Acoustics 101-Absorbers, Diffusers, Reflections, Room Dimensions and Subs

My A/V Room Rear 1D Quadratic Residue Diffuser

Introduction.

Let’s apply a little of your new found sound knowledge from the first post in this series. Remember, we are not designing a room to build, just gathering information that will help with that and most importantly the treatment of an existing room.

Sound Absorption in Porous Materials

Lets get this one out of the way first. As we have seen sound is absorbed as heat. Either directly by causing minute movements within a material or indirectly by causing the resonance and flexing of panels or some form of air resonator. For many of us the simplest type of absorber is just a wad of porous material, but not just any material. Two commonly used porous material in home theaters are Roxsul RW40 and Dow Corning 701/703, typically 4″ thick.

Besides its area and physical placement, all porous absorbing materials have two important  parameters that should be reviewed before selecting one:

1. Absorption coefficient – This shows how much of the incident sound is absorbed at different frequencies.
2. Flow Resistance – This defines how difficult it is for an acoustic sound wave to enter the material and ultimately become absorbed. Materials with high flow resistance like Roxsul cannot be very deep where as materials with low flow resistance like fiber glass can be. It is a function of the materials density and construction, and is measured in kPa*s/m2 or Rayls. (Sometimes referred to as Air Resistance, Air Flow Resistance or Acoustic Impedance)

Absorption Coefficient

No materials or designs of absorbers absorb all frequencies uniformly.  A materials absorption coefficient measures how much of the incident sound energy is absorbed. An absorption coefficient of 1 means that all incident sound is absorbed. A coefficient of 0 means no incident sound is absorbed – totally reflective. It is possible to have coefficients greater than one for some measurements as the edges of the material also absorb sound together with its frontal area.

See below for typical absorption characteristics for different types of absorber design.

See below for typical absorption characteristics for Roxsul and fiberglass with no air gap behind it.

Absorption Characteristic Comparisons

Clearly porous absorbers even 4″ deep, do not perform well at low frequencies, even though they are often used to create bass traps. Their physical installation and design has to be looked at if they are to perform well at absorbing the lower axial modes. Bass trap/absorber design and installation will be reviewed later.

Flow Resistance

A materials flow resistance or impedance, is a measure of how easily a sound pressure wave can penetrate the material. In simple terms easy penetration high absorption, low penetration low absorption. It is measured in pascal second per cubic metre (Pa·s/m³) or the rayls per square metre (rayl/m²).

Just because a material has a high flow resistance doesn’t mean it makes for a bad absorber. The design of the absober can be modified to allow the ingress of the sound wave into deep wads of it.

Roxsul Air Flow Resistance

Fiber Glass and Rock Wool Flow Resistivity Comparison

Materials like fiber galss 703 have relatively low flow resistance so can be several feet thick and therefore be very effective at absorbing lower frequencies.

Unfortunately nothing is quiet as simple as it looks with acoustics, and a materials acoustic absorption is actually a function of its flow resistance, density and how the fibers intermesh. Never the less, large depths of high resistivity materials are not conducive to good bass absorption.

See here for a simple porous absorber calculator.

Absorption Area

Another important parameter of sound absorption, besides the absorbers design, is its AREA. Presenting high surface areas to the incident sound wave provides rapid absorption of the wavefront. Take a look at every anechoic chamber in the world. They do not have their walls lined with many feet of solid absorber they use deep pointed absorbers that present huge surface areas and great depth.

A Typical Anechoic Chamber

In small rooms there is generally insufficient wall area, or space for enough large bass traps. So you need to get creative by using all the rooms corners, and selecting correctly designed and placed bass traps. In addition, using multiple subs will make a huge difference to a rooms bass performance. More on this later.

Room Dimensions

For the sake of simplicity, we shall only consider closed rectangular rooms and axial modes. Irregular shaped rooms, angled walls, vaulted ceilings and open rooms create other acoustic issues that are unique to them and cannot be reviewed here. (If you ever build a room from scratch make it rectangular, its acoustic performance is far easier to predict.)

Let’s first look at the standing wave patterns for two rectangular rooms. One is a cube (worst case) the other my A/V room (purpose designed).

