Indoor AcousticsPrinciples of Sound and AcousticsSound is a mechanical wave that is an oscillation of pressure transmitted througha solid, liquid, or gas, composed of frequencies within the range of hearing and ofa level sufficiently strong to be heard, or the sensation stimulated in organs ofhearing by such vibrations. Acoustics is the interdisciplinary science that deals withthe study of all mechanical waves in gases, liquids, and solids including vibration,sound, ultrasound and infrasound. A scientist who works in the field of acoustics isan acoustician while someone working in the field of acoustics technology may becalled an acoustical or audio engineer. The application of acoustics can be seen inalmost all aspects of modern society with the most obvious being the audio andnoise control industries.Studio Based AcousticsWhen youre trying to set up a studio on a limited budget, its all too easy toconcentrate on buying equipment rather than spending your hard-earned cash onthings that dont make a sound. A little money spent treating the room in which yourstudio is based, however, can often be a better investment. A lot of people find outtoo late that the acoustics of their chosen room cause problems, either by colouringtheir recordings, distorting their monitoring perspective or leaking sound and thusdisturbing their neighbours.Acoustic treatment, in the context of a recording studio, generally deals with theacoustic quality of the room from a listeners point of view. In other words, if youmonitor in a control room that has been designed using the correct acoustictreatment, what you hear is likely to be more accurate than the same recordingplayed back over the same speakers in an untreated room.Live RoomThe typical recording studio consists of a room called the "studio" or "live room",where instrumentalists and vocalists perform; and the "control room", which housesthe professional audio equipment for either analogue or digital recording, routingand manipulating the sound. Often, there will be smaller rooms called "isolationbooths" present to accommodate loud instruments such as drums or electric guitar,to keep these sounds from being audible to the microphones that are capturing thesounds from other instruments, or to provide "drier" rooms for recording vocals orquieter acoustic instruments.
Dead RoomRooms that have very little or no echo/reverberation of their own are considered tobe “dead”. The ultimate dead room is what is called an Anechoic Chamber. It is aspecial structure built in a way that it produces no echoes whatsoever. These areusually made for scientific/measurement purposes. You can make a quick dead-room by placing a microphone inside a closet filled with clothes/sheets. I have heardof some podcasters using a kid’s play tent covered with blankets to achieve this aswell. Even some cars make great dead-rooms to record in. The sound of a room thatis completely dead is very unnatural sounding. If you have ever been inside of ananechoic chamber, you can hear yourself breathing, possibly a soft ringing noisefrom your ears, and when you talk, your voice sounds very tiny and strange. Whenyou record in a closet, car, or tent, you will notice that your recordings sound kind offlat and muted.Surface TypesWhen a sound wave meets an obstacle, some of the sound is reflected back from thefront surface and some of the sound passes into the obstacle material, where it isabsorbed or transmitted through the material. Reflection and absorption aredependent on the wavelength of the sound. The percentage of the soundtransmitted through an obstacle depends on how much sound is reflected and howmuch is absorbed. We are assuming that the obstacle is relatively large, such that nosound passes around the edges.When a sound wave in air reaches the surface of another material, some of thesound is reflected off the surface, while the rest of it goes into the material. Forexample, when sound hits a wall, some is reflected and some passes into the wall.When sound reflects off a smooth flat surface, an echo or reproduction of the soundcan be heard. Echoes are more noticeable if the surface is far enough away to allowfor a time-lag between when the sound is made and when it is hear.
ReverberationReverberation is the persistence of sound in a particular space after the originalsound is produced. A reverberation, or reverb, is created when a sound is producedin an enclosed space causing a large number of echoes to build up and then slowlydecay as the walls and air absorb the sound. This is most noticeable when the soundsource stops but the reflections continue, decreasing in amplitude, until they can nolonger be heard. The length of this sound decay, or reverberation time, receivesspecial consideration in the architectural design of large chambers, which need tohave specific reverberation times to achieve optimum performance for theirintended activity.Sound ProofingSoundproofing is any means of reducing the sound pressure with respect to aspecified sound source and receptor. There are several basic approaches to reducingsound: increasing the distance between source and receiver, using noise barriers toreflect or absorb the energy of the sound waves, using damping structures such assound baffles, or using active antinoise sound generators.A room within a room (RWAR) is one method of isolating sound and stopping it fromtransmitting to the outside world where it may be undesirable.Most vibration / sound transfer from a room to the outside occurs throughmechanical means. The vibration passes directly through the brick, woodwork andother solid structural elements. When it meets with an element such as a wall,ceiling, floor or window, which acts as a sounding board, the vibration is amplifiedand heard in the second space. A mechanical transmission is much faster, moreefficient and may be more readily amplified than an airborne transmission of thesame initial strength.The use of acoustic foam and other absorbent means is less effective against thistransmitted vibration. The user is advised to break the connection between theroom that contains the noise source and the outside world. This is called acoustic de-coupling. Ideal de-coupling involves eliminating vibration transfer in both solidmaterials and in the air, so air-flow into the room is often controlled. This has safetyimplications, for example proper ventilation must be assured and gas heaters cannotbe used inside de-coupled space.
