Recording instruments and mixing tracks are different tasks and require balanced room acoustics. Instruments and drums produce a range of frequencies which are represented as musical notes. Think about the bass guitar and kick drum. They are low freq. instruments. The lowest note on a 5-string bass guitar is the low B at 31 hertz and a kick drum at 25 – 40 hz.
The wavelength of 31 hz is C/f = 1128.6/31 = 36.4 feet
The wavelength of 25 hz is C/f = 1128.6/25 = 45.1 feet
A wall will resonate at a given frequency when a sound wave acts upon it that shares the same frequency or is a harmonic of it. This is calculated in relation to dimension as f = C/2L.
Therefore a wavelength of 25 hz needs a wall of minimum length of L = C/2f = 22.6 feet to avoid this effect and to support low frequencies being recorded properly. That being said, larger rooms support higher reverb times that tend to give smooth low frequency response. Try to build your studio with at least 1 dimension that will support the lowest frequency to be recorded. Also, avoid equal dimensions or multiples of a dimension as these will cause modal build-ups of 200% to 300% of normal amplitudes.
A enclosed rectangular room is subject to standing waves which comprise a fundamental frequency and harmonics or nodes which are calculated for each room dimension up to the room low cut-off frequency. If the room dimensions are not spaced well or too small, then modal build-ups will occur. As an example, if a bass player hits the open low B string of 31 hz in a room that is 16′ x 13′ x 10′, then all surfaces in that room that resonate at 31 hz or its harmonics will vibrate because the minimum length of 18.2′ is just 16′. Therefore, the natural wavelength of the note cannot resolve and the sound energy will bounce around. The drum set will vibrate most likely as will the walls and if mics are live, they may feedback.
The room cut-off frequency (Lco) is very important to know to deal with troublesome mode build-up. It is related to room volume and reverb response. Modes (frequencies) above the Lco are handled by the room with a smooth response while modes below the Lco are of concern if they cause a build-up.
Lco = 20,000 * Sqr (Reverb Time / Room Volume) in hertz
Studio Design 2010 allows the user to enter in room dimensions of length, width, height and the reverb response for the room. It calculates all the modal frequencies for axial, tangial, and oblique modes and graphs the results for easy to see modal spacing and mode buildups.
Vist www.recording-studio-design-software.com for details, examples, and instructions on using this program.
Each room dimension is calculated as wavelength (fundamental frequency) and harmonics up to the Low Cutoff Frequency for the room. The number of modes and its color is shown in the text field next to each dimension. This makes it easy to see which dimension(s) are causing a mode buildup. Simply change a dimension to see how it effects the modal spacing to optimize your studio room. Enter your room dimensions for a rectangular room and the desired reverb response (RT60) you want the room to have. Press the calculate button to see your results.
Bandwidth is the range of frequency around the fundamental frequency (and harmonics)
that is depicted as a bell curve. If one mode bandwidth is within the mode bandwidth of
another mode, it excites that mode to the extent it overlaps the mode. This extra energy
is added to the excited mode. The graph will show all excited modes in blue.
BW = 2.2 / Reverb Time
Room Volume is Length x Width x Height in cubic feet.
The Ratio calculator converts the dimensions into a ratio where 1 equals the Height. This is useful when you have dimensions that provide good modal spacing and wish to resize the room using the same ratio. Enter a value in the ft. text field and select which wall to apply the change to. Press either the up or down button to recalculate the dimensions by the entered amount. Press the Apply Ratio button to update the project with the new dimensions.
The actual modal frequencies plotted are displayed in the listbox next to each graph. The modes for Length, Width, and Height are plotted as separate colors for easy identification when deciding which dimension to tweak to optimize modal separation or reduce modal build ups. The graph is plotted up to the Low cut-off frequency for the room. They are the modes of interest. Each dimension (fundamental and harmonic frequencies) are calculated up to the Lco.
The maximum “amplitude” is 300% which represents adding 3 modes of equal frequency which only occurs when all three dimensions are the same.
The Axial modal frequencies are given an amplitude of 100%. Tangial modal frequencies are given an amplitude of 50% and the Oblique modal frequencies are given an amplitude of 25%. The All modes frequency distribution graph plots all three modes together. The graph will show all excited modes in blue if their “amplitude” is over 100%. Axial modes are considered primary, while tangial modes have a second order effect and oblique modes have a third order effect. The tangial and oblique modes tend to “fill” in between the axial
modes.
The objective is to enter your room dimensions and desired reverb response then observe the graphs for mode spacing. Start by looking at the axial graph first. The modes for length are shown in green, width in burgendy, and height in tan.
If modes are shown too close to each other they will appear in blue. Blue is a problem and means a mode build-up will occur at those resonant frequencies. Walls, ceilings and floors are essentially resonating panel absorbers which have front and back panels, air space cavity, insulation, etc. They have a resonating frequency with harmonics related to its longest dimension and will resonate when the modes under the Lco are excited.
