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| {{For|the U2 B-side|The Unforgettable Fire (song)}} | |||
| {{Citations missing|date=August 2007}} | |||
| {{More footnotes|date=June 2016}} | |||
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| '''Bass Traps''' are acoustic absorbers or ]s which have the ability to capture low frequency ]. Bass traps are used to provide acoustic absorptive treatment in rooms and are common tools used in ]. Bass traps are particularly useful in the acoustic treatment of ]s, ], ], and other venues built to provide a critical listening environment. Bass traps provide a means to control room reverberations at low frequencies, a part of the audible bandwidth especially troublesome when critical listening venues are located in small rooms. Bass traps, like all acoustically absorptive materials, function by turning sound energy in a room's air volume into minute amounts of heat through friction. | |||
| '''Bass traps''' are acoustic energy absorbers which are designed to damp low-frequency ] energy with the goal of attaining a flatter low-frequency (LF) room response by reducing LF resonances in rooms. They are commonly used in ], ], ]s and other rooms built to provide a critical listening environment. Like all acoustically absorptive devices, they function by turning sound energy into heat through friction. | |||
| ==General description—types== | ==General description—types== | ||
| There are generally two types of bass traps: resonant absorbers and porous absorbers. Resonant absorbers are further divided into panel absorbers and Helmholtz resonators.<ref>{{cite web |title=Practical Acoustic Treatment: Part 2 |last=White |first=Paul |date=August 1998 |url-status=dead |url=http://www.soundonsound.com/sos/aug98/articles/practicalacoustic.html |archive-url=https://web.archive.org/web/20040815212324/http://www.soundonsound.com/sos/aug98/articles/practicalacoustic.html |archive-date=2004-08-15 |publisher=Sound on Sound}}</ref> | |||
| There are generally two types of bass traps: resonating absorbers and porous absorbers. By their nature resonating absorbers tend toward narrow band action and porous absorbers tend toward broadband action , though both types can be altered to be either more narrow, or more broad in their absorptive action. Both types are effective though the porous absorber has certain practical advantages in application as porous absorber type bass traps need not be specifically tuned to match the job at hand, and they tend to be smaller in size and easier to build than resonation type devices. For this reason most commercially manufactured bass traps are of the porous absorber type. | |||
| Both types are effective, but whereas a resonant absorber needs to be mechanically tuned to resonate in sympathy with the frequencies being absorbed, a porous absorber does not resonate and need not be tuned. | |||
| Examples of resonating type bass traps include ], and devices based on diaphragmic elements or membranes which are free to vibrate in sympathy with the room's air when sound occurs. Resonating type bass traps achieve absorption of sound by sympathetic vibration of some free element of the device with the air volume of the room. Such free elements in a resonating device can come in many forms such as the air volume captured inside a Helmholtz resonator - or a thin wooden panel held only by its edges . Resonating absorbers can be made from just about any material that can either form a stiff walled vessel or any membrane stiff enough to be susceptible to being induced to vibrations by impinging sound. | |||
| Porous absorbers tend to be smaller in size and are easier to design and build, as well as less expensive overall than resonant absorbers. However, the deep bass attenuation of a porous absorber is generally inferior, so its usefulness for attenuating lower frequency room resonances is more limited. | |||
| Porous absorbers are most commonly made from ], ] or other fibrous materials, and function through the existence of interstices in the medium which present small captured pockets of air to the room which when excited by sound pressure waves in the room's air volume are themselves induced to vibrate like small springs. The fibers which make up a porous absorber are typically loosely bound together so that the "air springs" within the interstices created by the fibers are free to vibrate one against the other. In a manner of speaking, a porous absorber is a concatenation of many thousands of small Helmholtz resonators, of varying sizes, which are free to interact by vibrating against each other. It is this enormous variety of potential vibrational actions both inside each discreet air cell formed by the interstices, and between the myriad cells, one to another as allowed by the loosely bound fibers, which causes a porous absorber to function efficiently over a broad range of sound frequencies. | |||
| Resonating absorbers tend to absorb a narrower spectrum and porous absorbers tend to absorb a broader spectrum. The spectrum of both types can be either narrowed or broadened by design but the generalized difference in bandwidth and tunability dominates their respective performance. | |||
