Which power amplifier is best suited to my speakers?
Unfortunately this is not a simple question to answer. The amplifier should be 'sized' based on the maximum rated input power of the loudspeakers and the duty cycle of the intended program material. For example, are the speakers to be used for touring? nightclub? small club/bar? A good rule of thumb is as follows: (a) If the program material contains short, controlled bursts of signal eg. recording studio or cinema, an amplifier of up to 4 times the speaker continuous power rating can be safely be used. (b) If the program material contains mildly compressed whilst still reasonably dynamic signals, eg. live bands or recorded music replay, an amplifier of up to 2 times the speaker's continuous power rating can safely be used. (c) If the program source contains highly compressed signal or is to be played at continuously high ouput for long periods of time, eg. loud bar, mobile DJ or nightclub, an amplifier equal to the speaker's continuous power rating can be safely used.
It is not good practice to use an amplifier that has a lower power rating than the speaker's continuous power rating because amplifier clipping may result in damage to the speaker's voice coil and components.
What is the best placement for subs?
In many small to medium sized applications it becomes more cost effective to run sub woofers from a Mono feed. In most applications audio below 200Hz is onmidirectional which defeats the purpose of Stereo imaging. This also facilitates the ability to place the subs together, creating a coupling effect, this coupling increases sub output 3dB for every sub coupled. Eg one dual 18" sub delivers 98db 1w/1m, place a second dual 18" sub on top and the output steps up 3dB to 101 db 1w/1m. A 3db output increase is also attainable by placing individual subs in corners or against solid walls. For each surface you increase output by 3 db, eg: on the floor (1 surface) +3db, against a wall (2 surfaces) +6db and in a corner (3 surfaces) +9db. This works well in bars, clubs and restaurants where space is limited; a smaller sub can still provide more than adequate output.
How to preperly set the wireless "gain"?
Before the audio connections are even made or with the PA system muted, simply sing or scream into the microphone as loudly as it will ever be used in performance in this application. (For guitar systems, turn the gains on the guitar to maximum and hit the hardest note that will be used in concert)
- Then adjust the gain on the transmitter until the audio meter peaks in the usable range.
- Make the audio connections and use the mixing board or amplifier to set the appropriate audio levels for the PA. For a guitar/instrument wireless system, use the receiver output level adjustment to match the “wired” instrument output level.
What is the difference between constant voltage system (70v/100v) and 'normal' system? And why should I choose one over the other?
Constant voltage systems step up the nominal voltage on the speaker line to a higher voltage, either 70v or 100v, to reduce the effects of cable losses. Costant voltage speaker present higher impedence to the line than 'normal' low impedance speakers. Higher impedance speakers draw less current from the amplifier, and since cable loss is proportional to current, cable losses are greatly reduced.
For example, let's consider two systems. System 1 is a traditional low impedance system. With 4 x 8 ohm loudspeakers connected to a 400W amplifier via 100m of 1mm2 speaker cable.
Total speaker load= 8/4=2ohms
Loss on speaker cable = 9db
Power delivered to the speakers = 49w
Speaker cable loss = 87%
The majority of our amplifier power is being wasted heating up the speaker cable! Now let's change to a constant voltage system feeding 4x 100v speakers tapped at 100w each.
Total speaker load= 100/4=25ohms
Loss on speaker cable = 1.2db
Power delivered to the speakers = 303w
Speaker cable loss = 24%
You can see that the constant voltage system is far more efficient. In fact we would have to increase the speaker cable to 30mm2 in the first example to even come close to the cable loss of the constant voltage system. That's a BIG cable, we are taling the stuff they use for welding cable !!!!
How do I stop my microphone from feeding back?
Feedback is caused by a complex interaction between the sound system, the room, and the microphone. Higher quality mics generally exhibit better feedback rejection. Directional mics such as Cardioid or Super Cardioid types are much better than Omni Directional types.
The following rules of thumb generally apply: (a) Try to keep mics behind loudspeaker locations. (b) Try to minimise talker to microphone distance and maximise mic to loudspeaker distance. (c) Only use as much mixer channel gain as required.
Equalisers and Digital Feedback Reducers when used properly can also help.
How to maintain the sound quality of your microphone?
Dust, perspiration, lipstick, moisture, etc. gradually but dramatically deteriorate any microphone's sound. However, all you need to do to maintain excellent sound for many years is clean your mic regularly. Use (a) methylated spirits or alcohol to clean the microphone body and the wire-mesh cap. (b) 70% isopropyl alcohol or soap suds to wash the windscreen and allow it to dry overnight.
What is the difference between cardioid and omni-directional microphone?
Whether you think of it as a mic’s directionality or it’s pickup or polar pattern, there are two basic types you need to understand – omnidirectional (all directions) and unidirectional (one direction).
This is the symbol associated with an omnidirectional mic. It can’t be aimed to isolate one area.
Best to use for: High fidelity recording or broadcast. Not generally used for sound reinforcement due to feedback limitations.
This is the symbol most often associated with unidirectional microphones. These are sensitive to sound coming from only one direction. The most common type of unidirectional microphone is called a “cardioid” because its pickup pattern is heart-shaped. It picks up most sound from the front of the microphone, less from the sides, and very little from the rear.
Supercardioid or hypercardioid microphones offer even greater sound isolation through narrower pickup patterns.
Best to use for: Live sound, since these mics isolate sound from one voice or instrument and can be aimed away from loudspeakers to avoid feedback. That’s why unidirectional microphones far outnumber omnidirectional microphones.
What is the differnce between different stage lights?
Stage lighting fixtures or luminaires can be broadly separated into two categories: floodlights, which illuminate a wide area, and spotlights, which produce a narrower controlled beam. A typical lighting rig will incorporate a combination of these fixture types to perform specific tasks. Some of the common types include:
Spots
- Ellipsoidal or Profile spots use convex lenses and having a gate at their focal point which enables the insertion of gobos or patterns which are then projected on the stage.
- Profiles deliver a hard edged beam and are generally used to highlight specific acting areas on the stage. Profiles are available in fixed angle orvariable (Zoom) formats.
- Follow Spots are manually operated high power ellipsoidal spotlights used to highlight and follow performers on stage.
- Fresnels are fixtures giving a varying sized, soft edged pool of light used to provide even coverage across the stage. Their name derives from their distinctive ridged Fresnel lens.
- Pebble Convex (or PC’s) are similar to Fresnels but use a plano convex lens with a stippled (or pebbled) surface on the planar side which maintains a soft edge whilst reducing excessive spill.
Floods
- Cyclorama fittings are asymetrical reflector flood lights used to throw an even light coverage over scenery or cyclorama’s from relatively close range. They may be single or multi-celled units as desired.
- PAR (Parabolic Aluminised Reflector) luminaires are used for high intensity fixed angle flood applications. Inter-changeable lenses (or lamps in the case of the common parcan) provide for a variety of mostly elliptical beam shapes. They are relatively low cost and coupled with high output, make them a popular choice for general lighting applications.
