Manteo, N.C.— Through wind and rain, stage fights, environmental acoustics and more, Point Source Audio mics have weathered all the challenges in The Lost Colony, the outdoor theatre production located on the Outer Banks of North Carolina. The audio team, confident in their PSA investment since 2013, recently added the new patent-pending CO2 Confidence Collectiondual-element headset mic to their collection of 24 active on set mics.

“We were excited to see a product that made the application of the dual elements simpler by combining those wire runs and keeping things as tidy as possible. That mic has been a great benefit to the production,” says Sound Designer/composer Michael Rasbury. “The [lead] character is most likely to have a mic issue due to the activity required by this role; stage combat; perspiration.”

Since 1937, The Lost Colony has been telling the story of the first English colonies in North America and their mysterious disappearance. Over the past few years, the audio systems have been overhauled, upgraded and redesigned, including the ever-important microphones.

Robust equipment to withstand elements including water—in the form of rain and perspiration— as well as wind and heat is invaluable for theatre, but especially for outdoor theatre. The associate producer Lance Culpepper explains: “To provide our audience the experience that they’re expecting, we really have to put a lot of our resources into ensuring that the audio equipment can sustain itself through all of these dynamic changes. That’s one of the reasons why microphones are so important to us. Prior to the Point Source mics, we saw a much shorter lifespan with the previous microphones and we had to send them out for servicing more frequently. We don’t see those problems with the Point Source mics.”

“I am beyond pleased with the addition of Point Source microphones to our production,” states A1 Joseph Reynolds. “The waterproof elements are paramount to an outdoor period drama, where the perspiration of the talent could compromise mic functionality. I especially enjoy the dual behind the ear headsets. It helps to have consistent placement and keeps the mic off the talent’s face.”

Placement is an important consideration as noted by Rasbury. “From a design perspective, the low profile of these microphones is key for getting the mic near the voice without detracting from the 16th-century costume design. Our show is also very physical in nature. We have a large company and large-scale fight choreography. Given the way these mics attach to both ears, they stay on the actor and in the correct place throughout the show.”

The other feature of the PSA mics that the team wants to highlight is that “the frequency response of these elements is excellent (20Hz to 20kHz),” says Rasbury. “Our show also includes gunshots, explosions, and loud actor responses; these do not distort at high SPL.” Reynolds adds, “The frequency response is tailored nicely to the full range of human speech and I don’t have to slice up every channel with a parametric scalpel.”

The CO2 Confidence Collection has two waterproof and frequency-matched elements, each measuring a tiny 3mm. Both elements offer built-in redundancy and integrate PSA’s first-to-market features including IP 57 waterproof rating and the “unbreakable” headset boom. Point Source Audio mics are engineered for durability and to endure challenging mic applications.

Culpepper appreciates the support from Point Source Audio. “They’ve been great to work with,” he says. “The communication has been excellent as well as customer service-wise. Knowing that we’re not just another account number with them means a lot to our organization.”

The audio team at The Lost Colony includes sound designer Michael Rasbury, A1 Joe Reynolds, and A2 Josiah Rodgers. Microphones supplied by Wake Forest, NC-based Provision Audio Video Solutions.

For more information about Point Source Audio and its products, visit www.point-sourceaudio.com.

AboutThe Lost Colony

First staged in 1937, The Lost Colony is the nation’s premier and longest-running outdoor symphonic drama. Written by Pulitzer Prize-winning playwright Paul Green and produced by the Roanoke Island Historical Association, a 501 (c)(3) nonprofit, The Lost Colony’s 2019 season runs May 31-Aug. 23, 2019 at Roanoke Island’s Waterside Theatre, located within the Fort Raleigh National Historic Site on the Outer Banks. For information, go to www.thelostcolony.org

About Point Source Audio

Point Source Audio (@PSA_audio) manufactures and distributes worldwide their SERIES8, EMBRACE, and CONFIDENCE collection of miniature microphones—a unique line of headset, earworn and earmounted microphones known for their robust bendable boom and waterproof features. The company also holds two patents for the EMBRACE concealable microphone as well as the patent for the world’s first modular in-ear comms headset that is supporting the hearing health for audio, lighting and camera techs using headsets everywhere from sports to space. Founded in 2004, Point Source Audio is headquartered in Petaluma, California. For more information call (415) 226-1122 or visit www.point-sourceaudio.com. Follow the company on Twitter at www.twitter.com/PSA_audio.

