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Sound Acoustics

By lesliechipango22 Mar 25, 2013 2083 Words
Indoor & Outdoor Acoustic Sounds

Indoor Acoustics
The Principles of Sound and Acoustics
Sound is the apparent vibration of air resulting from the vibration of a sound source (e.g. guitar sound board, hair dryer, etc). We can describe such regular vibration in terms of the sum of simpler vibrations (harmonics). In other words any periodic oscillation and hence resulting waveform can be described in terms of the sum of its harmonics. Each harmonic being a simple sine wave (often called a pure tone) with it’s own respective frequency and amplitude.

Studio-based Acoustics
When you're trying to set up a studio on a limited budget, it's all too easy to concentrate on buying equipment rather than spending your hard-earned cash on things that don't make a sound. A little money spent treating the room in which your studio is based, however, can often be a better investment. A lot of people find out too late that the acoustics of their chosen room cause problems, either by colouring their recordings, distorting their monitoring perspective or leaking sound.

Live Room
The typical recording studio consists of a room called the "studio" or "live room", where instrumentalists and vocalists perform; and the "control room", which houses the professional audio equipment for either analogue or digital recording, routing and manipulating the sound. Often, there will be smaller rooms called "isolation booths" present to accommodate loud instruments such as drums or electric guitar, to keep these sounds from being audible to the microphones that are capturing the sounds from other instruments, or to provide "drier" rooms for recording vocals or quieter acoustic instruments.

Dead Room
A lot of studios pride themselves in having a ‘dead’ room. What does this that mean? * It’s free and clear of ambient noise
* It has enough treatments in it to soak up any sound made in the room (so as not to hear an echo). When recording at home, sometimes it’s hard to get a dead room. Heating/AC vents, windows, neighbors, etc. all contribute to those ambient noises that you’re trying to keep out of your recording. You can also be the culprit with loose clothing, watches, a squeaky chair, computer fan, and even things like moving papers around.

Surface types and properties;
When a sound wave meets an obstacle, some of the sound is reflected back from the front surface and some of the sound passes into the obstacle material, where it is absorbed or transmitted through the material. Reflection and absorption are dependent on the wavelength of the sound. The percentage of the sound transmitted through an obstacle depends on how much sound is reflected and how much is absorbed. We are assuming that the obstacle is relatively large, such that no sound passes around the edges. When a sound wave in air reaches the surface of another material, some of the sound is reflected off the surface, while the rest of it goes into the material. For example, when sound hits a wall, some is reflected and some passes into the wall. If the surface that the sound wave hits is relatively smooth, more sound will be reflected than if the surface is rough. For example, more sound will be reflected from a smooth wall made of mud than a pile of dirt. The reason is that the rough or porous surface allows for many internal reflections, resulting in more absorption and less reflection. Some materials absorb sound more than others. Drapes and ceiling tiles are used to absorb unwanted sound and eliminate echoes. Music recording studios use sound absorbing materials on their walls to eliminate any undesired or outside sounds, when recording a song.

Reverberation is the persistence of sound in a particular space after the original sound is removed. Originally, the studio recordings were performed through the microphone quite remote from the sound source. This microphone, like the human ear, received the sound waves reflected from the walls of the studio room, and enrolled it into the recording. In the way the natural reverb of the studio was preserved in the recording. The effect was even more pronounced, when they began to use several microphones, respectively mixed with each other, this way even better results could be achieved. Sound proofing;

Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinomies sound generators. Since the early 1970s, it has become common practice in the United States and other industrialized countries to engineer noise barriers along major highways to protect adjacent residents from intruding roadway noise. The technology exists to predict accurately the optimum geometry for the noise barrier design. Noise barriers may be constructed of wood, masonry, earth or a combination thereof. An anechoic chamber (an-echoic meaning non-echoing or echo-free) is a room designed to stop reflections of either sound or electromagnetic waves. They are also insulated from exterior sources of noise. The combination of both aspects means they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results. Anechoic chambers were originally used in the context of acoustics (sound waves) to minimize the reflections of a room. More recently, rooms designed to reduce reflection and external noises in radio frequencies have been used to test antennas, radars, or electromagnetic interference.

