Home Studio Acoustics 101. Section 4. Diffusion.

If you walk into a room, clap you hands and hear “ta-da-da-da” after the initial clap, what you are hearing is the delayed reflections arriving from the surrounding surfaces. These sometimes tiny short reflections are called “flutter echos”. These reflections are problematic in that they often cause frequency specific phase shifting and colorations that are not sought after in recording or good listening spaces. These reflections can be tamed with absorption materials or diffusors.

DIFFUSION

Diffusion is the process if scattering. Sound diffusors are hard devices that diffuse or scatter reflections in a different direction or in a different time than if unaltered. Scattering into a different direction makes sense because it interrupts the ping-pong like reflections between parallel surfaces. Delaying the reflections is helpful because it smooths out the delays into more of a unperceived short reverb and analogous sound. These time variances are generally very minute, typically only a difference of nano or milli-seconds. Once again these come in various forms and sizes and can also be designed with a little bit of ingenuity. Here is a pro studio with some major diffusion in the control room.

COMMON EXAMPLES

A simple example of a diffusor is a book shelf stocked with books of varying depths. These varying depths allow an uneven surface that allows the sound hitting them to return not only in different directions but subtle time differences as well. Log cabin style walls are also diffusive due to their rounded shapes.

PLACEMENT

In my space I use absorption toward the back of my room to control reflections and diffusors at the front of my space, Specifically in the mix’s sweet spot. (The sweet spot is where you ears and the monitor speakers form to make a perfect, ear-height equilateral triangle.) Diffusors between the monitors and my ears help scatter any reflections away and out of time from the direct sound from the speakers themselves. You really cannot go wrong on placement of diffusers. A diffusive ceiling is great to help breakup the floor to ceiling reflections. The rear wall in a control room is another great place.

DO IT YOURSELF

The gray diffusors shown below were built with a 2 foot by 2 foot frame out of 1″ thick styrofoam. I used a yard stick to mark the various “depths” then cut them with a blade. Once cut, I used liquid nails to glue the pieces together and then fit them into the 1″ by 6″ wooden frame. I then used gobs of wall pain and a brush to make them look decent. Here they are hanging and alternating with absorption panels at a former location.

Behind my secondary monitor, you can see that there are two wooden diffusors. These were built with the help of my awesome father-in-law one day while “playing” in his shop. The idea behind the designs was to “confuse the heck out of any sound that hit them.” We had a great time joking about and designing them. Each 2 foot by 2 foot frame houses 10 narrow compartments. The bottom one is more of the varying depths idea and the top one is lots of wooden slats placed on angles with air gaps between them. I have them placed in front of my mix position to break up any reflections from the front wall and simply because they are just plain awesome. I love and miss you Kip Clark!

FINAL WORDS

Once again I want to say that it’s a matter of balance. A good blend of isolation, absorption and diffusion is what it takes to make a great recording space. Enjoy. Worship your Creator.

Home Studio Acoustics 101. Section 3. Absorption.

Whether you are recording an acoustic guitar with a quality microphone or you are listening to music in your favorite seat, reflections are at play. These reflections can be pleasing (reverb) but can also be problematic (short delays). As sound waves interact physically, they are also changing physically. Fundamentals and harmonics are doing all kinds of higher-ED math as they collide. These physical anomalies cause “cloudy” phasing issues, namely comb-filtering and the spots within the room that have frequency excess and deficiencies I spoke of in pervious posts. Most of these interactions whether direct (straight from the sound source) or indirect (reflected), alter our perceived clarity, smearing their clean, original tones. In order to clear up the studio or listening space, these reflections should be either reduced (absorbed) or scattered into different directions (diffused).

ABSORPTION

Absorption can be likened to “friction for sound” in that absorptive materials impede the sound from being reflected. All materials have some absorptive quality. The calculations for Absorption coefficients and Noise Reduction Coefficients are very sciencey. These technical specs are determined by the makeup, mass and thickness of a surface and are frequency dependent. The simplest way of understanding this is knowing that the thickness of the material is generally related to the wavelength of the frequency. Meaning, the thicker the material, the lower the frequency that can be absorbed. Higher frequencies, since their wavelength’s are shorter, are absorbed easier than the lower. More on this later. Typically thick, porous, soft materials are great for absorption. Closets full of clothes, carpets, plush-cloth-couches and thick curtains are examples of common absorptive surfaces. Here is a picture of a portion of our old coffee shop/studio. Notice the decorations.. ahem, the absorption materials?

