Article

Tuesday, September 27, 2011

Components & Techniques For Getting The Best Results With Portable PA

So many options, and so many of them great. Here's what to look for and how it can work in meeting your specific application needs

Technically, lot of sound reinforcement systems are “portable.”

Get together enough hands and/or the right heavy equipment, and almost anything is portable.

But in pro audio, the term portable PA refers to compact systems that can be easily transported in a small truck, van, or even the trunk/back seat of a car, and then hand-carried (or hand-trucked or rolled) into a venue and set up quickly by one person (if need be).

Most often, we’re probably first think of loudspeakers - compact, 2-way models that are also sometimes referred to as “speakers on sticks,” but there are other components such as powered and non-powered mixers in the genre, as well as packaged systems that come complete with stands, cables and even microphones. 

Portable PA systems have been around for decades, and while the basic form has remained the same, they’ve come a long way in terms of performance capability and feature sets.

A popular model in the 1950s was the Knight system, a total package that included a 32-watt power amplifier, a choice of microphones, and even the option of a 4-speed record player!

By the 1970s, the genre had really moved forward in terms of quality and output, with many of the larger manufacturers offering portable components and systems.

One that stood out was the TAPCO Entertainer system from Electro-Voice, which earned a well-deserved reputation as an excellent portable PA system, comprised of the 100M powered mixer and 100S loudspeakers.

In fact, Entertainer systems can still be found performing today, and the heritage of the system continues at EV, exemplified by the new ZXA1 loudspeaker introduced earlier this year.

The 2-way ZxA1 is powered by an integrated 800-watt, 2-channel amplifier module, and it also has built-in steep crossover slopes and woofer excursion protection as well as a switchable high-pass filter allows for use with a subwoofer.

Further, it’s outfitted with XLR microphone and line level inputs, along with an XLR line level output for daisy chaining several amplified loudspeakers or a sub.

Staying with modern portable loudspeakers, the JBL EON has undoubtedly become one of the world’s all-time best-selling models over the past decade or so, and these days it seems like every other event has a stage flanked by Mackie SRM loudspeakers on stands.

Other portable components continue to evolve to a higher standard as well. Soundcraft just released the Notepad Series of multipurpose mixers, with all models outfitted with the company’s GB30 mic preamp and four stereo line inputs.

The Notepad 124FX also has an integral digital effects processor, which has a feed from every input and over 100 effects, including a pink noise and test setting.

Meanwhile, QSC Audio GX Series amplifiers have become a staple in driving portable systems, featuring Class H topology that’s based on key elements of the premium PLX Series, while the recently introduced Crown Audio XLS Series amplifiers integrate advanced crossover, limiting and DSP. 

When it comes to packages, there are a plethora of packages.

The Yamaha STAGEPAS Series offers passive loudspeakers, powered mixer and cables. A nifty facet is that the powered mixer is actually housed in one of the loudspeakers, and is detachable.

There are dozens of choices in this genre, ranging from professional caliber down to more “prosumer” models that even incorporate CD and mp3 players and a wireless microphone receiver within the loudspeaker system cabinet.

Sometimes, just a powered loudspeaker about the size of a loaf of bread equipped with a mic input will do the trick, and these can also come in handy for spot monitoring in support of larger systems.

Plenty Of Choices
Whether it’s independent components or a package, every portable PA system usually includes these basic items:
• Microphones (and sometimes direct boxes)
• Mixer and power amplifier, or powered mixer, or mixer and powered loudspeakers
• A pair of loudspeakers
• Loudspeaker stands, microphone stands
• Mic, line-level and loudspeaker cables
• Optional items include floor monitors, direct boxes and a mic snake

Let’s look more closely at each component.

Microphones. Step one is figuring out how many mics (and mic inputs) are needed. Unidirectional dynamic mics work well for speech, singers, guitar amps and drums, while unidirectional condenser stand-mounted mics are a solid choice for acoustic instruments as well as singers. Presenters and singer can also be outfitted with headphone mics, and acoustic instruments with mini clip-ons.

Power Amplifiers. How much power do you need? If you’ve already got loudspeakers or are going to purchase them, follow the recommended power guidelines provided by the manufacturer. Levels of a powerful system can always be tuned down to match the application, so be sure not to go too light in terms of output.

That said, here are some very loose guidelines:
• Speech-only system in medium room: 50 watts continuous per channel
• Folk music in a coffee shop with 50 seats: 25 to 250 watts
• Folk music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 95 to 250 watts
• Folk music at a small outdoor festival (50 feet from loudspeaker to audience): 250 watts
• Pop or jazz music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 250 to 750 watts
• Pop or jazz music in a 2,000-seat concert hall: 400 to 1,200 watts
• Rock music in a medium-size auditorium, club or house of worship with 150 to 250 seats: at least 1,500 watts
• Rock music at a small outdoor festival (50 feet from loudspeaker to audience): At least 1,000 to 3,000 watts

As previously noted, some power amplifiers now also offer built-in DSP, and these packages are increasingly competitive from a price standpoint.

