Building a Better Bass Trap Read this before you decide to take the trash out to the curb By Barry Hufker, Scott Shepard Building the TrapCan is an inexpensive way to improve the acoustics of your studio, control room, or listening room. In its basic form, the TrapCan acts as a bass trap, absorbing excessive low-frequency energy. It can double as a polycylindrical diffuser to improve sound diffusion and reduce flutter echoes. Not only is the TrapCan an effective tool, it is also inexpensive and modular. You can build the traps one at a time until you achieve the desired result. With a little ingenuity, eight TrapCans can be built for about $250. Quite simply, the TrapCan is a bass trap built in a rigid rubber trash can. We hit upon the idea recently while trying to solve the acoustical problems of a small local studio. There was really no budget for any corrections but there was a definite need for an effective acoustical solution. Analysis of the studio’s resonant frequencies indicated that energy around 100 Hz was giving the room its “boomy” quality, while parallel walls reinforced other resonances and generated flutter echoes. The room’s 100 Hz “honk” was our first target. It must have been a fit of madness that made us realize a 32-gallon trash can is about the right height for a 100 Hz trap, and is usually cylinder shaped. Instantly the TrapCan was born! After buying a trash can to confirm its range of resonant frequencies (80–100 Hz), we built a prototype. Convinced that we heard a localized improvement with one trap in a room corner, we invited a class of advanced audio students from Webster University to participate in another subjective, unscientific test. The group’s assignment was to listen for any change in room sound based on the trap’s presence or absence. Initial listening tests proved the design successful so we built another seven units. Four of the eight traps were placed in room corners where mode pressures are at their greatest. The remaining four were positioned at a midpoint along each wall. With all the TrapCans in place, we again asked the audio students to evaluate the acoustics. All commented on an obvious improvement. Independent reports from others using the studio also confirmed our success. You can build the TrapCan in a variety of styles to suit different acoustical needs. Construction isn’t difficult, costly, or time-consuming. Finding your room’s resonances is only slightly more complex. Room resonances are more properly known as room “modes.” They are “standing waves” that occur when sound interacts with the room’s boundaries (walls, floor, and ceiling). There are three types of modes: axial, tangential, and oblique. The simplest modes (axial) occur between any two opposing surfaces, such as opposite walls. Once generated, a sound wave travels toward both surfaces, but is prevented from going further when it strikes the boundaries. The sound wave then reflects back along its original path, combining with the waves still heading toward the boundaries. Along the path there are pressure maximums where the mode is strong, and pressure nulls where it is essentially nonexistent. A standing wave forms at a frequency whose half-wavelength is equal to the distance between the two surfaces. Its energy increases over time, taking longer to decay than other frequencies. Multiples of the standing wave frequency also behave this way. As an example, if the standing wave is 50 Hz, then the room also has axial modes of 100 Hz, 150 Hz, 200 Hz, etc. Tangential and oblique modes are even more complex. They stem from the interaction between two or three pairs of boundaries. Fortunately, axial modes, although the most audible, are also the easiest to determine and attack. Note, however, that while parallel walls aid standing waves, modes are not eliminated in rooms with nonparallel walls. The modes are just made more complex. Room modes are not a problem if they are distributed evenly in frequency, and increase smoothly in number with increasing frequency. Unpleasant room colorations are created when two or three of these modes are clustered together around a single frequency, creating a noticeable “resonance,” such as a 100 Hz “boom.” Testing The Studio While computer programs are available for calculating a room’s modes, they tend only to be accurate with rooms having parallel boundaries. Begin by positioning a loudspeaker with good low-frequency response in Corner A. Place a high-quality microphone with a flat frequency response in front of the speaker at a short distance from it. Call this “Microphone A.” A second microphone of the same type as Microphone A should be placed in Corner B, directly opposite the loudspeaker and just a few inches above the floor. Face the microphone