Silencers and mufflers cover a wide range of noise reduction devices and must be considered one of the most powerful weapons available to the acoustical engineer. There is no technical distinction between a muffler or silencer and the terms are frequently used interchangeably; i.e., one manufacturer will use a silencer and another a muffler for the same basic configuration.
Despite the terms and myriad of configurations, the devices can be broken into three fundamental groups: absorptive/dissipative, reactive and dispersive. Absorptive silencers contain either fibrous or porous materials and depend on absorptive dissipation of the acoustical energy.
Reactive silencers contain no absorbing material but depend on the reflection or expansion of the sound waves with corresponding self-destruction as the basic noise reduction mechanism. The noise reduction of dispersive silencers usually comes from diffusing a high-velocity gas flow into smaller lower-velocity streams.
Some silencers combine the elements of two or more types for extended performance. In addition, other functions such as water separation, filtering, spark arresting, heat recovery or exchange, etc., may also be present. In any case, they are usually installed in pipes or ducts to reduce sound transmission from one section of a gas flow system to another.
With a basic understanding of the acoustical properties of each type, the noise control engineer can usually select a silencer or combination of silencers which will effectively provide noise reduction regardless of the character of the noise. Difficulty generally is found not with finding a silencer with adequate acoustical performance but with dealing with problems such as size, weight, aerodynamic pressure losses, etc.
Showing extreme promise as a noise control measure is a concept called active noise control. Here, noise reduction is achieved by generating an ‘anti-noise’ field which is superimposed on the source field. With careful attention to matching phase and sound pressure amplitude, a cancellation process ensues (interference), with resultant lower noise levels.
It should be emphasised that the basic approach of active noise control is not new, but it is only recently that the control theory and microelectronic components have reached the state of the art to produce practical results. We shall see that many of the inherent penalties of passive devices are avoided with the active noise control approach.
High-performance silencer designs generally combine both absorptive and reactive elements in their construction. However, before discussing the more complex combinations, each type will be broken down into its simplest form and the basic characteristics of each examined. It should be emphasised that even the basic forms cannot always be strictly divided as purely absorptive or purely reactive. However, the grouping herein will be on the basis of the major noise reduction mechanism.
Absorptive silencers are widely used in treating noise where large volumes of air or gas are moved at relatively low static pressures. We shall therefore begin our discussion of silencers with these types, their performance and their methods of selection.
The simplest form of absorptive or dissipative silencers is the parallel baffle. Basically, these baffles consist of an aerodynamically streamlined entrance and exit with perforated walls backed by highly absorbent acoustical material. The absorbing material is usually fibrous in texture, either glass or mineral wools.
The acoustical performance of parallel baffles depends primarily on three parameters:
1. Thickness of absorbing materials.
2. Spacing of baffles.
3. Length of baffles.
The effect of baffle thickness and spacing can be seen in Fig. 26.2. Note that the attenuation increases sharply at high frequencies as the spacing is narrowed. Note also that better performance at lower frequencies is obtained as the thickness of the absorbing material is increased.
With respect to the length of parallel baffles, the acoustical performance increases as length increases. Shown in Fig. 26.3 is the performance of baffles 6 inch thick, 12 inch on centre (50 per cent open area) for silencer lengths of 4, 8 and 12 ft. Note that the performance is not a linear function of length; i.e., doubling the length does not double the attenuation.
The nonlinearity is due principally to the rapid absorption of high-order transverse modes in the first few feet of the silencer, leaving only a plane-wave-type of sound propagation. The resultant plane wave motion presents essentially a grazing incidence to the absorbing treatment and hence little sound is absorbed.
The performance of absorptive silencers can be sharply improved if the line of sight through the silencer is blocked or eliminated. Various curves and staggered patterns have been designed and are commercially available. The increased performance of a blocked line of sight baffle configuration is clearly evident from the performance data presented in Fig. 26.4.
