Academy of Architecture for Justice

As an acoustic consultant, I work on a wide variety (very wide... exotic bird farms anyone?) of projects on a daily basis: projects ranging from new construction and renovation to mechanical noise and theatre acoustics. And with each new project, much like in the field of architecture, we must quickly gain a detailed understanding of the end goals specific to the user so that we can make informed decisions. We must, for the length of the project, become experts in the uses and functions of the buildings we are designing. Justice facilities are no different.

Each type of facility (and each individual facility for that matter) has its own set of acoustic demands unique to achieve optimum performance for all of its users. Below I have begun to outline those principles that begin to define the acoustic success of the three major typologies of justice architecture: Courtrooms, Detention Facilities and Offices.  Please refer to the Glossary at the end of this article for a detailed look at some of the acoustic terms and metrics referenced.

Courtrooms

        One of the most significant aspects of the acoustic design of courtrooms is the demand for speech privacy and intelligibility. Poor courtroom acoustics can be documents as far as 40 years back1, and more recently in 2010 a judge was forced to declare a mistrial in a case when speech was unintelligible in a historic courthouse in Mississippi2. Speech privacy will be dictated mostly by the Sound Transmission Class (STC) rating of the courtrooms perimeter walls. The division between courtroom and hallway, courtroom and jury room, and courtroom and council room are all particularly important. To maintain appropriate privacy levels, walls and wall assemblies (inclusive of any doors and windows) should be rated minimum STC 45-50 and have all necessary seals. It is also important to ensure that HVAC ductwork does not bridge adjacent rooms. The measures above will guarantee that conversations will not carry between spaces.

        In courtrooms where there are complex messages being conveyed and unfamiliar words being delivered, speech intelligibility is critical and the minimum Speech Transmission Index (STI) value to be achieved is 0.7. Excellent speech intelligibility with high STI can be obtained by making sure that background noise levels remain low, limiting echo conditions, lowering reverberation time, and choosing quality audio equipment. All of these actions will also help to accommodate AV and recording systems.

        Another unique difficulty specific to courtroom design is the combination of two uses with opposing acoustic needs; in this case, two areas in front of and behind the bar. There is a need to architecturally support speech at the bench and the well of the courtroom, and deaden sound in the gallery. While the courtroom as a whole is one singular volume, we can manipulate the surfaces and finish materials of each area to support speech in one and lower reverberation in the other.

Detention Facilities

        The most problematic aspect of maintaining proper acoustics in detention facilities is finding a balance between secure, durable materials that also have a high enough absorption coefficient to minimize excess reverberation and keep a low Reverberation Time (RT). This becomes especially critical for wall finishes that are easily accessible by inmates. Perforated metal panels, spray-applied acoustical treatments, and abuse-resistant sound absorbing panels are all viable options that will help reduce RT. A general rule of thumb for any space is that for a six-sided volume, at minimum, one of the six sides should be a material with a Noise Reduction Coefficient (NRC) of 0.75 or higher. Typically the easiest way to achieve this is on the ceiling with a high NRC acoustic ceiling tile.

        Reducing the RT will prevent any noise generated in a space from reflecting off surfaces and building up within a volume, but it is equally important to keep background noise down and lower the overall ambient noise in a volume. The biggest contributor to background noise is mechanical systems, particularly HVAC equipment and unlined ductwork. Reducing reverberation time, limiting reflections and echoes, and keeping background noise low will all keep the STI high and allow for open and easy communication.

Offices

        The office environments associated with justice facilities can get very noisy, very quickly. But there are some simple measures that can be implemented to keep noise at a minimum. The GSA has a set of facility standards that encourages proper office acoustics, but some of their other office design principles oppose strategies for proper acoustics. For example, the GSA encourages an open office plan with low, transparent cubicles for visibility and communication 3. These low, clear (read: hard and reflective) cubicles allow for sound to travel and reflect very easily.         

In office facilities, sound privacy is again critical; this is where sound masking becomes helpful. In this instance, a degree of background noise from the HVAC systems could be beneficial in masking the speech that would otherwise be transmitted between spaces. In areas where the HVAC system noise isn’t uniform or doesn’t meet the required background noise level it should be supplemented by an electronically produced noise spectrum. The optimum background noise level for an open office for speech privacy and masking is RC 40-50 N.

The furniture layout can also greatly help to increase speech privacy and lower unwanted noise.  Cubicle walls should be long enough (at least 6 ft) to prevent diffraction around the partition. Ideally, cubicles will have three walls, and the corners formed by the walls will trap sound and prevent diffraction. The layout of the cubicles should have desks facing inwards and away from other desks, and openings should be staggered to prevent direct sound paths across cubicles. Ceilings above workspaces should have a high Noise Reduction Coefficient (NRC) value and large lighting or mechanical fixtures shouldn’t be placed above cubicle partitions where they could reflect sound from one space to another.

All Facilities

In general, it is always important to consider potential problematic space and use adjacencies in any type of justice facility. Little things like placement of panic hardware can have a ripple-effect on design decisions affecting a much larger area than the room being considered. A great deal of attention should be given to the design, sizing and overall strategy for the mechanical equipment. In the case of a courtroom mentioned above, background noise should be kept at a minimum and an under-floor air supply system may be considered. In an open plan office mechanical noise can be beneficial to sound masking and the HVAC design may be less critical acoustically. Regardless, incorporating acoustics early in a project will prevent redesigns later in the design phase or costly noise remediation efforts once the building is occupied.

Acoustic Terms Glossary

A great resource for these and other acoustic issues has been provided by www.acoustics.com

The GSA’s Facilities Standards for the Public Buildings Service (PBS-PQ100.1) also has a rigorous set of guidelines with some basic definitions of acoustic issues.

Sound Transmission Class (STC) - STC measures the ability of a wall or wall assembly to prevent the transmission of sound.  For example a wall rated STC 40 will reduce a 100 dB sound on one side by 40 dB, to 60 dB on the other. This is important in both keeping background noise levels in a space low, and maintaining privacy in a space. It is also important to note that any penetration in a sound rated wall assembly (window, door, etc) becomes the weak link in acoustic isolation. For sound critical walls, windows should be impact-rated to raise STC values, and doors should have perimeter seals and thresholds to seals any gaps.