An Acoustically Poor Room Standing Wave Pattern

My A/V Room Standing Waves Showing My MLP

Comparing the two standing wave diagrams the cube clearly shows that the axial standing waves are all the same frequency and occur at the same points in space. My A/V room dimensions were deliberately chosen to try to spread out ALL modes in the most uniform possible way. While at the same time meeting a number of preferred acoustic requirements.

Mode bunching can clearly be seen in the following two tables. Table 1 for the cube shows a low count of all modes in vertical straight lines with significant bunching between 100Hz and 200Hz. Table 2 shows a much higher count of modes fairly well distributed all the way up to 350Hz.

Table 1 – An Acoustically Poor Rooms Modes

Table 2 – My A/V Room Modes

Click here for this popular Harman JBL axial mode room calculator.

When selecting room dimensions they should be chosen to ensure the most uniform spread of modes in order to reduce room colorations and provide easier room acoustic treatment. These dimensions also effect other acoustics critera that need to be taken into account when designing the room. An excellent tool for selecting a rooms optimum dimensions (my favorite) is Griggs Calculator found here. There are also numerous “golden ratios”, some of which are shown below and the research carried out by Professor Cox from Salford University here.

Typical Room Golden Ratios

EXAMPLE: For a room with a ceiling height of 8 feet, and using the Golden Ratio of 1:1.6:2.33, would create a room with a width of 8*1.6=12.8 feet and a length of 8*2.33=18.64 feet.

Seating Locations

The location of the main listening position or MLP is quite important. This position can be considered to be the ‘money seat’ for those of you, who like me, are a little selfish and want your position to sound the best. In larger home theaters it is much more about getting an even sound for all seats rather than the best sound for just one.

Looking at the above standing wave plots you ideally do not want to locate the MLP at either a pressure maximum or minimum on any room dimension. However, there are practical limits to just what can be achieved. Assuming that the screen is on the wall of the longest dimension, then you can adjust your MLP from the screen so that it is neither at a maximum or minimum, the same for the height for many of the modes. However, as most MLP’s are in the center of the width of the room you are stuck with what the width dimension presents you, and you can only improve the standing waves on this dimension by absorption and/or diffusion. Ultimately using EQ to trim the response. My MLP is 4’6″ from the rear wall and 3’4″ from the floor. Which is clear of any mode or antinode location except as described earlier, the width modes.

A single measurement at an antinode may result in excessive level reduction at that particular frequency when electronically equalized, adversely impacting other seats. A single measurement at a node can easily result in signal clipping and speakers being overdriven when attempting to bring these low levels up to match the average room level. This is why when equalizing a listening location it is better to average many readings around the MLP seat rather than at one specific location. Both our ears are never in the same location as the mic!

Optimizing the bass acoustic performance for multiple seats generally requires:

1. Good room absorption below 200Hz.
2. Two or more subs to help even out the bass response. More on this later.
3. Taking acoustics measurements at many locations during the rooms equalization so as to average the overall frequency response for all seats.

Optimizing the rooms response for multiple seats above 300Hz is generally straight forward and only requires the addition of relatively small absorbers and diffusers to control and diffuse reflections plus a good equalization system.

Room Reflections and Their Treatment

Stereo, ambisonics and multi-channel sound were created to provide the listener with the illusion to perceive the physical locations of all the sounds and instruments. In the case of multi-channel movie sound these effects are more directed to following the movies action. For music we try to create an image of the performance that shows the physical locations both between and outside the speakers and their relative depth and height positions. This illusion is very much part of both the microphone technique used and the way the reflections in your room interfere with the direct sound from the speakers. You do not need your eyes to see the musicians, your ears and brain are quite capable of creating this spatial illusion. Just close your eyes at a live concert.  For recording, once the engineer deploys more than one stereo mic it all becomes more complex to maintain the illusion. Each mic picks up multiple paths of the instruments sound reflections and starts to smear their timing information which is what your ears need to be able to create the spatial illusion.

