David Ross – Group 2 – Sound Reinforcement 2 – OC1
A sound reinforcement system is usually made up of microphones, amplifiers, loudspeakers and controls to mix the signals from the mics and other input sources. The purpose of this system is to reproduce audio to everyone who is in the area/venue, delivering a clear audio image, even to people who aren’t close to the sound source, giving the impression that they are close. The goal is to achieve this and to reproduce album quality sound to all areas of the venue. Effects such as delay and reverb are often used to achieve this.
In order to have all signals sent to the desk (usually placed near the rear of the venue) to be mixed, multi-core technology is often used. This is traditionally analogue technology which consists of having a large diameter cable, containing multiple copper wires. Each wire contains the signal from a different sound source. The sound source is connected to a stage box, from which, the multi-core is then sent to the desk where each source is individually split and plugged in to mic pre-amps or line inputs (shown below).
The digital age has brought about new technology which does the same job, but perhaps to a more efficient degree. Converting the analogue signals into a digital format at the stage box (also pre-amped there), the digital signal is then transferred through the digital multicore cable to a mixing console module and then to the mixing desk. The benefits of going digital are that the cable can be hundreds of metres long without any signal degradation, which comes with the analogue system. There are no chances of interference from the digital multicore and there is no hum/buzz that often effects an analogue system. The cable is also much lighter and thinner eliminating the bulk of the analogue cable, and making transportation a lot easier. The installation cost of a digital multicore is far cheaper and easier because of the lightweight, small cable. However, analogue multicore technology is still more commonly used and the initial cost is significantly cheaper.
Like multicores, mixing desks also come in the digital and analogue variety. Whilst both analogue and digital contain a common number of features, they also can differ greatly. They both typically have a range of faders, pan controls, and eq sections, but a digital desk offers more variation with high-spec channel strips and on-board, integrated effects, of which any number of these can be assigned to any number of channels. They also typically have a four band parametric eq, which is more versatile than most analogue desk eq sections. Settings on a digital desk also can be saved for later, allowing for easy switch between mixes for different bands etc. Digital desks may only have 24 physical channel strips but can contain 96 inputs arranged over 4 layers. Some digital desks, such as the Tascam DM4800 also offer a DAW control function. Signal quality is also pristine with digital desks.
Analogue desks are more common than digital desks and can be easier to set up and go, such as the Yamaha MG. They don’t have the versatility of on-board FX which a digital desk offers, and so outboard gear is usually required. Analogue desks, however are more affordable, compared to their digital counterparts.
Control surfaces differ to both of these in that they are connected to a computer and designed to directly control a DAW. These may look and feel exactly like a mixing desk, with faders and knobs, but they are in fact acting as a second mouse/keyboard. These are used in a situation where a DAW is used to mix.
Factors Affecting Quality of Sound
There are many factors which can effect the way sound behaves in an environment. These factors can affect the quality at which the sound is reproduced. An outdoor environment is free from surfaces and walls and so does not have any reflections of sound but there are still issues that can occur. Wind is a factor which can have an adverse affect. A high velocity wind can push the sound waves in a different projection from what is intended, as shown below:
However there is also the wind gradient effect to consider. This is typically where there is a bottom layer of low velocity wind and an upper layer of high velocity wind. This often occurs when the audience is sheltered by a wind barrier such as a wall. Sound above the barrier is refracted upwards and lost to the audience, where sound below the barrier is refracted down.
Humidity is another factor affecting sound. As sound travels through air, the air absorbs it and attenuates it. The dryer the air, the more acoustic energy is absorbed as dry air is more dense than moist air.
The same concept applies when considering the air temperature. The hotter the air, the faster the sound will be able to travel through it, as hot air is less dense than cold air. The inverse square law states that for every doubling of the distance from the sound source (in free-field conditions), the sound pressure will drop by 6dB. However, humidity and temperature can make this value fluctuate. There is also a gradient effect when considering air temperature as well. When there is a layer of hot air low to the ground and cold above (such as in the evening when the ground is still warm but the air begins to cool), the sound is pushed upwards away from the audience. In comparison, when there is cold air low to the ground and hot air above (such as in the morning when the ground is cool from the night before and the air begins to heat up), the sound is forced downwards and then eventually up again. Below is a diagram which shows this effect:
Sound indoors behave in a different manner because of the surfaces which it comes in contact with. The walls, ceiling and floor of a room are both reflective and absorbent. Every material absorbs sound. The amount of absorption is dependent on its construction and is defined by the material’s absorption co-efficient. This can be anywhere between 0.00 and 1.00 and it is frequency related. A material that has an absorption co-efficient of 0.00 absorbs no acoustic energy and therefore reflects everything. Respectively, a material that has an absorption co-efficient of 1.00 absorbs all acoustic energy and therefore reflects nothing. Reflection is the opposite of absorption. It is still defined be the materials absorption co-efficient. A reflective material has a low co- efficient. For example, glass has a very low absorption co-efficient, so it is a high reflector. Wall reflections can be reduced by angling speakers inward toward the centre of the room or by applying absorbent material panels to the walls. Transmission effects occur when sound passes from one medium into another. Similar to when light passes through glass – the light is refracted and the image is bent. In sound, a similar effect occurs. The speed of the sound is changed and the effected wave may change direction, dependent on the structure of the material it interacts with.
The following diagram shows how sound can be affected when it meets an obstruction.
