ISTIQ Noise Control Weblog

Room acoustics describes how sound behaves in an enclosed space.

The way that sounds behave in a room can be broken up into roughly four different frequency zones:

• The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone sound behaves very much like changes in static air pressure.
• Above that zone, until the frequency is approximately 11,250(RT60/V)^1/2, wavelengths are comparable to the dimensions of the room, and so room resonances dominate.
• The third region which extends approximately 2 octaves is a transition to the fourth zone.
• In the fourth zone, sounds behave like rays of light bouncing around the room.

Natural Modes

Computer plot of the application of criteria to a room with correct dimensions

Computer plot of the application of criteria to a room with bad dimensions

Typical acoustical absorption of the standard panels developed by Oscar Bonello

The sound wave has reflections at the walls, floor and ceiling of the room. The incident wave then has interference with the reflected one. This action creates standing waves (described at Standing wave) that generate nodes and high pressure zones. The mode spacing is a very important matter especially in small and medium size rooms like recording studios, home theaters, broadcasting studios and concert halls. Some methods were created from 1940 - 1981 to obtain the best mode spacing to avoid sound coloration, but none of them succeed, because the mode spacing is not only a geometric problem  An incorrect mode spacing in a room determinates a bad frequency response from a loudspeaker or a music player

In order to solve this problem, Oscar Bonello, professor at the University of Buenos Aires, created in 1981 the Modal Density concept that solves the problem introducing concepts from Psychoacoustics  This new Bonello’s Criterion as it was named, analyzes the first 48 room modes and plot the numer of modes in each one-third of octave. The curve must increase monotonically (each one-third of octave must have more modes than the preceding one). This Criterion is now the standard method of designing the room dimensions.

Reverberation of the room

After determining the best dimensions of the room, using Modes Density criteria, the next step is finding the correct reverberation time. A good explanation of the theory can be found at Reverberation. The reverberation time depend on the use of the room. Times about 1.5 to 2 seconds are needed for Opera Theaters and Concert Halls. For Broadcasting & Recording studios and Conference rooms, values under one second are frequently used. The recommended Rev Time is always functioning of the volume of the room. Several authors give their recommendations A good approximation for Broadcasting Studios and Conference Rooms is: TR[1khz] = [0,4 log (V+62)] – 0,38 TR in seconds and V=volume of the room in m3  The ideal RT must have the same value at all frequencies from 30 to 12.000 Hz Or at least, is acceptable to have a linear rising from 100% at 500 Hz to 150 % down to 62 Hz

In order to get the calculated RT in a room, several acoustics materials can be uses as described in several books. A valuable simplification of the task was proposed by Oscar Bonello in 1979. It consists of using standard acoustic panels of 1 m2 hanged from the walls of the room. These panels use a combination of three Helmholtz resonators and wooden resonant panel. This system gives a large acoustic absorption at low frequencies (under 500 Hz) and reduces at high frequencies to compensate the typical absorption of people, lateral surfaces, ceilings, etc

HVAC (pronounced either “H-V-A-C” or, occasionally, “aitch-vak“) is an initialism/acronym that stands for “heating, ventilating, and air conditioning“. HVAC is sometimes referred to as climate control and is particularly important in the design of medium to large industrial and office buildings such as sky scrapers and in marine environments such as aquariums, where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within. In certain regions (e.g., UK) the term “Building Services” is also used, but may also include plumbing and electrical systems. Refrigeration is sometimes added to the field’s abbreviation as HVAC&R or HVACR, or ventilating is dropped as HACR (such as the designation of HACR-rated circuit breakers).

Heating, ventilating, and air conditioning is based on the basic principles of thermodynamics, fluid mechanics, and heat transfer, and to inventions and discoveries made by Michael Faraday, Willis Carrier, Reuben Trane, James Joule, William Rankine, Sadi Carnot, and many others. The invention of the components of HVAC systems goes hand-in-hand with the industrial revolution, and new methods of modernization, higher efficiency, and system control are constantly introduced by companies and inventors all over the world.

