There’s a lot about PA systems that residential integrators need to know before entering the commercial sector. Anthony Grimani explains.

If you‘ve built a strong and reputable business in the residential integration sector, you may be tempted by the fruit of another, by which I mean the commercial or pro-audio integration sector.

The grass may look greener on that side of the fence, or you may just look at it as more work that can give you diversity or an opportunity to grow. But there are some technical practices and challenges that will potentially eat all of your profits if you aren’t equipped to deal with them.

Here is a short list of factors that you need to understand before you pack your cables and tools for an installation at the local discotheque.

Balanced audio lines
The most common form of interfacing in commercial and professional audio systems is balanced audio lines. You may recognise this as the extensive use of XLR connectors, 6mm Tip-Ring-Sleeve connectors, Phoenix screw down connectors, etc, but in each case you will see three wires interfacing component A to component B. This approach comes to us from the early days of telephony (late 19th century) and is in fact a brilliant way of running wires over a long distance while rejecting interference.

The three wires are usually arranged as a main pair twisted together alongside one un-insulated ground wire. The wire set contains these signals:
a)    Signal with plus polarity
b)    Signal with minus polarity
c)    Shield drain.

Signals a) and b) are identical, just opposite in terms of polarity.

At the receiving end of the wire run, which is the input of the product, the a) and b) signals are subtracted from each other. This means that any interference that would have fallen upon wires a) and b) is cancelled.

The intended signal is amplified by 2x (6dB), and any of the interference signals that landed on the shield gets drained out to a low impedance ground point.

Balanced audio lines are sometimes called ‘differential’ or ‘symmetrical’ as opposed to ‘single ended’ for non-balanced lines that are common in consumer audio. What you need to know is that when using an XLR connector, Pin 2 should be hooked up to the plus signal, Pin 3 to the minus signal and Pin 1 goes to the shield. A mnemonic I often use it ‘too hot’ to remember that Pin 2 is the hot signal. Then somehow you just need to remember that the shield goes to number 1.

When using 6mm phone plugs, the plus signal goes to the tip, the minus to the ring and the shield drain to the sleeve.

It will also happen that connections will go from balanced to unbalanced within a system and there are a few ways to deal with this. The best approach is to convert the signal through an isolation transformer if you want to avoid any possibilities of hum or noise.

That isn’t always practical, so you can do one of these two solutions:
a)    If the active balanced output stage is what is known as a ‘push-pull’ style, connect the plus terminal to the unbalanced plus terminal and hookup the ground terminal to the unbalanced input ground. That would leave the minus output floating. Some active products don’t actually like this, in which case try solution b).
b)    If the active product is a ‘transformer emulation’ type, you can either tie the minus output to ground and do the rest as per option a) above, or better yet you can insert a resistor between the minus output and the ground terminal at the output of the balanced product. This resistor should be the same value as the input impedance of the unbalanced product. How do you find out the value? Well, that can be a challenge, but I would start with 10Kohms. You can try measuring the input resistance with an ohm meter, you can try inserting a series 10Kohm resistor between a source signal generator and the input to see if the output signal drops by half (6dB) or you can even look at the manufacturer’s specifications, if you are lucky enough that they tell you!

You may also have to go from unbalanced to balanced connections. There again it’s always better to use a transformer, but you can also try to drive the plus terminal directly and connect the ground to ground terminal and the minus terminal. If you have hum, you can try lifting the minus terminal, or you can try lifting the ground while driving the minus terminal with the ground connection of the unbalanced source. All this can be challenging and will require some experimentation, as many factors will affect the hum conditions of the system. Be prepared to put some time into it during the commissioning process.

High SPL Sound Reinforcement speakers
Sound reinforcement speakers, aka public address (PA) speakers, can seem very different to typical residential speakers. They are built on different criteria. They have to play loud. They have to cover an audience in a specific area without spraying too much sound towards other surfaces or areas that would cause reverberation or result in feedback with microphones. They must be rugged. They must be cheap.

No wonder you may feel somewhat disoriented when working with them.

This is not all bad, and with some preparation you can get these speakers to sound quite good.

PA speakers typically have horn-loaded tweeters and that may make them seem low-fidelity. Not so these days. Horn design has improved tremendously in the last 10 years and you can get very good sound with a horn or waveguide in front of a tweeter. In some cases, a properly designed constant-directivity assembly can outperform a tweeter in a flat baffle by offering a constant spray of sound at all frequencies, rather than the typical narrowing at higher frequencies that is typical of tweeters, even the dome variety. Of course, another benefit of a horn system is the acoustic amplification available compared to an unloaded tweeter. This translates to less power required in the tweeter voice coil, and therefore better reliability. Also, a horn will allow the tweeter to effectively be used down to a lower frequency, which reduces the directivity errors in the crossover region since the wavelengths are now longer compared to the distance between the tweeter and woofer.