In modern day multi-track recording creating a well-defined left/right image is relatively easy (panned stereo), creating the illusion of depth AND relative height is far more challenging. I have no modern multitrack recordings that can compare to the depth and relative height placement of Sheffield Lab recordings. Many of which were accomplished with just one stereo microphone and a fill for the kick drum and double bass. Mixing and level control was then at the discretion of the band leader not somebody sat at a console adjusting numerous spot mics to balance the mix.

So why have I just spent two paragraphs explaining what happens during the recording? When we are talking about reflections in our rooms. What we are trying to accomplish is to create an aural illusion of the original soundstage. So anything that interferes with that creation should be dealt with.

The ear/brain is quite sensitive to how it interprets room reflections relative to the original direct sound. It is these reflections that can make or break the stereo, or multi-channel image illusion. So let’s look at some of the effects and what we should be thinking of when controlling these reflections.

The Precedence Effect

In short, when two identical sounds from different sources arrive at the ear with only a small time difference, the ear will localize the sound as coming from the earlier source only. Our ears determine the position of a sound based upon which ear receives it first and then its successive reflections. This arriving sound information, will also give us the perception of depth and spaciousness. Pretty simple!

Much work has been done by many researchers to show that in order to provide a stereo image with precise imaging and good depth perspective there is an optimal initial time window of up to 35mS, after which, room reflections are heard as  discrete echoes. This effect is often referred to as the Precedence Effect.  When a sound is followed by another sound separated by a sufficiently short time delay, listeners perceive a single auditory event; its perceived spatial location is dominated by the location of the first-arriving sound. The lagging sound reflections can affect the perceived location. However, their effects are suppressed by the relative level of the first-arriving sound.

The Hass Effect

A special application of the Precedence Effect is the Haas Effect. Haas showed that the precedence effect appears even if the level of the delayed sound is up to 10 dB higher than the level of the direct sound. In this case, the range of delays, where the precedence effect works, is reduced to delays between 10 and 30 ms.

Classical Hass Delay Chart

In a typical residential system there should never be any reflections that are higher in level than the direct sound.

Significant reflections within the first 2-5mS can create an effect called comb filtering. It is like a ‘whooshing’ effect sometimes called phasing as you move your head left to right. These reflections will come from furniture and acoustical paths that are typically just a foot or so longer than the direct path to the MLP and very often come from seat reflections especially if they are made of leather. You can see some early seat reflections in the following graph at 2mS. These can be reduced/prevented by using cloth seats or covering the tops and rears of leather seats with blanket absorbers.

My A/V Room L/R Reverberation Decay Curves – leather seats with no seat covers.

For now ignore the rise in reverberation time at 10mS. This is deliberate and will be discussed later.

Initial Time Delay – ITD

Classical Initial Time Delay Gap (ITDG) and Reverberation Decay Graph

During the ITD no reflections should ever exceed the level of the original sound. The length of the ITD is really a function of how big your room is and how well you can suppress early reflections. The ITD length and what happens to reflections during it is a very important parameter in the design of control and listening rooms.

Live End Dead End (LEDE) Control Room Research

These control rooms were very popular for many years but have recently fallen out of favor as newer diffusion designs have appeared. However, in their ‘hay day’ LEDE research provided a lot of valuable information as to how a listening environment should behave with regard to a rooms reflections. See here for a review of the LEDE technique by Don Davis. (My A/V Room is loosely based upon this design concept). The concept relies on a totally none reflective room front in order to prevent reflections arriving at the MLP within the reflection free time zone, and an ITD that is terminated with an added burst of reverb created by a Hass Kicker. This effect is controlled by the design of the reflective back half of the room.

You might ask why is all this done? You perceive a rooms size and acoustics by the length of the ITD and subsequent reflections. So if the mixing engineer, and you the listener, what to hear the space and reverb effects that were recorded you do not want to mask the recorded ITD with your rooms ITD. If yours in much shorter than the one recorded you will not hear the recorded acoustics as they were intended.