When it comes to sound absorbing materials, there are many commercial products available to help reduce reflections. Google it! Many of these work very well but they also can be very expensive. As you may know, I am kind of a DIY’er and for a DIYer, some of the best options for absorption can be items designed for use in construction. Rigid compressed fiberglass and other insulation materials can work wonders. I’ve had success using 2″ thick, 4 foot by 10 foot sheets of compressed fiber to line walls and doors. I have a good buddy who works in the HVAC industry and has been able to supply me with some of these materials at cost. One of my favorite products is a “green” solution to insulation made of recycled denim and cotton. This stuff is amazing and itch free. I’ve used it in 2 foot by 4 foot wooden frames as absorption panels. When designing Amusement Park Studios in Lubbock, we wanted to eliminate as much floor to ceiling reflection as possible in this rented, low-drop-ceiling building. The whole ceiling received this treatment as shown in this photo. The panels shown also house this material.

For the most part, 2″ to 4″ thick sound absorbing materials are very effective at controlling mid and high frequencies. Frequencies below 500Hz are the hardest to control due to their wavelengths. Remember: wavelength is determined by the speed of sound/frequency. (ex. 1130/100Hz= 11.3 foot wavelength). Bass frequencies tend to build up at walls, ceilings and floors. “Cornering” absorption materials or spacing them a few inches off from the wall to create an “air gap” helps effectively increase their thickness. [An example of this is displayed in the pic above located in the right corner.] Thicker materials or those purposfully engineered to battle these low frequencies can me employed to get the job done as well. These tools are commonly referred to as “bass traps”.

BASS TRAPS

Bass traps are absorbers with thicker design to help battle low frequency reflections. Used to help control inherent and active low frequency issues within a space, these can come in various forms. Following the same concepts of absorption these are usually placed in corners where the build up is greatest. Sometimes this can tamed with a large amount of foam or insulation materials in the corner. One of the cooler options (IMO) are resonant absorbers called “tube traps”. The ones I built for my space are a version of some that we built for the studios during my time teaching audio courses at South Plains College in Levelland Texas.

These “tube traps” are designed to resonate in response to lower frequencies (bass) creating a “vacuum” for these large waveforms. They are about 15 inches in diameter and 6 foot tall. The front side being absorptive is positioned in and the back semi-reflective (diffusive) side is turned outside. These are effective down below 100Hz and I have 11 of them. Here’s how I built these if you’re interested. Here is pic of what they look like. The gray ones.

PLACEMENT
Placement of absorbing materials can be just as important as having them in place. In my current space I have lots of absorption (tube traps) in the rear of my studio to reduce any reflections off the back wall. This way my brain is not any more confused than usual by rear reflections when mixing. This section of my room is where most recording take place as well. I can use my traps to encompass the musician and control these pesky reflections. Other great places for absorption materials are left and right of mix position (on the walls between you and your monitor speakers) as well as the ceiling above you to take on early reflections.
While controlling these reflections is important it is also good to remember that you want a space that sounds real. Back in the 80’s the idea with many commercial studios was to tame ALL reflective surfaces. This created the foam overkill revolution. In hindsight this was not the best idea. The key to good acoustics in a studio or listening space is to have a “controlled” room. Moderation. A controlled room is one that handles problematic reflections with a healthy balance of absorption and diffusion.

Home Studio Acoustics 101. Section 3.2 Absorption. Ryan’s TUBE TRAPS.

The inside diameter of these tube traps is 12″. The outside diameter is close to 15″. The width of the fiberglass is 1″. They come in 3 foot sections and must be purchased from an insulation distributer; a company that sells to contract HVAC companies. It’s pipe insulation and it’s compressed fiberglass covered in a paper wrap. You can use any diameter. As you know, the lower the Hz the longer the waveform. The bigger the ID the lower the frequency absorption capabilities of your tube traps. There is a good deal of science involved!

I got a 4 foot by 8 foot, 2″ thick styrofoam sheet from Lowes to make the inset “sandwiches” and internal support pieces. You will need 4 per tube trap. 1 for top, 1 for bottom and 2 glued for the middle connection point “sandwich”. These are roughly the circular size of the ID. Get a box of general construction liquid nails for all gluing.

