It’s time to re-visit the issues of acoustics for conference rooms. We are getting a number of phone calls from clients looking for help with the quality of their conference calls. Problems are in one of two areas: it is either a problem with the room or the equipment has not been optimized. Over the past few years fixed audio teleconferencing systems have improved, but with that improvement has come an increase it the complexity of the setup. Today’s systems all require a computer and detailed knowledge of how their software algorithms control the Digital Signal Processor inside the box. The rooms we are being asked to look at are large rooms that are too big for the tabletop “Polycom style” units.

Once you have chosen a room for teleconferencing, it is important to have it evaluated by an acoustic or audiovisual engineer. The goal is to optimize the acoustic quality of the room, making the conversations more intelligible for both your group and the people at the other end of the line. You may not know it, but there are several things lurking around in most conference rooms with the power to wreak havoc on your audio. These are known as STC, NRC, and NC ratings.

The first, STC, stands for Sound Transmission Class. This rating basically describes how soundproof your room is. The higher the sound isolation between one room and the next, the higher the rating. Construction of thick walls and a thick ceiling have the greatest effect on blocking exterior noises and boosting your conference room’s STC. This rating is especially important if you are as concerned about sounds (such as private conversations) being heard outside the room as you are about noise coming in.

The second rating, NRC, stands for Noise Reduction Coefficient and deals with how much sound the materials on the walls and ceiling inside the room absorb. Vocal echoes that may sound slight to you will be picked up by your microphone and amplified for those on the other end of the teleconference. The reverberation you hear in the room is really just a series of echoes building up until you cannot distinguish their source. Parallel walls made of hard materials are the most common source of these unwanted echoes. Rectangular rooms can also cause this problem by allowing sound from one end of the room to reflect back. One solution for defeating this type of echo is to “color” the sound in the room. This is achieved by covering portions of the ceiling or walls with slot finishing and perforated metals or woods. Another source of these echoes is glass. Large windows facing lobbies or looking over a skyline should be covered with cloth drapes to dampen sound reflection.

The last rating, NC, for Noise Criteria, indicates the level of background noise present in the room. This noise is usually made up of sounds that we have learned to tune out: computer fans, a nearby elevator, the rumble from air ducts, and many other commonplace office noises. While you may be blissfully unaware of these sounds, your microphone isn’t. This background noise will be transmitted to the other group and provide an unnecessary distraction that runs the risk of masking your voice. Close coordination between audio consultants and design/construction teams can identify and reduce the sources of this noise.

When you are convinced the conference room is at its acoustic peak, the final step is the installation of the microphones and loudspeakers. Luckily, this is much less involved than creating the room. The main thing to remember is that the microphones should be between one and three feet from the person speaking. If there is more than one person speaking, use more than one microphone. In those cases, it’s a good idea to also install an automatic microphone mixer. These are designed to activate only the microphone closest to the person speaking, eliminating the problem of several voices being transmitted at once.

While mounting a microphone on the ceiling may make the room appear more attractive, it puts the microphone out of its effective range. To make matters worse, most loudspeakers are ceiling mounted and their sound waves have a more direct route to the microphone than a speaker’s voice. Once the sound of the loudspeaker is picked up by the microphone, it travels through the system in a cyclical route while being amplified. The resulting high-pitched squeal is known as feedback. Anyone familiar with the annoying screech of feedback knows that once it begins, the conversation ends.

Another important thing to keep in mind is that a problem called “acoustic echo” occurs when teleconferencing in full duplex mode. Full duplex mode is when both parties are able to talk at the same time, like in face-to-face conversations. To do this, both parties set their teleconferencing unit on Full Duplex Mode. This leaves their mics open and speakers on, which lets the audio signals travel in both directions at once. The problem arises when the sound from their loudspeakers (your voice) is transmitted through their mics. Hearing the delayed sound of your own voice returning through your loudspeakers can be quite confusing. To remedy this, a device called an echo canceler can be used. An echo canceler looks at the audio coming through the loudspeakers, compares it to the audio going into the mics, and eliminates any matches from the transmission. To effectively cancel out “acoustic echo”, these units should be used at both locations.

Knowing the basics of teleconferencing will make you more comfortable with system. However, when your teleconference begins, don’t forget the most important detail–your notes!