In short, the findings of the LEDE work should be applied, if possible, to the reflections of your room to optimize the listening experience. They may be summarized as follows;

• There should be no reflections during the first 5mS. Before the first most significant reflection.
• After 5mS all reflections should be at least 20dB lower than the direct sound at the MLP for at least the first 10mS (the reflection free zone). In the case of a true LEDE design this window could be 20mS or more.
• The ITD should be terminated with a ‘Hass Kicker’. A burst of reverb that is approximately 10dB lower than the original direct sound.

LEDE Simplified Acoustic Performance

In order to achieve the above reflection free zone, the treatment of every first reflection point is often required, particularly for small rooms. See the following drawings for my A/V Room.

My A/V Room – First Reflection Point Treatment.

This treatment may be achieved by using broad band porous absorpers, inexpensive and simple and diffusers, more expensive and complex.

Excessive treatment of all primary reflections with large enough porous absorbers can result in the room becoming too ‘dead’. This is compensated for by the addition of diffusion at the rear of the room like that of a LEDE room. However, the timing of the reflections from these diffusers needs to be reviewed if they are close to the MLP.

Creating a Hass Kicker is not an essential part of many room acoustic designs, can be difficult in a none dedicated room, and will not be reviewed here. However, achieving the reflection free zone as described above is really a must.

We will review how to find all the first point reflections in the next post.

Rear Wall Reflections

Rear wall discrete reflections in small rooms often arrive at the MLP far too soon and can smear imaging. The solution to this problem is part of the Live End Dead End (LEDE) control room design.

Depending upon the rooms design the rear wall may be treated in one of two ways depending what you are trying to achieve:

1. Absorb sounds striking it
2. Diffuse sounds striking it.

While solution one will certainly prevent the negative interference effects of early arriving reflections, in my experience it tends to produce an uncomfortable acoustic ‘hole’ behind the listener unless the treated surface is immediately (a foot or two) behind the MLP.

NOTE 1: When placing ANY porous absorption it is essential that the bandwidth of the absorber is wide; ideally at 300Hz to 20KHz. Why? If you only absorb a small range of reflected sound frequencies the rooms acoustic performance will become colored and unbalanced as the absorbers will leave an unbalanced amount of reflected energy. So a reflection path of typically 8″ is required. In my experience this generally requires absorbers that are at least 4″ deep and often up to 8″ depending upon the incident angle of the sound.

NOTE 2. The deployment of too many absorbers can quickly cause small rooms to become too ‘dead’ and unpleasant acoustically. Reflection problems in many rooms can often be better dealt with using diffusers. These breakup the incident sound wave in to many wavefronts that vary in direction and time for 2D diffusers. This removes the strong single point refection while at the same time keeping the room reverberant.

When deploying solution 2, the diffused sounds should arrive at the MLP at least 20dB lower than the direct sound within the first 8-10mS. This ensures that they will not interefer with stereo imaging or depth perspective but will enhance the openeness of the perceived sound field. An acoustic delay of approximately 10mS means that the rear diffuser needs an acoustic  return path length of at approximately 10 feet to the MLP. When using QRD’s to diffuse sound this distance is also critical to stopping a condition called lobbing where the QRD acts like an acoustic lens concentrating their reflected energy into narrow beams (see Diffusers below). QRD design and deployment will be briefly reviewed in the next post.

My A/V Room – Front Center Reverb Decay at the MLP – Hass Kicker at 44mS, 10mS After Direct Sound.

My A/V Room Left Rear Reverb Decay at The MLP – Reflections at 58mS and 67mS are from the front room carpentry and speaker surfaces.

My A/V Room Left Surround Reverb Decay At The MLP

Absorbers, Diffusers and their Placement

Absorbers

There are two classes of absorber:

• Pressure based
• Velocity based

The terms ‘velocity’ and ‘pressure’ doesn’t describe their inner workings, but instead describe their optimum placement in a room. Pressure based treatments are most effective at areas of high sound pressure in the room, whereas velocity based treatments are most effective at areas of high particle velocity.

Velocity Based Absorber Placement

Pressure Based Absorber Placement

Velocity based absorbers would include:

1. Porous materials -all frequencies, particularly bass traps

Pressure based absorbers would include:

1. Sealed panels or membranes – for low frequencies
2. Resonant devices – for specific lower frequencies and general wider mid band and higher frequencies

Velocity Absorbers

The most common and popular bass absorbers are nothing more than a specific application of porous absorbers that are placed at points of high particle velocity. Specific problematic room modes can be dealt with using tuned, resonant sealed panels, that are placed  at points of high pressure.

