Modern Recording Techniques 7th Edition Ch 3 Studio Acoustics and Design - Posted on 2013-04-21 by Malecia Bynum

• The Audio Cyclopedia defines acoustics as a science dealing with production, effects and transmissions of sound waves; the transmission of sound waves through various mediums, including reflection, refraction, diffraction, absorption and interference. Acoustics is an artistic science that blends math with the art of intuition and experience.
• When designing or redesigning an acoustic space the following should be considered:
  • Acoustic Isolation: This prevents external noises from transmitting into the studio environment through the air, ground, building, or structure.
  • Frequency Balance: The frequency components of a room shouldn't adversely affect the acoustic balance of instruments and/or speakers. It shouldn't alter the sound quality of the original or recorded performance.
  • Acoustic Separation: The acoustic environment should not interfere with intelligibility and should offer the highest possible degree of acoustic separation within the room (often a requirement for ensuring that sounds from one instrument aren't picked up by another instrument's mic.
  • Reverberation: the control of sonic reflections within a space is an important factor for maximizing the intelligibility of music and speech. (No matter how short the early reflections and reverb times are, they will add an important psychoacoustic sense of "space" in the sense that they give our brain subconscious cues as to a room's size, number of reflective boundaries, distance between the source and listener, and so forth.
  • Cost factors: Not the least of all design and construction factors is cost. 
• Some common studio types include:
  • Professional music studios
  • Audio-for-visual production environments 
  • Project studios
  • Portable studios
• The professional recording studio is 1st and foremost a commercial business, so its design, decor, and acoustical construction requirements are often much more demanding than those of a privately owned project studio.
• An audio-for-visual production facility is used for video, film and game post production (often simply called "post") and includes such facets as music recording for film or other media (scoring, score mixdown, automatic dialog replacement (ADR), which is the replacement of on-and off-screen dialog to visual media and Foley (the replacement and creation of on-and-off screen sound effects).
• A project studio is usually intended as a personal production resource for recording music, audio-for-visual production, multimedia production, voiceovers and etc.
• Design and construction considerations differ in 2 ways:
  • Building constraints
  • Cost
• A symmetrical acoustic environment around the central mixing axis can work wonders toward creating a balanced left/right and surround image
• "Float" the rooms is a process that effectively isolates and uncouples the inner rooms from the building's outer foundations
• The reduction in the sound-pressure level (SPL) of a sound source as it passes through an acoustic barrier of a certain physical mass is termed the transmission loss of a signal; Transmission loss is frequency dependent and is expressed in dB
• Heavier acoustic barriers will yield a higher transmission loss
• Transmission losses will increase as the frequency rises (i.e.. Shutting a car door with the stereo playing or leaving out of the control room)
• You want to increase transmission loss when building a studio wall (reduce leakage) and this is best done by:
  • Building a wall structure that is as massive as is practically possible (both in terms of cubic and square foot density) 
  • Eliminating open joints that can easily transmit sound thorough the barrier 
  • Dampening structures, so that they are well supported by reinforcement structures and are free of resonances
• Some advice to consider:
  • If possible the inner and outer wallboards shouldn't be attached to the same wall studs. Stagger the studs along the floor and ceiling frame, so that the front/back facing walls aren't touching each other.
  • Each wall facing should have a different density to reduce the likelihood of increased transmission due to resonant frequencies that might be sympathetic to both sides. 
  • If you're going to attach gypsum wallboards (made of gypsum plaster pressed between 2 thick sheets of paper-aka drywall/plasterboard) to a single wall face, you can increase transmission loss by mounting the additional layers (not the 1st) with adhesive caulking rather than using screws/nails.
  • Spacing the studs 24 inches on center instead of 16 spacing yields a slight increase in transmission loss
  • To reduce leakage that might slip through the cracks, apply a bead of non-hardening caulk sealant to the inner gypsum wallboard layer at the wall-to-floor, wall-to-ceiling and corners
• Generally, the same amount of isolation is required between the studio and the control room as is required between the studio's interior and exterior environments
• A specially designed cavity called a soffit can be designed into the front facing wall of the control room to house the larger studio monitors. This superstructure allows the main, farfield studio monitors to be mounted directly into the wall to reduce reflections and resonances in the monitoring environment.
