Sound Diffusion : Diffusion is the scattering of sound in all directions so that discrete reflections are not heard. The intensity and flow of the sonic energy is equal in every location of the room. Although walls and ceilings in any space can produce discrete reflections, the problem is usually noticed in small rooms with loud sources such as loudspeakers or music practice rooms. The reflections from the room interfere with the ability to properly hear.
The solution is to add diffuser panels to the walls and ceilings of the room. These panels don’t absorb sound but rather reflect it in many directions. Diffused sound is the spatial and temporal reflection pattern of mid and late arriving reflections to the listening area.
A room that has sufficient diffusion will eliminate interfering reflections and maintain a natural ambience, It adds warmth and produces sonic images with more spaciousness (i.e.; more width, depth, and height and uniformly distributes the sound throughout the room). Diffusing surfaces have been used since antiquity in the form of statuary, coffered ceilings, columns and surface ornamentation.
Sound Diffraction: Diffraction is the act of changing the direction of a sound wave as it passes through an opening or around an object in its path. The amount of diffraction is determined by the sharpness of the bending of the sound wave. It increases with increasing wavelength and decreases with decreasing wavelength. When the wavelength of the waves are smaller than the obstacle no noticeable diffraction occurs. This is where diffusion takes over, by redirecting the waves. Our panels do both, depending on the frequency.
Comb Filtering : Comb filtering is the constructive and destructive interference between the direct sound and early reflections. Reflections cause time delays. This is because the reflected path length between the listener and source is longer than the direct sound path. Thus, when the direct sound is combined with the reflected sound, the listener experiences notches and peaks referred to as comb filtering. Using absorption panels will deal with comb filtering by removing energy from the room. It also deadens the room. Diffusion distributes the reflection over time, without absorption and thereby eliminates the comb filtering.
Room Modes: Sound waves consistently interfere as they reflect back and forth between hard walls. This interference results in tone quality at frequencies determined by the geometry of the room. This is particularly problematic at lower frequencies. That is because the step, or distance, between the next step is significantly greater than for higher frequencies. As the frequency goes up, the steps blend into a continuum, which no longer causes a sonic inconsistency. There are three types of room modes: Axial (two parallel surfaces), Tangential (4 surfaces), and Oblique (6 surfaces).
For most practical purposes calculating the axial modes for four room modes, the length, the width, the height and the diagonals of the room will give a good indication of which frequencies need special attention. Normally the first order, second order and third order waves are evaluated. As an example, if a room is 15’ wide, 9’ long and 7.5’ high the axial mode between two opposite walls is calculated by c/2X where c = the speed of sound (1,130 ft/sec) X = the distance between two walls. So, a 15’ wall to wall dimension results in a first order fundamental room mode of 37.6 Hz. The second order mode is 75.2 Hz and the third is 112.9 Hz. As the frequency increases, room modes are still present, but their number and density increase, so they are not perceived as a problem.
Modal Coupling : Modal Coupling is the acoustical joining of the loudspeakers and listener with the room’s modal pressure variations or room modes. Since conventional closed or ported dynamic loudspeakers are pressure sources, they will couple most efficiently when placed at a high-pressure region of the modal (or standing wave) pressure point. The loudspeaker placement will accentuate or diminish the coupling with the modal pressure variations at each of the modal frequencies. This is why in a non-linear room things sound so much different depending where you stand or sit in a room. This is also why one can increase or decrease the amount of bass by moving a loudspeaker. If you want to decrease the bass, move the speaker into the corner of the room, as close as you can. This moves the first cancellation notch to higher frequencies, where it can be reduced with porous absorption. Move the speaker away from the corner and the bass will increase.
Speaker Boundary Interference: (another term for modal coupling) Speaker Boundary Interference is the coherent interaction between the direct sound and the omni-directional early reflections of sound from the room’s adjacent boundaries. Since conventional closed or ported dynamic loudspeakers are pressure sources, they will couple most efficiently when placed at a high-pressure region of the modal (or standing wave) pressure point.
The loudspeaker placement will accentuate or diminish the coupling with the modal pressure variations at each of the modal frequencies. This is why in a non-linear room things sound so much different depending where you stand or sit in a room. To minimize the modal coupling effect it is very important to never place a speaker (especially a woofer) equidistant from the floor and two surrounding walls.
Slap Echo: A quick repetition of the original sound after the original sound has ceased. Flutter Echo: Short echoes in small reverberant spaces that produce a clicking, ringing or hissing sound after the original sound source has ceased.
Specular Reflections: Specular Reflections occur when sound is reflected in one direction, like from the loudspeaker off a nearby a wall. In other words, wherever the angle of incidence equals the angle of reflection, it is typically called a specular reflection. This is a common flat wall reflection. A specular reflection occurs over a very short period of time. Conversely, a diffuse reflection happens over a relatively long period of time.
Linear Room Response: Also called a Flat Response. Excellent Linear Room Response is the goal of every good listening/performing room. An excellent linear response means that for each and every frequency produced, at any given volume level, the room will effect that sound with the same relative characteristics, producing an “un-colored” sound. The sound will behave evenly anywhere in the room.
