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Innovations in audio reference monitors

The reference monitor speaker performs the same role for the audio portion of the program as the video monitor does for the images. It provides the neutral, uncolored “truth” about the clarity, quality, intelligibility, noise floor and other elements of the sounds that have been captured by the microphones. When mixing multiple audio tracks in post-production — blending dialog, ambience, sound effects and music to create the finished aural experience — these monitors allow the mix engineer to accurately place each of these elements using the tools of relative level, equalization for frequency shaping, spatial placement and reverb/effects to alter listener perception of the sources of those various sounds.

While circumstances differ somewhat between an engineer recording a band to create music tracks destined for airplay and one mixing dialog and other sounds for an interview, sporting event, documentary or commercial project, both trust the audio monitors on the console to serve as a reliable reference. For the former application, the audio track must stand by itself. The stakes are fully as high when audio accompanies and interacts with video images, creating an experience for the viewer where the sum is greater than its parts.

Chicago’s BAM Studios produces audio for television, feature films, corporate presentations and commercial ventures.

How have manufacturers advanced the art of monitoring beyond a woofer, dome tweeter and crossover in a rectangular box? What technical innovations have been introduced to enhance consistency, transparency and reliability? How have relatively recent breakthroughs in electronics, digital signal processing and computing been married with transducers and enclosures to create the latest audio reference monitors?

Performing a function

The key requirement is that the audio monitor is a reference and ideally reproduces any audio signal introduced through it without coloration, distortion or phasing anomalies, and with the transient characteristics and frequency balance of the original source (or at least as it was captured by the microphone). A sales engineer at one of the leading monitor manufacturers lamented that, in contrast with the exciting adjectives that can be used when presenting an effects device or certain other audio goodies, the description for monitors are words such as neutral and transparent, and the main goal is “preventing coloration.”

This fidelity to the source material is what gives an engineer the ability to make educated, critical judgments when building the soundscape. The neutral quality also can help prevent or delay listener fatigue as the engineer spends long hours evaluating minute changes to EQ, levels, panning and effects to best meld audio with video. And in a live setting, it can promote confidence.

An A1 audio engineer specializing in sports told me that in the rush to set up for an event, he doesn’t have the time to test or calibrate the audio monitors above the mix console in the OB truck. The smaller near-field active monitors he typically encounters in the compact space devoted to audio are usually approximated toward the engineer’s ears, and he accepts them as a neutral reference for his mix.

Depending on the size of the mixing location, the level of overall SPL required, and the amount and pitch of low-frequency energy within the program material, monitors of differing sizes are required. For near-field work with audio mainly in the vocal range, a pair of accurate two-way monitors with perhaps 4in to 5in low-frequency drivers would be sufficient. Large post-production facilities often have larger wall-mounted stereo or left-center-right monitors with subwoofers, and perhaps other mid-sized monitors in surround-sound positions or in more near-field positions on the console bridge.

Even while using transparent monitors for critical mixing, more mundane speaker systems are often used to demonstrate the integrity of the mix to clients. A major Chicago post house keeps a pair of computer monitors and a consumer LCD-TV wired in to check the final results.

Active monitors

When amplification moved from using external commercial amplifiers to being integrated within the monitors, a major step was taken toward accuracy and consistency in audio reproduction. Self-contained systems allow more precise matching of the characteristics of the transducers and the amplifiers that are driving them. Though the first attempts at this integration occurred perhaps three decades ago, these efforts have become much more sophisticated — and are mirrored in many of the professional and even consumer loudspeaker systems that are reproducing that finished mix.

Even most of the smaller professional powered monitors have a separate channel of amplification for the LF and HF drivers, and one manufacturer’s top-of-the-line control room monitor has two 1100W amps for each of the low-frequency cone speakers, a 600W amp for the midrange and a 300W amp for the high-frequency driver — with a frequency response of 21Hz to 20kHz, +/-2.5dB. A competitor’s four-way system provides six channels of amplification with a control panel that resembles a powered mixer.

The audio suite at WestCom Media Group, Germany, produces daily content for private and public televison.

The development of Pulse Width Modulation (PWM or Class D) amplifiers with an efficiency of over 90 percent is the technology that allows such powerful amplification to be contained within relatively small enclosures with minimal heat-sinking. Coupling a channel of amplification with each driver means that their individual impedances and power requirements can be more closely matched, and the relative frequency bands and levels fed to each one makes it more effective to precisely balance and blend their outputs.

