Encyclopedia of Home Recording: Acoustics
Understanding acoustics is helpful throughout the production process - from performing to recording, and even monitoring. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
The term acoustics refers to the properties that affect how sound moves around a room or space. From live performance to recording, the acoustics of a room have a significant effect on what we hear. The amount of reflected sound, when it occurs in relation to the original (or direct) sound, and the balance of frequencies in those reflections combine to make the acoustic properties of a space.
Factors that affect a room’s acoustics include:
- angles of walls/ceiling
- materials the walls/ceiling/floor are made from
- size/shape/material/placement of objects in the room
We rarely have control over all of these factors, but we usually have control over some. Keeping these factors in mind and strategically manipulating them can dramatically improve your recordings.
The prime component of acoustics is reverberation. Reverberation can be broken down into three elements (see Fig. 1). Sound waves that travel directly from the sound source to your ear (or microphone) are referred to as direct sound. Sound waves that reach your ear by way of reflection off of a surface are called early reflections. Early reflections generally happen within 10–100 milliseconds of the direct sound (depending on the size of the room and placement within it). The third element is reverberation, which is made up of sound waves that travel around the room reflecting off of multiple surfaces. The length of reverberation will vary depending on the size of the room and the nature of the surfaces in it. In a large cathedral, for example, sounds may reverberate for 4 or 5 seconds whereas a small room may only have a couple of hundred milliseconds of reverberation.
When treating a room acoustically, there are three main factors to consider: absorption, reflection, and standing waves. Absorption is the amount of sound energy that is absorbed by a surface. It is common practice to have rugs in a studio space that can be moved or removed as required. It is also common to use acoustic foam on the walls, ceiling, or on movable panels to absorb and break up reflections. On a budget, a similar result can be achieved using egg cartons. Their shape works very well for trapping high frequency reflections (see Fig. 2).
Low frequencies can be more difficult to control, so bass traps are frequently used to keep them in check. Bass traps absorb low-frequency energy that can build up in a room causing muddy sounding recordings. They come in a variety of styles, most of which are designed to be placed in the corner of the room.
A room with very few reflections is referred to as sounding “dead”. Conversely a “live” sounding room has many reflections. Both of these environments are useful in the recording process. Voice-over work is frequently done in a dead room to make the speaker seem very close to the listener, whereas many engineers prefer a live room for acoustic instruments or vocals because of the added high frequency “sparkle” it can add (much like the effect of singing in the shower).
Reflection is the opposite of absorption. Reflected sounds and their tonal characteristics have a significant effect on the sound of a recording and should be considered when choosing a room to record in or where in a room to place the performer and microphone(s). A basic empty room (four parallel walls with nothing on them) provides uninteresting reflections as sound travels out to the walls and then reflects straight back. These reflections can be made more complex (and therefore more acoustically pleasing) by use of diffusers (panels designed to create a variety of reflections), dividers (movable, freestanding panels), and by strategically placing available objects within the room for sound to reflect off of.
The material a surface is made from will affect the frequencies it reflects. Hard, smooth surfaces (such as wood or tile) will reflect most high frequencies, whereas soft or porous surfaces will absorb them. Mid-range and low frequencies will be absorbed to some degree, but not as easily as high-frequencies. The frequency range of the instruments being recorded should be kept in mind when considering reflections in a room.
Standing waves occur when a sound reflects off of parallel surfaces (such as walls, or floor and ceiling) then back into each other. Where the reflected sounds meet they begin to cancel each other out. This cancellation occurs in different locations for different frequencies, so moving the performer or microphone to different parts of a room can yield different sounds. In order to avoid standing waves and their effects, many studios are built in a way that specifically avoids parallel surfaces. Since it is rare in a home studio environment that we have the option of using a room without parallel walls, we must rely on diffusers, dividers, and other methods of breaking up reflections.
Acoustics in Your Recordings
When beginning a recording it can be a good idea to take a mental (or even written) inventory of the acoustic environments you have available to record in and their individual strengths and weaknesses. For example most bathrooms are full of surfaces that reflect a lot of high frequencies (tile, porcelain, glass, linoleum, etc.) and are reasonably small. This means plenty of bright, short reflections and may be a good choice as a location for recording vocals or acoustic instruments. If a room has too much reflected sound, blankets or carpets can be added to absorb some of the sound energy.
Another example of a possible recording space to consider would be a stairwell. The close walls and high ceiling will give a range of reflections, some quick (off of the close walls) and some longer (off of the high ceiling). A carpeted stairwell will be mellower sounding, whereas wooden stairs will sound brighter. This can be an interesting space to record percussion instruments.
A common way to take stock of the acoustic qualities of a space is to move around the room while clapping your hands, singing, or whistling to get a feel for its sound. Not only will different spaces have different acoustic qualities, but the sound of your recordings may differ greatly depending in where the performer and the microphone(s) are placed within an individual room or space. The proximity of walls and objects in the room can affect the reflected sound as well as the tonal content of these reflections (see Standing Waves). For this reason it is important to use care when choosing where to position the performer and the microphone(s).