Whether it’s audio, video, post, or editing, one aspect of good control room design involves the proper use and placement of acoustical materials, primarily absorbers and diffusors, and sometimes reflectors as well. Selecting the appropriate materials is helped by product specifications.

ABSORPTION COEFFICIENT

The design team for Discovery’s new audio edit suites in its Silver Spring, Md. headquarters used a combination of different diffusion and absorption materials to minimize detrimental reflections.

There are standards that describe how to measure and calculate these parameters. ISO 354 covers the absorption coefficient, while ISO 17497-1 and ISO 17497-2 address the scattering coefficient and diffusion coefficient, respectively. In addition, the AES information document for room acoustics and sound reinforcement systems, AES-4id-2001 (r2007) describes the “characterization and measurement of surface scattering uniformity.” These types of tests are generally conducted in labs designed for acoustical measurements.

The absorption coefficient indicates how much randomly incident sound energy is absorbed. Ideally this number is given for different frequency bands, such as one octave or one-third octave, because materials tend to have different absorption characteristics at different frequencies. Absorption coefficients range between zero and one, with zero being totally reflective and one being totally absorptive. The higher the number, the greater the sound absorption. An absorption coefficient of 0.6, for example, means that 60 percent of acoustical energy hitting that surface will be absorbed, and 40 percent will be reflected or scattered.

Sometimes we’ll see an absorption coefficient greater than one, and that can result from such factors as the size of the material being tested, the frequency range, the edge effect of additional absorption from exposed edges, the formula being used for calculations and the use, or not, of a reference reflector in the measurement protocol.

One method of determining the absorption coefficient involves making measurements in a reverberation room. The idea is to compare the measurement of the reverberation time of the room with and without the absorbing material. The reverberation time is usually derived from an impulse response measurement and is the time it takes for sound to decay 60 dB after a sound source is turned off. With absorption in the room, the reverb time should be less than that of an empty room. Many measurements are generally taken and averaged.

Fig. 1: To represent acoustical data, RPG Diffusors provides graphs with absorption, scattering, and diffusion coefficients plotted against frequency.

Other parameters, like room volume and air temperature and attenuation—which affect the speed of sound—are also measured. All of these are cranked through formulas to come up with the absorption coefficient (absorption per unit area).

The procedures for obtaining the absorption coefficient have come under review as standards for the scattering and diffusion coefficients were being developed. The value of the absorption coefficient can vary depending on the lab conducting the test and the nature of their reverberation room. Not all rooms are uniformly and totally diffuse; there are differences in what types of diffusing elements are installed and where they are placed within the room. There can also be differences in how the test materials are mounted and the attention given to the edge effect.

There has been a good argument for using a purely reflective surface as a calibration material, instead of the empty room, for the base measurement since both the reflector and absorber will be measured under the same room conditions. Also, a different formula than is commonly used for calculating the absorption coefficient seems to better indicate what the absorption will be when the material is actually installed in a control room.

These are some of the things to be aware of when looking at absorption coefficients and comparing materials. You may have to specifically ask the manufacturer for this level of detail and information on how the tests were conducted. Don’t overlook differences in absorption depending on how the material is mounted.

SCATTERING AND DIFFUSION COEFFICIENTS
Since the commercialization of diffusing products—pioneered by RPG Diffusors—is fairly recent (early 1980s), a standard approach to measuring and calculating the scattering and diffusion coefficients is also quite recent, having undergone many iterations in development.

When sound hits a diffusive surface, what isn’t absorbed is reflected back at many different angles (spatial diffusion). The reflections also can arrive at a listener at different times (temporal diffusion), and in that case absorption should be minimal. By comparison, a reflective surface follows the rule of specular reflections that the angle of incidence equals the angle of reflection. A diffuser, by definition, shouldn’t concentrate its scattered sound in the specular zone.

Diffusive materials differ in how they scatter sound. Some scatter sound in a way that favors certain angles and not others. This could be considered a poor diffusor since the sound is not scattered in a uniform way, even though it scatters sound away from the specular zone. A good diffuser scatters sound more evenly in both space and time.

This brings up the difference between the scatter and diffusion coefficient. The scatter coefficient indicates how much sound energy is scattered away from the specular zone. It doesn’t indicate anything about how evenly the scattered sound is distributed. According to the ISO standard, the scattering coefficient can only be measured on material that has an absorption coefficient less than 0.5.

The diffusion coefficient, on the other hand, indicates the uniformity of the scattering. The value of either of these coefficients ranges from zero to one. The greater the value, the greater the amount of scattering or uniformity of diffusion, depending on which coefficient we’re talking about.

