Blower Noise and Silencers

Blowers are among the noisier machines in a plant, and noise is a design issue from the start, because fixing it after installation is expensive and often adds resistance. High-pressure blowers run at high tip speed, produce strong tones at the blade-pass frequency, and radiate noise from the inlet, the outlet, and the casing. This guide explains where blower noise comes from, how it is rated, how speed controls it, and what silencers and enclosures can and cannot do.

Where Blower Noise Comes From

• Blade-pass tones. Each blade passing a point in the housing produces a pressure pulse, so the sound has a strong tone at the blade-pass frequency and its harmonics.
• Turbulence. Air leaving the blades, separating from surfaces, and passing through the housing creates broadband noise.
• Operating point. Running far from the best-efficiency point raises noise.
• Inlet and outlet conditions. Distorted inflow from bends or dampers close to the inlet increases noise.
• Casing breakout. Noise inside the housing radiates through the casing walls.
• Mechanical noise. The motor, bearings, belts, and drive add to the total, and vibration travels through ducts and structure.

The blade-pass frequency equals the number of blades times the rotational speed in revolutions per second. A wheel with 12 blades at 1,480 rpm has a blade-pass frequency of 12 × 1,480 ÷ 60 = about 296 Hz. Knowing it tells you which octave band the tone falls in and where a silencer must work.

Sound Power and Sound Pressure

Sound power level (Lw) is the acoustic energy the blower radiates, a property of the machine and its operating point, independent of the room or the distance.

Sound pressure level (Lp) is what you measure at a position, and it depends on distance, room absorption, reflections, and other sources.

Manufacturers rate blowers by sound power, and you calculate sound pressure for your site. In free field, for a source on a hard surface, a simple estimate is:

Lp = Lw − 20 × log₁₀(distance in metres) − 8

Example. A blower with a sound power level of 100 dB, heard at 10 m, gives 100 − 20 − 8 = 72 dB. At 20 m, it gives about 66 dB. Each doubling of distance lowers the level by about 6 dB in free field. Real sites with reflections, multiple sources, and buildings differ, so treat it as an early estimate.

How Fan Sound Is Rated

Fan sound power is determined in laboratory conditions. The ISO 13347 series describes methods using a reverberant room, an enveloping surface, or sound intensity, and AMCA 300 covers the reverberant room method for fans. In-duct sound power is determined under ISO 5136. Because the inlet, the outlet, and the casing radiate different levels, a single blower can have several sound power figures, depending on its installation category, free or ducted at inlet and outlet. Ask which installation category the sound data represents, and whether it is measured or calculated for your size and speed. See blower performance testing.

Speed Is the Strongest Lever

A common rule of thumb is that sound power changes by roughly 50 times the base-10 logarithm of the speed ratio, for the same blower in the same system.

Speed change	Approximate sound power change
Speed × 0.9 (10% slower)	−2.3 dB
Speed × 0.8 (20% slower)	−4.8 dB
Speed × 0.7 (30% slower)	−7.7 dB

Certified conversion rules are more accurate than this rule of thumb, but it shows why the most effective ways to cut noise are to choose a larger wheel at lower speed, or to reduce the speed with a drive when the full duty is not needed. Speed reduction also cuts power sharply. The trade-off is size, cost, and the pressure the wheel can still develop. See flow control for blowers.

Silencers

A silencer absorbs sound as air passes through it. Inlet silencers cut inlet noise, and outlet silencers cut outlet and stack noise. They have costs.

• Pressure loss. A silencer adds resistance, which must be added to the system curve, and it can move the operating point.
• Fouling. In dusty or oily air, the absorbent material can clog, so specify a suitable construction and provide access.
• Frequency dependence. A silencer’s insertion loss varies with frequency, so match it to the blower’s dominant tones.
• Space. Splitter silencers are large.

Enclosures and Lagging

Casing breakout noise is cut by acoustic lagging on the housing or by enclosing the blower. Enclosures need ventilation for motor cooling, and access for maintenance. Lagging on hot casings must also suit the temperature.

Vibration Isolation

Structure-borne noise travels through mounts, ducts, and pipes. Use flexible duct connections, anti-vibration mounts, and a heavy, rigid base, and keep the blower’s supports from resonating with its running speed.

Noise Limits

Limits come from occupational exposure rules, environmental permits, or client specifications. State whether a limit applies at the blower, at a work position, or at a site boundary, and at what distance, and whether it is sound power or sound pressure. Without that, the limit cannot be checked.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure. Give us your noise limit and where it applies, and we will advise on wheel size, speed, and attenuation.

Give Us Your Noise Limit

Send your noise limit, its location, and your duty, and we’ll advise on selection and attenuation. Contact us for a free quote.

Frequently Asked Questions