. It has a 70W power rating, a magnesium basket and similar resonant frequency to the WG version. You will have to change the woofer equalisation values from the 10µF and 12Ω values to 6.8µF and 6.8Ω 5W. The ports should be 80mm long each. Also note that the woofer hole cutouts in the loudspeaker baffle will need to be 145.5mm. The loudspeaker can be operated from a 100V line if connected via SC a suitable step-down transformer. September 1996  67