We know how to measure the frequency response of our audio playback systems thanks to various software packages available for free, along with fairly affordable calibrated measurement microphones. However, fewer of us know how to measure the clarity, or articulation of our rooms. Which measurement is more important?
Why is sonic clarity more important than a flat frequency response in your room?
by Art Noxon, ME, PE Acoustical
Contrary to popular belief, the big problem with bass in audio is not lumpy bass, standing waves, room modes, hot spots and suckouts. The big problem is sound masking, which is what happens when an unwanted sound overpowers our ability to hear and discern the fine details of a wanted sound.
Let’s take this idea to the extreme to get a better grasp on what is going on here. A thought experiment: Let’s move your HiFi system, carpet and listening chair from your house to a local racquetball court. Here the walls and ceiling are 7 layers thick and the floor is solid concrete, and they are rigid and unmovable. In this big room we pop a balloon and listen to the noise slowly decay. It takes maybe 7 seconds for the noise of the pop to completely disappear.
As the speakers play music, the sound we hear is a combination of the direct sound, the one that travels from the speaker to our ears, plus a cacophony built up out of the previous 7 seconds of music we have been playing. If we are sitting, say, 8’ back from the speakers, the built-up cacophony of diffuse reverberance we hear is at least 10 to 15 dB louder than the desired direct sound we wanted to hear. Under these circumstances, we cannot begin to hear the direct sound because the diffuse reverberance overpowers and drowns out the direct sound. This is an example of serious sound masking. Then we take a look at the frequency response of the music and it looks pretty flat.
The Other Extreme
Now imagine we magically left the racquetball court on our flying carpet and landed somewhere outside on a grassy field and played the same music. All we would hear would be is the direct sound, there being no reflections or diffusion, hence no possibility of a sound masking reverb buildup. Now we hear perfectly clear sound with vanishingly fast decay times.
Playing our system outside, we measure it to have a flat frequency response, with no EQ adjustments needed. Likewise, playing our system inside the reverb chamber we measure it to have a flat frequency response with no EQ adjustments needed. But inside, where decay times are long and the sound is 0% clear, it sounds terrible because we care more about having sonic clarity than flat frequency response.
Moving back into real rooms, we discover that in real rooms we do not have a flat frequency response nor do we have crystal clear clarity.
So now we have the opportunity to upgrade our audio playback room. Do we upgrade with an eye for improving the frequency response towards being more flat? Or, do we upgrade with an ear for improving the clarity of sonic events in the room?
When we EQ for a flatter frequency response we only change how loud certain bandwidths are. Notice that we cannot change the decay time of a bandwidth by using EQ, only acoustics can change reverb times.
We use test tracks to determine if we have any blurred bandwidths that are loud. If so, we use EQ to lower the volume of the slurred frequency ranges down a little, about 5 dB, just enough so they are not both loud and slurred at the same time (no one likes a loud drunk). While this improves the sound a little, remember EQ does not change the Clarity of sound in the system, it only changes how loud it is or isn’t.