properties. Excessive reverberation time, which can be calculated, can lead to poor speech intelligibility. Sound reflections create standing waves that produce natural resonances that can be heard as a pleasant sensation or an annoying one. Reflective surfaces can be angled and coordinated to provide good coverage of sound for a listener in a concert hall or music recital space. To illustrate this concept consider the difference between a modern large office meeting room or lecture theater and a traditional classroom with all hard surfaces. Interior building surfaces can be constructed of many different materials and finishes. Ideal acoustical panels are those without a face or finish material that interferes with the acoustical infill or substrate. Fabric covered panels are one way to heighten acoustical absorption. Perforated metal also shows sound absorbing qualities. Finish material is used to cover over the acoustical substrate. Mineral fiber board, or Micore, is a commonly used acoustical substrate. Finish materials often consist of fabric, wood or acoustical tile. Fabric can be wrapped around substrates to create what is referred to as a "pre-fabricated panel" and often provides good noise absorption if laid onto a wall. Prefabricated panels are limited to the size of the substrate ranging from to . Fabric retained in a wall-mounted perimeter track system, is referred to as "on-site acoustical wall panels". This is constructed by framing the perimeter track into shape, infilling the acoustical substrate and then stretching and tucking the fabric into the perimeter frame system. On-site wall panels can be constructed to accommodate door frames, baseboard, or any other intrusion. Large panels (generally, greater than ) can be created on walls and ceilings with this method. Wood finishes can consist of punched or routed slots and provide a natural look to the interior space, although acoustical absorption may not be great. There are four ways to improve workplace acoustics and solve workplace sound problems – the ABCDs. A = Absorb (via drapes, carpets, ceiling tiles, etc.) B = Block (via panels, walls, floors, ceilings and layout) C = Cover-up, or Control (background sound levels and spectra) (via masking sound) D = Diffuse (cause the sound energy to spread by radiating in many directions) Mechanical equipment noise Building services noise control is the science of controlling noise produced by: HVAC (heating, ventilation, air conditioning) systems Elevators Electrical generators positioned within or attached to a building Any other building service infrastructure