Building a Home Theater & Listening Room …. and getting the acoustics right
The room, measurements and analysis
First off all I find myself very fortunate to have a room that can be dedicated to music listening and watching movies.
The room is 3.2 meters wide, 4.9 meters long and had a height of 2.4 meters. So with a little more than 15 m2 it’s not a big room at all.
First problem is the width. It doesn’t make much room for placing the speakers, and the first reflections from the side walls almost coincides with the sound from the speakers, smearing especially the high notes. As the left side is entirely windows and the other side is hard concrete, doesn’t help at all but only makes matters worse.
It has been used for music and movie watching for a couple of years now, but now it’s time to do something about it.
The dream, the plan
The following changes are planned:
- New wooden floor on top of the concrete. This will be chipboard on foam as it will be covered with a thin carpet to finish it off.
- The windows on the left will be removed and replaced with a new window door an a single window in full height. The rest will be covered with gypsum board (drywall).
- First reflections must be treated
- Bass traps must be installed to improve the bass response that is quite dominated by standing waves
- The ceiling will have a 1.2 meter wide polycylindrical absorbers/diffusors in the rooms entire length, that will serve as both diffusor and a low to mid bass trap
- The front wall and the back wall will also be dominated by diffusing polys to minimise flutter and reflections
- The old 36″ TV will be replaced with a projector and motor screen
- A door will be inserted in the left side of the back wall to get better sound proofing to the rest of the house
- New power lines will be installed, to get clean power supply for all the equipment
I tried to add some .9 x 0.6 meter Rockwool (10 cm) plated at the first reflection points on the side walls. The result was both good and bad. The reflections was definitely stopped, but the sound became a bit dull and the sound stage was smaller. On the other hand, it was now possible to hear much more details in the music, which is of course not so surprising, and the tendency for the sound to be a bit harsh in the upper mid and treble was also gone. Another finding was that I had to turn up the volume some 5 db, to get the same perceived level as before.
Conclusions: I’m not so sure that the first reflections should be treated with massive absorption and that it will probably be something to tweak into place when everything else is in place. I think the treatment could be smaller absorption patches scattered over the wall to give a both diffuse and somewhat absorbed field. Another idea is to make some larger diffusers.
Click the drawing to get a larger view
Before starting the building process there are many questions that I would like to have answered, and I hope to be able to draw on some real life expertise on some of the acoustics forums on the net.
- What kind of absorbers should I build?
- Will the panel absorbers and poly in the ceiling be effective?
- Should the yellow panel absorbers be perforated (Helmholtz) instead to be more effective, and targeted the most dominant room modes?
- Should there be placed some broadband absorbers behind the main speakers?
- Do I need to go for more broad band absorbers?
- How do I treat the early reflections best, to get the best sound?
- Will the poly diffusers work and will they sound correct?
Analysis of the old room
Using the Eigenmode calculator on Hunecke.de we see that standing waves or room modes start around 35 Hz. There is 2 modes around 70 Hz, which is definitely audible, especially the (0 2 0) on 70 Hz which has a high pressure point in the middle of the room, exactly where the two main arm chairs are normally standing 🙁
Using the Loudspeaker Calculator on Hunecke.de shows the worst modes again being 35 Hz, 70 Hz and around 100 Hz (seen as peaks on the frequency response).
Also the reverberation time seems to lack some dampening from 1 kHz and below.
Using OWA Acoustics Calculator we get a similar analysis. From this it seems that both broad band absorption and more bass absorption is needed to get the reverberation time to under 0.4 sec. for all frequencies. ….. the question becomes how much?
Another simulation has been run, with the basic changes that will be done with the room, being, less window area, and the rest of felt side wall changed from glass to drywall. It seems that adding only 4 m2 of broadband absorber (or in the simulation a combination of 4.5 m2 of low freq, and 4 m2 of broadband absorber 500 Hz and up) will actually get the reverberation time down to an almost linear 0.4 second!
I did some actual measurements and they only confirm the above simulations.
Decay for left side including subwoofer.
Room modes clearly defined at 70 and 110 Hz.
Decay for full spectrum
The decay is a bit too slow and the area from 400 Hz to 2 kHz is obviously affected by hard early reflections from the “window wall”
Room size in feet (W:L:H): 10.4 : 16.1 : 7.9
Construction of the new listening room and home theater
The old wall covering has been removed, a new floating floor has been laid and the wall with the new door mounted.
Left: The new wall seen from outside, where the large windows used to be.
