In this part I will do the acoustic project for an empty room.
NOTE: I recommend you to read my first parts of the post.
For measurements of the room, I used Room EQ Wizard Software and miniDSP UMIK-1 USB Measurement Calibrated Microphone. For formulas and calculations please DOWNLOAD my acoustic calculations file. BEFORE, we start please watch this tutorial to learn how to use Room EQ Wizard.
After you finish the measurement in Room EQ Wizard you will find the Reverberation Time RT60 of your untreated room – now open my Acoustic calculations file and replace the “Dimensions” of the room and the “Untreated room RT60” column with the numbers from your RT measurement.
NOTE: PLEASE USE comma ” , ” instead of dot ” . ” when you write decimals in the acoustic calculations file.
Section 1: Dimensions project and their discussion
Length = 5,8 – Width = 4 – Heigh = 2.5
The dimensions need to match the Bonello principle so the number of modes must be non-decreasing; the number of modes in a frequency band must be greater than or equal to that contained in the band immediately previous. You can check HERE if your room dimensions match the Bonello Principle.
So with these dimensions, I m sure the principle is working.
Section 2: Modes analysis and their discussion
Using the speed of sound of 343.6m/s with the calculation ﬁle I have made, I found all of my axial, tangential, and oblique modes. ( Check The calculation file modes section, the Axial modes are colored in red )
So these modes will need to be resolved with the resonators that I will project later in the project. In the table, I also added the direction of the modes to be much easier later when I will project the resonators for each mode. To see where the most problematic modes are (under witch frequency I need to project my resonators) I have calculated the Schroeder Frequency (This is calculated with my calculation file).
REVERBERATION OF UNTREATED ROOM
So at 1 kHz, I have 0.85 seconds of reverb that is too high for this kind of audio production room and I will use it in the Schroeder Frequency formula to find the Limit frequency at 241.7 Hz. So under this frequency, I have some problems with the modes in the frequency response of the room, will be boosts and nulls so I need to resolve this problem.
I also calculated the Medium Time, Bass ratio, and brilliance of the room.
MEDIUM TIME: 0.87 – BASS RATIO: 0.97 – BRILLIANCE: 0.87
As we talked Studio Acoustics part 2/3 The Bass Ratio And Brilliance should be somewhere around 1.2 so I will need to fix this by finding the right amount (square meters) of absorption materials.
Section 3: Solutions of modes
In this section, I will speak about the solutions of modes for the room in the discussion.
For the ﬁrst mode at 29.5Hz I will not make any resonator because the Monitor system I will use in this room will not produce this frequency.
For the other modes of the room under the Schroeder frequency, I will project the resonators and also I will use some absorption materials already made found on the internet. So for the 43 Hz mode, I will project and place the resonator on the direction where that mode is. To project the membrane resonator I will use wood with a density of 420kg/m3 because at low density and bigger distance from the wall the resonator can work also on usual listening volume and with a thickness of 1 cm (0.01 meters). Please check Studio Acoustics part 2/3 to learn how to calculate the distance from the wall you need to place the resonators. You can also find some useful info HERE.
So after the calculation, I found that I need to put this resonator at a 47cm distance from the wall to have the resonance of 43 Hz. To resolve this mode I will need to put two of these resonators on the left and right walls of my room.
Using the same formula the same wood density of 420 kg/m 3 and the same thickness of 1cm I will ﬁnd the correct placement from the wall of each membrane resonator for all of my other modes.
68.7 Hz Mode will have one membrane resonator placed on the ceiling and at 18.1 cm distance from the wall.
59 Hz Mode will have two membrane resonators placed on the front and back walls of the room at 24.6 cm distance from the wall.
85.9 Hz Mode will have two membrane resonators placed on the left and right walls at 11.8 cm distance from the wall.
88.5 Hz Mode will have two membrane resonators placed on the front and back wall at 10.9 cm distance from the wall
For the remaining modes of the room, I will use some absorbers from the internet that have a big absorption coefﬁcient from 100 Hz to 250 Hz and will absorb at least 70% at my modes frequency and 90-100% at 100 Hz, 125 Hz, 160 Hz 200 Hz and 250 Hz. I will also try to place the materials on the surfaces in a mode that can also resolve my axial modes remaining, almost on every wall will be at least 4 meters square of this material that I will speak later in the project when I will make the calculation of the reverb with all my materials.
Section 1: Materials and the absorption coefﬁcient discussion.
For this project is requested a reverberation time at 1 KHz of 0.25 seconds. To get there I need to ﬁnd some materials that will absorb in the zones that I need to get from 0.85 to 0.25. After I have made some research on the internet I found the next materials to be good for my project.
Material 1 (Vicoustic Cinema Round Premium Black)
Product Technical Information Material – Acoustic Foam Product Dimensions – 60 x 60 x 7.5 cm = 0.27 Square Meters/piece – One box has 8 = 2.16 Square Meters per Box
This material will absorb very well from 315 Hz to 5 kHz.
Manufacturing Company Information can be found HERE
Material 2 ( Vicoustic Multifuser DC2 )
This one is a diffuser but it has a bit of absorption between 600 and 800 HZ and I’m going to use it because will also bring diffusion on top of the diffuser I will create later in the diffusion section.