You will open the paper that covers the pipe insulation and glue the seam closed. The resealable paper is helpful to close this up as the glue dries.
Once all 3 foot pieces are glued and dry, glue the middle support styro pieces together. These support “sandwiches” are used to join the 3 foot sections of pipe. Add glue to the inside of the bottom pipe where this support will go. Poke long nails into the sides of this so that it won’t slip down into the bottom section when the glue is still wet. 5 spaced, removable nails work great.
Once all has dried, remove the support nails and add glue to the top edge of bottom pipe section and the styro sandwich where the top pipe section will join. Add the top section of pipe. Allow to dry. Now you have a 6 foot section of pipe. DO NOT glue the top and bottom “caps” on yet. I then used a razor blade to cut the insulation’s paper “jacket” length-ways top to botton creating essentially a half jacket for the trap. This creates a side (back) that is more diffusive and one side (front) that is more absorptive. Use masking tape to keep this jacket in place as it will be flappy or even fall off without the tape. Tape it lengthwise to hold the jacket seam tot he compressed fiber.

Covering the tube trap is another beast in of itself. The fabric that I used is Guilford of Maine fabric which is acoustically transparent. There are many colors available. You will need about 8 foot lengths of material per trap. You will need to use a fabric measuring tape to measure the finished outside diameter for the tube trap. Measure and tack your material adding about .75 inch for slack as you will be sliding this over the tube like a sock and will need some wiggle room. Have somone who knows how to sew, sew your “sock”.

Cut off the access material with about 6 inches of excess beyond the seam. Turn it inside out and inch your sock down the tube trap. Once you have reached the end keep going until you have about 1 foot of excess material top and bottom. Fold this excess material into the top and bottom of the tube and add glue to the inside of the pipe, on the material. Then add the end caps. Allow to dry.
Curious as to what frequencies your tube traps will be effect to? There are calculation  you can do to find this info. Easier than that though is to gently drum on the sealed and finished sections of the trap. The resonant frequency your hear if the fundamental Hz of your trap. This is the lowest frequency your trap is most effective absorbing. Enjoy!

Home Studio Acoustics 101. Section 2. Isolation.

ISOLATION

Isolation is a big need and frankly one of the hardest things to achieve in a home studio. Basically you want to keep all unwanted noise out. Recording an intimate vocal or acoustic guitar while the trash truck outside of your house beating the trashcans into submission isn’t so desirable in your recording. Dogs barking and old hill-billy East Texas trucks aren’t welcome either.

Mass is a key to isolation. The more dense your construction materials the better because sound is more impervious to dense materials. This has to do with the molecular structure of the materials themselves. Not going there.

If you have the opportunity to have separate mix and recording rooms do it. This “isolates” the element being recorded from other noise producing elements such as computers, monitor speakers, engineers, etc. Typically these separate spaces are divided by a wall or two and sometimes include a double, non-parallel paned window for visual communication. Here is a pic of a studio I was blessed to help build the acoustics for in Lubbock. This is an example of the one of the isolated recording spaces.

In my case, I have not had the luxury of having 2 permanent separate rooms setup for recording purposes. Right now I rock a single room that I use for both recording and mixing. When in the same space as the musician, I monitor the recordings with headphones and try my best not to move and create chair and floor noises. I do use some home-made tube traps (more on this later) to create a “wall” around the instrument or vocal to help with isolation. Moveable walls called “Gobos” are easy to construct and can incorporate sound absorbing materials and diffusion as well.

If possible the desired construction of a space suitable for recording consists with the concept of double-wall construction. Essentially, a room within a room. This is actually 2 walls, doors, or window panes is with a small space between them. The purpose of this is that when one is hit with the sound and then travels through, the vibrations from the first wall are not passed on to the next because they are not touching. Also, this air space helps dissipate the passed sound by reflecting back and forth within the gap. In some cases the walls are purposefully not parallel to reduce resonance. Expensive noise isolating double-wall construction, doors and windows are not really an option in most home studio budgets. Commercial studios actually spend millions on construction dollars into creative ways to reduce potential problems from unwanted noise.

Most of the places I’ve used for recording have in been temporary and rented facilities. For me, it’s all about what I can to do to minimize the small things that are within my control. The outside door to my studio and windows both rattle at lower frequencies if not tamed with some inexpensive weather stripping from Lowes or Home Depot to fill these gaps. The doors in my space have weather stripping to help seal the room as much as possible too.

Another equally annoying noise is the heating and air conditioning units and the air they push through vents. Many suggest over adequate air handlers that super cool or heat the studio air with quick bursts. These units are placed at a distance for the recording space to help eliminate the mechanical noises associated with them. My favorite and obviously less expensive alternative is the handy “OFF” option on our HVAC system. When we are recording, I set it to OFF and when we are breaking or listening I move it back to the on position. Ask any small/home studio engineer and they’ll agree this is very common.