• It's important for the soffit to be constructed to high standards using a multiple wall of high mass design that maximizes the density with acoustically tight construction techniques in order to reduce leakage between the two rooms (sub-standard and standard don't work as effectively).
• Typical wall construction materials include:
  • Concrete: This is the best and most solid material, but is often expensive and not always possible.
  • Bricks (hollow-form or solid facing): (Often easier to place than concrete)
  • Gypsum plasterboard: Building multiple layers of plasterboard onto a double-walled stud frame is often the most cost-and design-efficient approach to reducing resonances and maximizing transmission loss. IT's often a good idea to reduce these resonances by filling the wall cavities with rock-wool or fiberglass, while bracing the internal structure to add an extra degree of stiffness.
• Studio monitors can be designed into the soffit in a number of ways:
  • The speakers inner enclosures are cavities designed into walls that are made from a single concrete pour (resonances completely eliminated).
  • Have studio monitors rested on poured concrete pedestals; inserts are cast into the pedestals that can accept thredded rebar rods (aka all-thread). By filing the rods to a chamfer or a sharper point, it's possible to adjust the position, slant and height of the monitors for final positioning into the soffit's wall framing. 
  • Use traditional wood framing in order to design a cavity into which the speaker enclosures can be designed and positioned.
  • *Extra bracing and heavy construction should be used to reduce resonances*
• For many recording facilities the isolation of floor-bourne noises from room and building exteriors is an important consideration. One of the most common ways to isolate floor-related noise is to construct a "floating" floor that is structurally decoupled from its subfloor foundation. Common construction methods for floating a professional facility's floor uses either neoprene "hockey puck" isolation mounts, U-Boat floor floaters, or a continuous underlay, such as a rubberized floor mat (or carpet foam pad).
• It is important for the floating superstructure to be isolated form both the underflooring and the outer wall. These wall perimeter isolation can be sealed with pliable decoupling materials such as widths of soft mineral fiberboard, neoprene, silicone or other pliable materials.
• Drum risers can be used to reduce low frequency leakage
• "Z" channels are often screwed to the ceiling joists to provide a flexible yet strong support to which a hanging wallboard ceiling can be attached.
• Window design and construction often varies and can range from being deep, double-plate cavities that are built into double-wall constructions to more modest prefab designs that are built into a single wall
• Access doors to and from the studio, control room and exterior areas should be constructed of solid wood or high quality acoustical materials, as solid doors generally offer higher TL values than their cheaper hollow counterparts.
• Whenever possible, double door designs should be used to form an acoustical sound lock. This technique dramatically reduces leakage because the air trapped between the 2 solid barriers offers up high TL values.
• Isolation rooms (iso-rooms) are acoustically isolated or sealed areas that are built into a music studio or just off of a control room
• Isolation booths (iso-booths) provide the same type of isolation as an iso-room but are often much smaller; Often called vocal booths, these mini-studios are perfect for isolating vocals and single instruments from the larger studio. 
• An equipment room that has easy access doors that provide for current/future wiring needs can add a degree of peace and quiet 
• Movable acoustic partitions (aka flats or gobos) are commonly used in studios to provide on the spot barriers to sound leakage.
• One of the most important acoustic design rules in a monitoring environment is the need for symmetrical reflections on all axes within the design of a control room or single room project studio. The center and acoustic imaging (ability to discriminate placement and balance in a stereo or surround field) is best when the listener, speakers, walls and other acoustical boundaries symmetrically centered about the listener's position (often an equilateral triangle). In a rectangular room, the best low-end response can be obtained by orienting the console and loudspeakers into the room's long dimension. 
• Care should be taken to ensure that both the side and ceiling boundaries are largely symmetrical with respect to each other and that all of the speaker level balances are properly set
• Speaker enclosures should be placed 1 to 2 feet away from the nearest wall and/or corner, which helps to avoid bass buildups that acoustically occur at boundary and corner locations. 
• Isolation pads can be used to reduce resonances that often occur whenever enclosures are placed directly onto a table or flat surface. 
