Mixers in PA Systems Part. 1

A mixer is an essential piece of hardware for capturing interviews that incorporate more than one talking head. Into the mixer you plug all the microphones and other sound sources you are harvesting during a single recording session. The mixer provides needed amplification, VU (volume) readouts that allow you to monitor input levels for each microphone, and controls for adjusting each input level until a perfectly balanced mix is achieved.

Mixers are rated by the number of channels (inputs) they support; a mixer that provides ports for plugging in four microphones, plus two stereo inputs and two line inputs will be labeled as an 8-channel mixer. The more expensive the mixer, the larger the number of input channels it supports and the greater control it provides for filtering and tweaking each incoming sound.

Most mixers used in podcasting productions are set up to output a single stereo track or two mono tracks to the sound card in your PC through a line-in or USB port. Yes, you can buy mixers that output multiple simultaneous channels. These are great for recording professional music sessions, but this level of control is usually overkill for most podcasting sessions.

Even the cheapest of mixers will however allow you to split your output into two separate mono tracks. Doing so is a great idea for recording sessions using two microphones to record a two-person interview. You use the mixer to bring each voice in separately and adjust them independently later on using a multi-track sound editor — a very useful trick in cases where one voice gets louder and softer and the other stays constant. Truth be told, 90 percent of the podcast developers I know routinely mix all the inputs together in the mixer and send out two identical mono tracks outto their PC.

Most mixers provide both XLR and 1/4 inch inputs for plugging in traditional broadcast microphones. Now that some podcasters are using USB mics, mixers that accept USB input are also becoming common. USB input is also useful for bringing in sound from USB-based recorders or MP3 players.

You need to make sure the mixer you select supports enough channels to connect to all the other devices you will need to plug into the mixer (CD players, turntable, midi out from your synthesizer, etc.). Give yourself room to grow — if you plan to use two microphones for a typical podcast, get a mixer that accepts input from at least four microphones. I can’t tell you how many podcasters I have talked to who underestimated the amount of equipment they would be using three months later.

Also make sure your mixer is powered (amplified). You plug the un-amplified microphones into the mixer; it boosts the incoming signal or each mic, mixes them together and sends out a powered output that is required for the line-in port of your computer. Some mixers also provide phantom power — a necessary feature for those of you using professional condenser mics that need to get their power from an external source.

Acoustics

Acoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound (all mechanical waves in gases, liquids, and solids). A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers.

Fundamental concepts of acoustics

The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.

The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of cause, both natural and volitional. There are many kinds of transduction process that convert energy from some other form into acoustical energy, producing the acoustic wave. There is one fundamental equation that describes acoustic wave propagation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.

The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse waves and surface waves.

Acoustics looks first at the pressure levels and frequencies in the sound wave. Transduction processes are also of special importance.