For any room the biggest challenge is bass absorption and there is no one simple solution for this. Unless of course you have lots of available unused space. Absorption at bass frequencies is generally achieved using large porous absorbers and/or resonate panels for specific problematic frequencies. Diffusers are rarely practical. Bass ‘piles up’ at room boundaries and corners so installing the correct type of absorbers in these locations will provide the largest amount of effective bass absorption.

Classic Porous Corner Bass Traps – Velocity Absorber

Purpose Designed Bass Trap – Velocity Absorber

Personnel Favorite – Purpose Built Studio and Control Room Porous Bass Trap – Velocity Absorber

The depth (D) of all the above bass traps determines the lowest frequency (wavelength) that can be efficiently absorbed. In the ideal world (D) would be equal to 1/4 the wavelength of the lowest frequency that you need to absorb. In practice this is rarely practical as a 40Hz signal has a 1/4 wavelength of over 7 feet. However, even depths shorter than 1/8 or less of the wavelength can significantly help control low frequency modes.

Understanding that the peak absorption occurs at the peak particle velocity for a porous absorber, we see in the following diagram how the peak absorption will change based upon the wavelength/frequency of the signal, air space depth and absorber thickness.

Absorption at Different Frequencies – Velocity Absorber

NOTE: Porous absorbers that use an air space behind the absorber will give rise to irregular absorption above the frequency that corresponds to its depth, so use the solid corner bass absorber.

All these porous absorbers are wide band, absorbing the entire frequency range and if extensively deployed without due consideration to mid band and high frequency absorption can cause a room to have too low a reverb time at mid band and higher frequencies.

Pressure Absorbers

The other common bass absorber is the panel resonant absorber. These rely on the mass/compliance of a panel or air mass in conjunction with a sealed air chamber to resonate and absorb energy over a band of frequencies. They are placed at points of high sound pressure, typically on walls or in corners. Rooms that only have one or two problematic bass standing waves can use resonant devices that are tuned to those problem frequencies.

Typical Resonant Membrane Absorber Construction – Pressure Absorber

Membrane panels have a resonance frequency that depends upon the depth of the trap (D) and the mass of the panel (M). Although quite large for bass absorption the Helmholtz resonator is another popular resonant pressure absorber.

Basic Helmholtz Resonator – Pressure Absorber

The basic Helmholtz resonator has a resonant frequency that depends upon the volume of the sealed container (V), the area of the opening (A) and the depth of the opening (L).

At mid band and higher frequencies perforated panel Helmholtz absorbers are quite small and easily tuned to absorb specific bands of frequencies.

A Panel Helmholtz Resonator -Pressure Absorber

The perforated panel Helmholtz resonator has a resonant frequency that depends upon the depth of the chamber (D), the diameter of the openings (d), the panel thickness (PT) and the panel % perforation.

It is essential that in all resonant designs that the enclosures are completely air tight or they will not resonate at the designed frequency and it will not absorb very well. Also the damping behind the membrane must not touch it or it will significantly reduce the absorption and alter the absorbers bandwidth. If the membrane is not rigid like drywall or plywood and is self damping like Revac or roofing materials (a limp membrane absorber) you can often omit the damping immediately behind the membrane and place it at the rear of the box.

We shall see how to calculate these absorbers resonant frequencies in the next post.

Diffusers

There are many ways to scatter, diffuse or breakup a sound wavefront. The technique used can vary from a simple domed geometrical structure to complex two and three dimensional structures. Like absorbers, diffusers have a diffusion coefficient between 0 and 1, indicating the degree of sound diffusion, 0=no diffusion, 1=100% diffusion. This coefficient, just like that for absorption, will vary with frequency.

My A/V Room – Side And Rear 1D Quadratic Residue Diffusers

These diffusers breakup the wavefront in two ways:

1. Spatially – the incident wave is scattered into multiple directions
2. Temporally – the incident wave is scattered into multiple directions but with different time delays.

