HIGH DEFINITION AUDIO

Sales growth of stereo television receivers, movie video rentals, home theater and high end automobile sound systems as well as the rapid acceptance of compact discs has raised the standard by which broadcast audio is compared. The old three inch internal speaker which filters out hum and buzz, minimized the impact of distortion, excessive compression and equalization, differences in level and masked the effects of poor mic placement is no longer the benchmark. Thus it can no longer be the scapegoat.

Newer receivers, with improved internal speakers, offer the added capability of easier connection to the home entertainment system. They also have the potential for improvements in actual sound and perceived picture quality. The standard is no longer "broadcast quality" but rather "CD quality."

High-definition or advanced television will provide improved picture quality along with the side benefit of "CD quality" sound transmission capability. However, the benefits of bigger and better pictures will be fully realized only with bigger and better sound. Many of these improvements can be implemented today, by simple electronic upgrades such as replacing the analog cart machine with a CD player, digital cart machine or workstation.

Others can be accomplished with improved production techniques and quality control measures. Greater attention should be placed on maintaining level, loudness and phase uniformity, eliminating over equalization or compression, correct alignment of analog tape machines and Dolby processors and insuring a system interface free of hum and noise.

The extension of the usable lower frequency limits and the expansion of available dynamic range to 90 dB and greater has placed more stringent demands on the electroacoustic monitoring system as well as requires an acoustic environment of equivalent capability. As video facilities are constructed or upgraded to accommodate HDTV with stereo/surround sound of "CD quality", attention should be placed on providing an acoustical environment which does not hinder production and in which quality judgments can be accurately made.

Improved Audio Monitoring

At every location where quality judgments are made, the audio monitoring system must be equal to the task. It must be capable of accurately reproducing the lower octaves. Assuming a suitable audio monitor loudspeaker and amplifier are chosen, mounting becomes a critical issue.

Because the sound must relate to the picture, they are placed symmetrically about the main video monitor. In most cases, this is at or near one or more corners. At low frequencies most loudspeakers radiate over 360 degrees. Some of the low frequency energy will be reflected back into the room, slightly delayed. Because of the long wavelengths involved, most of this energy will be in phase. Unless the speaker is designed with this type of mounting, the low frequency response will be boosted. Certain frequencies, however, will be canceled, depending upon the exact geometry involved. The worst case occurs when the loudspeaker is equidistant from two adjacent walls as well as the ceiling.

Professional recording studios attempt to circumvent this problem by soffit mounting the audio monitor loudspeakers. This forces the low frequency energy to radiate over a 180 degree angle rather than 360 degrees, boosting the low frequencies more uniformly. Some loudspeakers have internal equalization for these alternate mounting methods. If not, external equalization should be incorporated. A 6 dB boost in the low frequencies does not allow for quality monitoring.

In either case, when using audio monitors with extended bass response, it is important that the be decoupled from the structure of the room. This is necessary not only to prevent rattles but also to maintain good stereo imaging. Sound travels faster through most structural materials than it does in air. The structure borne sound will arrive before the direct sound.

Enhanced Sound Isolation

All on-air, production, and post-production facilities have the common requirements for the appropriate degree of sound isolation from the outside. A foley room or narration studio where live sound is being recorded requires a lower noise floor than a post production room where external noise is more a distraction. The distinction is important because the cost of each increment of isolation can increase construction costs geometrically.

Proximity to external stationary as well as transportation noise sources will establish the degree of isolation required from sources beyond your direct control. When choosing a site, be sure to evaluate every possible condition. External noise levels vary greatly from day to evening. In a multiroom facility, the proximity to high level, low frequency internal sources almost always demands a level of isolation exceeding that from any external sources.

The conventional method of specifying the noise level requirements in a room is the NC (Noise Criteria) curve. The data on which the NC curves are based was gathered from surveys of office workers on the effects of environmental noise on their ability to perform work and to communicate speech. The curves were developed for the purpose of providing criteria for reducing complaints to tolerable levels. They have been adapted by the building industry as a simplified method of communicating the allowable noise levels in a room.

NC 15-20 used to be the standard for studios and concert halls. Digital recording and Dolby SR have lowered the standard goal to NC 5-10, and even below. Achieving this low level of noise is extremely costly, each 5 point reduction in noise criteria translating to a minimum 10-15% increase in construction costs.