Parallel baffle-type absorptive silencers are available commercially from a large number of manufacturers. To meet a wide range of application, most manufacturers provide the units with face cross-sectional dimensions in increments of 6 inch and lengths of 3, 5 and 7 ft.
Furthermore, the individual modules can be assembled to fit almost any duct size or gas flow requirements. For design purposes, presented in Table 26.1 are typical attenuation values for parallel baffle-type absorptive silencers for common commercially available lengths of 3, 5 and 7 ft.
It must be emphasised that the acoustical performance of parallel baffles decreases sharply as gas flow velocity through the silencer reaches 3000 fpm. Acoustical performance corrections for flow velocity are usually supplied by the manufacturers in their design specification sheets.
Because there is very little flow resistance in parallel baffles, they can be applied readily to those installations where pressure losses are critical such as in the air-conditioning or ventilating systems, forced draft fans, gas turbine facilities, etc.
Another form of absorptive silencer is the tubular silencer. These silencers are really parallel absorbing baffles which have been wrapped around in order to interface simply with circular inlet or exhaust duct geometries. The spectral attenuation for tubular silencers is very similar to parallel baffles, i.e., good high-frequency performance but rather poor low-frequency performance.
For heavy industrial application involving gas flow at high-pressure velocity and temperature, the outer walls of the silencer and the adapter flanges must be constructed of heavy-gauge metal, i.e., a minimum of 10 gauges.
The acoustical performance of absorptive silencers is reduced somewhat in the presence of airflow or gas flow.
Another effect which must be considered in selecting or specifying silencers is the self-generated noise produced as the air or gas flows through the silencer and exits. Obviously, the level of such noise is the minimum noise level one can obtain.
The noise level is a first-order function of face velocity but can be substantially affected by silencer geometry. In the latter case, self-noise can be minimised by careful attention to good turbulence-free aerodynamic flow.
Self-generated noise is usually given in terms of sound power level for various face velocities. Shown in Table 26.2 is an excerpt from a silencer catalog showing the self-noise power levels for several silencers at a given face velocities. Note that the self-noise increases quite sharply with increasing face velocity.
In fact, in some bands, doubling the face velocity produced a level increase of 20 dB or more, representing a power ratio increase of 100. This substantial increase is not surprising and the noise associated with airflow is indeed a dynamic function of velocity.
Another factor, generally present, which provides design constraints on silencer selection, is pressure loss. Dissipative-type silencers are most often used where high volumes of air or gas are being moved at relatively low head pressures, i.e., in the range of a few inches of water. As such static pressure losses in the system must be minimised.
In particular, in air-conditioning or ventilating systems, pressure losses due to silencers must be a first-order design consideration. Most silencer manufacturers provide the static pressure losses for their equipment, again as a function of volume flow. In accordance with basic aerodynamic principals, pressure drop increases as the volume velocity increases.
Therefore, the only design guideline for minimising pressure loss is to assure that the flow velocity through the silencer is not increased. As a rule of thumb, if the open flow cross-sectional area of the silencer is 1.25 to 1.5 times the cross section of the duct, pressure losses will be minimal.
The effectiveness of splitter panels can be fully appreciated when we consider that a large jet engine in a well-designed test cell is barely audible at a distance of 100 ft. Generally all that can be heard is a very low-frequency rumble. It should be noted that the length of the inlet and exhaust silencers for these application ranges from 10 ft for a small gas turbine to 24 ft for a large jet engine test cell.
As shown in Fig. 26.8 is the octave band noise reduction measured across the inlet silencer of a small gas turbine test cell. In this installation the inlet splitter baffles were 6 inch thick on 12 inch centres (50 per cent open) and 7 ft long.
It should be noted that the chain saw used as a noise source was barely audible yet could be seen through the splitters less than 10 ft away. However, from the actual examples and illustrations, it is clear that noise from the loudest industrial equipment can be controlled to the level required with sufficient absorptive silencer treatment.