| Examples of resonating type bass traps include a rigid container with one or more portholes or slots (i.e. ]), or a rigid container with a flexible diaphragm (i.e. membrane absorber). Resonating type bass trap achieves absorption of sound by sympathetic vibration of some free element of the device with the air volume of the room. | |||
| Resonating absorbers vary in construction, with one type of membrane absorber using a springy sheet of wood that attaches to the enclosure only along the edges/corners, and another using a more floppy sheet of thin material stretched like a drumhead. A Helmholtz resonator can have one port tuned to a single frequency, or several ports tuned to either a single or to multiple frequencies, with round port, slotted port, or even perforated construction. Resonating absorbers often incorporate porous absorption internally to simultaneously lower the resonant frequency and broaden the spectrum of absorption. | |||
| Porous absorbers are most commonly made from ], ] or open cell foam that resists the passage of air molecules through the interstitial space.<ref>{{cite web |title=Introducing professional acoustic panels |publisher=namnak |url=http://namnak.com/%D9%BE%D9%86%D9%84-%D9%87%D8%A7%DB%8C-%D8%A2%DA%A9%D9%88%D8%B3%D8%AA%DB%8C%DA%A9.p74468}}</ref> Porous absorbers often incorporate a foil or paper facing to reflect frequencies above 500 Hz. Facing also improves low bass absorption by translating the physical compression of air at the facing into physical compression of the fibers that are in contact with the facing while also maintaining the resistive loss of air as it is driven through the bulk of the fiber by the facing. | |||
| ==Design concepts for building bass traps== | ==Design concepts for building bass traps== | ||
| ===Resonating bass traps=== | ===Resonating bass traps=== | ||
| Resonating bass traps will absorb sound with high efficiency at their fundamental frequency of resonance. As such, a knowledge of the frequencies of resonances which require damping is helpful before designing and constructing a resonating bass trap. This can be attained by calculation of the ] or by direct measurement of the room itself. | |||
| The key to build a resonating bass trap is to choose a style of device that will best fit the available space, and then match the resonance of the device to the frequency range you wish to absorb. In designing, building, and implementing a resonating bass trap one requires knowledge of the frequency of sound which requires absorptive treatment and then the device’s dimensions and/or the vibrational properties of the panel or membrane—must be matched to these needs. | |||
| ⚫ | Resonating absorbers can be broadened in the frequency range of efficacy to some degree by either introducing porous absorptive material to the interior of the vessel, by constraining the vibrations of the panel or membrane, or by installing an array of resonating devices each tuned to adjacent frequency ranges so that collectively the array functions over a broadened range of sounds. Such devices can be enormously effective over their tuned range, but can take up a great deal of space, especially when installed in arrays, and thus are sometimes not a practical solution. | ||
| ⚫ | |||
| ====Panel absorber==== | |||
| ⚫ | *1/8" plywood = 150 |
||
| ⚫ | *1/4" plywood = 110 |
||
| ⚫ | *3/8" plywood = 87 |
||
| ⚫ | A simple panel resonator can be built to hang on a wall by building a wooden frame, adding several inches of mineral wool to the inside, and fitting a sheet of plywood over the top attached only at the edges. A small gap should be left between the panel and the acoustic insulation so that the panel is free to resonate. Panel resonance can be enhanced by reducing the point of connection between the panel and the frame by means of narrow spacer material such as a loop of wire or welding rod run along the edge of the frame so that the panel is perched on a thin edge. Approximate full sheet plywood panel resonances when mounted on a 1"×4" frame 3.5" deep are: | ||
| ⚫ | Other common resonating bass traps are |
||
| ⚫ | *1/8" plywood = 150 Hz | ||
| ⚫ | |||
| ⚫ | *1/4" plywood = 110 Hz | ||
| ⚫ | *3/8" plywood = 87 Hz | ||
| ====Helmholtz resonator==== | |||
| ⚫ | Other common resonating bass traps are forms of the Helmholtz resonator—such as either a stiff walled box with a hole in one side , or a series of slats over-mounted across the face as a stiff-walled box forming narrow openings in the cracks between the slat members. | ||
| ===Porous absorber bass traps=== | ===Porous absorber bass traps=== | ||
| A bass trap generally comprises a core absorbent damp material, a frame, and a covering for aesthetic reasons. | |||
| * '''Core:''' Semi-rigid glass-wool or mineral-wool insulation boards or dense open-cell foam are typically used. | |||
| ] | |||
| * '''Frame:''' A steel exterior frame is preferred, although wood skeleton frames are common. The frame acts as a housing, allowing mounting to walls and ceilings, and also anchors the covering material. | |||