The digital processing can serve as a substitute for active crossovers and delays.

Crossover-filter presets for specific loudspeakers can make it a snap to set up a multi-way system.

High-pass filters prevent harm to loudspeakers, often due to powerful lows in music, mic-stand thumps or accidental DC at the amp output.

Mixers. Self-powered or not? A mixer that incorporates power amplification is usually easier to carry and set up - it’s more plug and play.

An increasing number of powered mixers also include onboard effects, and some offer a graphic equalizer, which is useful for tuning the frequency response of the loudspeakers in different environments.

An advantage of a separate mixer and power amp is that if either one fails, only one component needs to be replaced.

Stand-alone mixers also tend to be more ergonomic for the user, and there are more size options - no need to purchase a unit with 8 channels if the maximum number of channels needed isn’t going to exceed 4 or 5. 

Be sure that the mixer has enough balanced XLR mic inputs to handle any possible application you have in mind for the system. RCA jacks for CD and MP3 players come in handy.

Loudspeakers. Full-range loudspeakers for portable PA are usually 2-way designs, either powered or passive. Most applications require just two loudspeakers, but there may be times when additional loudspeakers are needed.

Flexible connectivity is something to keep in mind.

For example, the latest generation of JBL EON portable loudspeakers have an XLR output where the output signal is selectable, either the whole mix may be looped to another loudspeaker (or sent to a mixing console), or simply the primary input for traditional “daisy-chaining” of additional loudspeakers.

These also offer one XLR/ quarter-inch combo connector and additional quarter-inch inputs providing input flexibility and the ability to mix multiple sources.

Some applications may require a stage monitor or two, in addition to two or more main/full-range loudspeakers. An increasing number of portable cabinets offer a side that angles the drivers more steeply upward toward the performers.

Typically, full-range loudspeakers are available with a choice of 12-inch or 15-inch cone woofer that is ported, joined by a driver on a horn or waveguide.

Horn dispersion (6 dB-down points) is commonly 40 degrees vertical x 90 degrees horizontal, or 40 degrees vertical x 120 degrees horizontal, but again, we see an increasing variety of coverage patterns available.

Keep in mind that the overall goal is to focus as much direct sound on the audience (and off of surrounding hard surfaces). Thus the polar pattern is important, particularly for indoor applications and especially in highly reverberant spaces like gymnasiums and some worship sanctuaries.

The frequency response of the loudspeakers should be wide enough to reproduce the sound source accurately.

For speech only, 100 Hz - 12 kHz is usually sufficient, but a guitar-singer application is better served by 80 Hz - 15 kHz and a rock band really should have 40 Hz - 15 kHz or higher.

Those frequency limits are typically 10 dB down or less from the level at 1 kHz.

Of course, the flatter the response over the passband, the more accurate the reproduction.

Boosting The Boom
This brings us to subwoofers. Dynamic music performances can really benefit from extended low-end energy.

The majority of full-range portable loudspeakers also offer at least one companion subwoofer, typically loaded with a 15-inch or 18-inch woofer.

These usually include mounts to accommodate stand poles, and sometimes have wheels on their back floor edge for easier transport.

An option for music applications is a subwoofer/satellite configuration, where one or two subwoofers on the floor provide deep bass, while two compact satellite loudspeakers on stands provide the rest of the spectrum.

Because our ears don’t localize extreme low frequencies, all sound “appears” to come from the satellites. The advantage of this approach is not having to lift large, heavy loudspeakers for positioning on stands.

Column loudspeakers are having an influence on portable PA designs, and that’s no surprise considering that the Shure Vocalmaster system, one of the most popular portable systems in the 1960s-70s, also featured a column approach.

A recent example is the Fishman SA220, which incorporates six 4-inch mid-woofers in a vertical line topped by a 1-inch soft-dome tweeter, housed in a cabinet measure just over 5 inches wide by 6 inches deep.

It’s self-powered and comes equipped with two mic/instrument channels with high-quality preamps, each with 3-band EQ, phantom power, built-in reverb, effects loop, notch filtering and phase controls.

A popular configuration is to place the SA220 behind the performer/group, where it can serve as both mains and monitors. Another common set is one SA220 per performer.

The advantages are that the performer(s) hear the same mix that the audience hears, and the sound level is more constant with distance than with a woofer/horn system.

Further Considerations
These days, most powered PA loudspeakers are bi-amplified: they have one amplifier for the woofer and another for the tweeter.

Advantages of bi-amplification include:
•  Distortion frequencies caused by clipping the woofer power amplifier will not reach the tweeter, so there is less likelihood of tweeter burnout if the amplifier clips. In addition, clipping distortion in the woofer amplifier is made less audible.
•  Intermodulation distortion is reduced.
•  Peak power output is greater than that of a single amplifier of equivalent power.
•  Direct coupling of amplifiers to speakers improves transient response—especially at low frequencies.
•  Bi-amping reduces the inductive and capacitive loading of the power amplifier.
•  The full power of the tweeter amplifier is available regardless of the power required by the woofer amplifier.