Reverb Time: Reverberation time is the time it takes a for the sound level in the room to decay 60 decibels. Or in other words, the time it takes the sound to become inaudible after turning off the sound source. Depending on the purpose of the room design, different reverb times are desired. The following are good approximate times for different applications:
Speech (.4 to 1 second)
Music practice rooms (1 to 1.5 seconds)
Home Theater rooms (.5 to 1.25 seconds)
Live recording rooms for quartets & jazz (.75 to 1.25 seconds)
Live performance rooms for ensembles & contemporary music
(1 to 2.0 seconds)
Orchestral performance rooms (1.5 to 2.5 seconds)
Pipe Organ recitals (2 to 4 seconds)
In a normal sized listening room a sound wave has to travel about 10 feet on average between reflections off one of the six surfaces of a room. If the room has the typical home reverberation time of .6 seconds, a given wave will ricochet around the room some 70 times before becoming essentially inaudible.
Live End, Dead End: The Live End-Dead End room is one of the primary designs promoted since the early 1980’s. This is where the front end of the room has primarily hard surfaces and the rear wall and rear sidewalls are primarily covered with sound absorbing material. This makes the front of the room lively and the rear of the room quite dead.
Reflection Free Zone: This is the name given the primary listening area in Home Theater Rooms and Mix/Mastering rooms. This is where the direct sound wave from the loudspeakers hit the listener before any reflected sound waves. This is done by room geometry, speaker placement, and acoustic treatment of the walls and ceiling with absorption and diffusion.
Bass Build-up: There is a lot of sonic energy in low frequency sound waves. The walls of a room can actually act like drum skins. They can sympathetically pulsate with some low end frequencies. Sometimes the geometry of the room will cause constructive interference of certain frequencies. Either of these conditions can cause Bass Build-up.
There are several types of porous sound absorbers commonly in use today, such as fiberglass, mineral wool or polymer foams. There are also membrane type sound absorbers and Hemholtz absorbers. They are all used to absorb sound.
Absorbers work by converting the acoustical energy of sound waves into heat.
The efficiency of a porous absorber is highest when the sound is traveling at its highest velocity. This point is reached at ¼ of the wavelength, and thus varies with the frequency. Since porous absorbers rely on particle velocity, they have limited efficiencies at low frequencies when they are mounted at the wall surfaces.
At the wall surface, where most porous absorbers are placed, is where the particle velocity is zero. Unfortunately, this is where they are least efficient. However, this is where the pressure is at a maximum. To exploit this high pressure, a membrane type absorber can be employed. A membrane with high internal losses coupled with an air cavity which has a porous material near the membrane, will sympathetically oscillate with the pressure fluctuations at low frequencies, thus creating air movement through the internal porous material.
Sabin: A Sabin is a unit of sound absorption equivalent to 1 square foot having a coefficient of absorption of 1.00. This name comes from Wallace Sabine, generally considered to be the father of acoustics.
To calculate the amount of absorption in a room you take the number of square feet of each different material in the room and multiply it by its NRC (Noise Reduction Coefficient). Then, add these sums together. This will give you the total number of Sabins in that room.
How to determine how much absorption a given room needs to achieve a specific reverb time:
A) Determine existing reverberation time
T = V/20S
T = Reverberation time in seconds
V = Cubic volume in cubic feet in the room
20 = The constant
S = Sabins present in the room This quantity is obtained by multiplying the area of each surface by its absorption coefficient and arriving at a total.
B.) Determine acoustical absorption required
S = V/20T
S = Sabins (units of absorption required in the room)
V = Cubic volume in cubic feet in the room
20 = The constant
T = Desired reverberation time in the room
C) To determine acoustical absorption we need to add
Required Sabins (part “B”)
- Existing Sabins (part “A”)
= Sabins we need to add to the space
To determine how many actual square feet of a particular material, you have to look up its NRC and multiply it times the number of Sabins required.
Acoustic Treatments: Any device or object designed to affect the sonic characteristics of a room is an acoustic treatment. All acoustic treatment materials have published performance specifications. Sound absorbers have a rating known as a Noise Reduction Coefficient (NRC). These properties vary over different frequencies. Materials are tested at 125, 250, 500, 1000, 2000 & 4000 cps. An average of the middle 4 frequencies is known as the NRC.
Loudness of Sound: The loudness of a sound decreases with the square of the distance from the source. Sound travels about 730 miles per hour in all directions. The size of the room affects the listening experience. In any room, especially smaller ones, it becomes quite obvious how critical the space and loudness of sound affects the listening experience.
Bass Trap - Diaphragmatic or Membrane Absorber, Hemholtz Absorber:
Every audio engineer knows the importance of proper acoustic treatment. Without real bass traps, mixes that sound fine in your control room often sound boomy or thin when played elsewhere. Foam products and light-weight tubes absorb only the mid and upper frequencies. They do little to stop standing waves and acoustic interference that cause severe low frequency peaks and dips. And if you can't hear bass instruments accurately, it's impossible to create mixes that sound good everywhere.
Low frequency response variations as large as 35 dB are common, especially in smaller rooms. Worse, the peaks and dips change around the room. The sound is thin here. It’s too bassy over there. And nowhere is the response even close to flat. A lack of effective low frequency absorption also makes the bass range sound muddy and ill defined.
Once a room has been properly treated the clarity and articulation of bass instrumentsimproves enormously, so you can hear what you're mixing more accurately and with much less effort.
A Hemholtz bass trap or resonator is a device used to capture a very specific low frequency problem. It works much like blowing over the top of a bottle; a particular note is produced. If a porous absorber is placed in the neck of the bottle it will trap that resonant frequency. This is how the Hemholtz bass trap is constructed constructed. (also, see sound Absorption above)