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Signal Processing Shapes Your Sound
From compression and limiting to EQ and reverb, signal processing offers you the opportunity to sculpt the sound for the congregation. This Sweetwater Buying Guide includes information that can help you choose the right signal processing equipment for your needs. Since there's so much to consider when purchasing signal-processing gear, don't hesitate to call us at 1-800-222-4700 for more information.
Compression - The Ins and Outs
| A compressor smoothes the peaks and dips of the dynamic range. |
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Compression is perhaps one of the most high-profile audio tools in recording and live sound, yet it's often surrounded by a great deal of mystery. Beyond its capacity to perform some very necessary functions in recording, there have emerged a number of live sound usages as well.
Compressor:
A compressor is a device that reduces the dynamic range of program material. This is useful in the case of vocals or an instrument that changes from low levels - very soft - to high levels - very loud. Compression manipulates the dynamic range to cause the high levels to be somewhat reduced while bringing up the lower levels. This results in the perception of an overall higher average volume, allowing you to run the system at lower levels, and allows an instrument or vocal to "sit" better in the live mix.
Limiter:
A limiter is a device that stops the output of a signal from going above a predetermined level. An example of a limiter being particularly effective is the protection of PA speakers from overload and distortion. Limiting prevents clipping (distortion) in both digital and analog systems. The dbx 166XL provides both compression and limiting in one economical unit.
EQ Explained
An equalizer (EQ) has the ability to boost and/or cut the energy in specified frequency ranges by employing one or more filter circuits. Using EQ, you can enhance weaker frequencies and take the edge off of those that are too prominent.
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 Graphic EQ :
A graphic equalizer is configured to provide a graphic display of the EQ settings. Years ago equalizers were all rotary-knob based. When units began to arrive on the scenes that could EQ 15, 30 or even 45 frequency bands, it became difficult to see what was going on at a glance. So equalizers that used sliders instead of knobs were developed and quickly won the favor of engineers due to their improved ergonomics. Engineers liked how easy it was to see the overall EQ curve at a glance, but they also just liked using sliders more than knobs. The ONLY thing that makes an EQ "graphic" is its capability of displaying the curve at a glance. There are graphic EQs that have the same features as "parametric" EQs (see below), but most graphic EQs, however, only give you control of a pre-selected set of frequencies at pre-selected bandwidths. The dbx 231 is an EQ with 31 bands.
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Parametric EQ:
A parametric equalizer has several "parameters" for control of various filters that can be applied to audio frequencies. Parametric equalizers are most widely used in situations where very fine control over the audio signal is desired. In order for an equalizer to be parametric it must at least have control over gain, "Q" (the bandwidth of frequencies being boosted/cut), and frequency. In most cases each of these controls are on rotary knobs.
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When Should I Use a Gate?
Gates are a type of compressor (or expander) often used to automatically mute the signal from a temporarily unused mic or other input source. When one mic's signal level drops below a certain point, its input is cut ("attenuated") fairly quickly to eliminate the background noise it contributes to the mix. The number of open mics in your system reduces the total available gain before feedback, so keeping the input path as clean as possible is important. By adjusting the "gate" level (the point at which the signal is reduced or cut off) you can achieve a natural-sounding mix without excessive noise and bleed. A common use of a gate is to control the outputs of drum mics. Gates can also clean up vocal mixes by (for example) attenuating background vocalists' mics during those parts of the song that are "silent." This can eliminate a lot of track mutes and un-mutes while controlling extra open mics onstage. Peavey's Sanctuary Series of worship-specific mixers have an Automix feature that use gates to silence unused mics on, hands free.
What is Feedback?
| Feedback distracts from your message and annoys the congregation. |
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Feedback is the bane of almost all live sound systems. It can take a great performance and turn it into a painful and embarrassing experience for the speaker and performer, congregation, and sound operator. Acoustic feedback occurs in a sound reinforcement system when the signal output from the speaker is picked up by a microphone and re-amplified, creating a feedback loop. What results is an audible "squealing" or "howling" of the system. Historically (before automatic feedback elimination), feedback was removed manually using an EQ. When feedback occurred, the sound engineer would guess where the feedback was located, and pull down a fader to decrease the gain at that frequency. This method unnecessarily cuts out large portions of the frequency spectrum, which degrades the audio quality of the system. Unfortunately, another problem with manual feedback suppression is that by the time action is taken, it's already too late.
 How Does Feedback Reduction Work?
Up until a few years ago there was little that could be done electronically about feedback except rudimentary efforts using EQ. With the advent of Digital Signal Processing (DSP), automatic feedback elimination has been made possible. Feedback management systems such as the Feedback Ferret by Peavey employ an algorithm that automatically identifies and suppresses microphone feedback with minimal degradation of the audio signal.
 Understanding Reverb
| Apply reverb sparingly to avoid muddying the sound. |
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After equalization and compression, the most frequent effect live sound engineers rely on is reverb. In order to use this powerful tool to its optimum level, it's important to understand some basic concepts.
Reverberation is the continued sound that exists in a room after the source of the sound has stopped. We've all heard it when doing something like clapping our hands (or bouncing a basketball) in a large enclosed space like a gym. All rooms have some natural reverberation, even though we may not always notice it. The characteristics of the reverberation are a big part of the subjective quality of the sound of any room in which we are located.
You need to make careful choices when you enhance a room's natural reverberation with electronically generated reverb. Adding a touch of reverb to a music mix can add body and depth to the performance. But improperly applied, reverb can reduce intelligibility and "muddy up" the amplified sound. Fortunately, a number of high-quality reverb units are available at moderate prices, such as the TC Electronic M-One XL, that include hundreds of presets that are effective in a wide range of live sound environments.
 Understanding PA Management
PA management in short, is handling every detail of setting up and controlling a PA system for a worship service. It's the job of the audio volunteer staff to make sure that there is optimal sound and no surprises. This includes mic choice and placement, monitor mixing including EQ, notch filters, crossovers, and compressors for stage wedges, and stereo EQ and compression for in-ear monitors. Setting gain structure and controlling the mix and output to the mains requires tuning the system for room characteristics along with EQ, compression, limiting, and effects processing.
Generally, racks of equipment must be used from compressor/limiters, to multi-band graphic EQ, crossovers, delays to time-align monitors and house speakers, along with RTA (Real Time Analysis) gear to tune the room to avoid audio surprises while creating optimal sound conditions. Fortunately a technology to manage these aspects of sound reinforcement that was once only available to high-end touring systems is now available to everyone with units like the DriveRack PA and DriveRack 260. Now, the functions of racks full of gear can be had in a single unit at a very affordable price. A Wizard function allows easy setup of parameters such as automatic RTA, EQ, and feedback elimination.
At Sweetwater, we believe that there is no substitute for knowledge to get the most out of your gear and to avoid the pitfalls only learned after years of experience. To find out more about any of the products mentioned in this guide, contact your Sweetwater Sales Engineer today at 1-800-222-4700.
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