DIY acoustics testing, part 6: more interpretation tools
In this series, we’re going through all of the basics on how to run acoustic tests in a DIY setting.
In previous articles, we’ve gone over some basic plots you can use to visualize acoustics inside your acoustics testing software. Today, we're continuing our overview on how to interpret acoustics data.
There are many different graphs you can plot with Room EQ Wizard (our free acoustics testing software), and our focus in this article is on a graph that’s similar to the SPL/Phase graph we discussed previously. But, this new tool includes an added third dimension. It’s called the Waterfall plot.
The Waterfall acoustic plot
The Waterfall acoustics visualization in Room EQ Wizard
Just like in the SPL/Phase graph from previous articles, what you’re looking at here is a plot of the frequency response of your room, shown within the range of human hearing.
Frequency moves from left to right, from low to high, starting with the lowest frequency humans can hear on the far left (20 Hz) and moving up through the highest frequency humans can hear on the far right (20 kHz).
Amplitude in decibels (dB) is shown vertically, up and down along the Y axis of the graph.
In the waterfall plot, we add the third dimension of time on the Z axis of the graph (front to back, with time moving forward from the background to the foreground).
With this added third dimension, we can see the frequency response of the room, but also how different frequencies within our room are ringing out over time:
The X Y and Z axis of an acoustics waterfall graph
What to look for in the waterfall acoustics graph
I’ve marked some notable points, using this same example:
Common acoustics issues in untreated small rooms: lots of muddy ringing
Here, we can see that in this graph of a room with no acoustic treatment, a very large area of the sonic spectrum (20Hz - 8kHz) is ringing for a very long time (1470 milliseconds). This means that the room itself will continue to resonate for at least 1.5 full seconds, after sound has stopped playing inside of it!
Just like the inside of an acoustic guitar body, the inside of a room can resonate too.
And, in this case, we’re seeing a whole lot of muddy resonance bouncing around inside of this room! This amount of low end mud ringing out for this amount of time makes this a very inaccurate room to try to produce music in. It would be difficult to trust your ears inside this type of uncontrolled acoustic space, and you could expect a lot of inconsistency in your musical work as a result of working inside this room.
Luckily, we have many tools available to help control this kind of thing. And, we don’t need to worry about trying to create some sort of fictional perfect acoustic space, we just want to control the acoustics of our existing space as much as possible. Smart application of some common and affordable acoustics tools can make a really big difference in just about any room.
Solutions for muddy ringing
When you add bass traps to your room, you will see this type of muddy ringing in the low and mid frequencies improve. This will help a lot in clearing up the sound of your space.
A clearer sounding acoustic space helps you make much more accurate and consistent sounding musical productions. If you’ve ever been surprised by how different your mix sounds in the car compared to your studio, working inside a muddy and acoustically uncontrolled room is the primary reason why this is happening.
Here’s a waterfall graph of that same room, but after bass trap acoustic control has been added to every corner of the room. There is still some ringing, but it’s much softer and of shorter duration, especially in the low end. In the lowest part of the spectrum, around 30 Hz, we’ve reduced the ring in the room by a full 20 decibels. That’s a significant improvement!
A waterfall graph from the same room, after bass traps have been installed
Finally, here’s these two graphs overlaid on top of each other: the orange is the uncontrolled room, and the blue the same room with bass traps installed. This lets you more directly compare the before and after, and we can see some big differences in that Z axis that shows time:
Two waterfall graphs from the same room, overlaid before and after acoustic treatment
Again, we see much less muddy ringing overall, and especially in that bass area. Most of those sharp peaky bass resonances are gone, or greatly reduced. The room is also looking much clearer in the mid-range and vocal area.
This means much more accurate tracks that sound just like how you meant them to, everywhere!
Interpreting decibel changes on the waterfall graph
It’s important to note that the differences between the controlled and uncontrolled room measurements in this example might not look super visually impressive on the graphs, in terms of the amount of decibel change shown before/after. We’re looking at perhaps a 15-20 dB change in the amount of low end ring, after adding our bass trap acoustic treatment to control the low end.
Don’t worry about how that looks. Decibels aren’t a linear scale, and numerical decibel changes on a graph always look smaller than how they actually sound in the room. That’s because a numerical change of just 10 dB equals a 10X change in sound energy!
So, small numerical changes in the decibel response of a room when you measure it equal large changes in how that room will actually sound, in the real world.
That 15-20 dB reduction of the low end ringing we can see here means that we’re reducing the energy of the ringing area by about 150-200%, making that ringing appear more than twice as soft. That’s a significant improvement in how our room will sound, practically speaking, but it might not look super impressive or visually obvious when it’s shown on the measurement graph.
Another important item to note: these before/after graphs only show the differences when bass traps were added to a room, NOT after the complete holistic acoustic system has been installed. Bass traps are only one component of an acoustic system, and these examples only show what that one specific component is doing to the sound of the room.
In addition to bass trapping, there’s several additional acoustic control components that have yet to be added to these room examples, and these particular graphs are not showing those additional effects.
More before/after acoustic examples
For comparison, here’s another set of before/after waterfall measurements that were taken from a different room, with my notes on what to look for in these graphs at the end:
Before: an uncontrolled room with no acoustic treatment
After: the same room with bass traps added
Before/After overlaid: blue shows the room after acoustic treatment, red the initial state
Before/After graphs overlaid with notes
Now that you have a better sense of how to measure and interpret acoustic measurements, next time in this series we’ll continue to discuss different solutions to some common acoustic problems you’ll see inside recording studios.
