Signal Corps Recording: Room Analysis and Correction Arun Pandian The Steinhardt School New York University New York, NY 10012 akp264@nyu.edu ABSTRACT I currently own and operate Signal Corps Recording studio located in Dumbo, Brooklyn. My goal with this study is to use the theory I have learned in this class along with room analysis software to make improvements on the existing room tuning. I will restrict any modifications I make to the room solely to changing speaker placement, and I will then evaluate the results. 1. INTRODUCTION Signal Corps Recording has been operating at its current location for approximately 3 years. The tuning of the control room was done mostly by ear and has been an ongoing process as other engineers and I have utilized the space. The studio s construction is bare bones and the rooms are neither floated nor professionally acoustically isolated from the main building or surrounding rooms. But being that the studio is located in a commercial basement, more robust construction and expense did not make practical sense. Despite this, the current build and room tuning has not been an issue with any of the clients, producers, and engineers that have utilized the space since its inception. After taking this course, I realized that I could use my newly acquired theoretical knowledge along with software analysis to make improvements to the current setup by adjusting speaker placement within the room. 2. CURRENT SETUP The control room is currently setup in a LEDE style format. The dimensions are 20.92ft in length, 12.13ft in width and a height dimension of 8.75ft. Because the rear wall is over 10ft back from the listening position, I felt I could get away with more diffusion and less bass trapping in the back of the room. Most of the bass traps in the room are Mondo Traps made by Real Traps. See Figure 1 for the absorption coefficient comparison of a Mondo Trap placed in the corner. Figure 1. Absorption coefficient comparison of a Mondo Trap placed in the corner. [1] Please refer to Figure 2 for the following description of current control room layout. I have four Traps placed in each of the two front room corners behind the speakers. They are setup as two 4ft x 8ft arrays. I also have three bass traps behind the console setup in a 4ft x 6ft array and two 2ft x 8ft corner bass traps in the two back room corners. There are two large windows in the control room looking into the live rooms. There is a window to the left of the mix position, which has a velvet curtain that may be utilized during mixing to prevent any 1 st order reflections off the glass. There is also a window in the back of the room above the couch. The back window is surrounded by 3D wood diffusors. I am using ATC SCM25A Pros as the main monitors in the room. The left and right speakers are setup at a height of 3.81ft and 2.75ft from the back wall and 2.33ft from their respective sidewalls. The mix position is located exactly 7.92ft from the front wall and in the exact center of the room at 6.06ft along the width dimension. Copyright: 2015 Arun Pandian. This is an open-access article dis- tributed under the terms of the Creative Commons Attribution License 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Figure 2. Original control room layout. The left and right speakers are setup at a height of 3.81ft and 2.75ft from the front wall and 2.33ft from their respective sidewalls 3. ROOM MODES Table 1 displays the room mode distribution for the given dimensions of the control room (20.92ft X 12.13ft X 8.75ft). This table was put together using two online room mode calculators. [2][3] As is to be expected, the modal distribution correlates exactly with the room dimensions. I have color coded the modes to highlight areas of overlap between the axial, tangential and oblique modes which can cause issues in the room response. The spacing % shows significant overlap at various frequency clusters. 4. METHODOLOGY For my measurements I will be using the room analysis software called Fuzz Measure [4] and a Josephson C550H measurement microphone [5]. I will take measurements in four locations in the room (see Table 2), and the microphone will be placed at a height of 4.42ft corresponding to the Table 1. Control room modal distribution. Axial modes are highlighted in white. approximate height our ears are at when sitting in the mixposition, and down the centerline of the room width (6.06ft). All four measurements will be done for both the left and right speakers at every speaker position. I will also be listening to reference songs that I am familiar after every speaker position in order to obtain a subjective ear test and see if the measurements and ear tests match up.
Measurement Position 1 (Mix position) 7.92ft from the front wall Measurement Position 2 (Center of room) 10.5ft from the front wall Measurement Position 3 (Front of coffee table) 13.75ft from the front wall Measurement Position 4 (Front of couch) 3.33ft from the back wall Table 2. Room measurement positions. 5. SPEAKER POSTIONS Speaker Position 1 will be the original control room layout (see Figure 2). Speaker Position 2 will maintain the original speaker position, but with a lowered height of 3.48ft (see Figure 3). I maintained this height for the rest of the speaker positions. While I was taking room measurements, I noticed that the midpoint of the speaker woofers were located exactly halfway between the height dimension of the room which can cause speaker boundary interference to occur at that axial dimension causing a deep node at that frequency response. Speaker Position 3 will be based on the 38% Rule (see Figure 4). I obtained this position from an online speaker placement calculator that combines four options from the different websites on the internet [6]. There was only one position that made sense given my current room layout. It just so happened that this recommended option was from the Real Trap s website and that my current mix position was already in the exact location specified. So I decided to give it a try. Figure 3. Original control room layout with lowered speaker height. The left and right speakers are setup at a height of 3.48ft with dimensions of 2.75ft from the front wall and 2.33ft from their respective sidewalls Speaker Position 4 will be placed at the same angle in regards to the mix position as Positions 1 and 2, but brought closer in (see Figure 5).