When sound comes into contact with a smaller obstacle such as a pillar, it bends around the object. This is referred to as diffraction. If the object is small compared to the size of the sound wave, then there is very little or no effect as the waves bend round the object and to the listener, it’s as if it weren’t there. However, if the obstacle is large in comparison to the size of the sounds waves, then it creates a shadow behind it where the sound become quieter. Keeping this in mind, it is safe to say that this has more of an adverse effect on higher frequencies because there wavelengths are smaller and therefor will find it more difficult to bend around the object in question. Below is a diagram demonstrating how this applies to two different sized obstacles:
Standing waves are another issue for sound indoors. These are caused by the reflections of specific frequencies between parallel surfaces (usually a back wall). Near to the surface, the original waves and the reflections are almost in phase so the amplitude is increased here. Respectively, away from the surface, the waves are out of phase and therefore the amplitude is decreased. Some venues prevent this by creating non-parallel surfaces (false walls), or placing diffusers on the surface to break up and scatter sound in many directions.
People absorb sound more than reflect it, so the crowd in a venue is going to absorb a lot of acoustic energy. In order for all audience members, front and back, to hear the sound clearly, it is advised to have the speakers above the crowd and angled down towards them. This will minimise reflections from the ceiling and the back wall and also make sure the people at the back and the very front receive just as good quality sound as the people in the middle. The following diagram shows and ideal angle for this.
Externally and Internally Powered Speakers
There are two main types of loudspeaker. These are powered (or active) and passive. Powered speakers draw their power from a built in amplifier within the speaker unit, whereas passive speakers need an external amplifier to power them as they don’t have one built in.
Powered speakers give a more hassle free installation because there is no need for the external amplifier. There is also no risk of running too much or too little power to the speaker as the built in amp is already set at an optimum power level by the manufacturer when the speaker is constructed. They also tend to offer reduced distortion to their passive counterparts and it is easier to add more enclosures to the system as no calculations are required for amplification requirements. However, because all the components are contained together within one unit, repairs can prove to be more hassle and they also tend to be a lot heavier which can bring up issues with transportation or rigging. Cooling was in the past, also an issue but this problem has reduced as technology has become more advanced, but they can be prone to picking up noise.
With passive speakers, the separate parts mean that repairs and diagnosis of problems are made easier. They weigh less than active speakers so transportation and installation requires less effort. No internal amp also means that cooling is not an issue. They tend to be much cheaper than powered speakers, but this is because the amplifier, cables and crossover have to be purchased separately and so this may drive up the cost and become more expensive. Matching the power of the speakers and the amplifier is essential. Damage to the system will occur if it’s not accurate and it could be rendered useless. The abundance of separate equipment means more cables have to be used and so there is a higher risk of interference. Signal degradation could occur over long cable runs, depending on the types of cable used.
Radio In-ear Monitoring, Microphones and Instrument Interfaces
A more modern way in which artists can monitor their performance, is through in-ear monitoring. It gives them the freedom on stage to move around and wherever they are, still have the perfect monitoring sound. In-ear monitors are built in a 2 part system. The first part is a transmitter, sending the monitor mix (via radio) to a receiver. The receiver is usually a belt pack around the size of a mobile phone that the artist wears. This picks up the signal and amplifies it through a pair of earpieces. When using radio equipment, interference and dropouts can be an issue, and busy wavebands make finding clear frequencies difficult. Modern systems use carrier signals, which lock the receiver to the transmitter’s frequency to get around the interference problem. Dealing with radio signals, dynamic range compression and reduced bandwidth compromise audio quality. However, the technology has become so advanced that it can deliver a stable signal and therefor, high quality sound. Although the sound may not be hi-fi standards, many artists still choose this equipment for the freedom of consistency of sound wherever they move during their performance. The in-ear part of the system varies in price and quality greatly. The cheapest and less effective option is similar to earbud headphones, and the more advanced options are moulded to the artists ear and custom built. This option would be a comfortable fit and reduce background noise of a loud stage. However this means the artist is more reliant on the engineer to deliver them a balanced mix, as they are more detached from the live sound.
Wireless mics and instruments offer the same benefits of movement as in-ear monitoring as they basically cut out the limitations of cabling. They work in a similar way to in-ear monitoring, but in reverse. There are 3 main part to this system. The microphone itself picks up the audio source, a transmitter converts this to a radio signal and sends the signal to a receiver. The transmitter for wireless mics is usually built in to the handle of the mic, making it only slightly larger than a wired mic. Alternatively, in the case of the artist wearing a microphone headset (like at Eurovision for example), the headset is connected to the transmitter via a wire, and the transmitter is usually worn on their belt (like the receiver of an in-ear monitoring system). This would also be the case for wireless instruments such as guitars. The transmitter would be clipped to the players belt or their guitar strap. For instruments like saxophone or trumpet, the transmitter is often attached onto the instrument itself.
The receiver picks up the signal of the transmitter and converts it from a radio signal back to audio. The output of the receiver is identical to the electrical output of a standard mic or instrument signal. It is connected to the amplifier or mixing console in the usual input. Single antenna receivers use one receiving antenna and one tuner, similar to an FM radio. With these receivers there is a chance of momentary interruptions or dropouts in the signal as the artist holding the transmitter moves around the stage. A diversity receiver uses two separate antennas spaced a short distance apart and usually two separate tuners. A circuit in the receiver selects the better of the two signals, or in some cases blends them both. One of the antennas will almost certainly be receiving a clean signal at any given moment, so the chances of dropouts or interference are reduced. Most receivers require an AC power supply but some are able to use battery power. All transmitters require battery power.