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"ModeCalc is not meant to show how to treat an existing room."
"ModeCalc can help you design a new room that sounds as good as possible, or predict the low frequency behavior of an existing room."
"Playing music in a room with poor mode distribution is like listening through a 5-band graphic equalizer with one or two bands turned up all the way."
"When viewing the results look for an even spacing of the modes regardless of their color (good), and also look for multiple modes that occur at or near the same frequencies (bad)."
"Axial modes are the most important type simply because they are the strongest." |
OVERVIEW ModeCalc runs on all Windows computers, and displays the axial modes for any rectangular room using dimensions you enter as either feet or meters. ModeCalc can help you design a new room that sounds as good as possible, or predict the low frequency behavior of an existing room. The tutorial below explains the basics of room modes, and tells how to use ModeCalc and interpret its results. NOTE: ModeCalc is not meant to show how to treat an existing room. For treatment advice please see: Click HERE to download the Windows version of ModeCalc - our Graphical Room Mode Calculator (1.3 MB) that has a greatly improved interface and offers more information than the original DOS version below.There's nothing to install - just Unzip the files into any folder, then run the modecalc.exe program file. MAC USERS: We don't have a version of ModeCalc for Macs, but you can still get some benefit by clicking the image at left to make it full size. Then note the eight recommended ratios and build your room close to one of those. Top MODECALC TUTORIAL ModeCalc calculates and displays the first 16 axial modes, up to 500 Hz, for any rectangular room using dimensions you enter as either feet and inches or meters. It can help you design a new room that sounds as good as possible, or predict the low frequency behavior of an existing room. This tutorial explains the basics of room modes, and tells how to use ModeCalc and interpret its results. Please understand that ModeCalc is not meant to help you determine low frequency treatment for an existing room. Regardless of what is predicted (or measured using test equipment) the solution is always the same - as much broadband bass trapping as you can manage. Whether your current room happens to have favorable dimensions or not is irrelevant, unless you're willing to move the walls! Top ROOM MODES Room modes are natural resonances that occur in every enclosed space, and the frequency of each resonance is directly related to the room's dimensions. For example, a room 16 feet long has a mode at 35 Hz because walls that far apart provide a natural resonance at 35 Hz. Additional modes occur at multiples of 35 Hz because those frequencies also resonate in the same space. Wall spacing that accommodates one cycle of a 35 Hz wave also fits two cycles of 70 Hz, three cycles of 105 Hz, and so forth. When you play a musical note having the same pitch as a natural resonance of the room, that note will sound louder and have a longer decay time than other notes. Of course, this is undesirable because some notes are emphasized more than others, and the longer decay times reduce clarity. Therefore, room modes are important because they directly affect the character of a room. Although room resonances can be reduced by adding bass traps, they cannot be eliminated entirely. Top For this reason, rooms for recording and playing music are designed to have many resonances that are distributed evenly, rather than just a few resonances at the same or nearby frequencies. Playing music in a room with poor mode distribution is like listening through a 5-band graphic equalizer with one or two bands turned up all the way. A room with good modes is more like having a 31-band equalizer with all the bands turned up. The frequency response still isn't perfect, but all the small peaks combine to yield an overall response that's reasonably flat. Therefore, the frequency response of a room with many modes close together is flatter overall than a room that has fewer modes farther apart. Small rooms have modes that are spaced farther apart than large rooms because the first mode in a small room starts at a higher frequency. For example, when the longest dimension of a room is only 10 feet, the modes for that dimension start at 56.5 Hz and are 56.5 Hz apart. In larger rooms the first mode is at a lower frequency so subsequent modes are closer together. Therefore, a large room has a flatter low frequency response because it has more modes spaced more closely. Top The formula used by ModeCalc is extremely simple. For dimensions given in feet the first mode occurs at 1130 divided by twice the dimension. (1130 is the speed of sound in feet per second.) All subsequent modes are multiples of that result. When using meters the formula is 344 divided by twice the dimension. Twice the dimension is used because a room 10 feet long really has a total distance of 20 feet - the wave travels from one end to the other and back to complete one cycle. So for a room 10 feet long the first mode occurs at 56.5 Hz:
The second mode for that dimension is two times 56.5 or 113 Hz, the third is three times 56.5 or 169.5 Hz, and so forth to the tenth mode at 565 Hz. Top ROOM RATIOS The worst type of room shape is a perfect cube - say, ten feet long, ten feet wide, and ten feet high - because all three dimensions are the same and all three dimensions resonate at the same frequency. A 10 foot cube shaped room will have a strong inherent resonance near 55 Hz, which is the open A string on a bass. So when that low A is played it will sound much louder than other notes. Such a room also has a longer natural decay time at that pitch, so A notes will sustain longer and conflict with other bass notes that follow. A room whose dimensions are multiples of each other - for example, 10 feet by 20 feet - is nearly as bad because many of the same frequencies are emphasized. Therefore, the goal is to have a room shape that spreads the modes evenly throughout the low frequency range. This is done by designing the room with dimensions whose ratios of length, width, and height are as unrelated as possible. And here is where ModeCalc is useful because it tells you at what frequencies the modes occur and how close together they are. ModeCalc also shows you the ratios of the dimensions you entered, and lets you compare them to ratios commonly recommended by acoustic engineers and studio designers. Top USING MODECALC Instructions at the bottom of the screen explain how to use the program. Simply enter the Length, Width, and Height using the Tab and Shift-Tab keys to go between fields, then hit Enter to see the result. Up to 16 modes for each room dimension are displayed graphically so you can see where they occur and how they relate to one another. Each set of modes is shown in a different color, and when two or more modes occur near the same frequency the mode lines are drawn taller. Note that the graphic display portion of ModeCalc uses logarithmic spacing. This is how octaves and musical intervals are arranged, and is also how mode spacing should be viewed. You can also enter the room dimensions as meters and/or centimeters instead of feet by using m or M or cm or CM and so forth after the dimension values. Values can be entered with or without spaces, so '10m' and '10 m' both specify 10 meters. Likewise for feet and inches. Top Many rooms are not rectangular, and in fact having angled walls or a vaulted ceiling is desirable. Unfortunately, with angles there is no direct way to determine the room modes exactly. Modes still exist - they're just more difficult to predict. If the angles are not too severe you can average the dimensions. For example, if the ceiling varies from 8 feet to 10 feet high, you can use 9 feet when entering the height. When viewing the results look for an even spacing of the modes regardless of their color (good), and also look for multiple modes that occur at or near the same frequencies (bad). Also compare the ratios of the dimensions you entered with the recommended ratios, and compare your room's volume with the recommended minimum of approximately 2500 cubic feet or 70 cubic meters. Top To make it easier for you to identify modes that are close together ModeCalc draws those mode lines taller, which simulates the larger response peak that occurs. The normal line height is marked with a thin gray horizontal line. When two modes are adjacent, or at least close, both lines are drawn taller. The closer the modes are to each other, the taller the lines appear. This also lets you identify modes that fall on identical frequencies. ModeCalc draws all the lines for one mode, then the next, and the next. So if several modes are at identical frequencies one line will hide the other. If you notice that an isolated line is higher than usual, that means there are at least two modes at that same frequency. You can then use the Frequency Table display to see them all. Also note that modes naturally become closer at higher frequencies. Therefore, having taller lines toward the right side of the graph is normal, and does not mean your room will really have that rising frequency response. Finally, if you are using ModeCalc to check an existing room, please don't be discouraged by poor results. All rooms need bass trapping anyway, and poor modes can be improved enormously by adding a few more traps. You can also enter dimensions for a large room having one of the recommended ratios, such as 23.3 by 16 by 10 feet. Then you'll see how even with the recommended ratios the modes are still somewhat uneven, and two modes still sometimes occur at the same frequency. So unless you are willing to move the walls, just accept what you have, and maybe install a few more bass traps than you had planned for originally. Then relax and enjoy the music! Top A FEW WORDS ABOUT MODE TYPES There are two basic types of room modes - axial and non-axial. Axial modes occur between two parallel surfaces, where non-axial modes take a more circuitous route travelling much like a cue ball going around a pool table in a diamond pattern. The two non-axial mode types are called Tangential (the reflected waves touch four room surfaces) and Oblique (the waves touch all six surfaces). Every rectangular room has three axial modes, with one between the floor and ceiling, another between the left and right walls, and another between the front and rear walls. Axial modes are the most important type simply because they are the strongest. They contribute more to peaks and nulls, and modal ringing (extended decays), than non-axial modes. This is because the boundaries are parallel and so the distance between reflections is shorter too. Therefore, axial modes are the most damaging to accurate music reproduction, and the most important type to consider. Top |
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