| The keys concepts to efficiently building a broadband porous absorber are to choose the right material for the mineral fiber core, using a sufficient thickness of core material, choosing an appropriate fabric, and not overbuilding the frame work so as to yield an unwieldy device which cannot be easily installed for best performance. These fundamentals can be achieved by following a few simple rules: | |||
| * '''Covering:''' Porous fabric (similar to speaker grill cloth) is usually used. | |||
| ==Positioning== | |||
| # CORE: Use the best priced mineral fiber material your local market can provide—with some consideration given to material handling properties. If available at reasonable cost, semi-rigid fiberglass insulation boards in the 3 lb/ft³, or 48 kg/m³ density range such as Owens Corning 703 are an excellent choice this material will provide you a medium to work with that has good handling properties, durability, modest cost, and excellent acoustic performance. Mineral wool materials such as Rockwool in the 6 lb/ft³ or 100 kg/m³ range, or greater, will also work well, though this material is generally more floppy and less durable than fiberglass based alternatives . Use naked, panels, not foil, or paper scrim. These boards are typically easiest to source in 2" thickness, just stack them to achieve the desired overall size . However, thin paper or plastic facings increase bass absorption and reduce mid and high frequency absorption. This is often desirable in small rooms where substantial bass trapping is required, to avoid over-absorbing outside the bass range. See the Density Report link below for a comparison of various absorber densities with and without a paper facing. | |||
| # FABRIC: Any breathable fabric will work—literally put your mouth on the fabric and blow—if resistance is modest, the fabric will work fine, so just pick something that looks good to your eye and can be had at a reasonable cost. But also, to the extent appropriate for the location you intend to use the units, give some consideration to ease in cleaning/stain resistance, durability against wear and tear, and fire treatment. Upholstery and drapery fabrics work great or even muslin, craft grade felt or dyed burlap will work and are very cheap. If needed, most fabrics can be treated for stain or fire resistance, or breathable fabrics already having these properties can be found. | |||
| # FRAME: Here is where most do-it-yourself bass trap builders go overboard. Any material will work provided it affords you a means of attaching the upholstery, and mounting hardware. Depicted to the right is a frame that has all the characteristics needed—note that the lumber is quite modest in size, but that it has corner gussets for reinforcement—you could also add a cross brace or two, but don’t over build and add unnecessary weight, or occlude the sides or backside of the panels unduly. | |||
| Since low frequency resonances in a room have their points of maximum or minimum pressure in the corners of the room, resonant bass traps mounted in these positions will be the most efficient,<ref>{{cite web|url=https://www.soundonsound.com/sos/dec07/articles/acoustics.htm|title=Room For Improvement - Sound On Sound|publisher=}}</ref> while porous traps are most efficient at the points of high particle velocity such as 1/4 the desired wavelength away from the wall.<ref>{{cite web|url=http://www.acousticsinsider.com/why-bass-traps-in-corners/|title=The Real Reason You Place Bass Traps In Corners (It's Not Why You Think) — Acoustics Insider|publisher=}}</ref> Bass traps are typically used to attenuate modal resonances and so exact placement depends on which room mode one is trying to target. Bass traps typically combine structural mechanisms that can work at both positions of high particle velocity/low pressure (thick fiberglass) and high pressure/low particle velocity (membranes). | |||
| Keep the unit as light as is practical, and leave as much of the mineral fiber core exposed as is practical. Best practice would be to both glue and screw the frame together and a counter-sink pilot bit will help keep your screws from splitting the wood. As to hanger hardware, standard picture frame wire and eye hooks will do, or you can use light weight decorative chain as might be appropriate for the mounting position. For “hover” mounting on a wall (with a gap behind the panel) you can use a long metal hook for the hanger wire, and add screw-in door stops for stand-offs for the lower back side edges or place a spacer made of cardboard tubing or a small block of packing foam behind the panel and out of sight. | |||
| Porous bass trap absorbers need to be very thick to be effective at lower frequencies so they tend to be allocated either as diagonal wedges in the corners or as thick rectangular bulk behind false walls where they are out of the way and less likely to disrupt higher frequencies or room function. Air gap behind a porous panel absorber e.g. straddling a corner also helps to ensure it protrudes more into the room where there is more air velocity, improving its velocity-based absorption and extending its bandwidth while inducing some ripple in its absorption spectrum. Resonant bass trap absorbers need to be at a pressure maximum and tend to be thinner, so they are more conveniently and effectively positioned flat against a wall in a corner where the pressure is maximum, rather than straddling a corner where there is more velocity. | |||
