Subs and front speakers in place. Some initial acoustic absorption installed. Showing the front LHS/RHS rear panel damping in wall corners awaiting panel installation.
Now we have seen how the room was built, let’s review my home theater initial acoustic tests, design and acoustic treatments. Once the shell was complete and the doors installed and sealed I decided to see how the isolation performed, and what the rooms decay characteristics were like in its untreated form, before finalizing the rooms acoustic treatments and layout.
The Initial Tests
Unfortunately, during many of my tests I did not record the results to a file, so much of what follows is based on my memory and some old paperwork:
- Floor impact tests (a hammer) resulted in zero detectable sound transmission to the two adjacent rooms. Ear on the floor test.
- Using a generator, SVS subs, SPL meters etc. and the swept tone tests, resulted in all the expected standing waves below 200Hz spaced as expected; i.e. 30, 44, 60, 70, 89, 92, 120, 133 etc. (rounded values)
- Walls and ceiling were checked for resonances using a velocity transducer. I was unable to excite a resonance into any of the side walls at the SPL’s I could generate or using a simple impact (this was later found to be untrue). However, the ceiling was a different matter. It was a large damped flexible membrane and flexed from about 8Hz to 15Hz, peaking at about 10HZ with almost 1/8” movement (remember my earlier comment about drywall screws!). However, it was very heavily damped so removing the stimulus seemed to result in it coming to rest fairly quickly (this posed to be a problem later on). Also this movement, as I discovered, can show up on certain projected images, they move during extreme LF sound effects. This whole ceiling flexing issue is dealt with in a later post.
- The major disadvantage of having walls that have high mass is that, as I found out, they don’t flex very much and start to behave as concrete walls providing very little absorption to LF; one of the usual benefits of sheetrock walls.
- The room mode spacing and density was reasonable with little bunching and was, as predicted, from the room dimension ratios and acoustics criteria that I had chosen.
- The preverbal handclap test sounded like a really smooth, but bright, reverb chamber with approximately a 2 second decay. Surprisingly there was no audible slap echo between any surfaces. It did exist but was ‘hidden’ by the long reverb delay.
- Creating a white noise signal of 100dB 3’ feet away from the door inside the room resulted in a mid-band 33dB signal being picked up at a distance of 3 feet from the music door in the breakfast room. So my SRI of 60+dB seemed to have been achieved. Note that this is mid-band and is significantly reduced for low frequencies as transmission at these frequencies is really mass limited.
- HVAC noise measurements at this point were pointless as there was no plenum chambers or acoustical absorption.
Room Acoustic Design Approach and Considerations
This introduction is a brief, and hopefully quite readable, overview of how I derived the rooms’ dimensions and acoustic configuration. It would take a book to explain things in full.
Overview
Selecting your room dimensions, when possible, should not be underrated as they impact mode spacing and hence their bunching. They also impact a large number of other acoustic parameters that should be optimized if you are to extract the best acoustic performance from your room. All of my room sizing calculations were executed using ModeWiz by John C. Griggs. This program allows you to select a large number of desired room acoustic parameters and compare them to the idealized parameters for your room dimensions. You will need to register on Studio Tips Acoustics Forum in order to download the latest version of the program or you can get the earlier version from here.
Here are a few graphs using the JBL & Room Mode calculator applications. They were created just for this post to indicate what the room dimensions produced in terms of the standing wave patterns and the relative lack of those frequencies ‘bunching’, or appearing at the same frequency. Something you definitely do not want to happen.
Axial standing waves showing the maximum and minimum sound pressure levels within the room.
Axial Standing Wave Frequencies – Note lack of bunching.
All Room Modes
The room length axial standing wave pattern, in conjunction with the required minimum seating distance away from the rear diffuser, resulted in a main listening position (MLP) at approximately 5’ from the rear wall. This kept the MLP in a well-diffused sound field created by the rear diffuser and minimized any length node or antinode issues. Height and width position cannot really be easily adjusted so acoustic treatment and some EQ would be needed to deal with those frequencies. Ears at a height of 2′ 10″ just kept me clear of the 283Hz null. Care must be exercised when using quadratic diffusers as sitting too close to them, or using an incorrect sequence, will result in sub par performance creating more problems than they can solve.