At NC-20 the allowable noise level at 63 Hz is 51 dB SPL. While less than ideal, for many instruments and voice this may not be an insurmountable problem if, for example, the use of a high pass filter is tolerable. For Foley work, especially when the sounds being recorded fall in this frequency range and are at or below the background level, a filter won't help. When several channels are summed, the situation becomes even worse.

The human ear is less sensitive to low frequency sound. Most microphones are designed to be less selective. In passing through the electronic chain, any rumble picked up can increase distortion or even cause amplifier clipping.

The degree of sound isolation achieved in the field is dependent upon three factors: 1) the mass, stiffness and damping of the enclosing walls, ceiling and floor, 2) the airtight sealing of all penetrations for doors, windows, cables, and air ducts and 3) the physical separation from internal and external noise sources.

The first two are summarized in the STC (sound transmission class) rating system adopted by the building industry as a simplified means of comparing various barrier constructions primarily for residential and office applications. Because of this, test data for frequency bands below 125 Hz is not always available nor is it always reliable.

Wall, floor and ceiling transmission loss at low frequencies become much more complex to predict. The transmission loss is no longer governed by simply the mass, but such additional factors as damping, stiffness and panel dimensions come into play. Materials such as concrete and concrete block perform well in the low frequency range by virtue of their combined mass and stiffness. However, at 140 lbs./cubic foot, a 6" thick 12' high wall will weigh in at 840 lbs./linear foot. This is not a problem on grade, but in an existing high-rise office building will present some structural challenges. When increasing STC by adding mass reaches its practical limit, the only other method of increasing sound isolation is physical isolation.

Floating Floors

Floating or "room within a room" construction is usually necessary to prevent vibration and structure-borne sound from entering the room. Ease of access into the room and the law of gravity limits the physical separation of a floated floor from the structural floor.

There are three basic types of floated floors. The lowest cost and least effective is the use of a continuous underlayment. A continuous sheet of neoprene, fiberboard, or proprietary materials is laid down, covered with building paper or polyethylene, and concrete poured on top. Because of the limit static deflection, this type of floated floor is effective only in the mid and high frequency ranges.

The second type employs a neoprene or coated fiberglass fixed mount. These are placed at 12 to 24 inch centers and covered with plywood and polyethylene. Concrete is then poured over this form. A STC of 73 can be achieved with using this method.

For extreme low frequency vibration isolation, spring mounts are required. Although fixed springs have been used, the most common system is the raised slab system. Housed metal springs are placed on 3 to 4 foot centers with an integral steel reinforcing grid. Concrete is poured over the mounts and grid. After the concrete has cured sufficiently, about 30 days, the slab is raised by a process of slowly turning jack screws built into the mounts. Isolation down to sub-sonic frequencies can be obtained and STC ratings of 82 and greater are possible.

Inside Sources

Internal noise sources such as lights, dimmers and fans as well as the necessary penetrations of the enclosure such as cables and air-conditioning ductwork must be addressed. Dimmers, ballasts and transformers can be remotely located. When budget permits, DC lights and dimmers eliminate this potential noise source as well as significantly reducing the introduction of hum into electronic circuits.

Excluding doors and windows, air-conditioning ductwork represents the most significant penetration of an enclosure. Careful sealing and structural decoupling of these penetrations are crucial. Table 3 illustrates the importance of eliminating even the smallest crack in an otherwise adequate partition.

Air conditioning noise, especially low frequency noise, presents one of the most difficult challenges to providing a low noise floor. Conventional techniques such as fibrous duct lining and passive silencers are effective above 250 Hz. Passive silencers of sufficient length to control low frequency noise place severe static pressure restrictions on the air handling units.

Active noise control systems are becoming a viable solution to solving low frequency attenuation problems. An active noise control systems consists of a microphone that detects the noise as it propagates down the duct. A digital controller processes this signal, determines an appropriate canceling waveform and introduces this signal by way of a loudspeaker attached to the duct. A second microphone located just beyond the loudspeaker provides an error correction signal. Attenuation of 12-20 dB between 40 Hz and 160 Hz can be realized.

