AMERICA’s Sheet Metal Source

Since 1981

Sound Attenuators

Sound attenuators are a proven and effective method for reducing the noise generated by fans and other equipment. Also referred to as duct silencers, sound traps or mufflers, they are designed to reduce the noise transmitted from a source to the receiver.

For HVAC applications, they are commonly installed on intake and discharge sides of a fan or air handling unit. They can also be used on the receiver side of noise generating equipment (terminal boxes, air valves and dampers) and in areas outside the primary air system where they can reduce transfer noise between spaces.

Sound Attenuators


At Spiral Pipe of Texas, we have been making sound attenuators for over 35 years. Our products have been tested by an independent testing laboratory** for dynamic insertion loss, pressure drop and self-generated noise in accordance with ASTM Standard E477 test standards. We offer rectangular sound attenuators in high (type SA), medium (type MA) and low (type LA) pressure drop configurations. Individual modules can be assembled into assemblies to match most any size of duct. Our circular sound attenuators (type SC-C) are available in 12” through 60” diameter. We offer four configurations (models TGL, TGU, TGS and TGZ) of our transfer and return silencers. We can also make custom silencers using different materials and configurations, including flat oval shapes. Our basic sound attenuators are classified as dissipative silencers — metal construction with sound absorptive media (mineral fiber), covered by perforated metal to protect it from airflow erosion. For more information about sound attenuators and methods of HVAC noise reduction, you can refer to the 2015 ASHRAE Handbook — HVAC Applications (Chapter 48 – Noise and Vibration Control).

** testing performed at ETL Testing Laboratories, Inc., Cortland, NY (report# 483042)

Rectangular Sound Attenuators

22 ga. galvanized steel shells
22 ga. perforated galvanized steel baffles
4 lb/ft3 mineral fiber insulation
Module sizes 6×6 to 48×48 available

Round Sound Attenuators

Galvanized steel spiral pipe outer shell
Perforated galvanized steel inner shell
4 lb/ft3 mineral fiber insulation
Outer shell 8” larger than inside perforated liner
Spun metal head and cone shaped tail

Rectangular Sound Attenuators

Engineering Data

Spiral Pipe of Texas offers three models of rectangular sound attenuators — models SA, MA and LA. Each model is, in turn, offered in three standard lengths — 3, 5 and 7 feet long. Using our nomenclature, a model SA attenuator in a five-foot length would be a “Model SA-5”. We offer these different models and lengths because the selection of a sound attenuator requires some trade-offs between acoustical performance and pressure loss. Dissipative silencers operate by putting the flow of acoustic energy (and air) in contact with our sound absorptive media. When we increase the amount of contact — either by making the sound attenuator longer or decreasing the width of channels between baffles — we get more sound attenuation. But we also get a higher pressure drop.

Model Sound Attenuation Static Pressure Loss
SA Higher Higher
MA Medium Medium
LA Lower Lower

So, why would we ever be okay with a high pressure loss? It’s all about velocity and where we locate the sound attenuator. The actual static pressure is high or low relative to the velocity. If you locate your sound attenuators in a low velocity area — like a plenum — there may be very little real difference between the total pressure loss of either model, but you would still get a higher sound attenuation from a model SA.

Our sound attenuators have been tested in an independent testing laboratory and in accordance with ASTM Standard E477. For selection and comparison, our data is published for five face velocities — zero flow, 1000 FPM (+ and -) and 1500 FPM (+ and -). Self-Noise Sound Power Ratings are published for four referenced velocities (less zero flow where no self-noise is generated). Using our Air Flow Performance Data, you can obtain the applicable static pressure loss (Pd) if your actual CFM is known, using the following equations:

Rectangular Sound Attenuators from Spiral Pipe of Texas are constructed of 22 GA galvanized shells with 22 GA galvanized perforated baffles and 4#/ft3 mineral fiber fill. They will withstand a differential air pressure of 8” WG without structural failure. We offer a full complement of sizes from 6”x6” to 48”x48” and they can be arranged in an almost unlimited number of arrays. We offer other optional materials and constructions as well (please contact factory for information).

When ordering, please include model number and dimensions in the following order:

  Width Height Length Model
example: 24 24 60 SA-5

In an ongoing effort to improve our products, Spiral Pipe of Texas reserves the right to revise the design and specifications at any time.