Finishing the paint, and mounting the new window.
Materials for the first poly, between the main speakers, ready to be mounted.
Front poly installed. It covers the 3 main power lines coming down from the attic, and is half filled with Rockwool. Now it needs a little paint.
The framework for the ceiling poly has been mounted. It is a meter wide and will be half filled with mineral wool. The supports have been spaces unequally to broaden the resonance frequencies where it will act as a panel absorber.
Wiring for lights and speakers are being done …… hope I don’t mess this up 🙂
Down lights mounted in the side panels …. starting to look good 🙂
For the side panels I opted for two kind of Helholtz resonators.
At each end are placed a cavity of a little more than 20 litre with a large 110 mm port. At 17,5 cm deep this should give a resonant frequency of 71,5 Hz.
47 cm one port, minimum length = 16 cm => 73,7 Hz
17, 5 cm => 71,5 Hz
The middle part of the side panels
The middle part of the side panels are perforated by about 150 x 10 mm holes. This gives a broad (low Q) resonant frequency of 110 Hz
A lot can be read about the construction of corner absorbers, also termed SuperChunk on Studiotips.com. These are broadband absorbers, that if made big enough will also work down to very low frequencies.
I have opted to make a very small version to give some absorption behind the main speakers. These will not work very deep in frequency, but will help linearize the room response and give some general absorption in the whole audio frequency range. Remember hat these together with the corner cubes are the only real broadband absorbers in the room.
The corner placement is of course optimal as this is where the sound pressure is highest and the absorbers most efficient.
I like to get the absorption as far away from the listening position as possible. You have probably tried to walk past or stand beside an absorption wall (common in many office environments). This is a strange listening experience indeed, and the closer you go the more profound is the feeling. This I would like to avoid fully in the listening room. Pacing the absorption as far from the listening position as possible will make the room acoustic more uniform and the experience in the room much better.
The Chunk are only 15 cm deep (20 cm wide in the front) but more than 1,5 meter in height. The filling is a cover of a very heavy fibreglass floor plates (2,5 cm) (130 kg/m3) and a normal rockwool.
Various stages of the construction. Dark blue felt used as cover.
The purpose of the corner cubes is to provide a very broadband absorption where it is most efficient, in the corners where 3 planes meet (walls and ceiling). The corner cubes are made of medium heavy glass fibre Industrial Plates 2 from Isover (IP 2, 100 mm, 60 kg/m3), which is cut into 30 cm x 30 cm glued together using a spray glue, and covered with a nice black felt (bought at car modding shop, usually used to cover sub cabinets) on all 3 facing sides and grey on the rest.
I used plates that are covered with alu on one side, as this was what I could get. Don’t think it plays any role in this application, as long as it is not facing the room.
They are quite effective, and you will immediately hear the damping effect.
Not that difficult to build, and a very nice and good looking result.
As I have become the happy owner of a JBL HT1 THX certified speaker set, that I got for near nothing, I decided to build a podium for the 2. row (a coming sofa), and use it to double for the subwoofer casing. The subs are kind of old school, with pretty large ported cabinets (120l) and with 12″ units with relative small magnets and limited Xmax and a power handling of only 100W!!
But this aside, I can only tell that they will produce enough bass for modern movies, to satisfy most people. Anyway they will be supported by my Peerless 12″ XLS with 400W class-d amp, when it gets ready. The subs will only be used for movies and not for critical stereo listening
Old cabinets and speaker 12″ JBL unit.
Building the cabinet in 22 mm particle board ( …. ouch, quite heavy).
Room for the two bass cabinets, and a 19″, 3 unit rack for the amplifier. and 2 smaller unused hollows.
Wiring done, Acoustilux filled and the amp mounted (LAB Gruppen 300. 2 x 100 W RMS)
Speakers mounted, grilles mounted, carpet fitted, alu flight case angles and corners finishing it all off.
The Power Lines
To minimize the influence og washing machines, and other noisy equipment, I decided to lay 4 new power lines into the room. I’m really not a believer of reference power cords to all equipment, or any other snake oil rip off, but having a clean power line all the way from the house inlet, can’t be such a bad idea.
I installed 4 groups, 2 with 10 A (240 VAC) automatic fuses, and 2 with 13 A fuses. the two lines with 13 A fuses where laid with 2,5 mm2 cable and the other 2 with 1,5 mm2. Every line included an earth line.
Now I can rest assured that no noise from within the house will be disturb my listening.
Fuses and HPFI relay.