Loudspeaker cabinets come in both wood and molded plastic.

Wood cabinets are covered with either scratch-resistant heavy-duty paint or durable fabric that’s often called, in slang, carpet.

Molded plastic cabinets have also proven durable, can feature attractive styling, and are often lighter than their wood counterparts.

For example, a quality 12-inch, 2-way model with plastic cabinet might weigh about 25 pounds, while it’s wood counterpart could be double that figure. (You can come to appreciate lower weight after several nights of muscling loudspeakers on stands.)

Some plastic cabinets, however, have a tendency to “leak” low frequencies or resonate, possibly degrading both frequency and time response.

Most cabinets include a pole cup on the bottom that accepts a 1-3/8-inch or 1-1/2-inch pole for mounting.

The pole cups are vertically aligned, with some having a second angled cup to aim the loudspeaker down toward the audience when raised.

Two models in the new QSC Audio KW Series offer a proprietary Tilt-Direct pole cup mounting system.

A turn of a dial engages a 7.5 degree downward tilt of the loudspeaker, directing more of the acoustic energy toward the audience.

Moving & Mounting
Manufacturers of portable loudspeakers typically provide stands, either included or available as an option, and of course, these work well for the vast majority of applications.

Be vigilant about stands - they should be constructed of metal (usually aluminum), offer a solid tripod base and a secure collar, and be rated to comfortably handle more than the weight of the loudspeaker.

There have been some interesting developments in the world of stands as well.

The Ultimate Support Systems Air-Powered Series have an internal shock that lifts and lowers loudspeakers weighing 50 pounds and less with virtually no effort.

The Ultimate TeleLock Series has a collar that gives the user the ability to safely raise or lower the stand while a loudspeaker is on it. 

Both Air-Powered and TeleLock poles are also available for subwoofer mounting.

Portable PA loudspeakers can also be a good choice for permanent and semi-permanent installations.

For example, a church may find that its portable loudspeakers on stands are doing a fine job, so well in fact, that there’s the desire to get them out of the way and permanently mount them.

If you suspect this might be a future possibility, look for loudspeakers outfitted with mounting points that bolt onto steel cables or yokes that can be aimed as needed.

Don’t skimp on loudspeaker cables. Relatively low gauge cable is good practice, with #14 or #12 zip cord or zip cord, PVC or SJO cable of the same gauge seen most often.

Neutrik Speakon connectors are both more reliable than phone plugs (1/4-inch TRS), they lock into place, and pass more current.

To avoid tripping that could result in injury to a person or a loudspeaker or both, tape cables securely to the floor, using gaffe tape.

Extra cable length can be coiled and stored under the mixer or under the loudspeaker stand.

A very helpful item is a dolly, wheeled cart or hand-truck to transport equipment into venues. Consider lightweight tubular carts such as those from Ultimate Support Rock n Roller and Kart-A-Bag - being collapsible, they store easily in your car or truck.

Any additional racks and trunks should have casters for rolling, and lockable casters will keep them in place once positioned. Some trunks have trays and lid storage to make it easier to organize items.

Protective covers (usually available from the manufacturer or from Under Cover or Cloud 9) should be used to prevent damage of components, including for the loudspeakers. No one likes to see ugly, scratched-up cabinets, and it’s simply not a professional look. 

Placement Strategies
Typically, main left and right loudspeakers on stands are placed to either side of the stage/presentation area, aimed at the audience. (Figure 1, Option 1).

Make sure the loudspeakers are “in front” of the microphones, positioned toward the “dead” rear of the cardioid mic pattern, thus reducing the potential for feedback.

Raise the loudspeakers high enough to clear the crowd. Otherwise, people in the back will hear muffled sound because the crowd attenuates the high frequencies.

Also, raising the loudspeakers prevents blasting the front row of the audience.

Articulation is best if the direct-sound level is high relative to the reflected-sound level. This happens if you place the loudspeakers closer to the audience and take care to aim them.

An alternative to loudspeakers at either side of the stage is suggested by veteran audio consultant Ray Rayburn: Mount a single loudspeaker at one front corner of the audience, shooting across to the opposite corner (Figure 1, option 2).

Further, stack the two loudspeakers vertically (horn to horn) which narrows their coverage angle in the vertical plane. Clamp them solidly together.

The advantages of this arrangement include no comb filtering from hearing two loudspeakers at different distances, and a clearer overall sound with less reverb because of reduced ceiling reflections.

For sporting events, try to place loudspeakers so they aim across the playing field at the bleachers. This way, the players can hear what’s going on, and the people in the bleachers will absorb some of the sound and reduce reflections.

Final tips: wind screens on microphones outdoors really help eliminate noise, pop filters on microphones help prevent breath pops, which are particularly common among less experienced presenters, and a headworn mic allows a presenter to turn his/her head without getting off-mic.

Bruce Bartlett is a microphone engineer, sound mixer, recording engineer, and audio journalist. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”

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