Silencer is an important element of the ventilation system, and the comfort in the room depends on its correct calculation. In this article, we break down the calculation methodology using practical examples.
Basic Concepts • Fan noise level (Lw) — sound power level, dBA • Permissible indoor noise level (Lp) — per SNiP, dBA • Required noise reduction (ΔL) = Lw - Lp - margin (3-5 dB) • Frequency spectrum — fan noise at different frequencies (63, 125, 250, 500, 1000, 2000, 4000 Hz)
Step 1. Determine the required noise reduction Example: • Fan noise level — 72 dBA • Permissible noise in the office — 45 dBA (per SNiP 2.1.2.2645) • Safety margin — 5 dBA • Required reduction = 72 - 45 - 5 = 22 dBA
Step 2. Selecting the silencer length Approximate noise reduction values for impedance silencers: • Length 600 mm — 10-15 dB • Length 900 mm — 15-20 dB • Length 1200 mm — 20-25 dB • Length 1500 mm — 25-30 dB
For our example (22 dB required), a silencer with a length of 1200 mm will be suitable.
Step 3. Check the resistance The silencer creates additional aerodynamic resistance: • 600 mm — 10-15 Pa • 900 mm — 15-20 Pa • 1200 mm — 20-30 Pa
If the fan pressure is already at its limit, you can: • Increase the silencer cross-section (reduce air velocity) • Use two 600 mm silencers instead of one 1200 mm silencer • Choose a silencer with fairings (30% lower resistance)
Step 4. Consider the frequency spectrum Silencers perform differently at different frequencies: • Low frequencies (63-125 Hz) — require greater length (1200-1500 mm) • Mid frequencies (250-1000 Hz) — standard length (600-900 mm) • High frequencies (2000-4000 Hz) — short silencer (300-600 mm)
If the fan hums at low frequencies — take a longer silencer.
Typical errors 1. Silencer too short — does not provide the required noise reduction 2. Air velocity too high (>15 m/s) — the silencer itself becomes a noise source 3. No 3–5 dB margin — fan noise increases as the fan ages 4. Incorrect type — round silencers are required for round ducts, rectangular silencers for rectangular ducts
Conclusion Calculating a silencer is an engineer's task. If in doubt, it is best to contact the manufacturer with your system parameters (fan model, duct size, airflow rate, permissible noise level) — they will select the optimal model for you free of charge.
Need a calculation for your project? Write to sdhuiheng@outlook.com — an engineer will calculate the silencer for free within 24 hours.