Model SA

Rectangular Sound Attenuator

Dynamic Insertion Loss

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Net Insertion Loss in Decibels (dB)
SA-3 -1500 8 22 28 38 39 26 14
-1000 10 22 32 42 39 30 17
0 8 18 30 42 40 31 19
+1000 6 15 28 42 42 32 19
+1500 6 14 25 32 34 30 17
SA-5 -1500 15 33 46 47 41 35 26
-1000 13 30 45 53 50 47 26
0 12 26 42 53 58 49 27
+1000 11 23 40 52 55 49 29
+1500 9 21 40 50 50 44 29
SA-7 -1500 16 36 45 48 42 34 31
-1000 15 36 48 57 54 52 34
0 15 35 46 54 53 52 34
+1000 15 34 45 55 55 52 35
+1500 10 33 44 47 48 41 35

Self-Noise Sound Power Ratings (P.W.L.) — (dB re 10-12 watts)

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Self Noise Sound Power Levels in Decibels (dB)
SA-3
SA-5
SA-7
-1500 56 51 53 56 65 66 55
-1000 46 42 44 50 55 49 40
+1000 51 44 43 46 48 45 39
+1500 64 55 54 53 57 58 54
Face Area Adjustments Area (sq.ft.) 2 4 8 16 32
Adjustment – 3 0 + 3 + 6 + 9

Air Flow Performance Data

Model Static Pressure Loss (inches WG)
SA-3 0.07 0.12 0.18 0.24 0.38 0.49
SA-5 0.07 0.13 0.20 0.27 0.42 0.55
SA-7 0.10 0.18 0.27 0.36 0.56 0.73
Face Velocity FPM 365 490 610 705 875 1000

Model MA

Rectangular Sound Attenuator

Dynamic Insertion Loss

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Net Insertion Loss in Decibels (dB)
MA-3 -1500 9 17 23 32 39 20 13
-1000 7 15 21 36 40 21 13
0 6 14 20 35 42 24 16
+1000 6 13 19 34 42 25 16
+1500 6 13 18 34 42 25 17
MA-5 -1500 12 25 43 54 49 31 17
-1000 11 24 42 55 55 32 19
0 9 21 39 56 57 34 19
+1000 9 20 38 55 57 35 21
+1500 9 20 37 54 53 36 21
MA-7 -1500 15 30 36 46 48 32 18
-1000 15 28 47 57 56 36 20
0 14 26 46 57 58 38 22
+1000 14 25 46 58 58 41 23
+1500 12 25 43 57 56 42 25

Self-Noise Sound Power Ratings (P.W.L.) — (dB re 10-12 watts)

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Self Noise Sound Power Levels in Decibels (dB)
MA-3
MA-5
MA-7
-2000 58 55 57 57 64 64 56
-1000 51 43 44 46 46 36 29
+1000 43 37 34 37 35 25 28
+2000 56 50 49 47 54 54 49
Face Area Adjustments Area (sq.ft.) 2 4 8 16 32
Adjustment – 3 0 + 3 + 6 + 9

Air Flow Performance Data

Model Static Pressure Loss (inches WG)
MA-3 0.05 0.07 0.11 0.15 0.24 0.33
MA-5 0.07 0.10 0.15 0.20 0.32 0.44
MA-7 0.11 0.15 0.23 0.31 0.49 0.69
Face Velocity FPM 585 705 860 1000 1255 1490

Model LA

Rectangular Sound Attenuator

Dynamic Insertion Loss

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Net Insertion Loss in Decibels (dB)
LA-3 -1500 5 11 23 41 26 14 7
-1000 5 10 22 41 26 14 8
0 4 9 22 41 26 15 10
+1000 4 9 22 40 26 16 10
+1500 4 8 20 38 26 17 9
LA-5 -1500 7 15 31 47 40 18 10
-1000 9 16 33 50 43 21 13
0 6 13 30 47 40 21 11
+1000 6 12 29 46 41 22 13
+1500 6 12 27 44 41 23 13
LA-7 -1500 13 20 34 52 48 25 14
-1000 13 19 37 51 51 28 14
0 12 18 37 53 50 30 15
+1000 12 17 35 52 49 28 15
+1500 12 17 34 50 49 29 17

Self-Noise Sound Power Ratings (P.W.L.) — (dB re 10-12 watts)

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Self Noise Sound Power Levels in Decibels (dB)
LA-3
LA-5
LA-7
-2000 59 56 55 57 61 47 47
-1000 45 41 44 44 39 28 28
+1000 45 36 35 37 32 26 28
+2000 57 52 49 49 54 52 45
Face Area Adjustments Area (sq.ft.) 2 4 8 16 32
Adjustment – 3 0 + 3 + 6 + 9

Air Flow Performance Data

Model Static Pressure Loss (inches WG)
LA-3 0.02 0.03 0.07 0.09 0.14 0.20
LA-5 0.03 0.04 0.08 0.11 0.17 0.25
LA-7 0.04 0.06 0.11 0.15 0.24 0.34
Face Velocity FPM 490 610 860 1000 1255 1500

Model SC-C

Round Sound Attenuators

Engineering Data

Round Sound Attenuators from Spiral Pipe of Texas are double-wall construction with the outer shell of galvanized spiral pipe 8” larger than the inside perforated liner. The center absorber is designed with a spun metal head and cone-shaped tail for maximum attenuation and optimum pressure drop. The attenuator is filled with 4#/ft3 mineral fiber insulation.