toward the corner and call it “Microphone B.” Omnidirectional mics, such as the Bruel & Kjaer 4006, are ideal for this test. Unlike directional microphones, omnidirectionals don’t exhibit a low-frequency boost when placed near a sound source. The 4006 possesses a very linear frequency response, but other microphones will do. Once you have your mics placed, follow these three steps: 1. Send a 1 kHz tone into the audio console using a variable frequency oscillator and adjust for 0 VU on the board’s meter. This tone is the calibration signal. All other results will be compared to it. 2. Turn the loudspeaker’s amplifier on. Send the test tone to the speaker where it will be received by Microphone A. Return the signal from Microphone A to the console through a middle input. Route that signal to a multitrack bus for observation. Adjust this signal on the console until the bus meter reads 0 VU. If you can’t get enough level, increase the output to the speaker but be sure to reset the 1 kHz tone on the console for 0 VU. 3. Now bring the signal from Microphone B into the console. Route it to yet another multitrack bus for observation. Adjust this signal on the console for 0 VU. If you don’t have a loud enough signal, increase the level of the tone being sent to the loudspeaker until you do. If you increase the signal level going to the speaker, repeat steps one and two. Be careful not to overload the amplifier, speaker, microphone, or — most importantly — room frequency response. Now all three signals should be at 0 VU. The meter monitoring the oscillator’s input will reflect any change in level from the oscillator as you sweep through various frequencies. Microphone A’s meter shows the loudspeaker’s frequency response and will indicate any change in level due to frequency. These first two signals are the “control” and will reveal any flaws in the signal source, whether in the oscillator or speaker. If the oscillator or speaker level changes each time you select a new test frequency, start with the oscillator and adjust it again for 0 VU. Then go to the input for Microphone A and do the same. This eliminates any incorrect results due to imperfections in the signal source. With the oscillator and Microphone A signal always at 0 VU, the room’s response will be whatever difference is shown on the bus meter for Microphone B. 4. Being careful not to overdrive your system, begin the test by sending a low frequency (such as 50 Hz) through the loudspeaker. Again, be sure the meters for the oscillator and Microphone A read 0 VU. If they don’t, adjust them until they do. Now look at the bus meter for Microphone B and plot on the graph the change above or below 0 VU. You can continue to do this in whatever fashion you like. For instance, you can now tune to 51 Hz and repeat the procedure or increase the test frequency 5 to 10 Hz at a time. Modes tend to be more separated at low frequencies. They will be easy to miss with large increments in test frequency. It is best to be as precise as possible in choosing test frequencies, but you can overdo it. You will not be able to build a 125.33 Hz trap for instance. In fact, your trap will be resonant over a range of frequencies. Continue testing until you get to 300 Hz —the frequency at which almost all rooms are free of excessive resonant energy. You should now have a good idea of what the troublesome frequencies are. They are the large “peaks” on the graph. In addition to some “hot spots” you probably graphed a few “cold” ones. Just as the resonant frequencies were peaks, the gaps in the room’s response were valleys — unexcited frequencies. This is natural at low frequencies and isn’t a problem when they are distributed evenly though the low frequency spectrum. Problems occur when two or more modes are closely spaced, creating a “superpeak.” It’s the peak room modes you want to smooth out. They give the room its “sound” and color the recordings that are made in it. If the studio has closely grouped low-frequency modes (boominess), then a bass trap is certainly in order. The TrapCan in its standard configuration will help. For troublesome higher frequencies, you can modify the TrapCan to become a tuned “Helmholtz” absorber. Building The TrapCan While you can certainly build the TrapCan in a more sophisticated and costly manner than described here, the following construction details will give you an effective, attractive tool. Remember you’re doing all this for under $250! Here are the step-by-step instructions: 1. By reading the plot of room modes, determine the low frequency (frequencies) you want to control. It is important to keep in mind that it’s going to be tough to control anything much below 100 Hz. 2. Find the quarter-wavelength of the frequency. If there is a