If you're interested in pro audio or any kind of music recording and production, it's important to understand acoustic treatment and how it impacts sound in a room. When it comes to recording, acoustic treatment can actually be more important than the quality of equipment you're using. If you invest in expensive recording gear, you still won't be able to get high quality, clean recordings if the room you're recording in isn't optimized. Room acoustics can also impact playback and how you hear the music that comes out of your studio monitors.
Before discussing types of acoustic treatment, it's important to understand how sound travels in a room in the context of recording. When a sound is produced from the source (a voice, an instrument, etc.) it travels outward in all directions. Some of the sound waves travel straight to the microphone; this is known as direct sound. The rest of the sound that doesn't immediately reach the microphone bounces around off of various surfaces in the room; this is known as reflected sound.
Direct sound is never affected by the acoustics of a room, so it is always an unaltered, pure signal. Reflected sound will have altered tone and reverb that may be good or bad. Typically, reflected sound is undesired for recording since most rooms are not designed to produce a pleasant reverb, and instead lead to a muddy recording. Having a clean, unaltered recording also allows you to put any effect on it rather than working with a recording already colored by the room's reverb. For all standard production purposes, a good recording is one that is mostly direct sound. However, eliminating all reflected sound can lead to an "empty" or "dead" sound, so a small amount of reflected sound can add some color -- if it is reflected properly. That's where acoustic treatment comes in: treating the room will reduce the amount of reflected sound that reaches the microphone or scatter existing reflections to make them more pleasant.
There are two main ways to alter a room's reflections: absorption and diffusion.
Absorption techniques do just that -- they absorb sound rather than reflect it. This will reduce the reflected sound that the room produces, also known as deadening a room. Absorption treatments are generally made of a soft material like foam. There are two main types of absorption treatments: bass traps and acoustic panels.
Bass traps are thicker foam pieces that are typically placed along the corners of a room. They are specifically designed to absorb bass frequencies, although they are effective at absorbing mid and high frequencies as well.
Acoustic panels are thin and typically larger. They come in a variety of shapes and sizes and can be trimmed to your liking. Acoustic panels offer greater surface area with less material, however, they are almost completely ineffective at absorbing bass frequencies. The main advantage that acoustic panels have over bass traps is their placement. Panels can be placed anywhere in the room, typically in the middle of the walls, which makes them ideal for eliminating standing waves between opposite walls. Bass traps generally don't accomplish this because they're placed in the upper or lower corners of the room.
Rather than absorbing reflected sound, diffusors will scatter the reflections to create a more pleasurable reverb. Most small studio rooms have unpleasant reverb because the reflections gather in one spot in the room, which causes some frequencies to amplify and others to cancel out altogether. Diffusors protect the natural frequency balance by spreading out the reflections. Most diffusors look like large wood panels with square "cubbies" or holes of varying depth.
The biggest takeaway is that absorption and diffusion are not mutually exclusive; They should be used in combination to create ideal room acoustics.
One common misconception to note is the difference between soundproofing and acoustic treatment. These terms are sometimes used interchangeably, however they have completely different meanings. Soundproofing refers to reducing the sound that comes in and out of a room through the walls, doors, etc. This is typically accomplished by blocking the walls will dense, heavy materials like fibreglass panels and sealing any gaps in the windows and doors. Acoustic treatment only refers to the sound within the room and controlling reflections. Both of these processes are important for creating a recording space, but they serve different purposes.
When it comes to treating a room for acoustics, one of the most common mistakes is buying materials without doing a room diagnosis first. The proper acoustic treatment for each room is unique, and is impacted by factors like room size, shape, and building material. So, instead of buying a bunch of materials and filling the room with it, diagnosing the reverb in the room will help you use materials more effectively, and ultimately create better acoustics.
In order to diagnose your room, walk around the room and clap your hands as loud as possible. Listen closely to hear the reverb from the claps, and assess the sound from every area of the room. If you hear a harsh metallic ringing, your room will probably need more intense acoustic treatment. This typically occurs in smaller rooms with simple layouts. If you hear a more pleasant reverb, then you won't need as much treatment. In order to get a good comparison, try the clap test in other rooms as well. You can continue to test the reverb in the room as you treat it so you can hear the difference the treatment is making.
When it comes to choosing materials, the most essential treatment type is bass traps. This is because bass frequencies can be particularly intrusive, and there is no other treatment that can handle them. Bass traps also will reduce all frequencies to some level, so they should always be installed first. Then, you can move on to acoustic panels. Apply panels to the walls until you feel you've reduced the reverb enough.
The last step is diffusers. Diffusers can be pricey, so many people choose to omit them altogether, or opt for more panels instead. Additionally, some audio experts argue that the effectiveness of diffusers is largely reduced in small rooms, so you may find that the cost of the diffuser isn't worth the benefit. Remember that the diffuser is only used to naturalize the existing reverb in the room, not take it away.