MEASURING COEFFICIENTS
These coefficients can be derived by measuring the polar responses and comparing those of a reflective surface to the diffusive surface under test. A source loudspeaker emitting a specific test signal is aimed at a certain angle at the diffuser or reference reflector. Random incidence measurements can also be made.

The sound energy coming off the device under test is measured at multiple angles around the material. In practice, to reduce the measurement time, multiple microphones are placed on a boundary surface (like a floor) in a hemispherical arc pattern, with their signals simultaneously recorded. The next steps involve signal processing and computations to derive the coefficients.

Unlike the measurement of absorption coefficients which requires a reverberation room, measurements of the scattering and diffusion coefficients need to be measured in a room where any surface reflections don’t enter the measurements. This could be an anechoic chamber or a very large room with a reflection-free zone surrounding the test setup. Because of the size of some diffusor arrays, it can be difficult to find an appropriate measurement space.

The folks at RPG addressed this issue by developing testing methods that can use accurate scale models of the diffusors under test, with the diffusors placed on a boundary surface. The test signals need to be scaled up in frequency and the loudspeaker emitting the test signals and the measuring mics need to have appropriate frequency and phase responses at these frequencies. This allows measurements to be made in a more practical-sized space, with a smaller reflection-free zone needed around the test setup.

As an example of one manufacturer’s approach to presenting acoustical data, RPG provides graphs with absorption, scattering, and diffusion coefficients plotted against frequency (Fig. 1). They also provide tables with the coefficient values for each frequency band. In addition they often show a picture of the test setup.

When choosing acoustical materials, look for the test data on absorption, scattering and diffusion by frequency band, and ask manufacturers if you can’t readily find it.

Mary C. Gruszka is a systems design engineer, project manager, consultant and writer based in the New York metro area. She can be reached via TV Technology.

As the shift to 5.1 and beyond surround sound mixing rooms continues—with the potential for immersive audio on the horizon—there have been some new thoughts on the internal acoustical design. The ultimate goal of providing a neutral listening environment where critical judgments can be made on the quality of sound sources and the overall mix doesn’t change. Where the rethinking comes in is how to handle the internal sound field to accomplish this goal. So, all the usual suspects of good design—solid construction, adequate room volume, low-frequency modal control, symmetry, noise and isolation control, loudspeaker choice and placement and mix position placement—still apply. I’ve covered these topics in the past, and they have recently been revisited by my TV Technology colleague, Jay Yeary (Exploring Audio Control Room Acoustics).

THE RFZ CONCEPT
Since this new approach builds on current best practices, a brief review may be helpful. Ever since the concept of Live End- Dead End control room design emerged in the late 1970s, the idea was to keep early reflections away from the mixing position by making the front end of a control room absorptive and the rear diffusive. This concept was soon expanded and enhanced by thinking of a reflection-free zone (RFZ) around the mix position.

Absorption, while effective in controlling reflections, could end up creating a “dead” sounding environment, especially if too much was used. The RFZ was achieved not only by properly specified and situated absorption, but in the room geometry with splayed walls re-directing sound energy away from the mix position to the diffusive rear wall or walls. If the angles were correct, the splayed walls could actually be reflective, thus keeping more sound energy in the room.

At the same time diffusor science and technology took a giant leap forward in the early 1980s with the introduction of the Quadratic Residue Diffusor (QRD) by Dr. Peter D’Antonio and the company he cofounded, RPG Diffusor Systems Inc.

Studio C control room at Blackbird Studio Using diffusor concepts based on number theory as advanced by Manfred R. Schroeder, it was possible to predict how sound would scatter upon hitting the diffusor. This allowed for the design of units that provided more uniform scattering of sound in space and time within the designed frequency range than other forms of diffusion, such as poly-cylinders or pyramid shapes common in use at the time. (Since the QRD, there have been further diffusion improvements by RPG.)

Timing and the control of the diffuse sound level factored into the RFZ concept. Measurements, experimentation and the latest research on human sound perception and localization indicated that about a 20-millisecond time difference from the arrival of the direct sound from the loudspeakers to the first reflections would be needed to create an RFZ. This meant that the zone was effectively anechoic (without echoes or reflections) during this time period.

