Byte Gremlin

The DIY Path to Better Acoustics

Charlie

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I’ve recently completed my first pass at transforming the acoustics of my theater, and let me start by saying, the difference was mind-blowing. Roughly $300 later, movie dialogue is easier to understand, the operatic tones of John Williams are richer, and action sequences are more enveloping.

In short, I feel like I have a whole new theater for less than I spent on a single speaker.

$300 later, movie dialogue is easier to understand, the operatic tones of John Williams are richer, and action sequences are more enveloping.

While the actual process of treating my theater only took about a week, many months were spent researching different treatment methodologies and implementations. Ask a dozen acoustics nerds how to treat a room, and you’ll get a dozen different answers. My intention with this article is to thread the needle, providing a clear approach with concrete steps you can take to figure out your rooms problems and improve your sound, without exploding your head or your wallet.

That said, there is no “generic” treatment for a room. The acoustics of your space are unique, and any good treatment plan is going to start with some basic measurements of your space. You can’t fix your rooms acoustic problems until you know concretely what those problems are.

That’s where we’ll begin.

NOTE: Acoustic treatment should be the LAST thing you do to your home theater. What treatment you make, and where you place it is totally dependent on the placement of your speakers, and your main listening position. A common rule of thumb is, adding or moving anything lamp sized or larger will alter the sound in your space. Make sure you’re set for all the basics before ploughing ahead! If you’re not, check out the ByteGremlin Home Theater guide.

The Dual Nemeses of Sound Quality:

Before we get into buying equipment, making measurements and treating your space, you need to understand the two underlying problems room treatment aims to solve.

Uneven Frequency Response:

Just like light has different wavelengths we can see (i.e. all the colors of the rainbow), sound waves can be broken down into different frequencies. A bass drum will sound in a range of low frequencies, a flute will play at high frequencies, and human voices occupy a range in the middle-high region of audible sound. In a perfect world for a given volume, speakers would reproduce all frequencies of sound to be the same volume.

In the real world, the objects in your room (and the design of the speaker itself) cause some frequencies to be louder than others. A room with uneven frequency response can result in unnatural or muddy sounding audio, where the louder frequencies drown out the quieter ones.

Untreated Early Reflections:

The second denizen of home theater sound lies in the many paths sound travels as it makes its way from the speakers to your ears. You may visualize the sound from your speakers traveling in a straight line to your ears, but actually, sound emits from your speakers in all directions, with each part of the wave bouncing off whatever surface it runs into first. Many of those reflected sound waves bounce around the room before eventually hitting your ears. Your brain takes the earliest sets of these reflections and merges them with the sound waves that travel directly to your ears, which has the effect of muddying the clarity (and sometimes directionality) of the sound.

 

Part One: Measuring the Room

Acoustics is literally a science in and of itself, but for the purpose of treating a room, I’m just going to tell you how to take measurements of your room acoustics, and what to do with the series of squiggly lines that represent that data.

Gearing up for measurements:

For this guide, we’ll be using the go-to room measurement software for acoustics, Room Equalizer Wizard (REW). It’s free, it’s fantastic, and you can get it here. Note to Mac users: if you have any issues getting the software to load up, try using the betas.

For my room, I used the UMIK-1 USB microphone. These are pre-calibrated at the factory, and include a digital calibration file that’s custom to each mic sold. It’s affordable, hassle free to setup, and reliable.

You’ll also need some sort of stand to hold the mic in the correct position. The UMIK-1 includes a rotating microphone clip that screws onto a standard 3/8″ camera tripod mount, so if you’ve got a camera tripod in a closet, that’ll work just fine.

If you’re a millennial who scoffs at the idea of dedicated cameras (shame on you) and lacks access to a tripod, the best option is to buy a boom mic stand. If you go this route, you’ll also want a 5/8″ to 3/8″ adapter screw in order to connect the UMIK mic clip to the larger threads on the stand.