Independent duct systems for each critical room are necessary. Whether sheet metal, rigid fiberglass, or flexible, the ductwork should be routed and isolated so that it does not pick up any noise along its path or couple sound from one room to another. The ductwork should generate through vibration any noise of its own. Sheet metal duct, although having higher transmission loss that rigid fiberglass or flexible duct, can generate popping noises when it is pressurized or depressurized when it is not properly braced and damped. Square or rectangular duct is more susceptible to producing aerodynamically generated noise. Round or flat oval sheet metal duct minimizes both

of these problems. Flexible duct has very low transmission loss. It can generate crackling noises as it expands and contracts when the fan is turned on and off. Very often it becomes pinched, restricting airflow and generating noise. On the other hand it is very economical and does not transmit vibration as well as the other types.

To introduce the air into the room without adding noise requires a low outlet velocity. To attain NC 15 a outlet velocity of no greater than 250 ft./min. is required. Consequently, a large outlet area is required. Dampers within 20 feet of an outlet should be avoided as they reduce the net area of the duct and therefore increase noise. Grilles and diffusers must be sized so as not to restrict air flow.

Fine Tuning

Attention to detail is important in eliminating other potential internal noise sources. Any device with a fan should be relocated to a non-critical room if possible. Devices which must remain in the room should be treated on an individual basis to minimize their noise contribution. Light fixtures, air grilles and registers, and console and rack panels can be set into vibration at particular frequencies. These offenders can be easily identified with a sweep oscillator and damped with neoprene or foam.

In the studio, script stands or tables should also be investigated for resonance and covered with an absorbent material such as heavy felt or carpet. Chairs should be selected for quietness as well as comfort. Storage cabinets should be built so as not to rattle or resonate and prevent any items stored in them from doing so as well. Again, the best solution is to remove potential problem items from the room if they are not required.

Reverberation and Absorption

A reverberant room will be noisier than a non-reverberant room of the same volume. Sound is not absorbed when it strikes a boundary but is merely reflected back.

Unlike music recording studios where room character and moderate reverberation is desirable, for voice or Foley work, the room should be as transparent as possible. Voice and Foley studios for video are usually very small. Because of their low volume, traditional reverberation time calculations are not meaningful. However, any reflected energy should be made diffuse and of essentially uniform frequency content.

Reflection room resonance can be controlled by the addition of absorption with porous materials, diaphragms and resonators, geometric reflection control and diffusers. All of these techniques are to varying degrees non-linear with frequency. Porous absorbers become ineffective below 250-500 Hz, depending upon their thickness. Diaphragmatic and resonant absorbers are by design frequency sensitive. Most reflective surfaces become absorptive, usually with increasing frequency, and at some point can become diaphragmatic as well. Diffusive surfaces not only vary with frequency but also can be dependent upon orientation. They also usually become absorptive and diaphragmatic at certain frequencies.

At frequencies below 250 Hz, conventional techniques require a depth of porous material or a cavity equal to 1/4 the wavelength of the lowest frequency to be absorbed. At 20 Hz this is approximately 14 feet, at 40 Hz it is still over 7 feet. This consumes much real estate but for a critical listening environment the improvement in low frequency transient response is worthwhile.

Consider All Possibilities

Don't underestimate the importance of good sound. As video facilities are constructed or upgraded to accommodate HDTV with stereo/surround sound of CD quality, it is important to build an equally superior acoustic environment. This article has pointed out some of the important areas that should be given attention when constructing or remodeling a room for critical audio monitoring. Although fine tuning can only be done after construction, appropriate isolation must be built in. Correcting problems after the fact will always be costly.

 

(Sidebar) Is Home Room Noise the Limiting Factor?

It has long been assumed that home receiving and playback equipment as well as the home listening environment were limiting factors in chain. The CD and the proposed advanced television systems will eliminate the electronic portion of the chain from this consideration.

That leaves the home listening environment in question. Environment acoustics has received much attention over the past decade. Many communities have set criteria for the intrusion of transportation noise sources into the residential environment. Concern for energy efficiency has produced the side benefit of increased isolation for exterior noise.

In a recent AES paper, Cohen and Fielder [1] have readdressed the question of what is the appropriate background noise level for a recording facility. In a study of 27 home listening rooms, they found the rms average noise level to be NC 17. A substantial portion of the rooms measured had less noise than the average. These results agree with previous surveys referenced in the paper.

These conditions may not exist with kids, pets, kitchen appliances and HVAC units adding their contribution. But during serious viewing and listening situations, the noise levels measured are most likely reliable. More extensive research should be done to confirm this data.

[1] E. A. Cohen and L. D. Fielder, "Determining Noise Criteria for Recording Environments," J. Audio en. Soc., vol. 40, pp. 384-401 (1992 May).

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