Standard models are available in 12” through 60” diameter. They are sized to fit ductwork without other transitions. Other materials and flanged ends are available as an option (contact factory for more information).

Ordering Information: Give model number and inside diameter (in inches). Standard length is 2 X diameter unless otherwise stated. Overall length of the unit is 4” longer (including 2” slip collar at each end). If flanged ends are added as an option, the overall make-up length is (2 x dia.) + 4”.

Our sound attenuators have been tested in an independent testing laboratory and in accordance with ASTM Standard E477. For selection and comparison, our data is published for five face velocities — zero flow, 1000 FPM (+ and -) and 2000 FPM (+ and -). Self-Noise Sound Power Ratings are published for four referenced velocities (less zero flow where no self-noise is generated). Using our Air Flow Performance Data, you can obtain the applicable static pressure loss (Pd) if your actual CFM is known, using the following equations:

In an ongoing effort to improve our products, Spiral Pipe of Texas reserves the right to revise the design and specifications at any time.

Dynamic Insertion Loss

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Net Insertion Loss in Decibels (dB)
SC-C -2000 9 13 22 33 35 22 17
-1000 9 12 22 32 34 21 16
0 7 11 21 32 34 22 18
+1000 7 11 20 31 34 22 18
+2000 7 10 19 30 33 21 18

Self-Noise Sound Power Ratings (P.W.L.) — (dB re 10-12 watts)

Silencer
Model No.
Octave Bands 2 3 4 5 6 7 8
Center Frequency 125 250 500 1000 2000 4000 8000
Face Velocity FPM Self Noise Sound Power Levels in Decibels (dB)
SC-C -2500 60 57 59 59 62 55 48
-2000 56 52 55 56 56 46 39
+2000 53 49 45 44 42 34 30
+3000 61 58 55 53 53 50 44
Face Area Adjustments Diameter 12″ 18″ 24″ 36″ 48″ 60″
Adjustment (dB) – 6 – 3 0 + 3 + 6 + 8

Air Flow Performance Data

Model Static Pressure Loss (inches WG)
SC-C 0.027 0.04 0.11 0.17 0.245 0.33
Face Velocity FPM 1000 1500 2000 2500 3000 3500

Sound Attenuator Performance

Sound attenuators can be an effective method for reducing unwanted noise in HVAC systems and other applications. At Spiral Pipe of Texas, we have tested our products at an independent laboratory and in accordance with ASTM Standard E477 test standards. When designing a system using sound attenuators, it is important to understand the terminology, the purpose of the data we provide, and a few design criteria that will help you get the results you need.

Terminology

Noise – to put it simply, noise is unwanted sound. Sound attenuators are designed to reduce or eliminate noise. But how we define noise can get a little complicated.

Frequency – sound is a vibration — or “variation of pressure” — occurring as a frequency (cycles or fluctuations per second). The human ear can respond to a wide range of sound frequencies (the “audible range”) from 20 Hz to 20,000 Hz (Hz or Hertz is a unit of frequency corresponding to one cycle per second).

Octave Bands – we usually are not dealing with “pure tones” — sound occurring on a single frequency. For HVAC applications, we deal with what is referred to as “broadband noise”. Noise is usually “louder” (greater sound pressure) at different frequencies than others. Sound attenuators are also more-or-less effective at different frequencies. In addition, the amount we typically object to noise varies between frequencies. To simplify our understanding and treatment of noise, we divide the audible range into “octave bands”. If you’ve ever had musical training you will be familiar with the term “octave”. Basically, each octave is a doubling of frequency over the previous octave. We measure at the “center frequency” of each octave and nine octave bands cover the audible range. We use numbers 2 through 8.