Pro tip, if you’re using a home theater receiver with built-in calibration software like Audyssey, I highly suggest buying an angled camera mounting arm along with the mic stand. This will allow you to more precisely and easily line up your receiver’s included microphone (a process you need to complete seven or eight times per calibration on most receivers).

Now you’ve got the gear, we’re ready to take some measurements.

 

Taking measurements with REW:

Setting up your equipment with REW is pretty painless. First, connect your USB microphone to your computer, and connect an HDMI cord from your computer to your receiver, then launch REW. You’ll be asked if you have a calibration file for your mic. The UMIK actually has two files, one for measuring with the mic offset vertically by 90 degrees, and a standard file calibrated for measuring with the mic pointed right at the sound source. Pick the standard file.

Next, get your mic mounted on your stand. Place it in the center of your prime listing position, at ear height. If you’ve got a high backed sofa, you’ll want to cheat the microphone up and forward a few inches so it’s not right next to the fabric, as this can affect the measurement.

Power up your system, select whichever receiver input your computer is connected to, and head into the REW preferences panel. You may need to adjust your input and output. You want to make sure the program is sending multi-channel sound out to your receiver (as opposed to stereo), and that your Usb mic is selected (as opposed to measuring from the internal mic on a laptop).

For the most accurate results, you’ll want to crank the volume up loud, and minimize ambient noise as much as possible (turn off the HVAC, and politely ask your significant other to can it). Definitely grab some ear plugs for this.

For the most accurate results, you’ll want to crank the volume up loud

If your receiver includes room correction software such as Audyssey, YPAO, or ARC, make sure you disable this, as otherwise you’ll be measuring a skewed frequency range rather than the raw uncorrected sound. Also make sure you disable any EQ presets that may be enabled (such as “Vocal Boost,” Sci-Fi, etc).

In REW, use the level check button to make sure you’ve got the volume up nice and high without clipping (aim for a headroom of -5 DB) The level meter on screen will turn red if you’ve got the volume too loud.

You’ll want to grab an individual measurement from each speaker with your subwoofer unplugged, and then one final measurement of your front left and right channels with the subwoofer connected. For the individual measurements, ALWAYS point the microphone at the center of the speaker driver array. For the final fronts measurement, point the mic dead center.

REW makes it easy to zoom in and out of your sound range, and to view one measurement on any graph you want, so always take a measurement of the full frequency range your speakers emit (10hz to 20,000 is a good typical range if you’re unsure).

Finally, select “measure” in REW, adjust your settings as per the below screenshot, and hit measure. If you’re not bothering with earplugs, you can set a start delay for the measurement, allowing you to click measure and haul ass out of the room before the big SWOOSH.

By default, each measurement is named for the time it was made. It’s helpful to change these names to the name of the speaker being measured. Otherwise, the half dozen plus measurements will be impossible to differentiate, and therefore useless.

Once you’ve made your measurements and gotten a sore back from bending over to adjust your microphone a few dozen times, we’re ready to dive into all that data. Ahh… SCIENCE.

Finding your Room’s Problem Areas:

REW offers up a dizzying assortment of graphs, but for the treatment of a typical space, there’s only three data points we need to worry about. I always start by pulling up this data for my main left, main right, and center channel speakers, as the sound quality from these channels is going to define your soundscape more than any speakers behind (or above) your ears.

I always start by pulling up this data for my main left, main right, and center channel speakers

An important note: The measurements you’ve already taken hold all the information REW needs to generate a variety of different graphs. If you’re reading this as research before you dive into the process, you don’t need to do specific measurements for the below data. Just measure as recommended above, and you’ll be able to generate each of the graphs we’re talking about when you’re ready.

REW is complex software, and the interface isn’t super user friendly. I’ll offer some basic steps to follow to gather the information we need, but if you get lost or are looking for more in-depth REW instructions, check out the addition links at the end of this section.

The Data:

There’s three graphs we need to generate to understand our room’s problems. The first, the Time Frequency Curve, reveals any issues with Uneven Frequency Response.