The three functions of heating, ventilating, and air-conditioning are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs. HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution.^[1]

In modern buildings the design, installation, and control systems of these functions are integrated into one or more HVAC systems. For very small buildings, contractors normally “size” and select HVAC systems and equipment. For larger buildings where required by law, “building services” designers and engineers, such as mechanical, architectural, or building services engineers analyze, design, and specify the HVAC systems, and specialty mechanical contractors build and commission them. In all buildings, building permits for, and code-compliance inspections of the installations are the norm.

The HVAC industry is a worldwide enterprise, with career opportunities including operation and maintenance, system design and construction, equipment manufacturing and sales, and in education and research. The HVAC industry had been historically regulated by the manufacturers of HVAC equipment, but Regulating and Standards industries such as ASHRAE, SMACNA, ACCA, and AMCA, have been established to support the industry and encourage high standards and achievement.

The International Association of Plumbing and Mechanical Officials (IAPMO) published its first version of the Uniform Mechanical Code (UMC) in 1967. The UMC provides complete requirements for the installation and maintenance of heating, ventilating, cooling and refrigeration systems, while at the same time allowing latitude for innovation and new technologies. The 2006 UMC is supported by the Mechanical Contractors Association of America (MCAA) and Plumbing-Heating-Cooling Contractors National Association (PHCC-NA).

Most recently, the ICC has been established to create international standards that many countries, including the US, Canada, the UK, Australia and many others have been adopting.

Air-conditioning

Air Conditioning and refrigeration are provided through the removal of heat. The definition of cold is the absence of heat and all air conditioning systems work on this basic principle. Heat can be removed through the process of radiation, convection, and conduction using mediums such as water, air, ice, and chemicals referred to as refrigerants. In order to remove heat from something, you simply need to provide a medium that is colder — this is how all air conditioning and refrigeration systems work.

An air conditioning system, or a standalone air conditioner, provides cooling, ventilation, and humidity control for all or part of a house or building. The Freon or other refrigerant provides cooling through a process called the refrigeration cycle. The refrigeration cycle consists of four essential elements to create a cooling effect. A compressor provides compression for the system. This compression causes the cooling vapor to heat up. The compressed vapor is then cooled by heat exchange with the outside air, so that the vapor condenses to a fluid, in the condenser. The fluid is then pumped to the inside of the building, where it enters an evaporator. In this evaporator, small spray nozzles spray the cooling fluid into a chamber, where the pressure drops and the fluid evaporates. Since the evaporation absorbs heat from the surroundings, the surroundings cool off, and thus the evaporator absorbs or adds heat to the system. The vapor is then returned to the compressor. A metering device acts as a restriction in the system at the evaporator to ensure that the heat being absorbed by the system is absorbed at the proper rate.

Central, ‘all-air’ air conditioning systems are often installed in modern residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required. A duct system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts. An alternative to large ducts to carry the needed air to heat or cool an area is the use of remote fan coils or split systems. These systems, although most often seen in residential applications, are gaining popularity in small commercial buildings. The remote coil is connected to a remote condenser unit using piping instead of ducts.

Dehumidification in an air conditioning system is provided by the evaporator. Since the evaporator operates at a temperature below dew point, moisture is collected at the evaporator. This moisture is collected at the bottom of the evaporator in a condensate pan and removed by piping it to a central drain or onto the ground outside. A dehumidifier is an air-conditioner-like device that controls the humidity of a room or building. They are often employed in basements which have a higher relative humidity because of their lower temperature (and propensity for damp floors and walls). In food retailing establishments, large open chiller cabinets are highly effective at dehumidifying the internal air. Conversely, a humidifier increases the humidity of a building.

Air-conditioned buildings often have sealed windows, because open windows would disrupt the attempts of the HVAC system to maintain constant indoor air conditions.