Octave Bands 1 2 3 4 5 6 7 8 9
Center Frequency (Hz) 62.5 125 250 500 1,000 2,000 4,000 8,000 16,000

Sound Pressure – “sound pressure” and “sound pressure level” are two terms that allow us to quantify what our ears perceive as “loudness”. When you consider sound as a variation or fluctuation in pressure, sound pressure is the amount — or amplitude — of that fluctuation. The accepted unit of measurement for sound pressure is Newtons per square meter (N/m2). Our ears are receptive to a very wide range of these fluctuations in everyday life, ranging from about 0.00006 N/m2 (6 x 10-5), which is considered the threshold of good hearing, all the way to 6.3 N/m2 (a loud rock band) which is considered the threshold of discomfort.

However, the way we perceive these fluctuations as “loudness” is not linear. As amplitude increases our ears become less responsive to the increase. Doubling the sound pressure does not double the “loudness”. To simplify this, we have borrowed the term “decibels” (dB) from electrical and illumination engineering to create a numerical correlation between the changes in sound pressure and changes in perceived “loudness”. The equation is as follows:

P = the root mean squared (RMS) value of the sound pressure fluctuations in N/m2
Pref = a standard reference sound pressure equal to 2 x 10-5 N/m2

The following is a representation of some typical sound pressure levels and their decibel values.

Sound Pressure Level (dB) Source Subjective Reaction Sound Pressure (N/m2)
0 Threshold of excellent youthful hearing Threshold of hearing 0.00002 M/m2
10 Threshold of good hearing   0.00006 N/m2
20 Buzzing insect at 3 feet Faint 0.0002 N/m2
30 Whispered conversation at 6 feet   0.0006 N/m2
40 Quiet residential area (ambient)   0.002 N/m2
50 Window air conditioner (ambient) Moderate 0.006 N/m2
60 Conversational speech at 3 feet   0.02 N/m2
70 Freight train at 100 feet   0.06 N/m2
80 Computer printout room Loud 0.2 N/m2
90 Unmuffled large diesel engine at 130 fe et           Very loud 0.6 N/m2
100 Platform of subway station   2 N/m2
110 Loud rock band Threshold of discomfort 6.3 N/m2
120 Passenger ramp at jet airliner (peak) Threshold of pain 20 N/m2
130 Artillery fire at 10 feet Extreme danger 63.2 N/m2
140 Military jet takeoff at 100 feet You went too far 200 N/m2

Sound Power – sound is energy. And as the force of acoustical energy increases, we perceive an increase in “sound power”. If you grew up coveting high-end stereo equipment you should readily understand this concept. Both a cheap and high-end amplifier can be cranked up to an equally “loud” level. But there is a perceptible difference in the “sound power” coming from the one producing more energy. This “sound power” is expressed in terms of watts of power (remember wanting the amplifier with more “watts per channel). But as a practical matter, the range of sound power, expressed in watts, that we perceive in everyday life covers an extremely wide range — from as little as 0.00000000001 (10-11) watts up to 100,000,000 (108) watts. We needed a shorthand for this, so we once again borrowed the term “decibels” (dB). This describes an exponential increase in sound power with a standard reference sound power of 10-12 watts. Sound power level in decibels is determined by the following equation:

W = Sound Power in watts
Wref = 10-12 watts (standard reference)

Example Sound Power Levels:

Sound Power Level
(dB re: 10-12 watts)
Example Sound Power (watts)
10 dB  Human Breath 10-11
30 dB Voice in Soft Whisper 10-9
70 dB  Voice at Conversational Level 10-5
90 dB    Voice Shouting 10-3

Now, all of this may be starting to get confusing. We’ve just used the same term (decibels) to quantify two completely different things — “sound pressure” (loudness) and “sound power” (energy). And in HVAC, why do we even care about “energy” when all we want is to make the system “less loud”? The reason we care is that “sound pressure” only has a significance at the point at which it is measured. This “sound” is just a series of air pressure waves that is radiating outward from a noise source. And these pressure waves are a result of acoustical energy transfer from the noise source to the surrounding air. We need a way of predicting how that noise source is perceived at different locations and conditions. After all, our building occupants are not standing next to the fan.

In HVAC applications, our concern is that noise follows a “path” — our duct system — from the source to where it is perceived, and we want to know how much of that “sound energy” reaches the end. Some of it simply leaves the duct — what we refer to as breakout noise. This leads to the somewhat misleading claims made by fabric and flexible duct manufacturers that their products are “sound attenuating”. In reality, little sound energy was actually lost. These materials are simply not capable of containing and transporting the sound energy, so it “breaks out” soon after entering these ducts and into that surrounding space. Metal ducts are much more effective at containing and transporting sound energy — just as they’re more effective at containing and transporting air.

