Before we begin, let’s familiarize ourselves with the layout of the graphs. The horizontal axis represents frequency, a measure of the highness or lowness of sound; the lowest sound is on the left and the highest is on the right. The vertical axis represents the relative loudness of the sound. Essentially, we have a plot of what the room is doing to each frequency in the bass range. This is known as the room’s frequency response. From red to blue, each successive curve represents a later moment in time with the red line representing the room’s response at the instant the test began, and the blue curve representing the response approximately 1/3 of a second later. To appreciate this data, we must understand that each of these curves represents sound that persists in the room after the speaker has stopped sounding.
Ideally, all of our lines would be relatively flat, but under the influence of a room this is hardly the case. Rooms store energy at certain frequencies, a phenomenon known as room modes, and you can see several of these modes of vibration in frame one. The large peaks show sound ringing out like a bell long after the source had stopped sounding, more than a second for the strongest resonance. In addition to the obvious problem that these frequencies sound disproportionately loud, the ringing causes two other problems: loss of tightness and loss of clarity.
Tightness describes the response of the system in the time dimension. If you’re watching a movie and Jet-Li plants a kick on his assailant’s chest, it should sound like a crisp, solid thump, but if you’re listening in an untreated room, it will probably sound more like a ringing drum. Clarity describes how well we can distinguish sounds from other sounds. Let’s say the bass player on the record you’re listening to plays a note at one of our modal frequencies and soon after plays a slightly higher note. Since low frequencies cover up high frequencies, the two notes may sound as an indistinct blur, or the higher note may not be heard at all.
This study shows graphically how Ready Acoustics RT424 Broadband panels can correct the low end response of a room. Starting with an empty room in frame one, each successive frame shows the effect of adding one additional panel. Notice how adding panels simultaneously reduces the height of the peaks, reduces the time they take to decay, and broadens the frequency range they affect which makes them harder to perceive. All of these things move us closer to our goal: a room where every frequency is equally represented and where all frequencies decay at the same rate.