I Can Hear a Pin Drop
When an acoustical consultant walks into a room, the first thing they tend to notice is the acoustics. It can’t be helped, it’s second nature in much the same way a decorator would look at the furnishings. Occasionally someone else will make a comment such as, “The acoustics in that room were so good, I could hear a pin drop.” Or, at performance halls, “The sound there was fantastic, you could hear everything perfectly.” When acoustics are overlooked in the design of a building, comments often sound something like, “I couldn’t hear a thing they were saying. That place is just an echo chamber!”
The quality of sound within a room is typically referred to as acoustics. Good acoustics is very dependent upon the type of space and the sort of activities which take place there. Large atriums and lobbies are expected to be lively and full of sound. Theatres, on the other hand, need to be quiet so we can hear the person on stage reciting their lines.
Good acoustics centers around three very different issues. The first is Sound Isolation. Whether a room is large or small, noise and sounds from the adjacent spaces need to be kept out. How often have you been to a movie theater where you could occasionally hear the soundtrack from the neighboring theater? When this occurs, it’s because the partition assembly between the rooms does not provide enough sound isolation (or noise reduction) to adequately block the transfer of sound.
The comment “it was so quiet, I could hear a pin drop” actually describes the background noise levels within the room. This means that sounds generated by the building’s mechanical systems are quiet enough to allow you to hear a whisper or a pin drop. If the operating systems were louder, the sound made by the pin drop (or possibly by an actor on stage) would be overpowered by the background noise within the room. This second item, commonly known as Mechanical Noise and Vibration Control, becomes more important with each new machine added to a building, whether it’s an office copier, an air conditioner, or a movie theater projector. More machines mean more noise.
The third factor to consider is Room Acoustics. A main component of this is reverberation time. As a technical definition, reverberation time is the length of time it takes sound to decay 60 decibels. For a more tangible example just think about a bare room – no furniture or decorations on the walls, voices seem to bounce around, giving the room a very bright or lively sound quality. As we move furniture into the room, the brightness or liveliness of the room diminishes. This is because we have added absorption to the room, and by blocking or redirecting the path of the sound we have reduced the reverberation time.
Shorter reverberation times typically allow for better speech intelligibility, while longer reverberation times have the opposite effect. However, rooms with longer reverberation times tend to be considered better for music because it allows sounds to blend together, to fuse, to mix. This enhances the musical experience and creates the sense of ensemble that we hear within a concert hall. But this same blending reduces speech intelligibility by mixing consonants with vowels.
Exactly what is the difference between reverberation and echoes? Technically speaking, an echo is a discreet reflection heard bouncing off from a wall surface. Reverberation is basically a series of non-discreet echoes that have all been blended or fused together as the sound decays. Rooms that have dimensions of greater than 40 feet in any one direction should be reviewed for echoes. Are echoes bad? Not necessarily. There are very few rooms that you go into that will not have an echo. But for something to be heard as an echo, it requires three things: a source, a listener, and an acoustical path.
Give Him a Big Hand
When an acoustician walks into a room and claps their hands, they are listening for echoes. That hand clap acts as the initial sound source. As the sound travels away from the hand clap, it bounces off the different surfaces and returns back to the acoustician’s ears. They listen to how the sound has been affected among various acoustical paths. Echoes are perceived as reflections start to decay. Again, is that bad? Well, it’s only bad with typical use. If the room is not going to have a sound source in the location of the hand clap and a listener at the same location, then it won’t matter. The existence of an echo in that location is caused by the geometrical relationships of the space.
So, what does this all mean? No matter how large or small the room may be, it is important to understand the acoustical requirements. How is the space going to be used? Is it for speech? Is it for assembly? Is it to be used as a transitional space from one area to the next?
Great spaces like Union Pacific Station have hard interior finish materials which make them very reverberant. But the sense of energy as people move from one point to the next is refreshing. Although if you were to hold a lecture series in that same type of space, it would be quite difficult to hear what the presenter was saying. On the other hand, music performed in the same space would sound very full and very rich. As much as we would all like, very few of the spaces we actually get to work on are truly designed just for music. Communication, the dissemination of information, is usually more important.
Can echoes and excessive reverberation be treated? Yes, by identifying the offending sources, determining the reflection points, and applying either sound absorptive material or diffusing material (something that will spread the sound path into many different directions). But again, for a space to be properly designed, you must first understand how the room will be used.