To keep the room from being a truly anechoic space—which is very unnatural for listening—broadband diffuse, rather than specular, reflections would be allowed to arrive at the mix position after about 20 milliseconds. The level of this diffuse field would be quite a bit lower than the direct sound, around 30 dB, and decay fairly evenly over time. The QRDs, new at that time, provided this type of diffusion in a small space.

This level and spacing of diffusion didn’t interfere with the ability to critically evaluate the direct sound. Properly implemented, a control room with an RFZ provided good stereo imaging, and not necessarily in just one “sweet spot,” but across the width (or a good part) of the mixing console.

LEDR TEST SIGNALS
Imaging was not necessarily relegated to just the lateral plane. A test tape called LEDR, a trademarked acronym for Listening Environment Diagnostic Recording developed by Doug Jones, contained a series of specially designed synthesized and filtered sounds. When played back in a well-designed RFZ room over a pair of loudspeakers, these sounds would appear to travel not just left to right, but up, over and behind the listener in the RFZ.

These synthesized sounds simulate reflections off the pinna of the ear (the outer ear) combined with direct sound. This results in comb filtering in the frequency response. As sound moves around a head, the comb filters change, providing auditory cues as to location. This was brought to the audio community’s attention by Dr. Carolyn “Puddie” Rodgers in the late 1970s and early 1980s.

(There are other auditory localization cues that include level, phase and time of arrival differences between the two ears.)

Front and ceiling diffusive surfaces at Blackbird Studio The LEDR test signals are now online at www.audiocheck.net/audiotests_ledr.php. The effects can be heard on headphones and downloaded to check out a room. In a control room situation, if early reflections interfere with the direct sound, especially within the first 20 milliseconds of arrival, then the resultant comb filters can conflict with pinna localization cues, and the effects on the LEDR recordings won’t be heard.

The RFZ concept (and the LEDR test sounds) was developed in the age of stereo recording and monitoring. Could there be improvements for multichannel audio control rooms?

There are proponents of eliminating the reflection-free zone and instead create an overall diffuse sound field throughout the room, including the mixing position. There are different opinions as to the level and characteristics of this sound field so that the ability to critically perceive the direct sound isn’t hampered.

One recording studio put this concept into practice: Studio C of Blackbird Studio in Nashville, Tenn., which was designed by noted recording engineer, producer and audio equipment designer George Massenburg who collaborated with RPG on this project.

Studio C, based around a Digidesign ICON, has wide-bandwidth diffusive surfaces on all walls and ceiling. But these aren’t garden-variety off-the-shelf diffusors.

The large-scale, wide-bandwidth, two-dimensional diffusors were custom-designed and fabricated from a series of one-inch square (in cross-section) block pegs—each one a different height—that stick out from the wall in a specific arrangement. Over 130,000 different peg heights, with no two the same, were used to create the diffusive surface. The room treatment also included diffractals on the ceiling and low-frequency resonators (dampened metal plates).

The diffusion this treatment creates is very dense, with a level around 30 dB below that of the direct sound, and an even decay rate over time.

RPG is calling this concept “Ambechoic,” a trademarked name for ambient anechoic. The room as a whole has ambient energy due to the extensive wide-band diffusion, but at the mixing position it’s reported that there’s precise imaging and an extended “sweet” zone. In addition, monitoring can be done at lower levels.

Why does this work? It seems more research needs to be done, but the ear/brain integration time may play a pivotal role.

Studio C is a fairly large mixing room at more than 24,000 cubic feet, so the space was available to accommodate the diffusive surfaces with some of the elements sticking out more than three feet from the wall.

According to D’Antonio, “The Ambechoic design is really intended for larger rooms than broadcast because you have to develop low-level diffuse reflections, which is difficult when close to a boundary. 2D diffusors are also necessary to omnidirectionally distribute the energy. The idea is to provide a uniformly diffuse environment.”

Could this concept be adapted for smaller control rooms? It’s something to think about; and it will be interesting to see how this develops further as designers explore the possibilities.

Mary C. Gruszka is a systems design engineer, project manager, consultant and writer based in the New York metro area. She can be reached via TV Technology.

In the last two Inside Audio columns (Exploring Audio Control Room Acoustics, Part I, Exploring Audio Control Room Acoustics, Part II) we looked at some of the reasoning behind the design of high-end audio control rooms. This time we’ll take a look at what can be done when space for the control room is inappropriate and the budget inadequate. How do we create a decent control room when the space is too small; the floor is raised; the space above the drop ceiling is open; and the walls are as thin as those of a cheap motel?