Let us now consider other types of absorptive or dissipative types of silencer and their configuration.
Of the variety of silencers industry employs for noise attenuation, one is a duct silencer, which is designed to accommodate air flow in either direction of the duct. Figure 26.9 illustrates a typical duct silencer, the top and bottom of which are filled with a sound-absorbing inorganic material and covered with perforated galvanised steel sheeting.
Fig. 26.10 illustrates a circular duct silencer, which is filled on the inside with an inorganic material along the exterior walls and the inner shaft; this material is covered with galvanised steel.
i. Lined Ducts:
A very simple form of duct silencer can be fabricated by lining the interior of the duct with absorbing materials. Typically, the lining material chosen is acoustical-quality fibrous glass 1 to 3 inch thick.
Duct elbows or bends lined with absorbing materials can also provide significant noise reduction. Shown in Fig. 26.11 is the typical acoustical performance for a lined bend whose principal dimensions are in the range of 1 to 2 ft. To achieve this level of performance the absorptive lining must
1. Have an average absorption coefficient (NRC) in the range of 0.70 to 0.80.
2. Extend 4 to 5 times the principal dimension on both sides of the bend.
If the bend is 180° or switched back, an additional 5 dB can be expected at all frequencies above 250 Hz. Where pressure losses are extremely critical, the bend should be rounded with the possible inclusion of a treated guide vane.
Typical lining materials are essentially the same for bends as for lined ducts. In most cases, 1 or 2 inch acoustical-grade fibrous glass or mineral wool is used, being constrained and protected with perforated sheet metal or plastic film.
Illustrated in Fig. 26.12 is a plenum chamber lined with sound-absorbing materials. Plenum chambers of this type are frequently used as silencers in air-conditioning and ventilating systems and in test facilities to reduce flow velocity and turbulence. The attenuation of these devices comes from both dissipative and reactive effects, which renders mathematical analysis quite difficult.
Plenum chambers are often used where high-velocity gas is being discharged such as for gas turbines, burners, etc. In these installations where pressure losses are not so critical, a heavy-duty blast panel can be installed opposite the inlet to protect the chamber wall and lining treatment. If this panel extends from the inlet to the exit, the line of sight from the inlet to the exit is blocked and a sharp increase of up to 10 dB of additional attenuation is achieved above 1000 Hz.
For similar applications where a high degree of silencing is required, several chambers in series can be used. Here the resultant attenuation is approximately additive for noise above 500 Hz.
Another form of parallel baffles is the acoustical louvre. In this configuration, each louvre blade is filled with absorbing material. With the underside of the blade constructed of perforated sheet metal, noise passing through the air passage is absorbed in a manner analogous to a parallel baffle. Note that the performance is somewhat limited, especially in the low-frequency range.
There are many applications for these louvre units such as cooling tower inlets, mechanical room inlets, fresh air intakes, external building wall penetrations, etc.
The two principal features which account for the popularity of louvres are:
(i) Ease of installation; and
(ii) Relatively low pressure losses, i.e., less than 1.0 inch of water for face velocities less than 1000 ft/second.
These louvre units are usually available in modular sizes of 12 inch increments on each dimension. A bird screen is usually available as on option from the manufacturer and is strongly recommended for external (outside) wall installations.
Another form of absorptive silencer which has unique application is the stack-insert silencer. Often, noise-generating equipment such as fans, blowers, scrubbers, combustion chambers, etc., discharges both gas and noise into the atmosphere through a stack. The insert silencer is designated to be installed inside the stack, as illustrated in Fig. 26.14.
If a noise problem is anticipated, the insert can be installed in the stack at the time of construction. If after construction a noise problem is determined (most often the case), the insert can then be lowered into the stack and fastened.