My Right Speaker’s TFC Graph after some treatment

Generating this graph is easy. First, open REW and open up your set of saved measurements. In the center view panel, select the “All SPL” Tab, and uncheck all the channel selections in the lower panel, save for one (I’d suggest starting with your main Left, or main Right speakers).

This will pull up a spiky frequency chart, but we need to make some adjustments to make this view useful. First, pull up the “limits” on the righthand side, and set them as below:

We now have a useful range of frequencies and volume to look at, but the raw graph is a bit too spiky to draw conclusions from. Instead, we’ll smooth out the data, to give us a more practical graph. Head into “controls,” and select “1/12” under smoothing, and then select “apply smoothing.”

This chart gives you a baseline to judge the frequency response of your speakers and your room. Ideally, you want as much as possible of the frequency range to be within 10 DB of one another, but your room, your speakers, and your living companions will set limits on just how flat you can really make this graph. In my case, I didn’t see a huge change to my frequency response after treatment, but my biggest problem was related to untreated early reflections, and that’s where my room was dramatically improved.

To assess reflections, we need to pull up an ETC graph.

My Right Speaker ETC Graph After treatment. Believe it or not, this is BETTER

Instead of dealing with frequencies, the ETC graph shows the volume of different sound waves over time. Generate yours by selecting an individual channel measurement on the left, and then selecting the “impulse tab in the center view. Only Envelope (ETC) should be checked. Once again, we need to set limits as below (in this case, we’re limiting the range of time on the graph to a useful level.

ETC Limits Settings

An ETC graph for an ideal room would show the initial peak (which is the sound wave that hits your ears directly), followed by a huge drop-off in volume, as most early reflections would be tamed. A couple of early reflections can add character to the sound of the room, but when you have five or more peaks of similar volume to the direct sound, you’ve got problems.

In my case, I had a series of high peaks, almost as loud as my direct sound wave, arriving shortly afterwards.

Here’s my pre-treatment Right channel for comparison

For more info on how to look at this graph, check out this article from GIK Acoustics:

Unpacking ETC: Time-domain measurements & early reflections

In addition to showing us the state of our room’s reflections, the ETC graph can actually be used to identify which surfaces are causing the reflected sound. This takes some simple math, some string, and ideally, a friend.

Finding Reflection points:

Because the ETC graph is identifying the time it takes the sound to reach the microphone, it can also calculate the distance of the path of that sound. Once you’ve identified a spike (or reflection) in the graph you wish to identify, move your cursor to the 0ms spike (AKA the first and only direct sound), hold down the control key, and right click and drag to the reflection point. The red distance show in feet is the extra distance that sound wave traveled over the direct sound wave.

This is the arts and crafts portion of our experiment.

To identify what object or surface the sound is bouncing off of, you need to measure the distance from the speaker to the microphone, and then ADD the red distance number identified in REW. This is the total distance that particular reflected sound traveled to get from the speaker, to the reflection point, and then to your ears.

This is the arts and crafts portion of our experiment. Measure out and cut a length of string exactly as long as the path of the sound. Tape one side to the speaker, and have a friend hold the other end to your microphone. Next, pull the string taught, moving around the room until you find the point where the string both strikes and departs a surface at the same angle. This is the point that’s causing the reflected sound!

Before you treat the room, you’ll want to take some time and mark the earliest (and loudest) reflection points for each of your front three speakers. If you’re having trouble finding the place where the string lines up correctly, remember that it could be on the ceiling or the floor. We’ll address how to treat these problem areas later.

The final REW graph we’re interested in is called a waterfall graph, and it’s actually just another way of looking at reflections. Instead of looking at reflections over time, a waterfall graph shows you how loud (or how intense) the reflections are at different frequencies. It’s great for getting a visual overview of how much reverberation you’re getting in a room, and it’s particularly effective when diagnosing and treating problems in the bass range of your sound (typically 300hz and below). If you clap your hands and hear a pronounced ringing, this is the graph for you.

