Live Sound Reinforcement

Live Sound Reinforcement
Speakers, Amps and Crossovers

Speakers can be classified under two basic design types; Direct radiators Acoustical horns Speakers can be a combination of these two as well.

A speaker is a transducer that takes electrical signals and converts them into sound waves. All speakers involve a driver, which accomplishes the actual transduction of electrical signals into sound waves. Speaker drivers include a diaphragm that moves back and forth to create pressure waves in the air column in front, and depending on the application, at some angle to the sides.

The diaphragm is typically in the shape of a cone for low and mid frequencies or a dome for higher frequencies, and is usually made of coated or uncoated paper or polypropylene plastic.

All speaker drivers have a means of electrically inducing back-and-forth motion. Typically there is a tightly wound coil of insulated wire (known as a voice coil) attached to the neck of the driver's cone.

This cone, dome or other radiator is mounted to a rigid chassis which supports a permanent magnet in close proximity to the voice coil.

For the sake of efficiency the relatively lightweight voice coil and cone are the moving parts of the driver, whereas the much heavier magnet remains stationary. Other typical components are a spider or damper, used as the rear suspension element, simple terminals or binding posts to connect the audio signal.

Here is a cut away view of the components that comprise a speaker driver.

All speakers drivers are put into an enclosure to actually create a forward sound pressure, without the enclosure the speaker would have a cancelling affect from the energy in the back of the speaker.

At low frequency the speaker driver will flop out of control due to the acoustical load and the enclosure helps to eliminate this. Enclosures can be ported/vented or sealed, sealed enclosure are the most common and sub woofer enclosure are typically vented.

It is possible to reduce the enclosure size and still gain extra low frequency response by adding a vent or a port to the enclosure. PORTS

Direct radiators can be classified as tweeters, mid range or woofers and today we have sub woofers like those used in surround sound systems. HIGHS 2KHz-20KHz MIDS 200Hz-2000Hz LOWS 20Hz-200Hz 1st DECADE 2ND DECADE 3RD DECADE

A combination of these three drivers in one cabinet provides a full range speaker, some two speaker designs also fall under the full range category as well.

Speakers can be active or passive. What is the difference between an active speaker as opposed to a passive speaker?

Active speakers require power and have a built in amplifier where as passive speakers are not connected to a power source. Computer speakers are a good example of active speakers.

It should be noted that you NEVER plug active speakers into a power amplifier, it will blow the amplifier circuit in the active speakers and render them inoperative. Another term that is associated with speakers are crossovers, which can also be of the active, passive or a hybrid variety.

Passive crossovers are built into full range speakers and can be found in the speaker enclosure. AS YOU CAN SEE A PASSIVE CROSSOVER CONSISTS OF A RESISTOR, CAPACITOR AND INDUCTOR TO FILTER THE DIFFERENT FREQUENCIES.

Passive crossovers can be added to a set of different speakers to accomplish the same thing, divide the highs, mids and lows of the audio spectrum.

Active crossovers require power and are usually a separate piece of equipment that is added to live sound system.

Hooking up an active crossover. POWER AMP IN

Here is an example of a hybrid crossover system, the active crossover separates highs and lows and the passive crossover in the enclosure separates the highs from the mid range frequencies.

We’ve covered the basics of loud speakers and now we will take a look at horns and horn designs. Horns provide directional patterns that are better suited for large live sound applications such as concerts and other large venues. Horns are also more efficient than direct radiators.

Basic horn designs have a driver and narrow channel that flares out, this allows the driver to be much smaller as the sound waves achieve a higher level of sound pressure before leaving the mouth of the horn. HIGH FREQUENCY HORN

Not all horns have the distinctive visible flute (narrow channel), enclosures for horn type speakers can be used to fold the horn portion in such a way that they will fit into traditional enclosures. FOLDED HORNS IN AN ENCLOSURE

Folded horns utilize baffle designs to fold the sound waves through the enclosure thus minimizing the horn length and achieving the same frequency response, these designs are better suited for low frequency applications but can work for the mids and high frequencies too. The baffles comprise the throat or flute of a horn design

There are four basic horn types: Round exponential horn Straight exponential horn Multi cell horns Radial horns

Round exponential horns are primarily used as high frequency horns with a cone shaped pattern that are a well suited as stage monitors.

Straight exponential horns have a directional pattern that is rectangular and can reproduce most of the audio spectrum fairly well with exception of the higher frequencies.

Multi-cell horns consist of a series of exponential horns arranged in the multi-cell configuration with a common throat leading from the driver and is usually part of a permanent system.

Radial horns sometimes referred to as sectoral horns have a exponential curve rotated through a given horizontal angle that can provide better dispersion of high frequency sounds.

Horns provide directional patterns and sound dispersion to either keep the output from spreading out to far or to disperse the sound over a large area. Wide dispersion throughout a speaker’s bandwidth is difficult to achieve. Most high-frequency and bass/midrange drive units have good dispersion at the lower limits of their frequency range, but they start to beam as they reach their upper-frequency limits.

With high-performance high-frequency drivers beaming occurs beyond audibility, beaming from midrange drivers, however, occurs within the audible range. Speakers with beaming problems will not sound the same in all areas of a room, they may sound balanced in one area, but nasal, dull, or even harsh and shrill in other areas.

Midrange beaming can be reduced by lowering the crossover frequency. The high-frequency driver’s lower range will then provide wider dispersion and the bass/midrange driver’s output can be rolled off before its dispersion narrows.

Another type of speaker used in live sound is the monitor or stage monitor. This is a wedge shaped speaker that is placed in front of the performers during a live performance.

The speakers are directed at the performers in order that they may hear each other and be able to evaluate their music as they make it.

In order to drive the output signal to the speakers we need to use power amplifiers, as you know from a previous lesson amps make an electrical copy of an input signal and output it as a stronger signal. Amplifiers used in live sound will either increase the voltage of the signal or the power of an output signal.

For live sound we are primarily interested in power amplifiers to drive our speakers so we are also interested in the wattage. The majority of power amplifiers quote their output specifications into 16Ω, 8Ω and 4Ω impedance. The power level of an amplifier is stated with reference to the distortion level and the frequency range.

The most common form of distortion from an amplifier is harmonic distortion. Harmonics are integral multiples of the frequency of a fundamental note, for a given frequency its harmonic values result from multiples of whole numbers. For example, the harmonics of a 1KHz sine wave occur at 2KHz, 3KHz and so forth.

Harmonic distortions result when an amplifier creates harmonics for a wave form where they don’t exist. Harmonic distortion causes clipping which can result in damage to mid range and high range speaker systems by overpowering them.

Here is an example of what harmonic distortion looks like, the red lines are harmonics of the green sine wave and represent an unwanted signal that is unintened.

Live Sound Reinforcement

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