The goal is to get the best spacing possible while minizing modal build-ups and to identify those problem modal frequencies. Coefficients of absorbtion are measured for materials at 125hz, 250hz, 500hz, 1000hz, 2000hz, 4000hz. Example: If your Lco is 300hz then check for build-ups at 125hz and 250hz. Other hot spots can be dealt with by designing custom helmoltz resonators/absorbers for those frequencies. Studio Design 2010 allows the user to design helmotz resonators and save them into the acoustic materials database and then select them into the acoustic room design.
When the room dimensions are optimized, save the project and room name. Click the materials option button to begin adding acoustic materials representing your desired construction and materials you want to use. Studio Design 2010 has a database of more than 1100 acoustic materials and their respective coefficients of absorbtion. Even better, the user can maintain, edit, add, or delete acoustic materials from this database using the Acoustic Materials menu option.
The objective is to add materials one at a time into the grid and enter the area in sq.ft. for that material. Pressing the calc button will update the absorbtion calculations for Sabines and Reverb Time. By watching the Reverb results under each frequency of 125hz, 250hz, 500hz, 1000hz, 2000hz, 4000hz you will see the effect of adding the material.
Begin by adding walls, ceiling, and floor materials you will be using and enter the area for each entry. This gives you a baseline to start from. You can then add in other elements like bass traps, helmotz resonators, etc until your reverb totals are at the design reverb time. Selecting a category filters the “Select an Acoustic Material” list. For example, select
Walls to see all walls or further filter the list by type.
Use an SPL meter inside your space to baseline the road and ambient noise from outside to determine which frequencies are getting through and the amount of noise reduction required. This can be used to choose your wall construction. Walls are rated by STC (sound transmission class). If you need a 40 db reduction then choose as wall design with STC 40 vs STC 60. The cost of STC 60 walls (Double 2×4 walls with 2 layers 5/8″ drywall both sides filled with bat insulation) can be expensive.
Select type to filter by frequency band of absorbtion such as low, low mid, mid, mid high, high, or wide. When selecting a material from the drop down list, look at the coefficients as well as the current sabine and reverb totals, to pick a material targeted for the current results.
After selecting a type, the categories list will update to reflect records assigned to that type. Example: User selects Carpets and can search through all carpets to find one representing theirs, or filter by type to find carpets that have a specific band pattern. Refer to the reverb results. If the reverb time is higher at 2000hz and 4000hz then select a band type with emphasis in the high band. The material list will update and show materials that have coefficients of absorbtion with higher values in that range.
Use the acoustic materials sabine analysis to model your current studio, control, listening room or home theater. If its not in the materials database, then find it on the web and simply add it using the Acoustic Materials screen.
After a material selection has been made, enter the area into the white text field in Sq. ft. and press the calc button. A handy calculator is a click away. Click the calculator icon and it appears. It only multiplies but it does provide floor and wall areas calculated with the dimensions that are currently entered. Look over the different wall constructions and their COA’s. Walls really do absorb low frequencies! A common construction is 2 X 6 walls with 5/8″ drywall spaced 24″ on center filled with 5.5 inches of insulation.
Studio Design 2010 provides the ability to design perforated and slat-type helmotz and panel type absorbers and resonators by clicking the Absorber Design menu option. To address troublesome modes, design helmotz resonators tuned to these frequencies.
After designing low frequency helmotz absorbers, add them into the acoustic materials database, then select them from the materials pulldown list and add them into the design grid.
In addition to perforated absorbers, use slat-type and panel absorbers to give your studio a custom look. Build wall mount panels that can open and close with a hard reflective surface on one side and absorber on the other to allow altering the rooms reverb response. Distribute absorption and diffusors equally on all walls.
Use a reflective floor with carpet in some areas to reduce reflection.
Place low frequency helmholtz perforated absorbers into your ceiling’s construction. Alternate the different designs across the ceiling. For example, a typical new construction garage has 2×8 rafters overhead. If there is a second story, it also has a subfloor also, consider that the back of a panel. If there is no second floor, put down 5/8″ plywood. Fill the ceiling joists with bat insulation. A 2×8 is 7.5″ deep. Apply 3/4″ strapping across the ceiling joists to give some decoupling or use resilent channel. This gives an air gap filled with insulation of 8.25″. Cover the ceiling with 2 layers of 1/2″ plywood. Predrill the 4×8 sheets with the perforation patterns of the helmoltz resonators in the design then screw them to the strapping. Stain or paint them and cover them with a loose-weave fabric using a spray adhesive on the plywood. Use a panel thickness of 1″, air gap of 8.25″, hole diameter of .25″ – .375″ for the helmoltz designs to acheive the desired range of frequencies. Note, that not all ceiling panels need be perforated.
Diffusion is also required to breakup various direct, indirect, and random sources. Consider triangular panel wall mounts or QRD diffusors. Studio Design 2010 allows you to design qrd’s up to 43 periods. Place them in area’s that microphones will be recording, near drum kits, vocal booths, acoustic paino’s etc. to help distribute all sound sources evenly within the recording or mixing area.
Please visit www.recording-studio-design-software.com for more information.
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