Post Treatment Waterfall. This Graph reveals that my room could use more Bass Traps.

We open this graph by selecting our measurement on the left, then selecting the waterfall option in the center view (it may be hidden behind a double arrow). Again, we want to set our limits. If we’re looking for bass specific problems, set as below

An ideal waterfall graph will be fairly flat and smooth left to right. The three dimensional area reaching toward you is the amount of ringing/decay in the sound. Ideally you want this to drop off quickly, otherwise you’ll hear a nasty echo that’ll impact the quality of your audio.

So now you’ve analyzed your room, and you’ve got a rough idea of your problems. You may have lots of early reflections to tame, ringing base frequencies that are out of control, and/or huge troughs in your frequency response. It’s time to get our treatment on!

 

Looking for more detailed guidance on REW? Check out these resources:

Continued on Page 2:

Part Two: Treating the Room

There’s lots of conflicting methodologies on where to place treatment, and how much to use. The consensus I’ve found is that treating most of your early reflection points with acoustic absorptive panels is good, but treating EVERY reflection is actually detrimental to your sound quality, as it can make the room sound dead and unnatural. Finding the right balance comes down to your taste, so it pays to start small and add treatment as you go.

There’s three kinds of sound treatments, although there are variations of each type.

Finding the right balance comes down to your taste, so it pays to start small and add treatment as you go.

First off, you have your typical mid-high frequency absorber. This is a 2x4ft panel of acoustic batting, mounted in a wood frame and wrapped in acoustically transparent fabric. These can be purchased places like GIK Acoustics but you can cut costs significantly going the DIY route. More info on that below.

One of my mid-high frequency absorption panel

These panels can be hung on the floor or ceiling, and are good for dampening an over-active room, and for strategically reducing the volume of early reflections.

Next, we have diffusion. In some instances, you might find after treating a room with absorption panels that you still have problematic reflections, but adding more panels makes the room sound too “dead.” Another option is to place diffusing objects against the wall, breaking up the sound wave by bouncing it in several directions at once.

You can purchase acoustic diffusion devices, but you’ll find that many household objects can be used to create diffusion. The classic example is a bookcase staked with a random assortment of books, but you can get creative. Again, how much diffusion comes down to taste. For a typical theater room, placing some bookshelves on the back wall is a natural way to incorporate diffusion while satisfying members of the household with an interest in aesthetics. Whatever you do, DON’T glue egg cartons together and think it will work as diffusion. https://www.acousticfields.com/the-egg-carton-acoustic-myth/

Finally, we have bass traps.

Bass sound waves are very long (At around 10hz, they’re over 100 ft). In typical home theater room sizes, this creates a number of unique acoustical problems. Massive bass waves crash back and forth, building intensity and sometimes cancelling one another out in various points in the room. The best way to improve bass quality is to put your subwoofer in the ideal position, or use two subwoofers to increase the number of optimal listening positions. However, even an ideally positioned sub (or subs) will require significant treatment to sound their best in a confined space.

The solution is bass traps, which are deep moderately dense absorbers which absorb the lengthy bass waves, turning their sound energy into heat. Unlike mid-range absorbers, it’s hard to overdo it with bass traps, so go wild. Bass energy tends to concentrate in the corners of the room, so they’re typically built as triangles that fit into the vertical and horizontal corners where the walls meet the ceiling and the floor. Like other treatments, there are both professionally built and DIY options available.

General Treatment Recommendations

While no two rooms are exactly alike acoustically, here are some guidelines to get you started with placing your treatment.

Virtually all rooms will benefit from treating the front left wall, front right wall, front ceiling and front floor, typically two or three feet in front of your speakers (the exact spot will vary based on the size of your room and your listening distance, hence the measurements). Your aim is to place panels exactly over the point where those earliest and loudest reflections are bouncing.