Outdoor Acoustics
Sound bites; background atmosphere;
A sound bite is a short clip of speech or music extracted from a longer piece of audio. It is often used to promote or exemplify the full length piece. It may also be abbreviated as SOT. Before the actual term "sound bite" had been coined, Mark Twain described the concept as "a minimum of sound to a maximum of sense." It is characterized by a short phrase or sentence that deftly captures the essence of what the speaker is trying to say. Such key moments in dialogue (or monologue) stand out more strongly in the audience's memory and thus become the best "taste" of the larger message or conversation. As the context of what is being said is missing, the insertion of sound bites into news broadcasts or documentaries is open to manipulation and thus requires a very high degree of journalistic ethics. According to the Code of Ethics of the Society of Professional Journalists, journalists should "make certain that headlines, news teases and promotional material, photos, video, audio, graphics, sound bites and quotations do not misrepresent. They should not oversimplify or highlight incidents out of context." In filmmaking and television production presence (or room tone) is the "silence" recorded at a location or space when no dialogue is spoken. This term is often confused with ambience. Every location has a distinct presence created by the position of the microphone in relation to the space boundaries. A microphone placed in two different locations of the same room will produce two different presences. This is because of the unique spatial relationship between the microphone and boundaries such as walls, ceiling, floor and other objects in the room. Presence is recorded during the production stage of filmmaking. It is used to help create the film sound track, where presence may be intercut with dialogue to smooth out any sound edit points. The sound track "going dead" would be perceived by the audience not as silence, but as a failure of the sound system. For this reason presence is normally recorded - like dialogue - in mono, with the microphone in the same position and orientation as the original dialogue recording. In the sound edit, presence occupies the same track as the dialogue to which it applies.

Unwanted noise;
A Noise Gate or gate is an electronic device or software that is used to control the volume of an audio signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is 'open'. If the signal falls below the threshold no signal is allowed to pass (or the signal is substantially attenuated): the gate is 'closed'. A noise gate is used when the level of the 'signal' is above the level of the 'noise'. The threshold is set above the level of the 'noise' and so when there is no 'signal' the gate is closed. A noise gate does not remove noise from the signal. When the gate is open both the signal and the noise will pass through. They are commonly used in the recording studio and sound reinforcement. Rock musicians may also use small portable units to control unwanted noise from their amplification systems. Band-limited noise gates are also used to eliminate background noise from audio recordings by eliminating frequency bands that contain only static.

Unwanted Ambience;
As you edit dialogue, you’ll often need to cut out pieces of audio that you don’t want in the sequence. For example, the director may have given directions in between an actor’s lines, or the sound recordist might have bumped into something while shooting on location for a documentary. As long as there’s no dialogue happening at the same time, it’s pretty easy to cut out unwanted sounds. If you simply delete the sound, however, you’ll be left with a gap in your audio that sounds artificial. Since there’s always a low level of background noise, known as room tone, in any recording, a moment of complete silence is jarring. In order to edit out unwanted sections of audio without creating obvious gaps, it’s common practice to record a certain amount of room tone during a shoot. The recordist simply has everyone stand quietly for thirty seconds or so, and records the ambient sound of the room. If you’ve recorded some room tone during your shoot, you can capture it so that, as you edit, you have a long piece of “silence” that you can edit in whenever you need to cover a gap in the location audio. If, for some reason, room tone was not captured for a particular scene, but you have a gap you need to fill, you can try to copy a section from another clip in the same scene that has a pause in the dialogue, and paste it to fill the gap. If you have no pauses that are long enough to cover your gap, you can try to copy and paste a short pause multiple times. But there’s a chance that it will end up sounding like a loop, which will be too noticeable. In this case, you can use the following method to obtain a long section of room tone from a short copied pause in the dialogue.

Wind noise (Refer to your location recordings and essay)
Recorded wind noise is a blatant sign of an amateur. With just a little know-how, it's entirely possible to achieve good quality audio recordings, even with consumer-quality equipment. To prevent wind noise, all you need to do is prevent wind from striking the microphone's sound element. The black foam windscreens that come with most microphones helps to reduce noise caused by mild air movement indoors. Foam windscreens are made from special open-cell foam. "Open cell" means you can blow air through the foam's cells. These cells create an airspace around the microphone, dampening the effects of air movement, while still allowing sound waves to reach the mics sound element. Foam windscreens are usually fine for use in barely perceivable wind, and they certainly do a great job of protecting your microphone from damage, but the surface of a foam screen is just too dense to adequately dampen the strike of wind against your microphone the way a fur windscreen does.

Fur windscreens
Fur windscreens, when used over a foam windscreen, provide maximum wind noise reduction. A fur windscreen's synthetic hairs, being a soft and pliable material, soften the wind's effect on the mic. The fur breaks up the wind and dampens (quiets) it. Think of striking a solid surface, like a door, with your hand. It would be fairly loud, almost drum-like sound. Now imagine knocking on someone's door that is covered in a soft plush fabric like the synthetic fur. Instead of a reverberating beat, your knock would be silenced by the sound absorbing outer layer of fur.

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