So what do you have to watch out for in using horn-loaded PA speakers? First off, in specifying the speaker you need to pick one with the right dispersion. Some manufacturers offer a few versions of the same speaker with different coverage. They are shown as a horizontal angle by vertical angle. This means the full subtended angle to the -6dB attenuation point as compared to the sound directly in front of the speaker.

If a speaker is shown as a 90×40 coverage, that means that the sound will be half the level at 45 degrees off-axis horizontally as compared to axial sound. It also means that the sound will be half level at 20 degrees off axis vertically. You need to establish the necessary coverage for the audience area and match the specifications of the speaker. Try to stay clear of the -6dB, half level points because at that angle there is loss of fidelity. And also make sure to aim the speakers at the audience area so that they cover it correctly. I recommend playing pink noise through the speaker and walking around the area to verify and aim the speaker for best coverage.

Second, you need to also have sufficient directivity to reduce the audibility of sound reverberation in larger spaces. It’s not an easy task to figure out at first!

Third, you need to make sure the speaker will play loud enough for the intended presentation. Most professional product manufacturers will give you enough good data about the speaker capabilities that you can establish the limit points. Remember that the sound levels are usually shown at 1m. For outdoor use you can count on loosing 6dB for each doubling of distance. Indoor is more complicated, but a lot of people use the 3dB loss per doubling of distance rule since the sound reverberation will fill in the distance-caused loss.

In truth, it’s all quite a bit more complex than that but the 3dB rule is a good starting point.

Also, note that line array speakers loose less level with distance than horn-loaded point source speakers. This isn’t always easy to figure out either, but you definitely want to have an idea of the horsepower you need in order not to have reliability issues.

The fourth issue is that many of the PA speakers today are self-amplified. That is great because the amplifiers are matched to the speakers, the crossovers are active, there may be protection limiters built-in and the systems are overall more efficient. However, you will need to plan on having power points at the speaker locations and you will need to watch out for hum problems that come with displaced power connection points between source devices, mixers, signal processors and the speakers.

Ideally all this is plugged into the same power lines with identical grounds but this task may be beyond your scope of work. Be prepared to have audio signal isolation transformers on hand, and be sure to not hookup the balanced connection shield drains on both sides of the cable. Select the best ground point and drain the shield interference on that side only. Cut the shield connection on the other side if you are using factory-made pre-terminated interconnects. Hum is your worst enemy when it comes to the final commissioning process, and you need to have all the tools and knowledge ready to deal with it!

70V distributed speakers systems
A common scheme in large scale distributed sound systems is to implement what is known as a 70V line. This mysterious term refers to the fact that an ideal system is engineered so that the peak voltage is 70V, and that the amplifiers and speakers are matched up in a parallel arrangement so that the signal is properly distributed around the total area.

This is usually done with a transformer on each speaker that has taps so as to select the power dissipated and therefore electro-acoustic gain factor at that speaker. These systems have the reputation of being low fidelity, but that isn’t necessarily the case in a properly engineered solution. With decent quality transformers that don’t saturate at low frequencies, and with a properly engineered and matched system, you can have wide frequency response and reasonable dynamic range.

The engineering process is more complex than the basic rules-of-thumb would like to have you think, so be prepared to do some research and some math. The main benefit is that you can save a lot on wire expenses since the systems are running at high impedances, and you can have a bundle of speakers all strung together over a long run simply with 16Ga wire. The other benefit is that one power amplifier can run a large array of speakers all hooked up together without having to worry about Series-Parallel connection complexities. You just need to do some planning and pick good quality speakers with larger, more expensive transformers.

Documentation and cleanliness
While you can get away with putting together residential systems without worrying about wiring documentation or without getting too obsessive about the rack wiring cleanliness, you are asking for trouble if you try this in a commercial or professional environment. You have to plan on some documentation budget to fully define the wiring diagrams, system specifications and wire labelling. You also have to plan on extra time for equipment rack construction, wire management and lacing, and labelling of all the connections and wire runs.

The initial investment will pay off in the mid-to-long term since maintenance will be reduced and will be done faster if it is needed.

System failures are not tolerated in professional systems, and I find that most failures are actually due to faulty wiring rather than actual gear failure. Also, please worry about managing the thermal issues in your equipment racks. Sound reinforcement equipment can often run hotter than basic residential gear and is often subject to extreme environmental heat in small mechanical closets hidden in some corner of a building, far away from any decent ventilation.

Oh, and here is an interesting fact about fans and heat circulation: hot air is less dense than cold air. So the idea of extracting hot air, with passive make-up air is all backwards. You should push cool air into a rack or equipment closet. Or better yet, push cold air and extract the hot air with fans on both! Yes, I know, it’s all very complicated!