If you lack the time or cash to delve into treating your room, at least throw down a solid carpet.

Another rule of thumb is to avoid leaving large flat parallel surfaces untreated, as this will lead to slap echo (where high frequency sounds whip back and forth against the surfaces, unable to quickly dissipate). An easy recommendation for any room is to treat the floor! Thick carpet with a deep sound absorbing carpet pad will do wonders for any room. If you lack the time or cash to delve into treating your room, at least throw down a solid carpet.

Continued on Page 3:

Part 3: The DIY Route: Building Treatment

For my space, I built four mid-high absorbers and a pair of bass traps for the back wall. Due to construction limitations (One of my reflection points is ON our painstakingly stained entry door), and personal aesthetic limitations (Mrs. Gremlin wasn’t thrilled about putting panels on the ceiling) I’ve compromised in the placement and number of my panels.

Building Mid-High Absorbers:

The construction of these puppies is pretty straightforward.

Ingredients:

Required tools:

Safety note: Insulation fibers can do serious damage to your eyes, skin and lungs. Wear proper eye protection, gloves and a respirator when working with this stuff.

 

A Word on Acoustic Fabric:
Fabric is the one part of these devices that can provide aesthetic value to your space. There are tons of options that will work fine. Some people even print movie posters onto fabric to really spice things up! The only requirement is that sound be able to pass easily through the surface of the fabric without reflecting back. The classic way to test this is to hold the fabric over your mouth, exhale, and see if you can easily feel your breath move through it.

 

The build process is pretty easy if you’ve ever DIY’ed everything. Even if you haven’t you should be able to follow these steps and get a decent result.

If you haven’t bought exactly 2″ lumber, you need to use a table saw to cut all the lumber down to width. Those without a garage full of tools can pay a small fee to have all these cuts made for you at a Home Depot or Lowes.

My stack of assembled frames

Next, cut your horizontal pieces to 24″ wide, and your verticals to about 49-1/2″ (these should be 48″ plus the width of two of your boards).

Lumber laid out for assembly

Lay the lumber on a flat surface (a spare plywood sheet or a leveled garage floor work well). Affix the sides to the tops and bottoms, so that the fiberglass panel can be stuffed in, with its sides covered. I used a Kreg Jig with pocket screws for a quick and sturdy result, but nails will work too.

My Handy Kreg Jig

If you end up with rough corners, run a sanding block along them to even them out. You can also wrap them with some electrical or duct tape if you don’t mind creating a slight ridge.

Before you insert the fiberglass, you need to staple backing fabric to the back side of the frame. Whichever side of the frame was pressed against the flat surface during assembly should be used as the back (As it has to hang flat on the wall). You can technically use the same fabric for the front and back, but unless you score a deal on fabric, it tends to be more cost effective to buy cheapo fabric (or even an old bedsheet) for the back.

Using scissors, cut the fabric to roughly 25″x49.” Spread the fabric over the back of the frame, and staple it into place, affixing at opposite edges at a time to ensure you stretch the fabric evenly across the surface.

With the back secure, slip your fiberglass panel into the frame, pressing it gently so it ends up even with the frame edges on all sides.

Finally, cut your fabric for the fronts. I made these pieces WAY oversized initially, as its easy to cut off any excess later on.

Carefully place the panel onto the fabric face-down, then pull the fabric around the sides of the panel, and staple it in place on the backside. Again, take your time, pulling the fabric taught as you go. I folded the corners for a clean result. If you have a lot of excess that might impede the panel hanging flat on the wall, go ahead and snip it off with scissors.

Finally, screw two wall hangers into the pack panels. I placed mine about 8 inches down from the top. Pull a length of picture hanging wire through the two loops, and wrap it over on itself until it can withstand a solid tug.

Hang and enjoy!