| Another very effective porous absorber design is to stack triangles of mineral fiber in a corner to build a floor to ceiling wedge— frequently known as the StudioTips SuperChunk. See the external links below for more on various porous absorber bass trap designs. | |||
| Standard practice is to investigate the applicability of porous bass trap absorbers before investigating resonant or hybridized bass trap absorbers. Complementary methods to use in combination with porous absorption include drywall/stud construction of walls/ceiling and filled with insulation as a form of highly damped resonant bass trapping using the drywall itself as the membrane. The combination of inherently lossy resonant room boundaries and paper or foil faced porous bass trap absorption deep in the tricorners (where three room boundaries meet) is often sufficient to attain acceptable bass response even in listening rooms with somewhat problematic resonances. | |||
| ==Mounting== | |||
| Adjusting the listening position within the room boundaries and elevating the seating with a riser that is filled with porous absorption is one more method that can improve bass response without resorting to resonant bass trapping, while simultaneously improving home theater screen visibility. | |||
| The best place to mount bass traps is in corners. With porous absorber bass traps like the design described above, mounting the units straddled across the diagonal of a corner yields an extremely efficient unit both in terms of material costs, and space requirements. Slat type resonating bass traps are often installed on walls where they can be up and out of the way. | |||
| Small listening rooms suffer a paucity of low frequency resonances with gaps between them, so another complementary method is adding a subwoofer to drive the ] from an optimal physical location that minimizes ripple in the frequency response at the listening position. Yet another complementary method is splitting an existing subwoofer allocation up into multiple smaller subwoofers in spatially separated locations that increase the degrees of freedom available to tune the response with. Multiple subwoofers also tend to smooth the bass response across a larger listening area and are often easier to place for good bass response than a single larger subwoofer (or no subwoofer). | |||
| One can also use broadband porous bass traps in early reflection controls positions and thereby accomplish not only a diminution of the early reflections but also add beneficial additional low frequency absorption. | |||
| If the response is somewhat uniform across all listening positions using these methods, equalization can be used to shape the bass response to the desired target and further smooth out any remaining ripple. | |||
| ⚫ | ==References== | ||
| Some combination of porous absorption with these complementary methods is typically preferred for their simplicity, affordability, and convenience, but resonant bass traps are more effective for absorbing strong room resonances where the aforementioned complementary methods are inadequate or impractical, particularly when the geometry of the room causes problematic narrow-band resonances that affect the low bass and the composition of the room boundaries is highly reflective rather than acoustically lossy. | |||
| ⚫ | *Everest, F. Alton. ''The Master Handbook of Acoustics'', McGraw-Hill, 2000 (ISBN |
||
| ⚫ | *Kinsler, Frey, Coppens and Sanders, ''Fundamentals of Acoustics, Third Edition'', John Wiley & Sons, 1999 (ISBN |
||
| == Further reading == | |||
| ⚫ | *Everest, F. Alton. ''The Master Handbook of Acoustics'', McGraw-Hill, 2000 ({{ISBN|0-07-136097-2}}). | ||
| ⚫ | *Kinsler, Frey, Coppens and Sanders, ''Fundamentals of Acoustics, Third Edition'', John Wiley & Sons, 1999 ({{ISBN|978-0471847892}}), Section 10.8: “The Helmholtz Resonator”. | ||
| * https://www.soundonsound.com/sos/may06/articles/qa0506_3.htm | |||
| ⚫ | ==References== | ||
| {{reflist}} | |||
| {{DEFAULTSORT:Bass Trap}} | |||
| ] | |||
| ] | |||
| ] | |||
| ] | ] | ||
Latest revision as of 04:09, 16 December 2023
For the U2 B-side, see The Unforgettable Fire (song). | This article includes a list of general references, but it lacks sufficient corresponding inline citations. Please help to improve this article by introducing more precise citations. (June 2016) (Learn how and when to remove this message) |
Bass traps are acoustic energy absorbers which are designed to damp low-frequency sound energy with the goal of attaining a flatter low-frequency (LF) room response by reducing LF resonances in rooms. They are commonly used in recording studios, mastering rooms, home theatres and other rooms built to provide a critical listening environment. Like all acoustically absorptive devices, they function by turning sound energy into heat through friction.
