Lowest Mode
18.8 Hz
Total Modes
18
Speed
1125 ft/s
Problem Frequencies
Length
18.8 Hz
Width
28.1 Hz
Height
56.3 Hz
| Freq | Note | Dim | # |
|---|---|---|---|
| 18.8 Hz | D0 | Length | 1 |
| 28.1 Hz | A0 | Width | 1 |
| 37.5 Hz | D1 | Length | 2 |
| 56.3 Hz | A1 | Length | 3 |
| 56.3 Hz | A1 | Width | 2 |
| 56.3 Hz | A1 | Height | 1 |
| 75 Hz | D2 | Length | 4 |
| 84.4 Hz | E2 | Width | 3 |
| 93.8 Hz | F#2 | Length | 5 |
| 112.5 Hz | A2 | Length | 6 |
| 112.5 Hz | A2 | Width | 4 |
| 112.5 Hz | A2 | Height | 2 |
Inputs
Result
Length mode: 1125/(2×30) = 18.75 Hz. Width: 1125/(2×20) = 28.13 Hz. Height: 1125/(2×10) = 56.25 Hz. Each dimension produces 6 harmonics, totaling 18 room modes to manage.
Inputs
Result
At 72°F, speed of sound ≈ 1129 ft/s. Length: 1129/(2×80) = 7.06 Hz (infrasonic). Width: 1129/(2×50) = 11.29 Hz. Height: 1129/(2×30) = 18.82 Hz. Large rooms have very low fundamentals.
Standing waves form when sound reflects between parallel surfaces at specific frequencies. Formula: f = v / (2 × d), where v is speed of sound and d is room dimension. These resonances amplify certain frequencies, creating feedback when a mic picks up its own amplified output at those frequencies.
| Room Dimension | Fundamental Freq | Note |
|---|---|---|
| 10 ft | 56.3 Hz | A1 |
| 15 ft | 37.5 Hz | D#1 |
| 20 ft | 28.1 Hz | A0 |
| 30 ft | 18.8 Hz | D#0 |
| 40 ft | 14.1 Hz | Below audible |
For feedback suppression, use narrow Q (high value): 1/6 octave (Q ≈ 8.6) for surgical notches, 1/3 octave (Q ≈ 4.3) for moderate cuts. Wider filters affect more frequencies and can thin the sound. Start narrow and widen only if feedback persists.
| Filter Width | Q Value | Use Case | Sound Impact |
|---|---|---|---|
| 1/6 octave | ~8.6 | Precise feedback notch | Minimal |
| 1/3 octave | ~4.3 | Standard feedback fix | Low |
| 2/3 octave | ~2.1 | Room resonance | Moderate |
| 1 octave | ~1.4 | Tonal shaping | Noticeable |
Speed of sound changes with temperature: ~1125 ft/s at 68°F, increasing ~1.1 ft/s per degree F. A 30°F temperature rise shifts all room modes up by about 2.5%. Outdoor stages experience significant shifts between sound check and show time.
Feedback typically occurs at 250 Hz (muddiness), 800 Hz-1 kHz (honkiness), 2-4 kHz (harshness), and 8 kHz (sibilance). Room modes add venue-specific frequencies below 300 Hz. Most feedback issues are in the 200 Hz to 4 kHz range.
Place bass traps at room corners where modes are strongest. Avoid placing subwoofers at exact room dimension fractions (1/2, 1/3). Stagger speaker placement to avoid reinforcing single modes. Use the mode frequencies to target treatment placement.
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Last Updated: Mar 25, 2026
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