Outdoor acousticsSound bitesA 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 beabbreviated as SOT for sound on tape.Before the actual term "sound bite" had been invented, Mark Twain described theconcept as "a minimum of sound to a maximum of sense." It is characterized by ashort phrase or sentence that deftly captures the principle of what the speaker istrying to say. Such key moments in dialogue (or monologue) stand out more stronglyin the audiences memory and consequently become the best "taste" of the largermessage or conversation.As the context of what is being said is missing, the insertion of sound bites into newsbroadcasts or documentaries is open to manipulation and therefore requires a veryhigh degree of journalistic ethics. According to the Code of Ethics of the Society ofProfessional Journalists, journalists should "make certain that headlines, news teasesand promotional material, photos, video, audio, graphics, sound bites andquotations do not misrepresent. They should not oversimplify or highlight incidentsout of context.Background NoiseIn acoustics and specifically in acoustical engineering, background noise or ambientnoise is any sound other than the sound being monitored (primary sound).Background noise is a form of noise pollution or interference. Background noise is animportant concept in setting noise regulations. See noise criteria for cinema/homecinema applications.Examples of background noises are environmental noises such as waves, trafficnoise, alarms, people talking, bioacoustic noise from animals or birds andmechanical noise from devices such as refrigerators or air conditioning, powersupplies or motors.The prevention or reduction of background noise is important in the field of active
noise control. It is an important consideration with the use of ultrasound (e.g. formedical diagnosis or imaging), sonar and sound reproduction.Unwanted SoundA Noise Gate or gate is an electronic device or software that is used to control thevolume of an audio signal. In its most simple form, a noise gate allows a signal topass through only when it is above a set threshold: the gate is open. If the signalfalls below the threshold no signal is allowed to pass (or the signal is substantiallyattenuated): the gate is closed. A noise gate is used when the level of the signal isabove the level of the noise. The threshold is set above the level of the noise andso when there is no signal the gate is closed. A noise gate does not remove noisefrom the signal. When the gate is open both the signal and the noise will passthrough.The Attack, Hold, and Release functions of a noise gate.They are commonly used in the recording studio and sound reinforcement. Rockmusicians may also use small portable units to control unwanted noise from theiramplification systems. Band-limited noise gates are also used to eliminatebackground noise from audio recordings by eliminating frequency bands thatcontain only static.In audio post-processing, noise gating reduces steady noise sources such as rumblefrom LP records, hiss from audio tape, static from a radio or amplifier, and hum froma power system, without greatly affecting the source sound. An audio signal such asmusic or speech is broken up into many frequency bands by a collection ofoverlapping band-pass filters, and if the signal amplitude in any one band is lowerthan a preset threshold then that band is eliminated from the final sound. Thisgreatly reduces perceptible background noise because only the frequencycomponents of the noise that are within the gated pass bands survive.Unwanted Ambience
If you are building a studio on a budget, one of the most expensive things,depending on the room you are working in, can be sound dampening. You can’t sendoff a professional sounding voiceover with ambient noise cluttering the background.You’ve got to get rid of it.Fortunately, there are many different possible answers for you, ranging from veryexpensive to virtually no cost at all. Don’t jump and buy a bunch of pricey foamsquares until you have considered all the options:Isolation panels:These can range from £50 up to multiplehundreds, depending on your budget andthe level of isolation you are aiming for.There are several different kinds and makesof the isolation panels but as long as itworks that’s all that matters. You may findthat one is simply not enough, if theambient noise in your studio is above acertain level, but it will definitely help.The Harlan Hogan Porta-booth: This is a clever little contraption which, while helping your home studio, also doubles as a portable sound booth! Excellent for those who are “on the go”, working from laptops and hotel rooms. It’s conservatively priced, and is an excellent answer to getting rid of unwanted ambient noise.If you are on an even tighter budget, consider carpet remnants. This is a veryinexpensive way to get that quiet that your mic needs to perform it’s best. A creative
mind can come up with a way to wrap a carpet remnant around the back of yourmicrophone, which will significantly reduce ambient noise. You can also use carpetremnants hung on your walls to dampen sound.Recording out of doors obviously involves working with ambient noise all around;thats often why were recording outside in the first place. Stereo recordings capturea broad sound-stage area, with all the appropriate ambience in the appropriatethree-dimensional positions. However, if you are looking to record an individual,isolated sound you can have a problem. You cant ask the birds to stop singing, or getthe traffic on the road to stop for 10 minutes while you try to record the sound of aquiet mouse, for example! So the only option is to use very directional microphonesand work in mono, to try to exclude as much ambient sound as possible. You canthen mix your isolated mono sound with a separate stereo ambience to create apleasing ensemble.Wind NoiseRecording outside presents a whole different range of problems and challenges thanworking in the studio. The basic concepts and practices of mic placement areunaffected; you still need to position a mic where it can hear the best balance ofsound emitted from the source, and where the rejection null (or nulls) of its polarpattern reject the most unwanted noise. But perhaps the most obvious problemwith working outdoors is that of wind noise, and its crucial to take measures to keepthe air currents caused by wind away from the mic capsule. Simple foam windshieldscan help, but only to a limited degree. If you look at the setups employed byprofessional TV and film sound recordists, you will usually see them working withmicrophones mounted inside large zeppelin-shaped devices, often covered with fakefur: the proverbial dog on a stick.The main basket of the Rycote system is designed to set up an area of still air aroundthe microphone capsule, and the larger this volume of still air, the better the wind-noise attenuation will be, which is why effective windshields are inherently largethings. Moving air has to flow around the basket, keeping turbulence well away fromthe microphone and its mountings. However, pressure changes, including thosecaused by sound waves, still affect the pressure of the internal still air, and so themicrophone still hears the sound, without the rushing wind noise that wouldpreviously have rendered any sound unusable.