We’ll focus on four areas that will help overcome these issues and deliver a room that should function properly for professional audio work, and each can be adapted to fit the available budget. The information presented here has been gleaned through experience building audio control rooms in small spaces that now produce high-quality audio for television.

ISOLATE THE SPACE
Controlling sound transmission into the room is essential so the audio engineer can be certain that what they hear is direct sound from the speakers and not from exterior noise sources. We first must make sure the walls have enough mass to act as a transmission barrier and that all penetrations are sealed.

Walls need to span from the floor above to the floor below and should be extended if they don’t. Single layer walls require one or two additional layers of sheetrock to increase the wall’s mass. MDF can be used for the top layer instead of sheetrock to add mass without stealing space from the interior footprint.

Ideally, all adjacent room walls will receive the same treatment as the interior walls to minimize sound transmission between the rooms and to allow the sealed air space between the walls to act as an additional barrier. All openings, including those above ceilings and below raised floors, should be closed and patched, then sealed with an acoustic sealant or multiple layers of caulk.

If the budget doesn’t support the purchase of an acoustical door, then a heavy solid-core door with high-quality, properly adjusted acoustic seals will work as a less-effective substitute. It is best to purchase a real acoustical window if one is required, but if that is budget-prohibitive then an add-in acoustical window kit or, at a minimum, a double-pane industrial grade window can be used.

CONTROL STANDING WAVES AND MODES
Standing waves and modes are worse in small spaces because they are concentrated into just a handful of frequencies instead of being spread across a larger range as they would in large rooms. Modes are a characteristic of room dimensions, while standing waves are stationary modes below 300 Hz caused by reflections from the room’s boundaries.

A simple calculation for determining room modes is the formula 1130/2d=f. This breaks down as the speed of sound (1130 ft/s), divided by a single dimension of the room times two, equaling the frequency of the mode.

We need to calculate the fundamentals for height, length and width; then the harmonics by multiplying the fundamentals by two, three, etc. Charting these modes (Fig. 1) allows us to see which frequencies will cause problems in the room. Rectangular rooms generally give the best performance while cube-shaped rooms contain the nastiest modes because height, width and depth dimensions are the same, resulting in identical mode frequencies, all landing at the same location in the room, creating giant modes as well as cancelation of other frequencies.

Fig. 1

With a limited budget and no way to expand the space, our options for dealing with modes are limited. Experiment with placing traps in the corners, rear and front of the room until low frequencies seem under control. A significant concern in small rooms, especially cube-shaped ones, is that the modes and cancelations tend to be worse near the middle of the room; so make sure the mix position is located elsewhere.

CONTROL REFLECTIONS
Reflections interfere with our perception of direct sound from the speakers by summing or canceling some of the direct sound we hear. Mid- and high-frequency reflections are generally the most obvious and can be controlled by removing reflective surfaces near the speakers and placing acoustic absorbing panels on the room’s hard surfaces.

A good way to determine where to begin applying absorption is to stand behind the main speakers and see where they are pointed. Panels should be placed in their initial position then moved, added or removed systematically until reflections are minimized.

Once frequency response at the mix position seems even across the spectrum, panels can be mounted permanently. At this point there should be no need to add more absorption since it may cause the room to sound dead. If additional treatment is desired then diffusion should be used to disperse reflections rather than absorbing them. Placement of traps, absorbers, and diffusors is often more of a process than a one-time event and occasional adjustments may be required.

REMOVE INTERNAL NOISE SOURCES
One frustrating and entirely controllable element of building an audio control room is that, at some point, equipment will get installed and some of it will make noise. Hard drives and fans are typically the worst culprits so, along with CPUs, they should be moved to an equipment room and out of the control room.

Keyboard, video and mouse (KVM) extenders make this relocation of noise-making equipment fairly easy, but they can be expensive. If the budget doesn’t provide for the purchase of KVMs then an acoustic isolation cabinet can be used, or cables may need to be extended out of the room to an adjacent equipment closet.

A successfully built audio control room is one that insures the audio engineer can rely on what they hear while working in it. By focusing on isolation, control and noise elimination we can transform seemingly unusable spaces into effective audio control rooms. Still, it’s most important to listen critically as we make room improvements to help determine whether our changes will improve the room, make no difference or make things worse.

In audio engineering, the right decision often comes down to what we can discern with the most powerful tool in our toolbox, our ears.

Jay Yeary spends his days working for a large media corporation where he has the opportunity to work on audio control room projects of all sizes. He can be reached through TV Technology or via Twitter at @audiojay.