1 live sound reinforcement equalizers and other signal processing equipment

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Live Sound Reinforcement

Equalizers and other signal processing equipment

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Equalizers (EQs) allow the operator to finely tune the final product for presentation to the audience.

EQ circuitry is really a filter designed to filter out unwanted frequencies or enhance the desired frequency.

The simplest form of an EQ is tone control, the bass and treble on most car stereos allows you to either increase or decrease the lows or the highs of the incoming music.

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The bass and treble controls are essentially a 2 band EQ.

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EQ is defined as the change in volume of a frequency or a set of frequencies in a sound, kind of like turning the treble or bass knobs on your stereo system.

Using equalization you can mold an instruments sound to you're preference by boosting and cutting frequencies in the sound.

Lower frequencies consist of lower or deep pitch and higher frequencies consist of higher pitches.

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Equalizers fall under three main categories:

Fixed EQ Graphic EQ

Parametric EQ

Within these categories we can use different circuitry

to adjust frequencies within the equipment.

Shelving filters High pass/low

pass filters Peak/dip filters

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Fixed EQ's are about as basic as you get you'll find these in smaller mixers and they're designed to tidy up the sound rather than make delicate adjustments

Fixed EQ's will be set to certain frequencies and you can not change the frequencies at which fixed EQ's operate, they will only have one knob which sets the desired amount of Gain at the frequency labeled on it.

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Knobs labeled “High”, “Mid” and “Low” are usually a shelving EQ, meaning that they cut/boost everything below or above their named frequency (called the "corner" in this case).

Shelving EQs only boost or cut to a certain point and then level off creating a graph that looks like a shelf.

Shelving EQ graph

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We looked at the fixed EQ in the mixer lesson, this mixer arrangement has 3 bands, Hi, Mid and Low, to cut or boost these frequencies, you simply turn the knob left or right respectively.

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Graphic EQs are the most popular type of equalizers and provide slide controls to cut or boost around a center frequency.

A one octave EQ has 10 bands, a 2/3 octave EQ has 15 bands and a 1/3 octave EQ has 27, 30 or 31 bands.

Graphic EQs can utilize different types of filters such as shelving filter or high pass/low pass filters.

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Graphic equalizers can be used to change the volume of specified frequency points called bands.

The amount of bands on graphic EQ's can range anywhere from just a few all the way up to 31.

The more bands available the more control you have over the sound.

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The graph on the right is a low pass filter, it completely cuts the high end, the graph on the right is a shelving filter depiction, as you can see some of the high frequency remains creating what looks like a shelf.

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Here we have the opposing graphs, these are high frequency filters that cut the low part of the frequency.

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Switchable cut/high pass and low pass are simply filters that either pass the high portion of a frequency and cut the low portion of the frequency or vice versa.

THE FREQUENCIES LISTED ON THE SLIDE CONTROLS ARE THE CENTER FREQUECIES THAT CAN BE EITHER CUT OR BOOSTED.

WHAT DOES THE LEVEL CONTROL DO?

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Parametric EQ's provide even more control and flexibility because unlike graphic EQs, they allow you to set the location of the bands and the width of the bands.

When you boost the volume of a frequency you are actually raising the volume of frequencies around it as well.

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Being able to change the width of a band allows you to specify exactly what frequencies you are boosting or cutting on each band or raise/lower a wide range of frequencies with one band.

PARAMETRIC EQs ALLOW THE OPERATOR TO ADJUST THE WIDTH “Q” OF THE WAVE, THE GAIN OF THE WAVE AND THE FREQUENCY OF A WAVE FORM.

PARAMETRIC EQs PROVIDE MORE CONTROL OF THE FREQUENCY AND HAVE 3 KNOBS PER BAND AS OPPOSED TO JUST A SLIDE LIKE THOSE ON GRAPHIC EQs.

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Sound moves in waves, and the faster (more frequently) they repeat their up-down wave shape, the higher the pitch of the sound we hear. Lower frequency = lower pitch, and frequency is measured in Hertz (Hz).