Manufacturing Company Information can be found HERE.
Material 3 ( RPG Modex Plate) – One of the best panels I found they are a bit hard to find but it is worth to try and find some. Email or call the manufacturing company might be the best idea.
This plate panels are really good in the absorption of low frequency exactly where I need also for the reverb time and also for the rest of my modes so this panels will be placed in relationship with my remaining modes so with the calculation I found that I will need 16 square meters of this material in my room and to be placed in a mode that will also help my axial modes to be resolved I will put 4 square meters on each wall. I suppose to have 0.50 absorption at 50 Hz because this is what the manufacturers tell about the panels that will absorb from 50-250 Hz.
Material 4 ( DIY Membrane Resonators)
Here I consider all my resonators i buid earlier for the axial modes. I found on the internet how much absorption gives the wood I choose to make these resonators. Because I use in total 9 resonators in the calculation I will put 9 square meters of these materials. Their absorption coefﬁcient at low frequency. The maximum absorption will be at their resonant frequencies and to have something to put in my calculation form at 50 – 63 – 80 – 100 Hz I suppose 0.70 absorption coefﬁcient at these frequencies.
Section 2 Reverberation time
In this section, I will talk about the reverberation time of the room after I put all the materials. Into the Calculation FILE, you will have to replace the coefficient of absorption under each material, found in the technical info from the manufacturing company, if you are going to use the same ones i used then you can just leave them as they are.
ON THE RIGHT you will have to play with the surface numbers in a way that works better for your room looking at the REVERB GRAPH.
In my case I need.
Material 1 (Vicoustic Cinema Round Premium Black): 23 square meters
Material 2 ( Vicoustic Multifuser DC2 ) 10 square meters
Material 3 ( RPG Modex Plate) 16 square meters
Material 4 ( DIY Membrane Resonators): 9 square meters – because i’ve needed 9 resonators for my axial modes each of aprox 1 square meter
All my surface of the room is 95.73 square meters. After I put all the materials will remain 37.73 square meters of untreated surface. that will be summed in the reverberation time calculation.
The bass ratio of my room is 1.20 and the brilliance 1.16
OBTAINED REVERB TIME OF THE ROOM
Section 3: Materials placement & Diffusion
23 square meters of Material 1 (Vicoustic Cinema Round Premium Black) will be placed in the room surrounding the listening spot.
10 square meters of Material 2 ( Vicoustic Multifuser DC2 ) will be placed in the room surrounding the listening spot in relation with Material 1.
The diffuser I projected is designed to have a diffusion from 200 to 2200 Hz in the vocal range frequencies. And will be placed in the back of the listening spot on the back wall in the ears zone. 1.20 meters high from the ground. The diffuser I projected has the following characteristics.
The cells have a length: 7.8 cm and there are 11, the diffuser has 86cm Depth of the cells:
0.0 – 7.8 – 31.2 – 70.3 – 39 – 23.4 – 23.4 – 39 – 70.3 – 31.2 – 7.8
So in this case the diffuser has a 200-2200 Hz
You can project your diffuser HERE
Material 3 ( RPG Modex Plate) will be placed in a relationship with my remaining modes so with the calculation I found that I will need 16 square meters of this material in my room and to be placed in a way that will also help my axial modes to be resolved, I will put 4 square meters on each wall.
For the 118 Hz mode will be placed at least 2 panels. One on the front and one on the back wall.
For the 128.9 Hz mode will be placed at least 2 panels. one on the left and one on the right wall.
For the 137 Hz mode will use just one on the ceiling
For the 171.8 Hz mode, I will put one on the left and one on the right walls.
For the 206.2 Hz mode, I will put one on the ceiling.
For 16 square meters, I will need about 10 panels. Until now there are 8 in use so there 2 more left that I will put on the left/right walls.
Material 4 ( DIY Membrane Resonators): 9 square meters – placement is also in discussion at CHAPTER 1 – Section 3 -Modes Solutions
Section 4: Speakers placement
For this project, I chose the Genelec 8040B and Mackie MR 824. Genelec has a frequency response from 55Hz to 18Khz and Mackie are going more down to 35Hz. Because my room it’s not so big so I can’t place the monitors far from the front wall I will choose to put the monitors at 10cm far from the wall and to put absorption material that will absorb the 858Hz even if this frequency has great directivity. If the room was bigger the solution was to put the monitors very far from the front wall in a mode that the distance will be high enough so the cancelation to be under the lowest frequency reproduced by the speakers.
The listening spot
Avoid the center of the room because over there are most of the modes. Perfect symmetry is also needed from Left/Right walls. The front/back position must be at least 1/4 wavelength of the lowest frequency in the room so in my case 35Hz have a wavelength of 9.71 meters so the distance from the center to the listening spot must be 2.42 meters. Because I have 5.825 meters length the listening spot will be somewhere in the front of the room near the monitors but it is not possible to respect this distance in my room. So my listening spot will be at 2 meters distance from the Front Wall for Genelec and 2.5 meters for Mackie.