Digital Signal Processing

DSP algorithms for equalization, driver high-pass and low-pass for crossovers, delay compensation to time-align drivers within the enclosures and mains with subs, phase compensation, and other parameters are built into the circuitry and accessible via controls on the amp panel or remotely via control software. Many of the new monitors accept both analog inputs and digital signals adhering to the AES/EBU and S/PDIF standards, so the signal can remain digital from the console through the amplification. Some of the analog inputs contain precision analog-to-digital converters to process incoming signals into the digital domain.

Active crossover circuitry, in contrast to passive crossovers, responds more consistently to input signals at various levels and throughout a listening session, when the temperature of components can fluctuate — and produces less distortion. Steeper roll-off filters lessen the overlap between drivers, and the interaction of two drivers reproducing the same frequencies in differing physical locations in the enclosure. One system provides a movable 1/10th octave parametric filter to compensate for low-frequency room modes. Some monitors also include the ability to store and recall monitor settings, so that the particular parameters that are most useful for a certain room, type of mix or client can be consistently recalled. For film mixing, several monitors integrate a switchable Dolby X-curve setting that corresponds to the requirements for theatrical audio.

Transducer innovations

Many specialized materials technologies are used in reference monitors to achieve accurate linear response from a cone speaker or high-frequency device, and to optimize the weight-to-efficiency ratio of the transducers. Light yet rigid multi-layer cones mate with single or dual high-excursion voice coils powered by neodymium alloy magnetic structures. High-frequency drivers may be made of aluminum-magnesium or beryllium so that they are strong and rigid enough to accurately reproduce extremely high-frequency signals (even well above 20kHz) while providing extremely accurate transient response without breakup modes distorting their output.

One manufacturer specializes in inverted/concave HF dome drivers which in its experience yield an efficient, linear response with better dispersion and less “beaming” of the highest frequencies. Another’s trademark is its pleated ribbon HF (and in some models, MF) drivers that achieve flat frequency and phase response, and because of their three-dimensional structure have a greater surface area than dome HF devices. These drivers often use high-flux neodymium magnets for increased output.

Shown here is the audio booth in one of Total Production Services’ HD OB trucks.

Stiff cones that resist breakup distortion are made from a variety of materials, ranging from a sandwich of honeycombed Nomex with Kevlar on the inner and outer surfaces, to glass fiber over a foam core, glass beads over dense paper with a specially damped surround, and so on. For precise excursion control, one manufacturer’s latest models feature dual voice coils in a dual-magnetic structure. Several monitors offer time-aligned coaxial drivers, with their combined output emanating from the same location.

Advanced enclosure designs

Several monitor models have rounded edges to control the diffraction of sound waves around the cabinet and geometrically shaped baffles with integral horns to control the directivity of the transducers’ outputs. The goal is to provide a flat on-axis and off-axis frequency response, minimizing as best as possible the tendency for lower frequencies to spread more widely horizontally and vertically than higher frequencies — which can result in a less accurate sonic picture at the edge of the coverage pattern.

The baffles onto which the drivers are mounted are designed to be rigid, using cast aluminum or similar materials so that the enclosure is not excited into motion to color the frequency response. Other portions of the enclosure are similarly rigid so that they do not flex. Mounting points and feet damp any transferred vibration. To increase the low-frequency extension of these relatively small enclosures, innovative reflex ports use long internal tubes to reproduce bass without compression — sometimes with flares on the exit to minimize port noise.

Computer control and networking

Like so many other video and audio production systems, many audio reference monitors feature RJ45 connectors for networking and are supplied with control software. One model even has a USB connector to facilitate firmware updates.

The computer software allows the engineer to remotely adjust the configurations and parameters of multiple monitors and subwoofers, even across several different editing rooms. For example, the system can be remotely set for a stereo mix, LCR or 5.1 configuration. A pre-set for a particular client could be recalled and loaded, the LF or HF shelving applied or the X-curve placed across all monitors to audition the theatre mix.

Some reference monitor systems incorporate special software to perform room calibrations, using measurement microphones at the central listening position (and optionally at other locations). Test signals are sent to the individual monitors, and their frequency and phase response are measured. The software will then calculate the optimal delay settings and equalization to provide the most accurate overall response at the listening position.

Completing the mix

Like the decisions you make when selecting the best video capture, processing and monitoring equipment, finding the proper audio reference monitors for your application will enhance the quality of your production. A neutral reference lets you accurately hear the audio; detect and correct problems; evaluate changes to EQ, level and positioning; and determine the most satisfactory final mix. These innovations in audio monitoring will deliver more reliable and consistent results — and make you comfortable that the mix you hear is what you’ll deliver to your viewers.

Gary Parks is a freelance writer, formerly with EV, Vega Wireless, Clear-Com and Meyer Sound.