Jason Allen looks at input sources – how to capture your sound and how to get it where you need it to go.

At the very beginning of the design stage for any public address (PA) system, the question to ask is: “What are we amplifying?”

Will the system be used to amplify speech, singing, musical instruments, playback devices or a combination of any of those? Then, vitally, how many of each and how many at the same time? In touring sound this is known as ‘The Line List’ and helps dictate everything from the physical location of the input sources to the types of microphone used.

Speech, singing and all acoustic musical instruments will need microphones. Laptops and electronic musical instruments will need direct input (DI) boxes to accommodate for long cable runs and to deal with power supply issues. Most playback devices should be able to be connected directly to your mixer or digital signal processor (DSP) if they are physically close enough, but may need a DI if located too far away.

Microphones for speech
Almost every PA system in the world amplifies speech at some point. Determining the right microphone for your speech application depends on its particular physical use case. Let’s take the example of a school hall. The most common usage of the hall will be for assemblies at which one person at a time addresses the students from a lectern. If the lectern is a static feature that doesn’t move in or out, a permanent lectern microphone can be fitted.

Lectern microphones are goosenecked, flexible and designed to be plugged into a built-in XLR connection on the lectern. Most models are condenser microphones. A condenser microphone captures sound by running an electrical charge through a very thin, flexible diaphragm (usually plastic with a metal backing) and a stationary backplate. The 48V electrical charge, also known as phantom power, is usually supplied by the mixing desk or DSP. As the diaphragm is moved by air and the distance between it and the backplate changes, the amount of charge in the backplate varies due to an effect known as capacitance. This varying voltage is therefore an electrical representation of our moving air, and is the signal we amplify.

The diaphragm/backplate component is known as a capsule and in a condenser microphone it is tiny. This means it is easily moved by low levels of sound and can therefore pick up lots of detail or, in the case of our speech, people who are distant from the microphone. Other characteristics of lectern microphones include increased frequency output in the range vital for speech intelligibility (from 2kHz to 3kHz) and decreased response below the normal human vocal range, typically from 100Hz down.

Moving target
If you don’t have the luxury of a permanent fixed position for the main speech microphone, a radio microphone may be your best option. It’s really common in applications like school and community halls for these to be the only speech microphones supplied as they accommodate almost any use case the facility would need. Wireless mics come in three styles – handheld dynamic microphones and either clipped on or headworn miniature condenser types.

Dynamic microphones are the most common type of instrument and speech microphone on the market. The technology is old, proven and tough. Based on the very simple principle that moving a magnet along a coil of wire generates an electrical charge, the capsule in a dynamic mic does just that when pushed by air. The difference is that the mass that needs to be moved is larger and therefore dynamics are a lot less sensitive than condensers. That can be a desirable quality when mic’ing loud sources and also makes them less likely to produce feedback. For speech, it means the user needs to have a moderate grasp of microphone technique and have the microphone relatively close to their mouth for the best results.

Lapels and Headworn
In worship applications, lapel or headworn miniature condensers can be the best fit for the worship leader. These tiny mics are plugged into a small beltpack transmitter and are almost invisible to the audience. A lapel mic (or ‘lavalier’ as they’re sometimes known) can be clipped to clothing around the chest, pointing up at the mouth. A headworn mic can be almost as unobtrusive and often gets a better result than a lapel due to the proximity to the mouth and freedom from being interfered with by clothing or vestments.

The downside of these microphones is that they are very delicate. As every component has been miniaturised, even ordinary use will require more maintenance than a sturdy dynamic mic. Typical fail points are the small connector that joins the mic lead to the beltpack and the lead itself. Lapels and headworn mics are best kept reserved for use by more senior members of the user group in an application.

All wireless microphones need batteries in their transmitters. In the past, this meant having a lot of alkaline batteries on hand and generating a lot of waste. This isn’t desirable from an economic or environmental point of view. Try and buy a wireless product that comes with its own rechargeable batteries and charging cradles. It’s better for both the client and the environment in the long run.

Wireless Spectrum – the current state of play
Due to the recent switch off of analogue television in Australia and the government sell-off of the broadcast spectrum to telcos, the frequencies that wireless devices are allowed to operate on is in a state of flux. At the moment, allowable frequencies vary region to region, with a limited frequency spectrum usable throughout the whole country. Luckily, there are some great online resources for you to check legal operating frequencies before you buy a wireless product.