resonant band of frequencies, calculate the quarter-wavelength of a frequency in the middle of the band. The formula is: QW–1130/4f ...where QW is the quarter wavelength, 1130 is the speed of sound (at normal room conditions), and f is the frequency you are trying to control. As the equation shows, f should be multiplied by four before dividing it into 1130. For example, the quarter wavelength of 100 Hz is 2.85 feet. To convert decimal inches (i.e., .85), multiply the number by 12. In this example, the answer is approximately ten inches. The TrapCan needs to be at least as tall as the quarter-wavelength to be effective at the desired frequency (100 Hz). You can work the formula in reverse if you already have a can and want to know its resonant frequency. Multiply the can’s height by four and then divide that into 1130 to obtain the frequency. 3. If you want to duplicate the 100 Hz trap we built, buy a rigid rubber trashcan. It is important to get as rigid a can as possible. Be sure to buy the lid! Our can was bright yellow and came from a “wholesale club” warehouse. The cost was about $18 with tax. You can modify the TrapCan by building it in a larger can, but remember, it’s the height of the can that determines the resonant frequency. 4. Buy a paper “surgical mask” at the hardware store so you won’t inhale any fiberglass. You might also want to wear gloves to prevent handling it directly. If it has a paper backing you can leave that on. Place the paper side against the can. One roll of R-13 insulation will make two 32-gallon trap cans. 5. Line the wall. Tear a length of R-13 fiberglass so that it will line the interior walls of the can when the fiberglass is laid on its edge (side). If the can’s height requires it, place another piece along the can’s interior walls atop the previous piece. Adjust the fiberglass’s height so that the can’s sidewall lining is even with the top of the can. Two standard widths of fiberglass were just right for the trash can we used. 6. Tear a patch of fiberglass large enough to cover the can’s interior bottom. Place it so there are no gaps between it and the piece lining the can’s side. 7. Tear two sheets of fiberglass so that each is long enough to extend from the bottom of the can to within three inches of the top when stood vertically. Remove any paper backing. Stand the first sheet upright so that there is an airspace of a couple of inches on one side of the sheet and a larger space on the other. Install the second sheet, leaving a few inches between the first vertical sheet and the side of the can. 8. Tear another sheet large enough to lay across the top of the vertical fiberglass sheet and within the side sheets. 9. For a more finished look, lightly sandpaper the can’s exterior and spray it with paint. A textured paint (available in craft stores) offers the most attractive finish, giving a “stone” or “marble” appearance upon drying. 10. After allowing the paint to dry, cover the can’s top with a loose weave material, such as burlap. Neatly cut enough to tightly cover the top and allow a couple of inches to extend down on all sides. 11. Secure the burlap to the can using a child’s “Chinese jump rope.” Wrap the jump rope around the burlap and the can (at the top) as you would with a large rubber band. Finish trimming the burlap for a neater appearance. 12. Set the finished TrapCan into a corner of the room (where mode pressure is greatest). Put one TrapCan in each corner and experiment with the exact placement for maximum effect. Mark their final location so they can be easily repositioned if moved. The Rest Is Up To You Your imagination and your willingness to experiment are the only limits. There are still a great number of TrapCan variations. You can cut the can in half vertically and use it as a diffuser, or build a HelmHoltz resonator. Glue acoustical foam to one half of the can’s body for an absorbent side while leaving the other side reflective. Glue irregularly shaped wood to the bare can to aid diffusion and increase its visual aesthetics. Rotate the cans to change the amount of diffusion and absorption. -------------------------------------------------------------------------------- The authors thank F. Alton Everest, acoustic musician, author, and educator. His books make acoustics understandable, enjoyable, and have left a lasting impression on us. His creativity in solving acoustical problems was the inspiration leading to the TrapCan. -------------------------------------------------------------------------------- BARRY HUFKER is an assistant professor of Media Communications at Webster University, St. Louis, MO. SCOTT SHEPARD is the president of Bear Communication, St. Louis, MO, a firm specializing in acoustical consultation and sound system design.