Building a One-Way Mirror

I've moderated usability studies in about a dozen labs over the years in the US, Germany and England. I was heavily involved in building the usability labs for HumanCentric in Raleigh and at the Truist Financial Corporation headquarters in downtown Charlotte. These are my suggestions for building out a lab, focused on the glass, lighting, and sound proofing.

In a separate article, I discuss whether you need one-way mirror in your usability lab.

A one-way and a two-way mirror are the same thing. The term one-way refers to it being a mirror on only one side. The term two-way refers to its ability to be used in two different ways, either a mirror or a window, or from two different sides (but only one at a time).

What is a one-way mirror?

A one-way mirror is glass that has been coated with a thin layer of metal so that it's reflective, like a mirror. One side works as a mirror and the other side as a window. The terms one-way and two-way mirror are both used and mean the same thing. One-way mirrors are commonly used in usability labs and focus group rooms, but also in other locations like observation rooms in police stations, day care facilities, and reality TV filming locations.

Basics

The basic design is two panes of glass separated by an airspace. The pane on the participant side is mirrored, and the pane on the observer side is clear glass. There are two design issues involved: light and sound. Reducing light transmission is the easy part; reducing sound transmission is much more difficult.

Why two panes? Acoustics

If all you wanted to do was control light then you could use a single mirrored pane and be done. The second pane and air gap help reduce sound transmission. You want a STC of 35 as a bare minimum, 40-45 is better. There are many lab design techniques for reducing sound transmission between the study room and observation room. In this article, I focus on just the issues specifically related to the design of the one-way mirror.

Choosing your glass

Silvering - the glass used in one-way mirrors differs from regular mirrors in that the silver coating is thinner (generally 50% to 75% of mirror silvering), and it lacks the coat of paint typical on mirror glass. Tell your glass supplier that you want glass for an observation room and they will know what you mean. We used Mirropane. Be careful about the cleaning solutions you use on the glass because some can dissolve the coating; check the manufacturer's instructions.
Tinting- tinting helps further reduce light transmission. The mirrored glass we bought came pre-tinted. You can also buy untinted glass for the mirrored pane and then apply a tinting film to the unmirrored pane. This has the advantage that you can test out different tints and select one that has a good balance of visibility vs protection from light, but removing and re-applying tints is more expensive and time-consuming.
Tempering- Tempering makes glass stronger and for safety reasons is often recommended for interior windows, but you do NOT want to use tempered glass. The tempering process warps the glass, producing a fun-house effect. The warping catches your eye when you move, continually reminding your participants that they are being observed through a mirror.
Laminate - you can use either regular glass or laminate. Laminate is safer because it is more likely to stay in one piece if cracked, and it is slightly more soundproof than plain glass.
Thickness - the specific thickness of the two panes is less important than that the two panes be different thicknesses and/or materials to help reduce sound transmission. Different thickness/material panes have different profiles of the sound frequencies that they transmit, and so two different panes will block slightly different frequencies. You could use 1/4" and 3/8" panes, or 1/4" glass and 1/4" laminate.
Size - typical panes come in a maximum size of about 6 x 10 feet. You can connect multiple panes of glass with a thin ribbon of epoxy or silicone or similar substance and the seams will be barely visible from a distance.
Weight - glass is about 3.5 times as dense as gypsum (drywall) at the same thickness. Make sure your wall can support the weight of both panes. Larger openings will weaken the wall more.
IGUs - you do not want to use IGUs (insulated glass units in the US, or double glazed units in Europe). The maximum air gap of these factory-manufactured double panes is about 1.5", but you want a 2.5"-4" gap for sound insulation purposes (see below).
Argon and krypton - these gases are more dense than air. In home and building construction they are commonly used in IGUs to provide additional heat insulation. You don't want them in your usability lab because they have little value for sound insulation, both because of the sound transmission qualities of the gas and because they require factory-sealed IGUs with too small a gap between panes.
Triple panes - if a double-paned setup is good, triple-paned would be better, right? Wrong. You want to maximize the airspace between your panes of glass. Assuming your wall thickness limits how far apart you can place your panes, adding a third layer of glass means having two smaller gaps instead of one big gap. The added sound insulation of the third pane is a LOT less than the loss of sound insulation due to cutting your airspace into two sections.

