Article: World’s First Commercially Available Modal Suppression Device

Bill Flowers, senior director for Fisher rotary valves at Emerson, recently published an article in the June 2021 issue of Hydrocarbon Engineering. The article describes a newly patented, full bore, passive attenuator that provides broad spectrum sound level reduction with no pressure drop. The article is titled Sound Suppression and is summarized below.

High aerodynamic noise levels are a natural result of turbulent flow. As gas or steam flows through a control valve, velocity increases in the seating areas and then slows, creating pressure fluctuations and sound waves. Lighthill’s law states that aerodynamic noise varies as the eighth power of gas velocity, so high flow and high pressure drop applications get very loud, very quickly.

 High noise levels can damage hearing and destroy valve internals, so it must be addressed. The two main methods of noise reduction are source control and path control, with each varying in cost and effectiveness.

 Addressing the Source

Source control diminishes noise levels by eliminating or reducing sound using pressure drop staging or flow division. Pressure drop staging reduces the overall noise by dividing the total pressure drop over several steps, rather than taking the full drop at a single point. The reduction in velocity reduces the sound. 

An alternative source control method is flow division, which breaks up the single flow path into multiple flow paths. More paths reduce velocity and create lower levels of sound. 

In either case, source control often requires a more complex valve internal design, which is costly and prone to plugging. 

Upstream or downstream external diffusers combine both pressure drop staging and flow division, but they add cost and usually incorporate very small flow passages, which can plug. 

Sound Path Suppression

Path control is an alternate means of aerodynamic noise reduction. The author describes this method: 

Path control attempts to muffle the sound and keep it from radiating to the surrounding area. The techniques can be as simple as thick walled pipe, pipe insulation, or encasing the pipe with acoustic blankets or materials designed to absorb the sound. Alternatively, one can employ specially designed silencers which either absorb the sound or use resonant chambers to cancel the noise through destructive interference. A car muffler is an example of a resonant chamber silencer that is common in everyday use. 

Silencers work, but they usually create a high pressure drop and a tortuous path, and they may include sound absorbing materials which degrade over time. Acoustic blankets wear with exposure to the elements and are often damaged or improperly installed after maintenance, so their performance also declines over time. 

An Alternative Emerges

Both source and path controls do work, but as discussed, they each have significant costs and limitations. Unfortunately these were the only noise reduction choices available until a very different sound reduction technology entered the market. The new device has only recently been introduced, but it is based on a forty-year-old concept. 

In the early 1980s, a graduate student named Ali Broukhiyan published his honors thesis on a new method of control valve aerodynamic noise reduction called a “Modal Coincidence Suppression Device” (Figure 1).


Figure 1: This diagram shows the original concept of a modal coincidence suppression device, designed to reduce the aerodynamic noise produced by a control valve. The series of chambers are each sized to produce destructive interference across a small range of frequencies.

 The design consisted of a series of concentric rings around a pipe, with each ring designed to resonate at a particular range of frequencies. When installed downstream of a noise producing control valve, the device created destructive sound interference across a range of frequencies, reducing the overall sound level.

 Early trials of the technology were encouraging, but there was no way to economically construct such a device. The concept was abandoned and shelved for decades. 

Resurrection Through Additive Manufacturing

After being forgotten for years, the modal suppression device suddenly became a viable option as the author explains: 

The advent of additive manufacturing and 3D metal printing enabled Emerson to create complicated metal components that could never have been economically fabricated in the past. New and innovative anti-cavitation and low noise trims were the focus of early additive manufacturing efforts, but attention eventually turned to the modal coincidence suppression device. 

After several years of testing and refinement, Emerson developed and patented a means to fabricate a sound suppression device. Emerson’s WhisperTube will be released for sale in late 2021 and will be offered in sizes from 2” to 12”, with flange ratings of 150, 300, and 600lb (Figure 2).


Figure 2: The forged Whisper Tube (6” model shown) undergoes noise testing at the Emerson Innovation Center Flow Laboratory.


The new modal suppressor is similar to the original concept but incorporates several improvements (Figure 3). The inner pipe is a full bore perforated liner, and the new design includes a larger number of concentric cavities, providing an average 10dB noise reduction across a broader range of frequencies.

Figure 3: The modal suppressor consists of cylindrical chambers of varying sizes surrounding a full bore perforated tube. Each chamber generates destructive interference over a small range of frequencies to provide significant noise reduction across a broad spectrum.

 In addition, the rings have been modified to allow the device to be self-draining, so process liquids do not collect in the rings and negatively impact performance.

 A New Option for Noise Reduction

The modal suppression WhisperTube offers a variety of advantages over other noise reduction technologies. It is a simple, passive device offering a 10 dB average noise reduction across a broad range of frequencies (Figure 4).

Figure 4: Concentric rings of various sizes create broadband noise reduction as shown in this chart.

 This new design is full bore and generates no pressure drop. Less expensive butterfly valves can be paired with this device to provide a lower cost option, as compared to a typical globe control valve with low noise trim. Finally, the self-draining version can be utilized for steam and two-phase flow applications where liquid accumulation would adversely affect many other noise abatement designs. 

All Figures except Figure 2 are courtesy of Emerson. Figure 2 supplied from a public domain thesis.

 About the Author

Bill Flowers is the Senior Director for Fisher rotary valves at Emerson. He has global product responsibility for Fisher pipeline ball valves, eccentric plug valves, and all types of actuators. Flowers has been with Emerson for 33 years and has held roles in engineering, global sales, research and development, and product management.