• To maintain the frequency balance, the room should exhibit a relatively flat frequency response over the entire audio range without adding its own particular sound coloration. The most common way to control the tonal character of a room is to use materials and design techniques that govern the acoustical reflection and absorption factors. 
• One of the most important characteristics of sound as it travels through air is its ability to reflect off a boundary's surface at an angle that's equal to (and opposite of) its original angle of incidence.
• Avoid construction that include opposing parallel walls in its design. Such conditions give rise to a phenomenon known as standing waves. Standing waves (aka room modes) occur when sound is reflected off of parallel surfaces and travels back on its own path, thereby causing phase differences to interfere with a room's amplitude response. Room modes are expressed as integer multiples of the length, width and depth of the room and indicate which multiple is being referred to for a particular reflection.
• The distance between parallel surfaces and the signal's wavelength determines the nodal points that can potentially cause sharp peaks or dips at various points in the response curve (up to or beyond 19dB)  at the affected fundamental frequency or frequencies and upper harmonic intervals.
• (To prevent standing waves construct walls, boundaries and ceilings that are non-parallel).
• Diffusers are acoustical boundaries that reflect the sound wave back at various angles that are wider than the original incident angle. It along with non-parallel wall construction can reduce extreme, recurring reflections and smooth out the reverberation characteristics of a room by building more complex acoustical pathways.
• Flutter echo (aka slap echo) is a condition that occurs when parallel boundaries are spaced far enough apart that the listener is able to discern a number of discrete echoes (produces a "boingy" hollow sound). 
• Another factor that often has a marked effect on an acoustic space involves the use of surface materials and designs that can absorb unwanted sounds (either across the entire audible band or at specific frequencies). The absorption of acoustic energy is the inverse of reflection and occurs when only a portion of the incident acoustic energy is reflected back from a materials surface. 
• Whenever sound strikes a material, the amount of acoustic energy that's absorbed relatively to the amount that's reflected can be expressed as a simple ratio known as the materials's absorption coefficient.
• Many designers agree that a balance of 25% absorption and 25% diffuse reflections is a good ratio that can help preserve the "life" of a room, while reducing unwanted buildups
• The absorption of high frequencies is accomplished through the use of dense porous materials, such as fiberglass, dense fabric and carpeting
• Low frequencies are best damped by pliable materials, meaning that low frequency energy is absorbed by the material's ability to bend and flex with the incident waveform.
• Another absorber type can be used to reduce low-frequency buildup at specific frequencies (and their multiples) within a room. This type of attenuation device (aka bass trap)  is available in a number of design types:
  • Quarter-wavelength trap
  • Pressure-zone trap
  • Functional trap
• The quarter-wavelength trap is an enclosure with a depth that's 1/4 the wavelength of the offending frequency's fundamental frequency and is often built into the rear facing wall, ceiling or floor structure and covered by a metal grating to allow foot traffic.
• The pressure zone trap absorber works on the principle that sound pressure is doubled at large boundary points that are at 90º angles (such as walls and ceilings)
• Originally created in the 1950s by Harry F. Olson (former director of RCA Labs), the functional bass trap uses a material generally formed into a tube or half-tube structure that is rigidly supported so as to reduce structural vibrations
• Reverberation (reverb) is the persistence of a signal (in the form of reflected waves within an acoustic space) that continues after the original sound has ceased. The effect of these closely spaced and random multiple echoes gives us perceptible cues as to the size, density and nature of an acoustic space.
• The reverberated signal itself can be broken down into 3 components:
  • Direct Sound
  • Early Reflection
  • Reverb
• *The last set of signal reflections makes up the actual reverberation characteristic*
• Reverb is considered to be the time that's required for a sound to die away to a millionth of its original intensity
• Reverb times can range from 0.25 sec in a smaller absorptive recording studio environment to 1.6 sec or more in a larger music or scoring studio
• (Recreating room reverberation) An acoustic echo chamber is an isolated room that has highly reflective surfaces into which speakers and mics are placed. The speakers are fed from an effects send, while the mic's reverberant pickup is fed back into the mix via an input strip of effects return.