The sound-reduction is obtained through absorption. Here the basic construction of the insert is a sheet metal shell, enclosing acoustical-quality fibrous glass or mineral wools. The sides of the shell are perforated to permit sound wave impingement on the absorbing materials. Since the stack itself provides an outer containment shell, the configuration resembles the tubular silencer.
The acoustical performance of commercially available insert silencers is also shown in Fig. 26.14 along with aerodynamic pressure loss characteristics. The first two digits of the model number refer to the stack diameter in inches, i.e., 24 inch refers to a 24 inch diameter stack.
Note that the acoustical performance is appreciable, exceeding 20 dB in most models for the critical, hard-to-control 250-Hz octave band. Equipment discharged through a stack is generally rich in lower- frequency acoustic energy and in the 250-Hz band in particular.
Note also that pressure losses also are appreciable and cannot be ignored. However, these loses can be reduced considerably by installing the insert at least five stack diameters below the top of the stack.
Installation of these insert silencers is relatively simple. When the stack insert is to be installed at the top of the stack, it is supplied with a beam on the trailing edge that extends over the edges of the stack. After the stack insert in centered in the stack, this beam is either welded or bolted to the top of the stack.
The lower edge of the insert is provided with a load-carrying circular support extending completely through the leading edge. This support is sized for a ‘snug’ fit with a suitably sized rod or bar. After dropping the insert silencer into the stack, the rod or bar is passed through the support and welded.
7. Reactive Silencers:
Reactive-type silencers generally consist of one or more expansion chambers wherein, as sound passes through, attenuation is achieved through reflective self-destruction. Probably the most common example of a reactive silencer is the automobile muffler. As we shall see, the acoustical performance of reactive silencers is rather selective spectrally and hence in most applications the silencer must be designed or tuned to the discrete frequency character of the noise.
In short, these silencers are most effective in applications associated with rotary equipment generating noise with dominantly discrete frequency character. Some common examples include – internal combustion engines, compressors, rotary positive displacement blowers, vacuum pumps, etc.
To increase and provide a wider range of attenuation, most manufacturers and suppliers of reactive silencers put several chambers in series. Generally, the critical length L of each chamber is varied such that for those areas in the spectrum where the attenuation vanishes for one chamber, another is at or near peak. In this way, those regions of little attenuation or holes are filled and the muffler has wider application.
Silencers are commercially available in a wide range of sizes and with a variety of options. For even wider applicability, reactive chamber silencers with absorptive passages are also commercially available. With this combination, the attenuation is at a high level and nearly uniform over the entire audio spectrum.
The number of specialised silencers and their applications are far too numerous to cover in detail. However, a few are worth mentioning in that their application and basic construction illustrates imaginative design approaches.
A clever variation of the parallel baffle silencer is the tuned-dissipative silencer. Here, each baffle contains cavities which can be designed or tuned for optimum acoustical performance at selected frequencies.
Each cavity also is treated with absorbing material along one of its sides. In this way, the width of spectral attenuation near the tuned frequencies can be increased. These silencers have been especially effective in reducing noise of large, high-volume fans.
An added feature is that the acoustical performance of these silencers does not degrade by the buildup of particulates over the perforated sheet metal facing as is the case for conventional parallel baffle units. As such, they are popular in ‘dirty’ applications such as coal, wood an oil-fired power plants; sewage and solid waste treatment incinerators; coffee roasting exhaust fans; and so forth.
Wherever high-pressure steam or gas is vented or exhausted, extremely high-intensity noise levels result. Examples of such installations are steam boilers, relief and purge valves, gas process vents, switch valves, compressor blow offs, autoclaves, etc. The sudden expansion of the gas produces high velocity turbulent jet flow with corresponding broadband aerodynamic noise. The level and character of the noise from a steam vent, for example, can resemble a small jet engine and the impact on a quiet neighbourhood can be, to say the least, disturbing.
To control the noise from these vents, one basic design principle must be observed. The gas must be allowed to expand smoothly inside the silencer and the exit flow velocity must be reduced. To achieve this design criterion, a series of 180° switchback expansion chambers in conjunction with absorbing materials is employed.