A "band" of EQ is a specific range of frequencies that has its own controls. Low and High are two separate bands; 80 Hz and 3000 Hz are usually in separate bands; and if a single knob or set of controls adjusts a chosen frequency range, that is a band.

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When you turn an EQ knob to a certain Hz number, or adjust the slider labeled with a certain Hz, that number is called the "center" frequency because it's the center of a bell-shaped range of frequencies that get affected by that knob or slider, you aren't just turning up or down that one frequency, but also a lot of frequencies on either side of it.

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Here is an illustration of one band of EQ boost, and how the range of frequencies affected by it increases and decreases as you adjust the bandwidth ("Q"):The wider the bandwidth the more the frequencies surrounding the center frequency are affected by the cut and boost.

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Often called resonance, the Q parameter, defines the steepness or width of each filter depending on its type.

The width of the filter, or 'Q', becomes narrower the higher the number selected.

The Q parameter is often referred to as resonance, if you listen to the sound produced by such a filter, it is similar to an object (glass for example), resonating at a particular frequency, making it louder.

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We’re now going to look at other signal processing equipment.

Compressors and limiter circuits assist in keeping the signal level within a workable dynamic range, in other words it proportionately reduces the amount of gain beyond a given signal level.

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The purpose of a compressor or a limiter is to make sure the signal level does not go beyond a certain threshold.

A limiter typically has a higher ratio threshold than a compressor and has a gain reduction ratio of 10:1.

The two main applications for a compressor / limiter in live sound are limiting speaker volume, and evening out speech variations from a microphone.

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Assuming that your power amplifier rating is well matched to your speakers, and no one will be operating the system at excessive volume levels, you should have no need to limit the amplifier

In some cases however, this can happen. If it does, the amplifier may produce a high level distortion which can damage the speaker.

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Even if it doesn't affect the speaker, the excessive sound pressure level can cause short and possibly long term damage to ears.

A good solution to this, is to install a compressor / limiter between the mixer and amplifier.

When it is adjusted properly, it will have no affect on the sound until the volume reaches a predetermined level. At this point the unit will prevent the level from increasing beyond that point. This way, the amplifier will not distort.

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So in essence a compressor or limiter is really an automatic volume control that kicks in when the signal suddenly gets very loud.

This happens when a singer increases the pitch in their voice to a very high volume, if left unchecked it would produce a distortion.

The compressor will reduce the loud noise by a certain ratio, typically 2:1, 4:1 or 8:1.

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Threshold is set a specific dB level (the level not to exceed), the ratio is set low at first (1.5:1 or 2:1) to reduce the signal peak below the threshold, the attack is how fast the compressor responds to the peak overload and release is how slow the compressor releases the signal ratio back into the mix.

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Noise gates and downward expanders reduce the gain below a certain threshold in order to reduce or eliminate unwanted noise when there is no program signal, but allow the desired signal to pass through the system.

Noise gates and downward expanders are very similar and are relative to compressors and limiters in the amount of ratio applied to the mix.

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Downward expanders offer a higher ratio of noise suppression than a noise gate.

Noise gates and expanders can be incorporated into compressor units as an all in one device and operate in the same way.

You would set the threshold, the attack and release time which is in milliseconds the same way you do on a compressor.

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The idea here is if you have a lot of microphones on stage you might pick up interference or feed back when they’re not in use, the noise gate eliminates the noise until the mic is actually used.

Other types of signal processing equipment are the delay/echo and reverb units, the Xenxy 1202FX mixer has these types of circuits built into it, however they can be purchased as separate units.

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A delay and reverb unit stores an original copy of the input signal for a certain length of time and then reproduces the signal as needed.

The delay unit will return the signal at specified time intervals which is determined by the operator.

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A reverb unit is designed to return the input signal numerous time at very closely spaced intervals.

1 live sound reinforcement equalizers and other signal processing equipment

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