Figure 4. 38% Rule layout. The left and right speakers are setup at a height of 3.48ft with the dimensions of 4.48ft from the front wall and 4.06ft from their respective sidewalls 6. MEASUREMENTS AND ANALYSIS Figures 6-13 show the full frequency response curves for both speakers at all four measurement points. As can be seen by the graphs, the lowered speaker height position produces the smoothest response at all frequencies. It also sounded the best with musical references. Both the 38% rule position and the closer layout sounded uneven and the stereo width was much too small to be appropriate for mains monitors in a commercial studio environment. The frequency graphs seem to backup the uneven response I was hearing. Figures 14 and 15 show the T20 reverberation calculations at the mix position for the left and right speakers for all the measurement positions in the room. All the new speaker positions seem to exhibit a better response than the original location of the speakers. Because the new lowered height position sounded the best and the other new positions proved to be undesirable, I Figure 5. Closer layout. The left and right speakers are setup at a height of 3.48ft with the dimensions of 3.67ft from the front wall and 2.79ft from their respective sidewalls decided to restrict my comparison of the low-end frequency response to only the original layout and the new lowered height positions (see Figures 16 and 17). I have only included the measurements taken in the mix position for purposes of this paper. I believe the lowered height of the speaker has reduced some speaker boundary interference that was occurring in the 0,0,1 axial mode. The right speaker does seem to exhibit more of a node issue than the left and also exhibits higher low end T20 times and ringing. I think this variance is due to the differences in inner wall construction. The right wall has brick and concrete inside it and the left wall is an all-wood construction. Therefore I think there is more bass transmission and loss on the left through the wall. Figures 18 through 21 show the waterfall graphs for both speakers and once again compare the original speaker height to the new lowered height position. The waterfall graphs demonstrate that there is reduced modal ringing in the new lowered height positions. The waterfall graphs of
the other new speaker positions, demonstrated even less modal ringing, but as stated before the uneven frequency responses and reduced stereo width make them undesirable. The ETC graphs exhibited no real issues, and as such I chose not to include them in the paper. The only interesting thing I noticed is that you can see the increased reflections of the mixing console in the 38% rule and closer in positions reflected in the ETC graphs. 8. CONCLUSION Both my subjective ear tests and the frequency analysis indicate that the lowered speaker height position is indeed an improvement over the original speaker height. I am going to leave the speakers in that position. For further analysis in the future, I would like to purchase some ASC Monitor Stands, which are designed to reduce speaker floor transmission and speaker boundary issues (see Figure 22) [7]. I think they will help to correct the boundary issues even further. 9. BIBLIOGRAPHAPHY [1] E. Winer, RealTraps - Product Test Data. http://realtraps.com/data.htm. [Accessed: 14-Aug-2015]. [2] A. Melcher, Amroc -The Room Mode Calculator. http://amroc.andymel.eu/amroc_andymel_eu_calcul ator.php. [Accessed: 12-Aug-2015]. [3] B. Gold, Bob Golds Room Modes Calculator. http://www.bobgolds.com/mode/roommodes.htm. [Accessed: 12-Aug-2015]. [4] SuperMegaUltraGroovy. http://supermegaultragroovy.com/. [Accessed: 15- Aug-2015]. [5] Josephson C550H / C550L - audiomeasurements.com. http://www.btnproductions.com/audiomeasurements /?page_id=1181. [Accessed: 15-Aug-2015]. [6] N. Audiophile, Speaker Placement Calculator. http://noaudiophile.com/speakercalc/. [Accessed: 12-Aug-2015]. [7] ASC MonitorStand for Control Room and Mix Environments. http://www.asc-studioacoustics.com/products/monitor-stands/. [Accessed: 14-Aug-2015]. Figure 22. Acoustic Sciences Corporation monitor stands