My Design Approach
There are several approaches to the acoustical design of a space such as this. When I used to design recording studio control rooms I used an outer and inner shell. The space between the two being used only for LF absorption while the inner shell was geometrically designed together with appropriate acoustic treatments to obtain the optimum sound field at the MLP. This position could not be too narrow as these rooms often required to sit 2 or 3 engineers at the mixing console and several performers at the rear desk and other room locations. Dual shells are a common approach but requires significant space, which I did not have.
I also designed an acoustic space known as ‘The Box’ for the UK Independent Radio market. This was a self-contained modular kit that used various absorbers built into each panel and was comprised of wall, door and window modules that could be built up to form studios and control rooms of virtually any size. The control room modules, when assembled, met the 1988 Independent Broadcast Authorities (IBA) acoustic specification for independent radio. My room was not going to use this approach and as the woodworking for the panels required quite sophisticated milling equipment and assembly techniques.
After weighing up my options I decided to build a room that was as close as possible to a live-end dead-end room – LEDE. Essentially, the front of the room is acoustically dead so that you only hear the speakers with little to no other reflections interfering with the sound during the initial time delay gap (ITDG). ITDG refers to the actual time gap, at the MLP, between the arrival of the direct sound from the speakers and the first sound that is reflected off the surfaces. This effect can be achieved by geometric room boundary design and/or the addition of absorbers. Reflections at the MLP during this period being typically 20dB down. The rooms rear is made live in order to create sufficient RANDOM reflections and a diffuse sound field to enhance the sound as if in a larger live room. This is usually achieved by using quadratic residue diffusers (QRD).
Meeting the ideal requirements for such a room where the MLP doesn’t get a significant reflection from any surface for about 20mS was never going to be possible, the room was far too short and the rear wall far too close. (Remember that I also needed to be able to view movies; so moving the MLP closer to the front of the room was not practical as I would be far to close to the screen and front speakers). Early reflected energy can reduce the ears ability to create a clean stereo image and depth perspective. Fortunately, this did not pose a problem with the final listening results at an ITDG of 8mS.
If you can find a copy of Don Davis’s article ‘THE LEDE CONCEPT’, published in AUDIO magazine AUGUST 1987, it makes a very good introduction into LEDE design concept. Also see this link to Gearslutz.com.
There are plenty of very readable and relatively non-mathematical documents on room and studio acoustics such as this one from Ethan Winer and, of course, what many consider as the the ‘bible” on acoustics, the Master Handbook of Acoustics by Alton Everest. For those readers who want a little more academic approach the BBC Research and Development Department has a vast range of white papers on acoustics and its application to control rooms and studios.
Acoustic Treatments
The room point treatments were arranged to minimize all first reflections form all speakers at the MLP. Remember that these absorbers need to be deep as you need to absorb both low and high frequency energy in an attempt to keep whatever is reflected a uniform frequency response. This is obviously not practical below about 80 Hz. Treatment locations were determined using computer aided design (CAD) software. They were located on all walls, ceiling and, yes, the floor…pretty extreme yes. Well my wife thinks so!
In order to create the rear rooms diffuse reflections, and make it live again, three 1D quadratic diffusers (QRD) were used for the rear and sidewalls and 2D quadratic diffusers were used for the ceiling. These devices cause reflections to be “sprayed” out over a wide angle and diffuse the sound so that it has no correlation with the original signal. The wall QRD’s were all designed and made by me using QRDude, a great little program. The ceiling 2D QRD’s were purchased from Vicoustic, model Multifuser DC2.
