Acoustic Absorption - Why It Gets Worse Before It Gets Better
Updated: 20 hours ago
Most people assume that treating a room acoustically is a simple, straightforward process.
And honestly, in many ways it is.
Add absorption.
Reduce reverberation.
The room sounds better.
From a 30,000-foot view, this is pretty much the process in a nutshell.
But if you have ever treated a reflective room a little at a time, you may have noticed something strange:
As you add acoustic absorption to the walls, the room acoustics can actually start to sound
“worse.”
The broad wash of reverberation starts disappearing, and suddenly you begin to hear things you didn’t notice before: a discrete echo from the back wall, a slap from the ceiling, or a ping-pong flutter between two hard surfaces.
To the uninitiated, it may seem as though the acoustic treatment made things worse by somehow creating new issues.
The truth is that it did not.
It actually exposed them.
The Reality of Reverb
We tend to talk about “reverb” as though it were a singular sound the room adds. We often describe a room’s level of reverberation in generic terms such as “wet” versus “dry,” or by describing how “live” or “dead” the room sounds.
In reality, reverberation is an enormous collection of reflections grouped closely together in time.
So closely, in fact, that they sound like a singular, continuous sound.
When you make a sound in any space, that sound travels outward from you in all directions. The direct sound travels straight to the listener, but the sound traveling in every other direction encounters other surfaces. It strikes the floor, ceiling, walls, glass, furniture and anything else in its path.
Some of those surfaces reflect a significant portion of that sound energy in another direction. Those reflections then strike other surfaces and reflect again. This continues over and over, with some of the sound energy being lost with each interaction, until the remaining reflected energy becomes too weak to be perceptually significant.
Because of the physics of sound propagation, a single sound created in a reflective, or “live,” space can very quickly become an enormous number of delayed copies of the original sound arriving from many different directions.
Some of those reflected sounds arrive at the listener almost immediately after the direct sound. These are generally referred to as early reflections.
Others arrive later.
And some arrive much later.
Some of the reflections may have bounced off only one surface before reaching your ear. Others may have traveled through a much more complicated path around the room before finally making their way back to you.
When enough of these reflections arrive at your ear close enough together in time, your hearing no longer identifies them as individual sounds. Instead, they perceptually blend together and you hear them as a continuous decay.
We refer to the result as reverberation.
Reverb is not one reflection.
It is what a huge number of reflections sound like when they become too dense for us to hear individually.
Aural Resolution
A useful way to visualize this is to think about a digital photograph.
Look at a high-resolution image from a normal viewing distance and you can’t see the individual pixels that make up the image. You see a continuous picture.
In reality, that image is made up of thousands or even millions of individual pixels packed tightly together.
A standard 1920 × 1080 HD image, for example, contains 2,073,600 individual pixels.
Reduce the resolution far enough, however, and something changes.
The image begins to look pixelated.
Suddenly, you can see the individual pieces that make up what previously appeared seamless.
A reverberant room behaves in a similar way.
In a highly reflective room, there are so many audible reflections arriving so densely that they blend into a relatively continuous reverberant field.
In the midst of all those reflections, your auditory system no longer perceives each one separately.
You simply hear:
Reverb.
Now start adding acoustic absorption.
For the sake of the analogy, think of this as reducing the “resolution” of the reverberant field.
Each time sound strikes an absorptive surface, more of its energy is absorbed instead of being reflected back into the room. Reflections die away sooner. Many of the weaker reflected sounds that previously contributed to the reverberant field no longer remain strong enough to be perceptually significant.
The effective density of audible reflections decreases.
The acoustic image starts to become “pixelated.”
And that is where things get interesting.
You start to see — or, more accurately, hear — some of the individual pieces that made up the previously seamless whole.
Those remaining discrete reflections start to stand out.
Abra Echo Cadabra
Imagine a room with a large reflective back wall.
Every time someone speaks, that wall produces a strong delayed reflection.
But initially, the rest of the room is also highly reflective.
That back-wall reflection arrives surrounded by hundreds or thousands of other reflections. Your ear may have difficulty distinguishing it from everything around it.
You simply hear a reverberant room.
Now acoustic treatment is added to the side walls and ceiling.
The overall reverberant field becomes quieter and less dense.
But the back wall remains untreated.
The reflection coming from that wall may not have changed very much.
What changed is everything around it.
Suddenly you hear:
Hello … hello … hello.
The treatment did not create that echo.
It removed enough of the competing reflections for your auditory system to recognize that particular reflection as a separate event.
This is essentially a form of acoustic unmasking.