General description—types
There are generally two types of bass traps: resonant absorbers and porous absorbers. Resonant absorbers are further divided into panel absorbers and Helmholtz resonators.
Both types are effective, but whereas a resonant absorber needs to be mechanically tuned to resonate in sympathy with the frequencies being absorbed, a porous absorber does not resonate and need not be tuned.
Porous absorbers tend to be smaller in size and are easier to design and build, as well as less expensive overall than resonant absorbers. However, the deep bass attenuation of a porous absorber is generally inferior, so its usefulness for attenuating lower frequency room resonances is more limited.
Resonating absorbers tend to absorb a narrower spectrum and porous absorbers tend to absorb a broader spectrum. The spectrum of both types can be either narrowed or broadened by design but the generalized difference in bandwidth and tunability dominates their respective performance.
Examples of resonating type bass traps include a rigid container with one or more portholes or slots (i.e. Helmholtz resonator), or a rigid container with a flexible diaphragm (i.e. membrane absorber). Resonating type bass trap achieves absorption of sound by sympathetic vibration of some free element of the device with the air volume of the room.
Resonating absorbers vary in construction, with one type of membrane absorber using a springy sheet of wood that attaches to the enclosure only along the edges/corners, and another using a more floppy sheet of thin material stretched like a drumhead. A Helmholtz resonator can have one port tuned to a single frequency, or several ports tuned to either a single or to multiple frequencies, with round port, slotted port, or even perforated construction. Resonating absorbers often incorporate porous absorption internally to simultaneously lower the resonant frequency and broaden the spectrum of absorption.
Porous absorbers are most commonly made from fiberglass, mineral wool or open cell foam that resists the passage of air molecules through the interstitial space. Porous absorbers often incorporate a foil or paper facing to reflect frequencies above 500 Hz. Facing also improves low bass absorption by translating the physical compression of air at the facing into physical compression of the fibers that are in contact with the facing while also maintaining the resistive loss of air as it is driven through the bulk of the fiber by the facing.
Design concepts for building bass traps
Resonating bass traps
Resonating bass traps will absorb sound with high efficiency at their fundamental frequency of resonance. As such, a knowledge of the frequencies of resonances which require damping is helpful before designing and constructing a resonating bass trap. This can be attained by calculation of the room's modes or by direct measurement of the room itself.
Resonating absorbers can be broadened in the frequency range of efficacy to some degree by either introducing porous absorptive material to the interior of the vessel, by constraining the vibrations of the panel or membrane, or by installing an array of resonating devices each tuned to adjacent frequency ranges so that collectively the array functions over a broadened range of sounds. Such devices can be enormously effective over their tuned range, but can take up a great deal of space, especially when installed in arrays, and thus are sometimes not a practical solution.
Panel absorber
A simple panel resonator can be built to hang on a wall by building a wooden frame, adding several inches of mineral wool to the inside, and fitting a sheet of plywood over the top attached only at the edges. A small gap should be left between the panel and the acoustic insulation so that the panel is free to resonate. Panel resonance can be enhanced by reducing the point of connection between the panel and the frame by means of narrow spacer material such as a loop of wire or welding rod run along the edge of the frame so that the panel is perched on a thin edge. Approximate full sheet plywood panel resonances when mounted on a 1"×4" frame 3.5" deep are:
- 1/8" plywood = 150 Hz
- 1/4" plywood = 110 Hz
- 3/8" plywood = 87 Hz
Helmholtz resonator
Other common resonating bass traps are forms of the Helmholtz resonator—such as either a stiff walled box with a hole in one side , or a series of slats over-mounted across the face as a stiff-walled box forming narrow openings in the cracks between the slat members.