Note that as the gas exits the first chamber it impinges on an impact plate constructed of heavy-gauge steel. To reduce the velocity of direct impingement, a perforated diffuser plate backed by scrubble is also included. In the next chamber, the gas flow velocity is reduced to approximately one-half its original velocity.
In addition, the noise levels are reduced as the sound waves pass in close proximity to the absorptive treatment contained behind the perforated sheet metal. In the next chamber, the flow velocity is again reduced and the sound again is exposed to absorptive treatment. Here, however, the treatment is thicker than in the first chamber and hence better low-frequency noise attenuation is obtained.
The flow finally enters a large reflow or plenum chamber designed to reduce turbulence and lower the final exit velocity. Because of the relatively high pressures and unusually high temperatures typical of vents, the containment shell of the silencer must be of heavy gauge and the absorbing materials capable of withstanding temperatures of 1000°F or greater.
The acoustical performance of these silencers varies, but attenuation of more than 40 dB above 250 Hz is common. One point that cannot be over-emphasised, however, is that the gas flow velocity at the silencer exit must be sharply reduced or very little attenuation will be achieved. That is, the noise will be regenerated if the gas flow velocity is returned to its original value.
A good design criterion for effective attenuation is to keep the exit velocity for those vents under 100 ft/second or a Mach number of 0.1. To achieve this, a single plenum chamber in series with the vent silencer will further reduce the exit velocity and provide additional noise reduction.
In some applications, not only is noise a problem but often water, carbon particles, sparks, etc., are entrained in the gaseous discharge. In this configuration there is a stationary element which imparts a rotational movement to the gas flow. The heavier water, sparks, carbon particles, etc., are centrifugally thrust outward and collected, while the gas proceeds on through the inner chamber. Silencing is accomplished in the usual manner by employing reactive or absorptive mechanisms.
By combining effective silencing with separation in one vessel, it is possible to produce a very compact unit and simultaneously solve two problems.
Another example where silencing can be combined with another requirement is the filter silencer. Here the silencer shell can be designed to include a dry and/or wet filter. The acoustical performance of these combinations is quite good, usually in the range of 10 to 20 dB in the critical low-frequency range; below 1000 Hz.
Probably the most common applications are on the inlet to reciprocating compressors, internal combustion engines, vacuum pumps, rotary positive displacement blowers, etc. The acoustical energy of these machines is often concentrated in the 125 to 1000 Hz range.
The noise from electric motors is dominantly due to the cooling air fans. This is especially true for the explosion proof, totally enclosed fan-cooled (TEFC) types whose rotational speeds are typically about 3500 rpm. To control the fan noise, an absorptive-type silencer can usually be mounted on the end bell of the motor.
Here the cooling air enters the silencer through the end screen and passes through the silencer and exits over the motor-cooling fins. These air passages are typically constructed of absorbing materials which reduce the fan noise as it passes through the silencer. The attenuation of these devices is typically in the range of 6 to 10 dB at frequencies of 1000 to 8000 Hz, where the fan noise is concentrated.
More specifically, resultant sound pressure levels at 5 ft with the silencers installed are typically in the range of 85 to 88 dBA for motors up to 100 hp. Other common applications of motors in this horsepower range include drive motors for pumps, compressors, blowers, fans, expanders, centrifuges, etc.
Silencers of a similar design can be applied to the inlets of gas or oil burner intakes such as are common in the petrochemical industry and for drying oven applications in the areas of food processing.
Dispersive-type silencers are used dominantly in the control of noise associated with small high-pressure relief or exhaust valves such as found on pneumatic controls, solenoid valves, air cylinders, clutches, brakes, air-driven hand tools, shop air wands, etc. The noise source is usually high-velocity air exhausted as a burst to the atmosphere. Thus the character of the noise is impulsive in duration and with a spectrum rich in broadband high-frequency noise energy.
