SOUND SYSTEM THEORY
Connections:
Sound: Periodic changes in air pressure. Sound requires changes from low to normal to high to be perceived as a sound wave.
Audio signal: Changes in voltage that represent sound.
Transducer: Changes one form of energy into another corresponding form.
Transducers:
- Microphones convert acoustical energy into electrical energy.
- Multitrack: Record and playback heads convert signal to and from magnetic energy for storage.
Speakers: Convert signal back to sound, electrical energy back to acoustical energy.
Recording multiple performers:
When using a single mic, loudest performers will have to be placed at a greater distance from the mic in order to lower their level in the mix.
The greater the distance from the sound source, the more high frequency detail is lost, and the more ambiance (reflected sound or reverb) is emphasized.
Something that can be done to improve control when recording several players at once is to use several mics, baffles, placement of musicians, and to use a mixing console with several inputs.
Acoustic instruments:
Produce sound through physical vibration.
Electric instruments:
Use a pickup to convert vibration of strings in a magnetic field into signal
Electronic instruments:
Produce signal only, using an electronic tone generating circuit and require an amplifier or speaker to be heard.
Mic preamp:
Boosts mic level signal to line level signal. Electronic instruments, as well as tape machine run at or close to line level.
Connecting line level devices such as tape recorder outputs, effects devices and some electronic instruments to mic inputs will cause a problem. The mic preamp will distort. What is needed is anpther input on each channel that accepts line level.
Any audio device can distort. We use a line level input when the instrument is electronic and no coloration from an amp or speaker is desired.
Electric instruments direct tend to be more accurate in reproducing their full dynamic range. Because of their low voltage, they would probably be connected to the mic input.
The cables that array our signal, which is electrical energy, are made of wire. They can also pick up variations in electro-magnetic energy creating unwanted noise.
Cable length is important because the longer the cable, the more noise it will pick up.
When connecting electric basses and guitars directly to the console, we may have two problems:
- Noise pickup
- Impedance mismatch
Unbalanced system (semi-pro): An inexpensive system designed for cable runs under fifteen feet. Most electric or electronic instruments are unbalanced.
Balanced system (pro): Developed to minimize noise pickup and can be used with cable runs up to 1000 feet. Most pro recording equipment is balanced.
Because of the multitude of routing and re-routing involved, balanced systems are better equipped to handle the accumulation of noise.
Two types of electricity:
- Alternating current (AC): Alternates between positive and negative voltage.
- Direct current (DC): Either negative or positive but never changing.
Audio signal is AC. It represents changes in air pressure to changes in voltage.
Impedance:
Resistance and reactance in an AC system.
All substances resist flow of current to a different degree. The amount they resist is called “resistance”. The resistance and impedance are measured in ohms. Written as Z
Musical instruments such as guitars and basses are generally in Z. Guitar pickups are high Z because of the wire and magnets used, size and number of turn of wire coiled around the magnets. Pro recording systems are low impedance to minimize high frequency loss.
If we connect equipment whose impedances do not properly match, loss of level and tonal change occurs.
Professional mics range from 50-250 ohms.
Mic preamps may have an ohm rating of 3000.
Line outputs on some equipment (especially older devices) are around 600 ohms.
Line inputs are usually 10 ohms.
Impedance mismatch: A direct box or DI is used to convert hi Z to lo Z and balanced to unbalanced.
Two types of DI:
Passive:
Requires no external power, uses a transformer.
Rugged and durable.
Eliminates ground loops well.
Lots of level.
If cheap transformers are used, tonal change occurs.
Active:
Requires battery power, AC power or phantom power.
More expensive.
Does not isolate ground loops well.
No signal loss.
May offer other options such as gain control, tone control, or tube coloration.
Electrical ground: Third pin in electrical outlet is connected to the earth, either by a cable from the breaker box to a copper rod driven into the ground, or by connection to a cold water pipe.
Ground loop:
Electrical current changes at 60 cycles per second hum.
60cycle hum produced by voltage flow through the electrical ground of two or more components.
May be caused by devices who’s signal lines are connected to two circuits.
Ground lift switch:
Breaks ground connection on signal lines.
A direct box may have a ground lift switch to break the grounding connection from the instrument to the console.
Only use the lift when necessary.
The DI has both a lo Z balanced out and a hi Z unbalanced out. This allows for connection of the instrument to the mixer and instrument amplifier simultaneously. Generally it is sent to a mic preamp for added boost.
In order to balanced, the output device must be balanced (or a DI used), the input device must be balanced, the cable connection must be balanced.
How a balanced system works:
- Balanced: Two conductors.
- Unbalanced: One conductor.
EX: If signal is at +2 volts and the noise floor is at +.5 volts, the value of the noise is +2.5 volts.
In a balanced system, the output is split into equal but opposite voltages by the mic and output on both conductors.
EX: If signal on each is +7 volt, both carry that plus noise on each conductor. The noise is picked up by both conductors at equal level AND polarity. Note that the cable did not reverse polarity.
Cable carries signal only. Combining the two signals would cancel the signal out, so before this is done we flip the polarity on the cold conductor.
At a balanced input the cold conductor’s signal is polarity reversed and then combined with the hot conductor. The noise has already been polarity reversed when the both conductors are combined. The noise gets cancelled. Combining the noise would cancel each other out, while the signal adds.
Ex: +.5v (+)-.5v = 0v
Reversing the signal down the cable then picking up noise, then reversing again and canceling out the noise.
Phase cancellation: Two signals of different polarity will cancel when combined fully or partially.
Snakes or multipair cables: Several shielded pairs of cable sharing the same jacket. These tend to be more cost effective and smaller than separate cables. These are often used when several signals are to be sent to the same destination from a common location in the studio.
Multi pin connectors: Often used when snakes are utilized. One snake may use a single multi pin connector instead of many single connectors.
Amplifiers: Amplifies voltage, boosts signal.
Power amp: A power amp raises a line level signal to speaker level.
Two categories of power amp:
- Vacuum tube or valve
- Transistor or solid state
Tube amps are warmer in the midrange and add a desired musical coloration.
Since the amp and speakers output does not go to tape, any desired coloration of the signal should occur before the output of the stereo buss, during recording or mixdown. The power amp should be as accurate as possible and not color the sound the way a guitar amp does. We want an accurate picture of what is being recorded and should not allow the amplifier to distort, regardless of its type.
Differences in power amps:
Valve:
Tubes
More expensive
Tubes wear out
Tubes are costly to replace
Distorts gradually
Transistor:
Solid state
Less expensive
Rugged
Harsh distortion
Runs cooler
Vacuum tubes emphasize harmonics that are pleasing to the ear. Vacuum tubes are most useful in equipment that affects the signal being recorded.
Solid state amps are most often used in the studio as they offer the best combination of cost, performance and reliability.