Fortunately for us in our efforts to reduce “noise”, much of that acoustical energy can be lost — or attenuated — as it is transported through the duct system. Some is lost through breakout, and this is greatly affected by the shape and rigidity of the duct (rectangular duct walls vibrate much more readily than round or flat oval duct walls — which can be a bad thing). Friction and reflection account for a significant portion of the rest. The use of sound-absorptive media — such as fiberglass insulation or liner — will also greatly decrease acoustical energy. Information on attenuation of various types of ducts can be found in the 2015 ASHRAE Handbook — HVAC Applications (chapter 48 “Noise and Vibration Control”).

When our duct system cannot dissipate enough of our acoustical energy between the noise source and our target space, an effective option can be the use of a sound attenuator. The sound attenuators offered by Spiral Pipe of Texas are known as dissipative silencers. They use a sound-absorptive media (mineral fiber or fiberglass) as the primary means of attenuating sound. A baffle or series of baffles disrupts the path and increases the amount of contact between the sound path (or airflow path) and the transported sound energy. The absorptive media is covered by perforated metal, and in some cases fiberglass cloth or polymer film, to protect it from erosion by airflow. By disrupting the airflow, sound attenuators have an effect on air flow and system pressure.

Narrower paths between baffles and attenuator walls increase the loss of sound energy, but also increase the loss of duct static pressure. Specially designed nose pieces and tail cones can decrease the static pressure loss by reducing air turbulence. The location of a sound attenuator can also greatly impact both attenuation and pressure loss. Sound attenuators should generally be located as close to the noise source as possible, but far enough away to have a uniform, laminar profile of air flow. One common mistake is to locate a sound attenuator too close to a fan. The result is system effect — highly turbulent air with much greater pressure drop and irregular flow profile. Not only is the attenuator less effective at absorbing sound energy, the turbulence can cause the sound attenuator to generate more noise than it attenuates. The recommendation is to locate the sound attenuator with 3 to 5 diameters of straight, unobstructed duct on both inlet and discharge ends, or located at the plenum discharge/inlet with unobstructed duct at the ducted end for 3 to 5 diameters.

Now that we’ve covered some of the basics of acoustics and noise control, we need to go over how to use the information published by sound attenuator manufacturers. Spiral Pipe of Texas has had their sound attenuators independently tested with procedures outlined in ASTM Standard E477 as recommended by ASHRAE. We strongly encourage using only sound attenuators tested consistent with these procedures, and recommend independent testing for consistent and verifiable results. Our testing was performed at ETL Testing Laboratories, Inc. (Cortland, NY), a widely respected facility, and results are available upon request. Using ASTM Standard E477, performance is measured and reported as “insertion losses”.

Because these employ a substitution technique — the sound attenuator is substituted for an equal length of standard duct — we can have reasonably precise values for dynamic insertion losses, airflow-generated noises and airflow static pressure losses (see 2015 ASHRAE Handbook chapter 48 for deviation and confidence results based on round-robin testing). The following information is published for sound attenuators from Spiral Pipe of Texas:

Dynamic Insertion Loss – these are presented as a chart showing the net insertion loss (in decibels) of a sound attenuator in the second through eighth octave bands. These are “sound power” decibels – the ones referring to watts of acoustic energy that are attenuated. This insertion loss is “dynamic” because performance changes under different flow rates and conditions. We publish our net insertion loss in decibels in five flow conditions — two negative velocities, two positive velocities and no flow.

Self-Noise Sound Power Ratings – as previously stated, sound attenuators disrupt the flow profile of air and the resulting turbulence creates noise. This is not an issue when there is no flow, so we publish noise created in four flow conditions — two negative velocities and two positive velocities.

Face Area Adjustment – size impacts the self-noise generation of a sound attenuator, but all potential sizes are not listed. A face area adjustment table is provided to make the needed corrections. Determine the face area of the sound attenuator (ft2). Then take the applicable “adjustment” from the table and add it to each octave band number for self-noise sound power ratings.

The adjusted airflow-generated Self-Noise Sound Power Ratings represent the noise floor — the lowest level achievable, regardless of high insertion loss values. It is logarithmically proportional to the sound attenuator’s cross sectional area, but it generally does not vary with silencer length.

Air Flow Performance Data (static pressure loss in inches WG) – the actual static pressure loss does change with both length and velocity. Since sound attenuator model numbers are associated with length, values are given for each model. Static pressure losses are presented in the tables for six reference face velocities. If you know your actual flow volume (CFM) you can determine your actual static pressure using the following two correction equations:

Face Velocity = CFM / Area (ft2)

Actual Static Pressure Loss = Pd = (Face Velocity/Referenced Table Velocity)2 X Table Pressure Loss

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