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A bass note that booms at the desk and almost disappears a few steps away is a reason to investigate the room. A room-mode calculation can identify frequencies to check. It cannot tell you the level, decay time or cause of every peak in a recording.
\nThe Room Mode Calculator lists axial, tangential and oblique modes for an ideal rectangular room. Begin with the three lowest axial frequencies. They are easy to verify by hand and show how strongly the dimensions affect the result.
\nTake a room with internal dimensions of 4 m long, 3 m wide and 2.5 m high. Assume sound travels at 343 m/s, an approximate value near 20 °C. A first axial mode fits half a wavelength between one pair of opposing boundaries, so its frequency is sound speed divided by twice the distance.
\n| Direction | Distance | Calculation | First axial frequency |
|---|---|---|---|
| Length | 4 m | 343 ÷ 8 | 42.875 Hz |
| Width | 3 m | 343 ÷ 6 | 57.167 Hz |
| Height | 2.5 m | 343 ÷ 5 | 68.600 Hz |
For this model, the next axial modes in the four-metre direction occur at 85.750 and 128.625 Hz. These are integer multiples of that direction’s fundamental. They do not establish that the room’s complete set of modes forms one harmonic series: the other dimensions contribute different sequences.
\nMeasure between the main internal boundaries, rather than using the outside dimensions of the building. A small measurement error shifts the estimate. For example, changing the length from 4.0 to 3.9 m raises its first axial frequency from 42.875 to about 43.974 Hz. Recording dimensions to many decimal places does not make an irregular room behave like a perfect box.
\nThe complete rectangular-room equation is:
\nf = c/2 × √((nL/L)² + (nW/W)² + (nH/H)²)
Here L, W and H are the dimensions in metres; c is sound speed in metres per second. Each n is a nonnegative whole number. The all-zero combination is excluded. Sengpiel Audio’s room-mode reference presents this model and the three mode families.
\nA mode labelled 1,0,0 varies along the length only. The label 0,1,0 denotes variation along the width. A 1,1,0 mode includes both length and width and is called tangential. A 1,1,1 mode includes all three dimensions and is oblique. The numbers describe a spatial pattern; they are not a ranking of which problem matters most.
\nUsing the example dimensions, 1,1,0 gives about 71.458 Hz. It is neither 42.875 + 57.167 nor their average. The squared terms combine before the square root. That matters when checking whether a nearby measured peak is consistent with more than one possible room pattern.
\nA mode describes a possible standing-wave pattern. A frequency response describes what a particular source, room and receiving position do together. They answer different questions.
\nAn ideal first axial pressure pattern has pressure antinodes at the two boundaries and a pressure node halfway between them. That statement concerns this one mode. It does not mean that the centre of any real room is silent, or that sitting at a quarter of its length guarantees a flat response. Other modes and reflections remain present.
\nSource position affects how strongly a pattern is excited; receiver position affects how strongly it appears in the result. Absorption and boundary losses affect its decay and strength. The Room EQ Wizard simulator documentation explicitly includes sources, receiving positions and surface losses when generating a response. A dimension-only frequency list leaves those quantities unknown.
\nKeep this distinction when looking at the calculator’s plot. Each dot locates a predicted frequency. The three rows separate mode families. There is no decibel axis, and a dense group of dots is not a measured loudness peak.
\nSuppose the length and width are both 4 m. The first axial modes in those two directions share 42.875 Hz. They are separate patterns with the same frequency, rather than one duplicate entry to delete.
\nSimple dimension ratios also produce coincidences further up the spectrum. In a 4 × 2 m floor plan, the second length mode and the first width mode both occur at 85.750 Hz under the same sound-speed assumption. This is a reason to inspect the distribution carefully. It is not enough evidence to grade a room, predict its bass decay or promise that changing one dimension will solve its response.
\nThe calculator preserves these separate rows in its CSV. When sorting a measurement against them, retain the mode indices as well as the rounded frequency. Rounding two nearby frequencies to the nearest whole hertz can make them look exactly equal when they are not.
\nSave the dimensions, assumed sound speed and frequency ceiling before testing. Make one measurement near the normal listening position, then repeat at a nearby position using the same playback level and measurement settings. A feature that changes with position gives different evidence from a feature that stays unchanged everywhere.
\nFor an informal listening reference, the tone generator supplies a sine wave. Start quietly. A browser volume setting does not establish sound pressure at your ear, and turning up a frequency that seems missing can create excessive levels elsewhere. A listening test also cannot supply a calibrated decay-time measurement.
\nUse suitable measurement software, a microphone with appropriate calibration data and a repeatable procedure when the result will guide treatment or substantial changes. Professional assessment is useful when the boundaries are uncertain or the work requires a reliable specification. The calculator is a planning aid for that comparison.
\nFinally, compare like with like. An L-shaped room, a large opening into another space or a sloped ceiling violates the rectangular rigid-wall assumptions. An exported list can still document the simplified model you tried, but extra decimal places cannot resolve the missing geometry. Preserve the measurement alongside the model instead of replacing one with the other.
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