Acoustic Designfor the Home Studio By Mitch Gallagher
Acoustics Defined • acoustics. noun. 1. The scientific study of sound. 2. The characteristic way in which sound carries or can be heard within a particular enclosed space, for example, a concert hall. • No matter how cool your gear, improving acoustics will result in the biggest sonic improvement you’re going to get. • Good acoustics sound and feel better, and allow you to hear more accurately.
Acoustics versus Sound Isolation • Quality of sound in a room does not address isolation. • Stopping the transmission of sound from one location to another is a different issue altogether. • Soundproof is like waterproof - doesn’t happen without a hefty investment. • Most acoustic treatment materials have little or no sound isolation value.
Basics of Sound • Characteristics of Waves • Frequency • Amplitude • Wavelength • Phase
Sound Waves • Sound travels in waves radiating outward from a vibrating source.
Frequency • How fast the wave vibrates. • Determines the pitch • “A-440” means that the note A has a frequency of 440 Hz, which means 440 cycles, or wave vibrations, per second
Amplitude • Volume or “level” of a sound is measured using decibles • A decibel is the smallest volume change that a human ear can perceive without a reference. • Because the range of levels is so wide, the decibel scale is logarithmic to keep things manageable.
What does it mean? • You don’t have to make much of a change dB-wise to hear a relatively large change in volume. • A 3 dB increase or decrease is quite a bit. • A 10 dB increase is twice as loud.
How much force the wave exerts thru air. Measured in micro Pascals. Sound Pressure Level SPL
Wavelength • Measurement of a wave’s physical length. • Higher the frequency the shorter the wavelength. • Lower the frequency the longer the wavelength. • Wavelength in combination with phase affects where there will be problems in a room
Phase • Describes the relationship of waves or signals in time • Phase is extremely important because waves that are out of phase by even a small amount can cancel each other, resulting in tonal changes. • Phase problems occur when sound bounces around within a room; the reflecting waves interfere with each other destructively, causing all sorts of problems.
Reflection Control • Sound waves can be reflected by surfaces in a room and bounce around. • Sound waves can be partially absorbed and bounce around a bit less. • Or they can pass through or around the surfaces in the room and exit to the outside.
Any of these behaviors can be problematic • Sound waves bouncing around can interfere with each other. • Sound waves escaping the room can interfere with others tranquility. • If sound can get out, sound could get in.
How Sound Behaves in a Room • Frequency of the Sound Wave. • The shape and dimensions of the room. • Materials of construction and coverings. • Placement of doors, windows, and contents.
Sound Wave Frequency • At lower frequencies wavelengths are long, and sound waves have a tendency to bend around objects in the room, to pass thru lighter materials, and to spread out in an omni-directional pattern. • At mid and high frequencies (100 Hz and up) they are directional, bounce off hard surfaces, and are absorbed by softer materials. • Bounces if a surface has a dimension equal to or greater than a sound’s wavelength. • Reacts to softer surfaces like a ball as well.
Sound inside a room bounces off hard surfaces just like a ball thrown against a hard wall. More surfaces mean more bouncing.
Thrown against a soft surface… • It will stop dead and fall to the ground. • Or bounce off slower. • How much absorption occurs depends on the materials.
But Sound Waves Are More Complex • A vibrating source transmits waves in multiple directions. • Loadspeakers will have dispersion specifications. • You hear sound directly from the source as well as sound waves reflected off nearby surfaces. • Different frequencies have different dispersion characteristics.
First Reflections are the Worst! • Sound travels at about 1.14 foot per millisecond. • Sound waves that arrive after one bounce in less than 20 milliseconds will cause the most problems. • That equals a round trip of 20 feet.
Early Reflections Evil Sidekick… Flutter Echo • Upper mid- and high-frequency waves reflected between two parallel surfaces makes a rattling fast echo known as flutter echo. • Clap your hands in any small room with parallel bare walls… yuck.
Reverberant Decay • Once the sound gets past the first reflections, the remaining reflections will tend to mush together, creating reverberation. • Too much reverb is usually bad. • RT60=time required for reverberation to drop below 60 dB. • RT60 as well as the reverberant frequency response is very important. • Bright ringing reverb, or low booming reverb are equally undesirable.
So what can we do about it? Absorption and Diffusion. Yea!!!!
Absorption • Primary method for reducing mid- and high-frequency reflections, flutter echo, and undesirable reverb. • Soft materials placed at the main reflection points • As sound waves pass thru the material some of its energy is converted into heat.
Best Absorptive materials • Open cell foam. • Glass fiber. • The thicker the better. • Thicker absorbs more lower frequencies.
Is there a such thing as too much absorption? Yes… since absorption affects mainly hi-frequencies too much can create a dark, dead, or booming, acoustic space.
Whats the Trick? • Enough absorption to control the mids and highs. • Coupled with good bass control. • Create a balanced response with an even reverb across the entire frequency range. • Proper placement of absorption.
Diffusion • Scatters the sound wave to reduce first reflection problems. • More or less any irregular surface. • Smoothes out the reverb. • Very scientific materials for ultra critical rooms.
Low Frequencies An entirely different proposition…
Room Modes and Standing Waves • In a room low-frequency waves reflect between walls and create standing waves, a.k.a. room modes. • A mode creates resonance and longer reverb decay at that frequency. • A mode will also generate modes at each octave above itself.
A room’s modes are determined by its three dimensions. • All rooms have modes • A small room is worse because of the way modes are spaced • You can improve the spacing by making sure the room dimensions aren’t related.
Three types of Modes • Axial - between two surfaces (the biggest mode problems) • Tangential - between four surfaces • Oblique - between all six surfaces
Bass Varies Throughout the Room All the surfaces and spatial factors of a room contribute to it’s modal response.
Bass also tends to build up near surface boundaries. Uhh… near the floor, and in the corners.
Figure it Out Assuming a perfectly rectangular room - its fairly easy to figure out where the modes will occur. The fundamental frequency of the mode (F) equals the speed of sound (1130 feet per second) divided by twice the room dimension (D).
Bass response can be shaped with Bass Traps of which there are two types: • Tuned Absorbers • Broadband Absorbers
Tuned Absorbers • Slatted or Helmholtz devices which function as resonators. • Panel or membrane (say a thin sheet of wood) traps. • Both are designed to solve problems at specific frequencies… they are tuned. • You need to have the modes identified.
Broadband Absorbers • More common. • Pourous. • Large thick absorbers at appropriate locations. • They catch mid and highs too. • But they must be very big to catch lows.
Can you Trap too much? • Nope • But too much broad band absorption could deaden the highs.
Room Dimensions The size and volume, shape, and ratios of the three room dimensions have a major impact.
Rules of Thumb • Worst room dimension are a perfect cube • 2nd worst is a square room with a different ceiling hight • Also bad two or three dimensions are multiples of each other or of the same number • There are some good ratios
The Golden Mean • Ratio of 0.168 • 16’ x 10’ = ratio of 16:10 • 24’ x 15” = ratio of 24:15 • If all three dimension come close to the Golden Mean - for example 24’ x 15’ x 9’ modes will be distributed in a good way.
If you can’t get the Golden Mean… Bigger is Better Rectangular is Better