With a dispersive silencer installed over the exhaust vent, the high-velocity air is released into an expansion chamber and subsequently dispersed at a much lower velocity through porous sintered metal or slots. It should be noted that these silencers are usually available with standard pipe thread connectors for easy direct installation and can be mounted in any orientation.
One problem that these silencers present is a tendency to clog, with a corresponding increase in back pressure. In most installations, the back pressure is intolerable, especially for pneumatic controls, clutches and air-driven tools. As such, the diffuser elements require frequent cleaning with a solvent or replacement.
As an alternative approach, one can adapt a ½ or ¾” diameter flexible hose to the threaded exhaust orifice and vent the air into an overhead 2″ diameter pipe header running the length of, say, the press room or moulding room. In this way, dozens of exhaust ports can be vented into a common expansion chamber. The ends of the header pipe are left open, reducing back pressure buildup and eliminating further maintenance.
The concept of reducing noise by actively adding acoustical energy is not new. Paul Lueg demonstrated that noise in a duct could be reduced by introducing sound waves out of phase with the source.
It is well beyond the scope of this text to discuss in detail either the theory or the electronics of active noise control systems. The input microphone converts the varying sound pressure of the source to an analog electrical signal. The signal is then sent to the controller where a new signal is generated and adjusted for input microphone-to-loudspeaker spacing.
The new signal activates the loudspeaker, producing destructive interference with the undesired sound. One might ask, ‘Is there time for the controller to activate the loudspeaker?’ The answer is obvious when one notes that the sound waves are travelling at the speed of sound in the duct and the electrical signals are travelling at the speed of light (almost instantaneously) between the input microphone and the loudspeaker.
Now the precise adjustment or time delay that is calculated by the controller depends to the first order on the speed of sound in the duct medium, i.e., air or other gases. Since there are generally gas flow and often dynamic changes in gas temperature that affect the speed of sound, an error microphone is included to monitor the resultant field after cancellation.
This microphone essentially provides feedback to the controller to modify the time delay and optimise the cancellation process. Thus, the active noise control system provides a powerful engineering noise reduction measure. This is especially true in the lower frequency ranges (20 to 400 Hz) where passive devices are least effective.
The practical industrial applications of active sound control are really in their infancy when compared to passive methods. However, effective applications in ducts have been demonstrated and applications to vehicular noise are showing steady progress. The impetus here is for truck mufflers, in that back pressure loss penalties are always present with passive devices.
With active control, penalties vanish and the overall engine efficiency is significantly improved, with corresponding savings of fuel. Another unique application is with headsets. Here, selective noise reduction of sirens on emergency vehicles has been effectively demonstrated. In short, where the geometry of the propagation is small or two-dimensional, such as in ducts, active control can be considered.
In summary, where air or gas is either moved, conditioned, or vented, some form of noise control is usually required. Areas such as these are common not only in industry but in our daily and recreational living. As such, one recognises that silencers, mufflers and active noise control provide probably the most formidable weapon available to the acoustical engineer in the fight against noise.
Silencers are used in many applications. After the silencer system was installed, microphones were positioned in the test cell and also around the exhaust silencer and the inlet silencer. These microphones were hooked; up to a tape recorder and the output performance points were recorded; instrumentation data is illustrated in Table 26.3.
The amount of noise reduction achieved by the exhaust and inlet silencers was determined by calculating the difference in dB level between the microphones within the test cell and the average of the microphones outside the cell near the silencers. The range of attenuation values for the engine operation from 1000 pounds to 4100 pounds thrust is illustrated in Fig. 26.24.
The arithmetic average of the values at each octave band was determined in order to obtain single attenuation values for the inlet and exhaust.
Demountable noise suppressors and demountable test cell noise suppressors are widely used by US Air force noise control. Use of this system has led to greater hearing protection for technical personnel, who now show a higher working efficiency and morale.