Spatial Diffusion (simplified)

Spatial and Temporal Diffusion

Well designed diffusers will exhibit both spatial and temporal dispersion and may be either:

1. One dimensional 1D – the sound is only diffused into one plane such as left/right or up/down
2. Two dimensional 2D – the sound is diffused into two planes both up/down AND left/right

There are subtle acoustic differences and uses for these two styles of diffuser but there is a great deal of overlap and often it may just come down to the users preferences.

Typical diffuser designs fall into the following catagories:

1. Geometrical – These are simple geometrical shapes like domes and cylinders
2. Reflection Phase Gratings – There are sets of different height blocks and wells based upon a mathematical sequence.
   1. Quadratic Residue – QRD
   2. Maximum Length Sequence – MLS
   3. Primitive Root – PRD
3. Binary Amplitude – These are 2D reflection gratings and are designed on a binary sequence; a hole =0 or 1, a blank =1 or 0.

Basic Geometrical Diffuser

A 5 Section 1D QRD Diffuser. Barker Sequence +1,+1,+1,-1,+1.

Vicoustic Multifuser DC2 – 2D PRD Diffuser

Simple Binary Amplitude Diffuser – 2D

Image Credit – Vicoustics. DC2 Diffusion and Absorption Characteristics

All diffusers have a low and high cut-off frequency beyond which they no longer diffuse the incident sound energy, but either reflect it and/or provide some degree of absorption. They all also have a minimum seating distances in order for the diffusion to be effective. Badly designed QRD diffusers or sitting too close to one can result in lobbing. A condition where by the diffusers act as a lens concentrating certain bands of frequencies into areas radiating from its surface called lobes. See below:

QRD Lobes For Different Panel Configurations

In the above diagram the thin lines are for a single panels and the thicker lines are for multiple panels and preferred panel sequences. Note the reduction in discrete lobbing for multiple panels using the correct panel sequence. Also the need to sit a sufficient distance away from the panels in order to be in a uniformly diffused region.

Subs and Their Placement

In small rooms and even sometimes large ones getting enough low frequency absorption installed can be very challenging. Equalization at these low frequencies can flatten the rooms frequency response but frequency response alone DOES NOT make a room sound good. One of the major measurements that really impacts a rooms bass performance is its decay time. If it takes too long for a sound to decay the room will become colored and sounds will become muddled and muddy despite the rooms apparently flat frequency response. These two parameters, flat frequency response and fast decay time are not directly linked. You can have a flat frequency response with very long decays, just as you can have a very lumpy frequency response with fast decays. How do you achieve both? Lots of bass absorption, multiple subs with optimum placement and a good equalization system.

My A/V Room Sub Waterfall

The above graph shows the sub decay response of my room. Note how all room modes above 10Hz have decayed by at least 30dB within the first 300mS. The preferred LF decay is generally taken as being at least -20dB in the first 160mS across all bass frequencies; which this graph shows. The long decay at 15Hz is the resonance (ringing) of the room walls and ceiling construction. Not a decaying room mode. (This ‘floating’ room within a room is a separate isolated structure and is not attached to the main house structure.)

Many automated room equalization programs cannot optimize decay time while at the same time optimizing frequency response…..well none that I have used. See here for my experiences equalizing four subs using Audyssey XT32.

The addition and placement of additional subs in a room can significantly improve both axial modes and overall bass uniformity, but bass absorbers are still a pre-requisit for a good low frequency acoustical  performance. Multiple subs are NOT a replacement for the lack of bass absorption, although they will help smooth out the rooms frequency response making electronic equalization easier.

NOTE: Almost everybody places sub(s) on the floor, so using subs to improve height axial modes doesn’t work unless you raise them off the floor to the appropriate height. Something that is generally not required and is highly impractical for most of us, even with a dedicated and purpose built room.

Analysis of this topic is quite complex so I will just provide some basics to get you going, together with a few links for further information.

