1. Indoor Acoustics
Principles of Sound and Acoustics
Sound is a mechanical wave that is an oscillation of pressure transmitted through
a solid, liquid, or gas, composed of frequencies within the range of hearing and of
a level sufficiently strong to be heard, or the sensation stimulated in organs of
hearing by such vibrations. Acoustics is the interdisciplinary science that deals with
the 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 is
an acoustician while someone working in the field of acoustics technology may be
called an acoustical or audio engineer. The application of acoustics can be seen in
almost all aspects of modern society with the most obvious being the audio and
noise control industries.
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 and thus
disturbing their neighbours.
Acoustic treatment, in the context of a recording studio, generally deals with the
acoustic quality of the room from a listener's point of view. In other words, if you
monitor in a control room that has been designed using the correct acoustic
treatment, what you hear is likely to be more accurate than the same recording
played back over the same speakers in an untreated room.
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.

2. Dead Room
Rooms that have very little or no echo/reverberation of their own are considered to
be “dead”. The ultimate dead room is what is called an Anechoic Chamber. It is a
special structure built in a way that it produces no echoes whatsoever. These are
usually 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 heard
of some podcasters using a kid’s play tent covered with blankets to achieve this as
well. Even some cars make great dead-rooms to record in. The sound of a room that
is completely dead is very unnatural sounding. If you have ever been inside of an
anechoic chamber, you can hear yourself breathing, possibly a soft ringing noise
from your ears, and when you talk, your voice sounds very tiny and strange. When
you record in a closet, car, or tent, you will notice that your recordings sound kind of
flat and muted.
Surface Types
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.
When sound reflects off a smooth flat surface, an echo or reproduction of the sound
can be heard. Echoes are more noticeable if the surface is far enough away to allow
for a time-lag between when the sound is made and when it is hear.

3. Reverberation
Reverberation is the persistence of sound in a particular space after the original
sound is produced. A reverberation, or reverb, is created when a sound is produced
in an enclosed space causing a large number of echoes to build up and then slowly
decay as the walls and air absorb the sound. This is most noticeable when the sound
source stops but the reflections continue, decreasing in amplitude, until they can no
longer be heard. The length of this sound decay, or reverberation time, receives
special consideration in the architectural design of large chambers, which need to
have specific reverberation times to achieve optimum performance for their
intended activity.
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 antinoise sound generators.
A room within a room (RWAR) is one method of isolating sound and stopping it from
transmitting to the outside world where it may be undesirable.
Most vibration / sound transfer from a room to the outside occurs through
mechanical means. The vibration passes directly through the brick, woodwork and
other 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 amplified
and heard in the second space. A mechanical transmission is much faster, more
efficient and may be more readily amplified than an airborne transmission of the
same initial strength.
The use of acoustic foam and other absorbent means is less effective against this
transmitted vibration. The user is advised to break the connection between the
room that contains the noise source and the outside world. This is called acoustic de-
coupling. Ideal de-coupling involves eliminating vibration transfer in both solid
materials and in the air, so air-flow into the room is often controlled. This has safety
implications, for example proper ventilation must be assured and gas heaters cannot
be used inside de-coupled space.

4. Outdoor acoustics
Sound bites
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 for sound on tape.
Before the actual term "sound bite" had been invented, 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 principle 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 consequently 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 therefore 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.
Background Noise
In acoustics and specifically in acoustical engineering, background noise or ambient
noise is any sound other than the sound being monitored (primary sound).
Background noise is a form of noise pollution or interference. Background noise is an
important concept in setting noise regulations. See noise criteria for cinema/home
cinema applications.
Examples of background noises are environmental noises such as waves, traffic
noise, alarms, people talking, bioacoustic noise from animals or birds and
mechanical noise from devices such as refrigerators or air conditioning, power
supplies or motors.
The prevention or reduction of background noise is important in the field of active

5. noise control. It is an important consideration with the use of ultrasound (e.g. for
medical diagnosis or imaging), sonar and sound reproduction.
Unwanted Sound
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.
The Attack, Hold, and Release functions of a noise gate.
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.
In audio post-processing, noise gating reduces steady noise sources such as rumble
from LP records, hiss from audio tape, static from a radio or amplifier, and hum from
a power system, without greatly affecting the source sound. An audio signal such as
music or speech is broken up into many frequency bands by a collection of
overlapping band-pass filters, and if the signal amplitude in any one band is lower
than a preset threshold then that band is eliminated from the final sound. This
greatly reduces perceptible background noise because only the frequency
components of the noise that are within the gated pass bands survive.
Unwanted Ambience

6. 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 send
off 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 very
expensive to virtually no cost at all. Don’t jump and buy a bunch of pricey foam
squares until you have considered all the options:
Isolation panels:
These can range from £50 up to multiple
hundreds, depending on your budget and
the level of isolation you are aiming for.
There are several different kinds and makes
of the isolation panels but as long as it
works that’s all that matters. You may find
that one is simply not enough, if the
ambient noise in your studio is above a
certain 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 very
inexpensive way to get that quiet that your mic needs to perform it’s best. A creative

7. mind can come up with a way to wrap a carpet remnant around the back of your
microphone, which will significantly reduce ambient noise. You can also use carpet
remnants hung on your walls to dampen sound.
Recording out of doors obviously involves working with ambient noise all around;
that's often why we're recording outside in the first place. Stereo recordings capture
a broad sound-stage area, with all the appropriate ambience in the appropriate
three-dimensional positions. However, if you are looking to record an individual,
isolated sound you can have a problem. You can't ask the birds to stop singing, or get
the traffic on the road to stop for 10 minutes while you try to record the sound of a
quiet mouse, for example! So the only option is to use very directional microphones
and work in mono, to try to exclude as much ambient sound as possible. You can
then mix your isolated mono sound with a separate stereo ambience to create a
pleasing ensemble.
Wind Noise
Recording outside presents a whole different range of problems and challenges than
working in the studio. The basic concepts and practices of mic placement are
unaffected; you still need to position a mic where it can 'hear' the best balance of
sound emitted from the source, and where the rejection null (or nulls) of its polar
pattern reject the most unwanted noise. But perhaps the most obvious problem
with working outdoors is that of wind noise, and it's crucial to take measures to keep
the air currents caused by wind away from the mic capsule. Simple foam windshields
can help, but only to a limited degree. If you look at the setups employed by
professional TV and film sound recordists, you will usually see them working with
microphones mounted inside large zeppelin-shaped devices, often covered with fake
fur: the proverbial 'dog on a stick'.
The main basket of the Rycote system is designed to set up an area of still air around
the 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 large
things. Moving air has to flow around the basket, keeping turbulence well away from
the microphone and its mountings. However, pressure changes, including those
caused by sound waves, still affect the pressure of the internal still air, and so the
microphone still 'hears' the sound, without the rushing wind noise that would
previously have rendered any sound unusable.