The websites www.readyfordigital.com.au and www.frequencyfinder.com.au, operated by Jands and Sennheiser respectively, allow you to check by region before you buy. You will also want to scan the RF environment you are installing into to make sure there’s nothing unexpected being generated by a neighbour. The more expensive wireless products have the ability to tune themselves around any problems in the spectrum, but they tend to be at the top end of the price scale. Often you can pair your wireless receiver products with a software scanning application on your computer to get a picture of your RF environment. Ask your wireless product distributor about your options.

Rhythm Section
There are a huge range of options on the market for mic’ing up instruments. You’re going to need a very clear picture on what kind of musical instruments will be used regularly in your application and what kinds might be used intermittently. Let’s take the example of an evangelical church with an active contemporary music service. The band for a modern service in this church will involve a drum kit, electric bass, two or more electric or acoustic electric guitars, one or more keyboards, several backing vocalists and a lead vocalist.

Drum kits are their own special case. A drum kit is made up of several different instruments with different frequency and response characteristics. The kick drum requires a specialised kick microphone to capture its high sound pressure level (SPL) and low frequency energy. The high hat cymbal usually requires a small diaphragm condenser that can also take high SPL. Dynamic mics are usually used on snares and toms. A pair of condensers are usually used to capture cymbals. Many microphone manufacturers sell drum microphone kits that should cover you for most applications.

Electric guitars and basses are either mic’ed at the amplifier, connected directly via DI or a combination of both. A traditional electric guitar played through an amp will be covered by a dynamic mic positioned close to the cabinet. It’s common for an electric bass to be connected to a DI, then its signal split to the mixer and to the bass player’s amp. Electric acoustic guitars, synthesisers and computers are usually connected to a DI and then either sent to the mixing desk and back to foldback, or split to an amp. Vocal microphones are usually hardwired dynamic microphones or wireless dynamic microphones. Some top end vocal microphones are condensers especially designed for stage use, but they are expensive and you might want to reserve them for the lead vocalists. 

Going direct
More and more DIs are being required on the modern stage. Where once there may have been two or four required for a band’s keyboards and maybe a guitar, it’s now common for up to eight to be needed to accept inputs from computer audio interfaces and guitar processors with direct outs.

Many keyboard players have moved their synthesisers ‘into the box’. Instead of carrying around bulky hardware synths, lighter controller keyboards now connect to laptops running digital audio workstation (DAW) software loaded with software synths, offering a huge range of sounds. These DAWs can also run loops, samples and backing tracks. A mid-level audio interface can have up to eight outputs that a mixer will need to accept. There are rack-mount DIs on the market made especially for this purpose.

While it is true that some electronic instruments produce an output level that can happily be connected directly to a multicore or stagebox, it’s almost always a good idea to have a DI handy for the other functions it provides. A DI can provide an ‘earth lift’, which eliminates the hum caused by a ground loop. Ground loops are often caused when part of an electronic instrument’s power supply or wiring provides an additional electrical path to ground. Another handy DI function is a built in pad, which can be switched in to reduce the signal level form the instrument by 10 or 20 dB, making for a more usable gain structure at the mixer.

Playback and line sources
DVD and CD players, iPods and the line outs from wireless receivers are most often physically located near the mixing desk or rack-mounted with the DSP unit. This is because they are either used by the sound operator or controlled by the DSP. As such, you’ll be using short cable connections directly to line inputs and shouldn’t need a DI. Check what kind of output your wireless microphone devices have before you decide on a mixing desk or DSP – most wireless receivers have both mic and line output, but some only have one option. That will affect how many line inputs or mic preamps your mixing device will need.

Cabling and other considerations
All of your hardwired instrument microphones and DIs will need XLR microphone cables. Depending on the size of your performance area and the amount of channels you’ll have, these cables will then in turn either be connected to small dropboxes of six to 12 inputs that connect to an analogue or digital multicore, or for smaller setups, plug straight into the multicore inputs. It’s good to have a few different lengths of mic cable to hand, between two and 10 metres, and more cable than you need. Sadly, a few of them will probably ‘follow people home’ from the venue every now and again.

If the system is using condenser microphones that require 48V phantom power from the mixing desk and are going to get connected and disconnected regularly, make sure that the people doing this are trained to do so with the phantom power turned off. Unplugging and replugging a phantom powered device (some DIs are too) with the power active not only causes a nasty ‘bang’ though the system that damages speakers, it eventually destroys the microphone preamp in the mixer. Some preamps are more robust than others, but will fail eventually. The users need to either disable phantom power on the channels being changed, or just plug and unplug with the mixer turned off completely.