Open Plan Acoustics
Office Noise Reduction
Sound Masking for Offices
Office Sound Control Solutions
Noise Reduction for Open Plan Offices and Spaces
In the open plan office, the acoustical performance of panels, ceilings, floors and walls must be controlled if speech privacy is to be maintained, and the overall noise level is compatible with the intended use of the space. (Conference areas, executive offices and general office areas each have different sound level and privacy criteria).
With Open Plan Spaces the importance of good acoustics in an office environment has grown with the increasing use of systems furniture. We are surrounded by sound at all times. Some sounds are pleasant and soothing; others annoying and distracting.
Sound affects the way we feel and behave. In the working environment, sound is particularly critical. Complaints in the office often involve the lack of speech privacy, high noise levels and the distraction of, overheard extraneous conversations. These complaints risk becoming more pronounced when traditional office walls are replaced by partial height panels, which allow sound to circulate more freely throughout a space.
Speech Privacy in the Office
In considering what is and what is not good open plan acoustics, it is important to appreciate the difference between “hearing” and ‘understanding”. The muffled sounds from an adjacent work space generally are not as distracting if they cannot be understood. Of course, if they cannot be understood, you have confidential speech privacy even though some degree of distraction may exist. The overall ambient sound level is also important.
We create a quiet environment by dealing with the paths that sound takes between the sources (equipment and people) and the receivers (people).
In the office, people and equipment are sound sources. People also are receivers of that sound which is transmitted.
There are three paths by which sound travels: a direct path which is the straight line between the source and receiver; a reflected path which occurs as sound bounces off various surfaces; and a defracted path which involves sound bending over the top and around the sides of partitions. The control of sound in an open office requires consideration of all three paths.
To stop the direct path of sound, we erect barriers (system wall panels) which stop sound from passing through. The STC, or Sound Transmission Class, of a partition measures the ability of a barrier to stop sound from passing through it. A material of an STC of 21 will prevent 21 decibels from passing through.
The most sound reduction that can be expected between workstations is 21 decibels, because sound will defract, that is, bend over the top and around the side of partial height partitions. An STC of more than 21 is not generally an advantage.
Acoustically absorbent panels absorb rather than reflect sound. The reflection of sound off hard surfaces is called reverberation. The absorption of sound, on the other hand, actually refers to energy conversion.
Sound is created when something resonates i.e. pushes against the air and retracts creating waves of dense and rarefied air – fluctuations in air pressure.
Sound is a form of energy, and energy cannot be destroyed. It can be dissipated as it spreads out over distance or converted into another form of energy. Acoustical panels convert sound energy into mechanical energy. As sound waves impact the material, the material responds by vibrating. Those vibrations (“wiggles’) are then dissipated as a minute amount of heat. The ability of a material to convert sound energy to mechanical energy is measured in a test that provides the Noise Reduction Coefficient (NRC). An NRC of 70 means that a material absorbs 70 percent of the sound that hits it. The reciprocal, 30 percent, is returned.
The ability of a material to absorb sound determines its acoustical capabilities. The most effective sound reduction in a office environment is achieved when the higher frequencies of human speech, those that lend intelligibility, are those which are absorbed. These are often referred to as the articulate speech frequencies.
The NRC is a simple average of the material’s absorption of sound at frequencies of 250, 500, 1,000 and 2,000 Hertz (Hz). Hertz cycles per second (CPS), and frequency all refer to the number of fluctuations per second which determines the pitch of a sound.
Our ears are more sensitive to certain frequencies – it is no accident that these frequencies are the same as those of human speech. It is the higher frequencies of human speech (1,000 Hz through 3,000 Hz) that provide intelligibility. It is these frequencies which must be considered most closely in developing an acceptable open plan environment. There is also a test which provides an Articulation Index (AI) This test measures the intelligibility of sound (usually the human voice) by giving a higher value to the articulate speech frequencies.
In evaluating an NRC rating for open plan acoustics, it is important that the higher absorption coefficients are at the higher frequencies (the articulate frequencies of human speech).
Two different materials may have the same NRC but, the one which absorbs more of the intelligible (higher) speech frequencies is a more effective material for controlling sound in an office environment. In fact, the reflection of low frequencies may be an advantage in that it permits an ambient sound level that can reinforce the background masking sound.
It is important to have background masking sound to overwhelm that sound which is not absorbed or blocked by acoustical panels as well as that sound which is detracted. That sound can be masked artificially with electronically produced sound evenly distributed throughout a space, usually through speakers above the ceiling. It cannot be masked with the low frequency sounds of speech, heating, ventilating and air conditioning equipment, or other background sound.
Effective sound masking has the following characteristics:
- It is uniform throughout the space – no “hot spots” or “dead spots.”
- It is the correct volume – louder than what you don’t want to hear but, not so loud as to interfere with the conversations you want to have. One should never feel compelled to speak over it or strain to listen.
- It has the correct tonal qualities. A “humm” not a “hiss.” It is never obtrusive – it disappears and may be thought to be the normal background sound of a well designed HVAC system.
- It is not noticed when it is on but, is missed if turned off. (You never turn it off !) Background music is not effective as masking. It is another specific signal which may or may not be a distraction – depending on the individual and /or the task.
To be effective, the masking level should be 4 to 10 decibels louder than the level of incoming intrusive speech from adjacent work stations. See our page on Sound Masking for more information.
Putting the Pieces Together
In an open plant office, the STC (Sound Transmission Class) and NRC (Noise Reduction Coefficient) must be balanced to achieve good speech privacy, while background sound levels are comfortably maintained.
The combination and interaction of the STC and NRC is measured as the Noise Isolation Class (NIC). A furniture system with a NIC of 21 will reduce sound by approximately 21 decibels between work stations. Developed in the 1970’s, this criteria has fallen into somewhat of non-use; probably though lack of understanding of it by those who should be using it in the design and evaluation of space. Each component in the open plan can be tested under this criteria; however, an educated evaluation of traditional properties (NRC, STC, etc.) can suffice.
It is important to recognize that all of the components in an open plan office – ceiling, walls, floor and partitions – contribute to or detract from the overall acoustical performance of the space. Each component can only compensate so much for the poor performance of another component. When too many compromises are made, the overall performance suffers.
“NO ONE COMPONENT CAN DO THE JOB. IT CAN ONLY DO ITS JOB!”
While carpeting reduces some airborne sound; it’s greatest contribution is in the reduction of “foot-fall” sound. Carpeting stops sound from starting – impact noise.. It’s use on walls is not recommended for noise control.
Performance criteria for open plan offices include:
- Ceilings NRC .75 or higher
- Partitions STC 21 or higher
- Partitions NRC .70 or higher
- Walls NRC .80 or higher
- Floor (Carpeted) NRC .15
Spacing between people also is important and should be maximized with the layout of the office. “Lines of sight” between people should also be minimized. If you can see someone, you haven’t dealt with the direct path of sound. Separate noisy activities from quiet ones.
Flat lighting fixture lenses should be avoided or replaced. From a purely acoustical perspective, the best lighting in the ceiling is: no lighting in the ceiling, ambient or indirect lighting. The next best is fixtures with deep parabolic diffusion lenses (eggcrates.)
This basic understanding of sound, its measurement, and how to control it will help us to create the most functional acoustical environment in open plan and other office space.