The Bass Crawl – One Sub

Single subs are frequently placed in room corners. This is because a corner restricts the area of the subs wavefront providing it with effectively more acoustical output as the sound pressure is not radiating into a sphere. This is particularly true at its lowest frequency range, where the room gain or the boundary effect takes hold. Depending upon the room geometry and seating positions one corner may be better than another. Experimentation is required here.

A common way to determine a single subs best placement is to do the ‘bass crawl’. Put the sub where you sit, play some bass heavy music and crawl around your room on your hands and knees in order to determine the smoothest and cleanest sounding bass location. This is where you put your sub. Yes, I know it sounds ridiculous but it does work. Unfortunately you may end up with a location that is just not practical or acceptable to your better half!! So that corner maybe looking really good at the end of the crawl.

Preferred Sub Placement Positions. -Two or Four Subs

Based upon the number of subs that you have, there is a choice of optimal sub locations that will provide you with the smoothest bass response.

Multiple Sub Placement In Reverse Order Of Performance

Remember, and this is VERY important. When using more than one sub all the subs technical performances must be the same. DO NOT mix different designs of subs like sealed and vented and do not mix different sized subs. Ideally do not mix manufactures and models at all. Why? There are several reasons:

1. As all subs have to be set to give the same SPL at the MLP it is easy to overdrive the smaller less capable sub for both level and low frequency causing a significant rise in distortion and possible damage to the lesser sub.
2. If your using auto EQ like Audyssey it will set the combined performance of the subs equal to the lesser sub in order to protect it due to its higher frequency LF roll off.
3. If you mix sealed and vented subs their GROUP DELAYS will be very different making combined equalization extremely challenging and rarely optimal. (see here for my expereiences).

Besides correctly matching all speakers SPL’s at the MLP, you must also set the time the arrival of the sound at the MLP from each sub to match each other and all your satellite speakers.

Room Equalization – A Brief Comment

Equalizing a room for best acoustic peformance is more than just getting a flat frequency response. The rooms decay must also be uniform and meet certain preferred criteria as discussed earlier. Achieving a flat frequency response while technically correct has been shown to be less pleasing to many HT listeners than a gradual 10dB fall from 20Hz to 20KHz. See below.

My A/V Room Calibrated Reference Frequency Response LHS Flat

My A/V Room Preference Frequency Response With 10dB Roll Off -L/C/R

However, flat is flat, and that sets your reference point giving you somewhere to start from and return to. Preference is a whole different issue and is simply what the user prefers to hear, which may or may not be close to what was heard in the studio control room.

Equalizing for both frequency and time (decay) issues is complex, particularly at low frequencies. In a simplistic world a rooms low frequency response can be shown to be composed of a number of resonant frequencies having nodes and antinodes.  This is the area below Schroeder’s critical frequency where the rooms acoustic performance is dominated by standing waves. These discontinuities cause the irregular frequency and time (decay) responses. If we can create exactly the opposite nodes and antinodes for the rooms low frequency response, that is dominated by these room modes (standing waves), both the frequency and time responses will be optimized. The LF waterfall response that you see above was created by doing exactly that.

With modern laptops and software this process is quite simple using free programs like Room Acoustic Wizard (REW). Some automated equalization programs also claim to be able to equalize both frequency and time anomalies. I am yet to be convinced by this; Audyssey XT32, in my experience, certainly doesn’t!

Room Acoustic Software Programs & Further Reading

There are literally hundreds of free acoustics programs available on line. Many are very basic just calculating room modes and predicted required room absorption, others are are very sophisticated in that they can provide optimized room sizes according to a number of acoustic criteria that you select. Below are a small selection of acoustics programs and further reading:

• QRDude – Diffusers
• REW
• Porous Calculator
• Griggs Room Calculator
• Harman/JBL Modes
• Pro Sound Training
• GIK
• Acoustics Insider
• Ethan Winer

So now we know how to choose materials, where to place the different types of absorber design and where to place additional subs. In the third and final part we will see how to determine primary reflection points and examine how to build several different types of absorber and QRD diffuser.

See post 1 – Acoustics 101 – Sound Basics here.

See post 3 – Acoustics 101 – Designing Porous,  Resonant and Reflective Sound Treatments