Treating a small space is always challenging, especially for bass absorption as it ALWAYS requires significant space and treatments. Initially I decided to try my hand at using panel absorbers for the bass absorption (this was what ‘The Box’ was designed around). However, while their design did exactly what they were supposed to do, the amount of energy that they took out of the room was not sufficient as I could not get a large enough area of them. Note: A great deal of care has to be taken constructing these panel absorbers in order to ensure that the panel boxes were completely air tight or they will not perform as designed. Also the absorber that is placed just behind the panel that damps it must not touch it at any point or it will reduce the panels absorption performance.
Ultimately, I retuned to very deep, wide band bass traps, similar to what I used for recording studios. The bass traps for this room are up to 3 feet deep and form the front, and all corners of the room where the maximum bass energy exists. They are created from Roxul slabs that are spaced apart (not packed together) in order to allow the sound pressure wave to better penetrate the absorber. This also significantly increases the surface area for the sound wave to strike. The flow resistance of Roxsul is quite high and solid slabs 3 feet deep would not provide good absorption.
Remember that at a high level, absorption is a function of three things:
- The type of material
- The depth of material or resonant device – Resonant devices rely on other physical parameters depending upon their design
- The area of material
In my case I had already opted to use Roxul, so for the mid bass and mid range I opted to increase the surface area of the passive absorber without increasing its wall area in order not to have too much of the hard surfaces covered in absorbers. This would give rise to a very dead and rather unpleasant listening environment. These absorbers were built in similar manner to the Mulitfuser DC2 QRDs but with blocks of Roxsul having a maximum depth of 6”. They certainly seemed to do the trick.
This was the intial room layout. It was updated later based on acoustic measurements.
The above wall, ceiling and floor plans were how I originally thought the room would look. It was pretty close to how it ended up. However, when it was acoustically measured a number of issues arose, primarily early discrete reflections, that caused me to make some small changes to several of the surfaces. These changes will be reviewed in a later post.
Room Acoustic Treatment and Speaker Placement
The following is a highly abridged version of the design that occurred over a 6 month process.
The MLP was selected based on the room dimensions and projection requirements, see above, so all speakers were placed at the optimum positions/angles relative to it. I did ensure that the two sets of surrounds were exactly the same distance from the MLP, you will see why later. Genelec recommends that the acoustic axis of the relevant speakers be directed to the listener. All surrounds were angled and placed according to current recommendations and practices. The front subs were then located 1/3 of the rooms width in from each wall as there was no other suitable location. However, I did reserve space for two smaller subs under the rear corner large surrounds. More about the addition of these subs in a later post. Correct placement of height speakers is often an issue for many home environments, even a custom one, so they were placed as high as possible and angled appropriately.
The screen and projector placement was based on its zoom capacity and the largest 2.4:1 screen that I could install. This was a 115” Da-Lite tensioned electric dual mask screen that also supported a smaller 16:9 ratio (now discontinued).
I wanted an analytical room with a relatively short flat RT time of no more than 0.25 secs down to at least 63Hz (hopeful as always). However, I choose to go for 0.2 secs due to a personal preference. This is a short RT time so in order to create a pleasant diffuse sound field at the MLP I used a 6” deep QRD (sequence +1,+1,-1, +1) on the rear wall to handle the front speakers and 3” deep QRD (sequence +1, +1)on the side walls to handle the side film surrounds. QRD orientation was to set to achieve the desired acoustic spread. The rear ceiling Vicoustic 2D diffusers were added at a later date due to financial restrictions.
Front RHS panels installed into absorbers.
As stated earlier I initially designed resonant panel/membrane absorbers for the primary modes. However, while they did exactly what I expected them to do, their absorption values were far too low and only managed to reduce peaks by about 3dB and had a limited effect on the LF decay time. They also posed more absorption construction problems for the middle and higher frequencies. This work was a terrific waste of time and money..…but I continue to learn!
Rear panel damping prior to panel installation.
Rear panel absorbers installed and ceiling/Wall boundary absorption.