Porous absorber bass traps
A bass trap generally comprises a core absorbent damp material, a frame, and a covering for aesthetic reasons.
- Core: Semi-rigid glass-wool or mineral-wool insulation boards or dense open-cell foam are typically used.
- Frame: A steel exterior frame is preferred, although wood skeleton frames are common. The frame acts as a housing, allowing mounting to walls and ceilings, and also anchors the covering material.
- Covering: Porous fabric (similar to speaker grill cloth) is usually used.
Positioning
Since low frequency resonances in a room have their points of maximum or minimum pressure in the corners of the room, resonant bass traps mounted in these positions will be the most efficient, while porous traps are most efficient at the points of high particle velocity such as 1/4 the desired wavelength away from the wall. Bass traps are typically used to attenuate modal resonances and so exact placement depends on which room mode one is trying to target. Bass traps typically combine structural mechanisms that can work at both positions of high particle velocity/low pressure (thick fiberglass) and high pressure/low particle velocity (membranes).
Porous bass trap absorbers need to be very thick to be effective at lower frequencies so they tend to be allocated either as diagonal wedges in the corners or as thick rectangular bulk behind false walls where they are out of the way and less likely to disrupt higher frequencies or room function. Air gap behind a porous panel absorber e.g. straddling a corner also helps to ensure it protrudes more into the room where there is more air velocity, improving its velocity-based absorption and extending its bandwidth while inducing some ripple in its absorption spectrum. Resonant bass trap absorbers need to be at a pressure maximum and tend to be thinner, so they are more conveniently and effectively positioned flat against a wall in a corner where the pressure is maximum, rather than straddling a corner where there is more velocity.
Standard practice is to investigate the applicability of porous bass trap absorbers before investigating resonant or hybridized bass trap absorbers. Complementary methods to use in combination with porous absorption include drywall/stud construction of walls/ceiling and filled with insulation as a form of highly damped resonant bass trapping using the drywall itself as the membrane. The combination of inherently lossy resonant room boundaries and paper or foil faced porous bass trap absorption deep in the tricorners (where three room boundaries meet) is often sufficient to attain acceptable bass response even in listening rooms with somewhat problematic resonances.
Adjusting the listening position within the room boundaries and elevating the seating with a riser that is filled with porous absorption is one more method that can improve bass response without resorting to resonant bass trapping, while simultaneously improving home theater screen visibility.
Small listening rooms suffer a paucity of low frequency resonances with gaps between them, so another complementary method is adding a subwoofer to drive the room resonances from an optimal physical location that minimizes ripple in the frequency response at the listening position. Yet another complementary method is splitting an existing subwoofer allocation up into multiple smaller subwoofers in spatially separated locations that increase the degrees of freedom available to tune the response with. Multiple subwoofers also tend to smooth the bass response across a larger listening area and are often easier to place for good bass response than a single larger subwoofer (or no subwoofer).
If the response is somewhat uniform across all listening positions using these methods, equalization can be used to shape the bass response to the desired target and further smooth out any remaining ripple.
Some combination of porous absorption with these complementary methods is typically preferred for their simplicity, affordability, and convenience, but resonant bass traps are more effective for absorbing strong room resonances where the aforementioned complementary methods are inadequate or impractical, particularly when the geometry of the room causes problematic narrow-band resonances that affect the low bass and the composition of the room boundaries is highly reflective rather than acoustically lossy.
Further reading
- Everest, F. Alton. The Master Handbook of Acoustics, McGraw-Hill, 2000 (ISBN 0-07-136097-2).
- Kinsler, Frey, Coppens and Sanders, Fundamentals of Acoustics, Third Edition, John Wiley & Sons, 1999 (ISBN 978-0471847892), Section 10.8: “The Helmholtz Resonator”.
- https://www.soundonsound.com/sos/may06/articles/qa0506_3.htm
References
- White, Paul (August 1998). "Practical Acoustic Treatment: Part 2". Sound on Sound. Archived from the original on 2004-08-15.
- "Introducing professional acoustic panels". namnak.
- "Room For Improvement - Sound On Sound".
- "The Real Reason You Place Bass Traps In Corners (It's Not Why You Think) — Acoustics Insider".