Mounting

Orientation of the mirror - you'll want the mirrored side of the mirrored pane to close to and facing the participant. It's tempting to put the mirrored surface facing into the airspace to protect it from scratches and cleaning fluids. However, glass is slightly reflective, and this may result in a double-image effect where you can see reflections on both the glass and silver sufaces at the same time.
Gap between panes - the larger the gap, the better sound insulation. At less than 2. the air acts like a solid and transmits more sound. Every extra inch of gap adds an extra 3dB of noise reduction. Sound studios may use a 6. to 10. gap, but this is necessary because they use angled panes, and is achieved by building two separate walls. You'll want to maximize the gap between the panes, within the contraints of the thickness of your walls. For a usability lab, 2.5. to 4. should be sufficient. When discussing this with your glass contractor, they may speak in terms of gap between the glass frames, which isn't exactly the same as the gap between the panes.
Angled panes - angling glass prevents reflecting sound back to its source and slightly reduces sound transmission. In a recording studio, controling sound reflection is extremely important, but it's not an issue for a usability lab. More important is that angling the pane reduces the volume of air between the panes. Sound studios, with a 6. to 10. gap, can afford to loose some air volume. If you are installing into a typical office wall, maximizing the volume of air between the panes is more important than an angle. If you have a wide gap and would like to angle one pane, a 1/4" difference between the top and bottom of the unmirrored pane should be sufficient.
Wall framing - you can frame out, plaster and paint the opening for the one-way mirror before installation begins. Paint the frame a medium or dark color so that participants don't see it illuminated with the light from the participant room. Make sure you run all your cables (power, network, audio/video) before you frame the mirror space.
Sound absorbing material - you can fill the gap between panes with sound-absorbing material like neoprene and foam. We decided this was unecessary.
Glass frame - your choice of frame should be guided by the ability to seal all air gaps around the panes and to maximize the gap between panes. We used a snap-in frame that only needs mounting screws on one side so that it can be placed closer to the outer edge of the frame.
Seal adjacent panes - if you need to purchase multiple panes to fill in your space, seal the seam with a ribbon of epoxy or silicone. It will block sound and be barely visible.
Seal gaps - when installing the frame, caulk around the inside and outside of both frames to make sure there are no air gaps to allow the passage of sound. Close any seams in the framing with caulk or epoxy.
Dust and prints - be extra careful that there are no fingerprints or construction dust on the pane interiors or frame between the panes, because once that mirror is up you are never going to be able to clean it!

Lighting

One-way mirrors work when the participant side is bright and the observer side is dark, it's as simple as that. If the moderator wants to turn down the lights because they have a headache that day and the observer wants brighter lights to write on paper you risk turning your one-way mirror into a window and allowing your participants to see into the observervation room.

Participant room lighting should be as bright as you can make it without giving the moderator and participants a headache.

Observation room lighting is more complicated. Here are some tips:

Dimmable lights - you'll want to be able to dim the lights during sessions, but your observers may go crazy if they can't have bright lights between sessions or during setup.
Blackout curtains - cover any windows with blackout curtains. True blackout curtains are extremely expensive and not necessary for most rooms. Our labs are in a wooded area and don't get much direct sunlight, so we used drapes we bought at a home linens store, and added light-blocking liners. Mount the drapes several inches higher than the top of the window to minimize vertical light spillage. Velcro can help attach panels together and close gaps between the curtains and the wall. Pick a dark color for the drapes.
Paint - paint the walls of the observation room and the gap between the panes a medium or dark color. We used gray.
Doorways - be very careful of doors that can be seen from the participant room that might be opened during a session. Some strategies:
- Place the door on the same wall as the one-way mirror, if your observation room is wider than the participant room.
- Keep the hallway outside the door dim by turning off the lights or removing bulbs
- Most doors can be hung to open on either side; hang the door such that the door itself blocks the hallway from view when opened
- Build a wall to block sight of the door. In our lab you enter into a short hallway so the door doesn't have to be closed at all
- Hang a curtain on the door
Ceiling lights - avoid lights that can be seen from the participant room. Mount the lights above the one-way mirror so that they are blocked from view by the wall, or use deeply recessed canister lights.
Monitors - orient monitors in the observation room away from the one-way mirror, and turn down the brightness.
LEDs and other point sources - avoid lights in the observation room like LED status lights and backlit LCD displays. You can cover lights with static-cling tinting film sold for tinting your own car windows. Watch out for backlit logos on the lid of your laptop.
Light-colored objects - avoid placing anything light-colored near the one-way mirror because the light spilling in from the participant room will increase its visibility. This includes observers wearing light-colored clothing, white note paper, and white notebook computers.

Additional sources

How to Build a Small Budget Recording Studio From Scratch (book)
Sound Studio Construction On a Budget (book)
Aurelex Acoustics: Practical Guidelines for Constructing Accurate Acoustical Spaces and many other user guides, FAQs and blog articles on the Aurelex website
Auralex Acoustics YouTube channel

You may also be interested in another article of mine, Does your usability lab need a one-way mirror?