The spaces above and below the two Genelec 1038 rear surrounds and below the two side panel absorbers are for the HVAC returns and the addition of the future rear subs. The spaces behind the Genelecs were acoustically lined with rigid fiberglass and Roxsul to act as return plenum chambers.
All these panel/membrane absorbers were removed after numerous tests and replaced with broad band LF traps/absorbers in all corner locations, across the entire front of the room (floor to ceiling), part of the rear wall and at the ceiling/wall junctions on all four walls. This gave me reasonable absorption all the way down to 30Hz, due to the depth of the traps, and should, therefore, hopefully control the lower room modes better than the panel absorbers. I did not use deep sold “wads” of absorption as Roxsul has a relatively high flow resistance and I needed to increase its effective surface area for absorption. What was required for good LF absorption were deep traps with a high surface area (ever been in an anechoic chamber?). Once all the LF absorption was installed a quick listen to some bass heavy music sounded cleaner than it had with the panels. I just hoped that it would bring the peaks and rooms LF decay to a range that EQ could handle, and eventually it did.
The addition of all the internal woodwork had made the walls very rigid and LF absorption from the walls flexing seemed to be minimal as they now seemed to be behaving more like solid concrete walls. During my later tests a second lower frequency mode along the length of my room was discovered, more on this in a later post. However, I now believe it is a function of the outer walls of the space as its frequency corresponded to their distance apart.
The flat shelf surface under the screen had most of its area removed providing acoustic access to the absorption below it, and the rack room was lined with 1” rigid fiberglass. This treatment served three purposes:
- It absorbed any fan noise from the players
- It ensured no secondary decays from the rack room
- It provides the sound absorption required to turn it into a plenum chamber
Behind each corner Genelec 1038, where the HVAC inlet and exhaust ducts terminated in the floor, the space was lined with a mix of 1” rigid fiberglass and 3” Roxsul to create the required plenum chambers.
Front Wall Absorbers with some of the cut outs just visible in the shelving above the center speaker.
Front Wall absorbers and RHS Absorption. The large corner absorbers were later ‘slotted’ as can be seen in other absorbers.
Front LF absorbers and Wall absorbers covered with the retracted Da-Lite screen clearly visible.
Rear Corner Broadband LF Absorbers
Rear Room Absorption Installed. Showing absorption above and below the Quadratic Diffuser.
The sidewalls and door were treated with broadband absorbers to reduce the predominant modes and remove all flutter echo, which had now become very apparent. This occurred as the masking effects of all the higher order room modes were now gone. Absorbers or diffusers were placed, as determined by my custom CAD program, at the primary point of reflection for all the speakers.
In order to maintain the maximum possible room symmetry at ear level a dome diffuser was placed opposite the rack, so now both walls had hard diffusing surfaces. Absorption was placed above this diffuser to reduce flutter echo and help absorb reflections coming off the rack opposite. The dome diffuser was later replaced with a passive broadband absorber, as it created a number of strong primary reflections at the MLP.
This just left the ceiling treatment, and me getting tired and short on funds. I decided that I would leave the ceiling treatment until after the room was complete. I had installed extra thick high-density carpet underlay, the carpet and seating. These would all impact the floor to ceiling mode absorption so I wanted to see what might need to be added after I got the room measured and equalized and actually listened to some music.
This is what the room looked like before I added the front ceiling absorption and rear Vicoustic diffusers, and adjusted some of the wall panels.
Front Of Room With the Screen Retracted.
An acoustically transparent hessian cloth was used to cover all traps and speakers etc. The front of the room using black cloth in order to prevent light being reflected back to the screen and the viewers eyes which would have reduced contrast ratios. The ceiling was originally painted dark grey, again to reduce light refections, but it made the room look like a battleship! So dark brown was settled upon for the ceiling and red was selected for the walls, as from experience, reflected light with a red hue seems to be less distracting and problematic when color balancing a video projector.
Rear of Room with seating in Position and all absorbers covered.
Side and Rear Quadratic Diffusers.
With the room acoustics and finishing all done, for now, the next post will review the equipment and cabling.