Distance from the source of the noise is great too and make off the road, out of the city studios desirable. The best way for me to avoid outside nuisances is to plan recording sessions during quite times. In the strip mall space we had for Studio Java/Dixon Productions a few years ago, this was in the evenings typically after 7. Then and on weekends. Fortunately, this is also some of the best hours for musicians. In my space now (our house) this is great time as well except for the noises that permeate from the adjacent rooms, my girls. I do my best to plan sessions when they can leave for a few hours or over night. Seems to never fail though that when I do plan sessions time at off-peak hours, turn off the HVAC system, etc… dogs that I’ve never heard before in my life seem to have barking competitions and birds outside our windows chirp with excessive joy.

Isolation is a beast to conquer correctly in a home studio environment. For me it’s all about the small things I can do such as weather stripping and planning sessions at quieter times of the day. Many times ABSORPTION is a great solution to help minimize “leakage”. Thick curtains is a great example of reducing and therefor helping isolate the outside world.

ABSORPTION will be covered in section 3.

Home Studio Acoustics 101. Section 1. The Basics.

I realize that this post series might not interest many of our usual readers but I’ve been thinking about writing this for quite some time and I will keep it light. Acoustics is a science and there are actual degrees awarded in the subject. A lot of what is spoken of here is heightened to higher levels in commercial studios. This series of posts however is intended to give general advice for the ever growing home studio environment and I am no means an expert on the topic. Enjoy!

Most do not understand what makes a “recording studio” suitable for recording. Obviously to most there is the equipment such as microphones, cables and recorders (computers and software programs these days), but one of the most important elements (besides a great band) to a great sounding recording is a great sounding room.

What does that mean? Well, that means that the “room acoustics” or the way a room responds to the sound within the room is pleasing and or controlled. When you open your closet door and talk, it sounds different. When you are in a large room with hard floors, walls and ceilings, you can hear more of the “room” and it’s largeness. Your brain was created to be able to recognize the basic size of a room without you having to see it. Pretty cool huh!

BASIC CONCEPTS OF SOUND

“Sound is a result of the pushing and pulling of air molecules.” The audio spectrum is broken up into measurable frequencies from 20Hz (very, very low) to 20,000Hz (very, very high) and everything we humans hear resides in this range.

The fundamental or root frequency producing the “boom” of a kick drum sound is around 75Hz. The “shick” of a shaker is about 4,000Hz. That annoying tone you hear in the Emergency Broadcasting Tests is pure tone of 1000Hz (or 1KHz). These frequencies and their associated waveforms push and pull air molecules while interacting with their harmonic frequencies (multiples of the fundamental) and the room on a path to your ears. Your ears contain frequency specific tiny hair-like receptors (cilia) that then take that acoustical energy and transduce (or change) that energy into electrical impulses that your brain perceives as “sound”. Amazing huh?

Ever wonder why it’s really only the bass you hear from the car behind you at the red light? Bass (low) frequencies are physically longer that those higher in our audible spectrum. The length of the frequency (wavelength) is derived by taking the speed of sound (1130 feet per second at 68 degrees) and dividing it by the cycles (positive “push” and negative “pull”) per second. This means that the physical wavelength a low frequency such as 100Hz is nearly 11.5 feet long! A very high frequency like 10,000Hz (or 10KHz) is less that 1.5 inches. You hear the bass frequencies further than their counterparts because they are physically longer  and therefore travel further. In the same sense, the intimacy of a whisper is identified by the detailed high frequency “spit” noises that accompany the proximity of another’s mouth. Moving further from the whisper loses that detail. Cool huh?

In a studio it’s imperative to control the sound to help “clean up” whatever you are trying to record or hear. Due to the physicality of sound and its behavior in a room or “the acoustics”, this can be tricky to accomplish. Ever notice that in some spaces like a standup shower, that some notes you sing resonate with more intensity than others? This is due to the physical length of the frequency and the physical dimensions of that space “fitting” together. Drums along with the tuning of their heads are intentionally designed with specific dimensions to make these resonant frequencies accomplish the desired tone. Sometimes this interaction is negative (null), sometimes its positive (node). While entertaining in concept, these interactions can really mess up recording or listening space.

Some of these issues can be avoided in construction. Non-parallel surfaces (wall to wall and ceiling to floor) help eliminate the inherent “standing waves” or resonant frequencies of the room because the waveform is reflected off at a different angle and path than which it traveled. Most commercial studios are engineered and designed in this manner. For the home studio/listening room or theater, the best way this is achieve a good sounding room is to control the way the sound interacts within the space you have. This can be achieved by using physics to our advantage by making surfaces non-reflective or at least non-reflective on the same plane. This introduces the concepts of absorption and diffusion. But before we go there though, lets talk about ISOLATION, a key element to a quite space.