Acoustics is where the physics and aesthetics of sound meet. It’s how we ascribe quantitative and qualitative meaning to a sound’s or environment’s properties. These distinctions allow us to appreciate why places are lauded for their acoustics, to demystify microphone placement, and to visualize how sound navigates its environment.
Physics and sound
What we refer to as sound is actually a collection of complex sound waves interacting with their environment and your hearing. I love the sensation of stepping into a space and peeling away the layers of ambient sound.
Sound waves: A crash course
Sound waves emanate from the sound source, they are portrayed as a 360-degree cycle, and are measured in cycles per second (Hz). The basic wave form types are sine, square triangle and sawtooth waves.
You can easily demo these waves by firing up a synthesizer plugin on your DAW of choice and listening to how each one sounds and how they combine together. Try pairing this with an oscilloscope plugin as you will be able to see the shapes and cycles in action as you switch wave types.
When describing a space’s acoustics we are referring to the traits determined by its mix of design, materials and treatment. These, in turn, determine how the sound waves behave within the space. That behavior is a combination of reflection, diffusion, absorption and the room’s overall frequency response.
Too much reflection introduces long reverb tails. Too diffuse and the sound feels like it is escaping, but too absorbent and the room sounds dead. An aggressive response in higher frequencies translates to shrill overtones. On the other hand, failing to absorb lower frequencies can lead to bass loading and muffling.
Acoustics and the recording space
I had the chance to study acoustics during school and later put theory into practice by helping build and run a recording studio. We spent a lot of time looking for different acoustic solutions that would work in our space and budget. This included the rooms’ shape, design, materials and acoustic treatment.
There will forever be a need to balance design and cost, but that should not mean losing out on quality.
There will forever be a need to balance design and cost, but that should not mean losing out on quality. Strip away the equipment and furniture and you should be left with a good sounding albeit emptier space. Its baseline acoustics is one of a room’s greatest assets because this quality has an impact on how well the equipment performs. Bad sound does not sell!
Sounds need room to breathe in order to be heard properly. Stuffing too much sound in a small space quickly gets painful. This happens when sound waves literally start crashing into each other in a destructive manner. I think back to all the small boxy cinemas, narrow underground music clubs, project studios and rehearsal spaces I have been to. Content is always king, but it shouldn’t come at the expense of damaging the audience’s hearing.
What you don’t want to do is cheap out on the space and then try to plug the gap with more expensive equipment. Garbage in, garbage out. Our recording studio build prioritized the quality and completion of the space, only buying necessary equipment that we did not already own. Buying new equipment is much easier than building new walls.
Acoustic construction techniques
When designing a space for acoustics, you will want to contain leakage, lower the noise floor and create a predictable frequency response. This will keep your customers and neighbors happy. Keep in mind that if sound can escape, it can also come in.
We followed industry design practices by adapting and emulating them into DIY solutions. At a high level this includes:
- Addressing sound leakage
- Heavy boundary walls, offset stud, variable density drywall, multiple types of insulation
- Exterior doors used on all rooms for a better seal
- Expanding foam used to seal gaps between the walls, ceilings and beams
- Reducing parallel surfaces by angling walls in
- Suspended/drop ceiling using staggered arrays of bass traps
- DIY acoustic treatment: adjustable bass traps, foam panels, Auralex SonoColumns
- Multilayer floors: plywood, carpet pad, carpet
- 400 pounds of tabletop glass for a two-pane angled control room window
- DIY bass traps with Owens Corning 703 cores, on a wooden frame, wrapped in fabric
The materials that make up a space serve two other crucial tasks besides holding the building up. They absorb and reflect sound and even come with coefficient ratings describing the performance for materials of different types, sizes, and densities. The scale is based on a maximum value of 1.0, the smaller the value the less absorbent the material is at that range. Now you can find out exactly how absorbent ¼” drywall is at 500 Hz!
Audio playback: Let’s build a home theater!
I hope the build spiel above on the acoustics of a recording space provided some insight into what is possible. All those concepts scale across nicely to the acoustics of a home entertainment space and to other applications where sound quality is important. They are reflected in the build below.
You have just completed a new and generously proportioned extension to your house. The space has incorporated the following acoustic and construction techniques:
- New concrete slab, plywood floor layer, carpet pad, carpet
- Existing boundary walls have new wood frames/studs, mechanically separating the room from the rest of the house
- All walls use ⅝” drywall
- Offset walls to eliminate parallel surfaces
- Walls are insulated with Owens Corning 703 to a height of six feet, the rest is standard rolls of pink insulation
- Heavy walls like these will require less surface treatment
- Vaulted ceiling
- Corners deleted with 4″ Auralex SonoColumns
- DIY bass traps
- Exterior door used for entryway
The material design above tackles several key factors that you’re looking for in a room intended for sustained high levels of sound, low noise floor, higher dynamic range, flatter frequency response.
The multilayer floor and walls provide mechanical isolation from the rest of the property by reducing physical anchor and contact points. The floors and walls are more separate and will produce and receive less noise in the form of sound and mechanical vibration. The insulation of the walls will provide good lower frequency absorption on both sides. You want to keep your sound in and other sounds out.