The echo was there all along.
You can just pick it out now because it is no longer blending into the crowd of all the other reflections.
Why the Room Can Temporarily Sound Worse
This is also where the whole “resolution” analogy kinda breaks down.
With an image, higher resolution generally gives you more detail and makes it easier to recognize what you are seeing.
A highly reverberant room causes the opposite effect.
The greater the density and persistence of reflected sound, the harder it becomes to recognize the original source clearly. More reflected energy and a longer reverberant decay can have the perceptual effect of blurring the original sound.
As we add absorption, we reduce that blur.
The original sound becomes clearer — but so do the acoustic problems that remain.
This creates an unintuitive stage during acoustic treatment.
At first, the room may be extremely reverberant. It sounds muddy and uncontrolled, but the reflections are so dense that very few individual problems stand out.
The acoustic image is too “blurry” to hear the specific issues.
Then absorption is added.
The reverberation decreases.
The “image” becomes clearer, and the remaining issues become much more obvious.
A flutter between parallel walls suddenly stands out.
A ceiling reflection becomes noticeable.
A rear-wall echo that previously disappeared into the reverberant field becomes distracting.
Acoustic problems that were previously buried in the blur have now become obvious enough to draw your attention.
The room has technically improved acoustically, yet to someone listening casually, it may actually seem more “echoey” than before.
This is the awkward middle ground:
Uncontrolled room → exposed reflections → controlled room
If you’ve ever tried to drastically change your hair color, you know the middle stage can sometimes be the most visually objectionable part of the process.
Acoustics can have a similar awkward phase.
At this stage, there are no longer enough significant reflections to completely mask the individual problems, but the remaining problems have not yet been controlled.
Flutter Echo Makes This Easy to Hear
Flutter echo is one of the easiest examples.
Stand between two large, parallel reflective walls and clap your hands.
Instead of a smooth decay, you may hear a rapid series of repetitions as sound ping-pongs back and forth between the two surfaces.
In an extremely reverberant room, that flutter may be partially buried inside everything else happening acoustically.
Treat much of the room while leaving those two parallel surfaces reflective, however, and the flutter will suddenly become far more obvious.
It can seem as though the new acoustic panels caused the flutter.
What actually happened is almost the opposite.
The treatment cleared away enough acoustic clutter for you to hear it.
Your Brain Is Constantly Organizing the Room
Our auditory system is remarkably good at interpreting reflections.
It uses differences in arrival time, direction, level and frequency content to help determine where sounds are coming from and what part of what we hear belongs to the original source versus the surrounding environment.
When reflections arrive densely enough, we tend to perceive them as a reverberant field.
When a reflection becomes sufficiently isolated in time and strong enough relative to the surrounding sound, it becomes easier to identify as a separate event.
That means changing the acoustic environment can change our perception even when a particular reflection itself has not become any louder.
Sometimes you hear something more clearly not because it became louder, but because you removed everything that was hiding it.
Better Acoustics Is Not Just Less Reverb
This is also why good acoustic design involves more than covering a certain percentage of the walls with absorptive panels.
Reducing the total amount of reverberant energy is important.
But so is controlling where the remaining energy goes.
Strong reflections from the rear wall, ceiling or parallel surfaces may need to be addressed even after the room’s overall reverberation time has fallen significantly.
The goal is not necessarily to eliminate every reflection or make a room acoustically dead.
Useful reflections can add spaciousness, energy and musical character.
The ultimate objective is control.
We want the reflections that remain to arrive at appropriate levels, times and directions for the way the room is being used.
To do that, we use a combination of acoustic materials and techniques. Some absorb sound energy. Others diffuse it, redirecting reflections and breaking up strong reflection paths.
But that’s a story for another time.
The next time you find yourself in a great-sounding room, listen a little more closely.
Instead of hearing the room as one uniform acoustic character, see if you can pick out the individual pieces that create it — the direct sound, the early reflections, the decay, and the specific surfaces contributing to what you hear.
Once you start hearing a room that way, “reverb” stops sounding like one thing.
You start hearing the pixels.
Ready to Take Control of Your Room’s Acoustics?
Great acoustics aren’t simply about reducing reverb. They’re about creating the right balance of absorption, reflection, and diffusion for the way your space is actually used.
Whether you’re dealing with excessive reverberation, poor speech intelligibility, distracting echoes, or a room that simply doesn’t sound the way it should, Pierson ProAVL can help identify the problem and design a solution around your space.
Let’s make your room sound as good as it was designed to look.
Talk to a Pierson ProAVL Specialist ↓
717-323-6367
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