For the purposes here, Sound is waves in air. These waves may be the result of an object, such as a musical instrument, rapidly compressing and decompressing the air around it. The distance between an area of compression and its adjacent/contiguous area of decompression is a single wave. The length of a wave, or the distance between compressions, determines its frequency since the speed of the sound wave is relatively uniform.
Not to confuse the above definition but to be precise, sound also travels through liquids (like water) and solids (like the walls or windows of a building). It is important to note this because sometimes reducing unwanted sound maybe difficult because sound can move through conventional types of barriers.
When considering the qualities of sound there are two things that are most important: frequency and amplitude.
Frequency is the pitch of a sound, and is determined by the space between sound waves. Short distances between waves give a higher pitch and longer waves give a lower pitch. Pitch or frequency is measured in Hertz (Hz). Named for Heinrich Hertz (1857-1894) the measurement simply means cycles per second. In this case a cycle is one wave. Since the speed of the waves is fixed at (speed of sound) the number of waves occurring in a second determine the sound's frequency.
Amplitude is the volume of a sound wave. The larger the compression and decompression of the air from the normal atmospheric pressure the louder the sound is. Amplitude is measured in decibels (dB). Unlike the Hertz the decibel has no fixed unit, but rather is a measurement of relationships between sounds. However, there is a convention as to sound pressure level . The threshold of hearing is measured at .0002 microbar. This measurement is one of atmospheric pressure, it means that the faintest sound perceivable to young human ears is 1/20000th the pressure of the average atmospheric pressure measured at sea level. The "threshold of hearing" is listed as 0db on a conventional sound level chart. Another common reference is the " threshold of pain" which is listed as 140db on a conventional sound level chart.
Acoustic sound can be converted into an electronic signal. The electronic signal, as transformed from an acoustic signal by a microphone, can also be thought of as being comprised of waves just as acoustic sound is.
Sound Reflection and Absorption
Different freuquncies of sound travel through air differently. High frequencies (above 500Hz or so) tend to be very directional and radiate from their source like a beam of light from a flash light or a laser. Low frequencies of sound (below 500Hz or so) tend to spread out from their source like a pebble dropped in a pool of water. The different natures of high frequency and low frequency sound must be considered when recording sound. Directionality of the source sound to be recorded should affect microphone selection and placement. Low frequency sound sources may require different microphone specifications than a high frequency sound source depending on what the overall goal of the recording is. Because of lower frequency sound's ability to spread and reflect like waves in a pond the low frequency reflections occur readily. Reflected sound may or may not be desirable in a recording. It may not especially because of the added problems of some kinds of reflected sounds.
Reflected sound is such a common occurrence we, as casual listeners, fail to notice how much of the sound we hear is reflected off of some surface after it is initially emitted from its source. Our brains have a very sophisticated method of deciphering the reflected sound from the direct sound, but because even the best contemporary recording equipment makes these reflected sounds very obvious upon playback, much must be done to control or at least minimizes reflected sound when recording for motion pictures. Some surfaces reflect sound better than others. Of these surfaces, they diverge into types by how they reflect different frequencies of sound. For example: a thin pane of glass may reflect high frequencies very well but will allow low frequencies to pass. Sheet rock, a common wall material in houses, will absorb many of the high frequencies but will reflect mid, and low middle frequencies, and depending on how the material is attached to the support structure of the building, it may or may not reflect low frequencies. A basic rule of thumb is that plush soft materials absorb high frequencies and large massive amounts of porous material absorbs low and mid frequencies. (more mass is required to the lower the frequency of sound) Hard surfaces reflect sound but again mass plays a factor. Thin hard surfaces will only reflect higher frequencies the thicker, and more dense, the hard surface the lower the frequency of sound it will reflect.
Room Tone in an enclosed space's tendency to reinforce particular sonic frequencies. This term also refers to a particular room's unique character of light fixture hums, appliance buzz, and other sorts of anomalous noise.
A phenomena called standing waves, which is a kind of resonance, can be a particularly large problem when recording in small to medium sized rooms that have sonically reflective surfaces. If the walls, ceiling, and or floor of the room reflect medium to low frequency sound, the sound waves may have a tendency to build up, reinforcing each other to the point of amplifying sound frequencies, that the room is sympathetic to, that make a sound recording unusable for motion pictures. When a sound wave is the same length as the distance between two parallel walls, or any two parallel surfaces, like the floor and the ceiling, and the surface can reflect that wave the wave will double back on its self creating a much louder sound than emitted from the source. When this occurs the wave of that frequency is said to be "standing." This naming is due to the fact that as the wave travels from one surface to the other and is replaced by another wave, the succession of waves give the impression of motionless pressure building in localized regions of the room. A example of this phenomena is the resonant tone one hears in a tiled bathroom on in a shower stall. Much of the unusual sound in these environments in natural reverb but in addition some notes, pitches, particular frequencies, have waves that are of equal length or harmonic length, of the distances between the surfaces in that space. Try humming a note in your shower stall at home. Raise the pitch gradually, and then lower it gradually. You will notice that there are points in the ascending and descending pitch that the volume increases significantly. These pitches where the volume increases are also called the resonant frequencies of that space and are the standing waves previously mentioned. In a larger space these frequencies would be lower in pitch and would require more energy to be as noticeable but are still very present unless a sound absorptive material is used to reduce the reflective surfaces and stop the standing waves. And even though the resonant tones are not as noticeable to the naked ear in a larger room they will be noticeable upon playback. And once they are recorded there is no good way to remove them. So, much attention must be given to preventing resonant tones, or standing waves, from being present in a room that will be used to record sound for motion picture.
Also, check the general AUDIO page of this website.