The angled ceiling eliminates a parallel surface to the floor and gives the room more height than it actually has, which itself contributes to its character. The same applies to the offset walls, great for the side channel speakers!
The corner reduction is key; it makes a huge difference and is magical when you find the right product for the job. I had a really positive experience with the Auralex SonoColumns. It sounded like we deleted the corner!
I think these bass traps have achieved near-mythical status, but they really do work in terms of broadband frequency absorption and can be packaged in an aesthetically pleasing manner. We used Burlap for the fronts and sides and Muslin for the backing. I can’t wait to build more again at some point!
I am not going to be discussing specific equipment here and will be using a high-end generic representation of a 7.1 surround system. Amp and speaker shopping is a joyful and personal experience, I’ll leave that to your capable imaginations.
We installed conduit, cabling, and wiring terminals during construction. No wireless surround speakers here! A 65-inch to 85-inch television happily fills the center. The cost to brightness ratio is worth it these days.
There has been a rise of self-calibration systems that use incredibly clever signal processing to compensate for the environment and positioning. Just like stereo speakers, surround sound setups benefit from good placement. Your positioning should be based around your listening position.
The main and center speakers are head height, with the front left and right-angled at 30 degrees relative to your listening position; the center speaker is located at 0 degrees. A large wall-mounted television might make the center channel harder to place at head height, depending on the screen’s distance and mounting height.
I have never owned tower speakers before. This is a prospect I am cherishing because of their potential as stereo and surround speakers. They naturally sit at head height, look good, and offer a sound dynamic package.
The side speakers are wall-mounted — good thing you put in those wall speaker terminals! Mount them about two feet higher than head height. The angle range is between 90 and 110 degrees. The rear speakers are two feet above head height angled between 135 and 150 degrees.
I have not mentioned a specific room size or speaker distance because those will depend on your needs and space availability. Surround sound angles and distances do differ from stereo configurations, but you will need to maintain set distances for each pair of fronts, sides and rears. The side and rear speakers can be further away compared to the front channels.
Speaker and room calibration
The same calibration technique used for stereo speakers apply here and at similar levels. Studio monitoring is around 80 to 85 dB, but popular surround sound ranges are 70 to 80 dB. Adjust this to taste since you certainly don’t want to suffer from hearing fatigue or damage. Using a decibel meter set capable of “C” weighting and slow response. We will be using Pink Noise for our calibration tone, sweep through each speaker and confirm they reach 82.5 dB.
If you want to go deeper you can use a calibration microphone and some analysis software to sweep the room. Fuzzmeasure is still around and offers a free demo. You can use this room profile to adjust and test different panel arrangements to alter frequency response. This is also an exercise in physically tuning your bass levels and response.
Just about every amplifier comes with some sort of self-calibration functionality; they even provide a small calibration microphone. I recommend an A/B comparison only once you have completed the manual calibration above. The self-calibration process asks about the location and distance of speakers and performs its own sweep of the room. On its own, this makes a poor substitute for good placement.
The sine wave sweep will produce a frequency response that it corrects against or tries to flatten out. I really do not see the frequency correction as being applicable here since the whole point of the exercise was to design a space that sounds good. This is more aimed at living room setups thrown together in a hurry.
Recording and capture
Let’s expand on some of the earlier content on sound waves; otherwise, all I have said is that you need to physically see sound!
Microphones come in many shapes and sizes, with different polar patterns that define their directionality and on/off-axis frequency response. Dynamic and condenser microphones bring different levels of sensitivity to the table, the latter being the more sensitive. These are the traits that make microphones tick and determine their texture.
If microphones are brushes then the space is your canvas — albeit one that is subject to changes like content and location. Understanding those changes is the key to picking the right tools and making good placement decisions.
The same applies to continuing the sense of space throughout the mixing stage.
Let’s run through some examples.
You need overdubs and have set up a microphone. Do you use an omnidirectional microphone in the center of the room? Or, do you use a cardioid closer to an acoustically treated wall or surface? I would opt for the directional cardioid. All while asking why is there no vocal booth? The point here is that you do not want a diffuse or wet vocal recording, you want a dry track with no room tone.
Instead of plugins you strike out on an adventure and decide to record live reverb by setting up a sole monitor speaker. The room of a choice? A small tiled bathroom with a snappy fast envelope reverb. This really applies to capturing larger ambient sounds. An omnidirectional microphone is the easiest choice, but you can also experiment with stereo pairs.
It’s like putting the acoustics of a room in a to-go bag! The impulse response software will sweep the room with a 20Hz to 20 kHz sine wave, recorded by a calibration microphone. The sweep is canceled out, leaving you with the room’s sonic snapshot. These short sonic gasps become the impulse response.
The same technique applies to modeling outboard equipment, speakers guitar cabinets. That is how some plugins are made!
In practical terms, acoustics manifests itself as appreciating and understanding a good sounding space. Microphone and speaker placement benefit greatly and form the beginnings of a better routine. Your knowledge and understanding will continue to grow over time.
It is important to revisit concepts from time to time as you develop new insights. Get used to questioning some of your habits. Experience also teaches us better ways of delivering that quality quickly and consistently!
- Absorption Coefficients
- Speaker angles and image
- Calibration levels
- Convolution reverb