Live sound engineering is part of the performance. The purpose is to deliver the best sound possible from the stage to the customers. To accomplish this the engineer has to take into account the room, the crowd, the instruments to be used (some, like horns and drums have large “acoustic footprints” while electric guitars and keyboards don’t) and the vocal techniques of the singers. Here we’ll take a quick look at the tools a sound engineer uses to deliver the performance.

Microphones

A microphone is a special kind of transducer. It uses sound (waves of fluctuating air pressure) to create and then amplify a similarly fluctuating electrical current. While there are many types; Dynamic, Ribbon, Condenser, Electret Condenser, in most club or live music settings Dynamic microphones are preferred. What they might lack in sensitivity and fidelity (the ability to accurately convert and represent the sound) they make up for in ruggedness and cost. Besides, current sound mixing technology can often mask or even correct a microphone’s shortcomings.

A simple dynamic microphone consists of a diaphragm with an attached coil of wire, a magnet, and signal amplifying circuitry connecting to the coil. Sound waves move the diaphragm, which in turn moves the wire coil through the magnet’s field producing a small fluctuating electrical current. The amplifying circuitry is needed to boost that fluctuating electrical signal to a useful level.

Even though dynamic microphones are robust, as far as microphones go, they can still be easily damaged. Obviously you don’t want to play catch with one (or dodge microphone) but they can also be damaged by loud sounds. Sustained loud noise can permanently deform the diaphragm causing a “built-in” unwanted distortion.

There are a wide variety of dynamic microphones in use for live music, most fall into just two main categories; vocal and instrument. Vocal mics often have the distinctive “ice cream cone” look. The round ball covering the business end is a shield or filter that minimizes the popping or puff sounds generated by some vocalists. Instrument mics can come in different shapes and sizes and are usually chosen to match the frequency and volume range of the instrument being miced—most obvious is the bass mic used for kick drums. It’s size and shape allows for a larger diaphragm, more responsive to lower frequencies. Performance mics typically have a cardioidal pickup pattern. This mean they pick up sounds pretty well within a sixty-degree angle either side of the nose of the mic, and it purposely doesn’t pick up the sound behind the mic.

Guitar Pickups

Electric guitars operate much like a dynamic microphone but instead of a vibrating diaphragm initiating the response the vibrating metal strings serve that purpose. Beneath each string is a small cylindrical magnet wrapped in a coil of wire. The magnet’s field polarizes the steel string allowing it to in turn contribute to the magnet’s field. When the string vibrates the field changes and a current is induced in the coiled wire. Like in a microphone, that current is then amplified to a usable level.

Acoustic guitars can use a variety of methods to produce a response signal. Steel string acoustic guitars can have pickups similar to electric guitars. Nylon string guitars can use piezoelectric transducers as pickups. These very specialized crystals change their electrical resistance when stressed (vibrated). The changing resistance is easily converted to a usable electronic signal. Finally, a guitar can be mic-ed either indirectly by placing an instrument microphone near the sound hole or directly using it’s own internal microphone.

Often (more often with electric guitars and electric basses) musicians will put their own amplifier between their instrument and the venue’s sound system. In these cases they’ll plug the guitar into the amplifier and either mic the amplifier’s speaker or connect the venue’s line to an output plug. This allows them better control over intended distortion and other effects.

Other Stage Outputs

As mentioned above, guitarists sometimes use their own amplifiers on stage. These small amplifiers are really combination power-supplies/speakers with their own effects, distortion and volume controls. The power-supplies input the relatively small electrical signal from the instrument and boost it to a level suitable for driving the speaker and, as I previously indicated, they also have signal-output connections so they can be plugged directly into the venue sound system—or a microphone can be placed in front of their speaker.

Other “electronic” instruments are able to output an electric current signal to the sound system but have little or no acoustics footprint. These include keyboards, synthesizers and an assortment of stylized instruments that plug directly into the sound system.

Cables

For 99% of the situations, the sound engineer uses one of two cable types to connect instruments and microphones to the sound mixing board (the next stop along the path to the speakers); XLR tipped (three pronged, one end male the other female) shielded or ¼ inch mono-tipped shielded cable—usually male on both ends. Often the majority of connections to a soundboard are the XLR type (most microphones also use this type of connection). Guitars and guitar amps use the ¼ inch cables. If a soundboard does not have enough ¼ inch connections, the cable can be jacked into a conversion “D/I Box” which takes the signal from the ¼ inch cable and outputs it to XLR cable.

It’s very important to use only shielded cables. At a cable’s core are three or more separately insulated wires that either carry electrical signals or act as a grounding wire. Around this core is some more insulation and then a highly conductive sheath. This sheath is the “shielding” and it protects the electrical signals from the effects of ambient EM radiation—radio signals, cell phone tower signals and such. The most outer portion of the cable consists of another layer of insulation. Cables can also be “balanced” to eliminate the 60 Hz hum produced by powering the system through our public grids—the socket in the wall which leads to the public utility lines. Within these type cables, the signal carrying wires are braded with another wire that transmits the 60 Hz hum 180 degrees out of phase. The two “hums” now cancel each other without interfering with the desired signal transition.

Cables can be relatively expensive and easily damaged so Sound Dude carries a mallet in his back pocket to whack the toes of anyone his sees standing on his cables.

Sound Mixing Board

The mixing board is the sound system’s traffic cop. It accepts all the signals either directly from the stage or through a standalone pre-amp, allows the sound engineer to manipulate those signals and then sends the finished product to the amplifiers/speakers and/or recording devices. A number of sound effect devices such as compressors or distorters might work as stand-alone hardware in conjunction with the board or they might be contained within the board. This last is the case with most high-end digital sound mixing boards.

A typical sound mixing board will have a column of dials atop a fader control for each channel—24 or 36-channel boards are pretty common for live music venues. Each instrument, each singer, each input should have its own channel. When that’s not possible the engineer can use a small on-stage mixer to group certain instruments or mics and then send just a single signal to the main soundboard from those instruments. The column of dials on a mixing board control pre-mix signal gain, EQ’ing functions and monitor-speaker volume levels. A dial also acts as the coarse gain control for the main speaker’s volume with the fader being the fine control. Each channel will also allow for a number of inputs and outputs—as will the board in general. The channels will allow for either ¼ inch or XLR inputs plus input/output for stand alone effects hardware—compressors or distorters. Outputs to amplifier/speakers (monitors, mains, subs) can be either directly from the board or through a separate speaker’s controller.

The third article in this series will cover the sound mixing board in more detail.

Amplifiers/Speakers

The final step along this path is getting the music to the speakers. A venue's sound system will include monitor speakers (so the band can hear themselves), main speakers (which broadcast middle and high frequencies pretty well) and subwoofer speakers (to handle the low end frequencies—the bass). Some venues will have multiple sets of these last two types with pairs set at some distance from the stage. The signal to these speakers will be time-delayed so their output coincides with the sound coming from the stage. Speakers should be matched to the venue. Small PA speakers will work for an acoustic performance in a modest-sized coffee shop but won’t be enough for your local rock club, and whole stacks of mega-sized speakers will be needed for an outdoor jam blast.