 

Building Semi-Permanent Corner Bass Traps:

The principals of bass traps are very similar to the above absorbers. The big difference is thickness. Bass frequencies are far too large to be significantly absorbed by a 2″ panel. As a result, bass traps tend to be 12+ inches deep (really the deeper the better). They’re also almost always triangular for corner placement as this positioning allows them to absorb quite a bit more bass energy.

In my case, my room and personal circumstances only allowed for two traps in the back corners of the room. Rather than build a full triangular frame, I opted to permanently install the base trap into the wall in stages.

Required Materials:

Required Materials:

I started by taking some scrap cherry plywood I had, and cutting them into 12x12x~17 triangles. I used scrap pine ply for the bottom pieces.

Mounted bottom of trap

Next, I screwed L brackets upside/down into the wall, so that the bottom triangle could be supported by the bracket and also sit flush against the wall. Secure to the brackets with screws (Note: I screwed up on this part, not factoring that I lacked clearance underneath the bottom boards to insert screws from below. A clean solution to this issue would be to use bolts, but I opted to recut the bottom triangles slightly smaller and flip the L brackets back upright, securing from above).

Next, repeat this mounting process for the top triangle. I test fit the top plywood, then stained it before affixing it in place.

The front edge of the stained top is covered by front of the trap

From here, we proceed into the messy part. The core of the bass trap is a stack of Rockwool triangles that fill the space and provide the sound absorption. These come in rectangular batts that need to be cut to shape.

It’s critical that these triangles be stacked freely, without being compressed by the front cover of the trap, or between the top and bottoms. As a result, I undersized them to ensure they would comfortably fit the space.

My acoustic triangles being stacked into place.

To aid in the cutting process, I cut some spare plywood into a template that was about a 1/4 inch shorter on both sides that would touch the wall. You could undersize them even more if you’re building a larger trap.

Safety note (AGAIN): Insulation fibers can do serious damage to your eyes, skin and lungs. Wear proper eye protection, gloves and a respirator when working with this stuff.

To cut, place the template over the batt on a flat surface, then use a sharp utility knife to score and slice through the batt. It usually takes a couple of strokes per edge. If you’re looking for a shortcut, some people recommend using an electric carving knife (thanksgiving turkey style) to slice the batts more easily, but be prepared to get a new blade afterwards).

Once you’ve cut a LOT of triangles, carefully stack them in place between the top and bottom mounted plywood triangles. You’ll likely have a little empty space at the top. That’s okay.

It’s time to build front frames to keep the fiberglass in place and provide a decorative finish.

My Fabric Courtesy of Ikea

I used a table saw to cut angles into the side pieces of my frames. Feel free to cut a somewhat narrow angle. All that matters is that the frame edges sit flush against the wall; they don’t have to match perfectly.

As with the mid-high absorber frames, assemble these on a flat surface. This time we’re placing all the wood flat on the table. I used pocket screws and my kreg jig to secure the top pieces to the sides; you could also use metal joiner brackets in a pinch.

With the frames assembled, it’s once again time to wrap in acoustically transparent fabric. I used the same fabric for the bass traps and my mid-high absorbers. The wrapping process is the same. Pull tight… staple… repeat.

Finally, press the frames into place, using a nail gun (or standard nails and a hammer) to secure the frames into the wall. Be sure to hold the frame flush with the top plywood, to cover the raw edges of the plywood piece.

Repeat for as many bass traps as you have room for!

Putting it all together:

Go forth and enjoy your new creations! As you place these panels, take your time to experiment.

Ultimately, whatever sounds best to your ears is what you should stick with!

If you’ve bought a measurement microphone, take measurements with and without your panels to see what they’re doing. Try moving the panels into different positions and compare the sound of your system. Ultimately, whatever sounds best to your ears is what you should stick with! If you look at my graphs, you can still see lots of hypothetical issues, but I placed to taste within the limits of what I could correct, and I’d advise you to do the same.

Thanks for reading, as always. If you found this information useful, please consider using this site’s product links. Your purchases may earn ByteGremlin a small commission, and it doesn’t cost you a dime.

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