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Bass Trap Myths
by Ethan Winer
This is an expanded version of my article in the August 2016 issue of audioXpress magazine.
I've been writing about bass traps and acoustic treatment for more than 20 years. I had hoped by now most people would know what bass traps do and how they work, but I still see several myths repeated online and in print that need to be disproved. First, understand that all acoustic problems are caused by reflections from the walls, floor, and ceiling. These problems include response peaks and nulls, excess reverb and ambience, comb filtering, individual "early" and "late" reflections, and modal ringing at bass frequencies. When the reflections are removed the problems go away. Some of these acoustic problems are similar, such as comb filtering which is a series of peaks and nulls at related frequencies. Likewise, modal ringing, or room resonance, is similar to reverb in that sound waves linger after the sound source stops, but it occurs only at individual low frequencies. Room modes are resonant frequencies related to the room dimensions, and modal ringing is one important acoustic problem that bass traps improve.
Bass Trap Basics: Reflections are the root of all evil
Whether an indoor sound source is a person speaking or singing, or a musical instrument or loudspeaker, our ears receive a combination of the direct sound from the source and its reflections off the walls, floor, and ceiling. Note that reflections are heard as discrete echoes only when they arrive much later than the original sound. In rooms substantially smaller than gymnasiums or auditoriums, reflections change the tonal balance rather than create "hello, hello" type echoes or reverberation.
Bass traps are specialized acoustic absorbers you put in a room to reduce peaks, nulls, and ringing at frequencies below around 300 Hz. When placed on the walls and ceiling, bass traps absorb sound rather than let it be reflected back into the room. There are two types of bass traps: tuned traps absorb a narrow range of frequencies, whereas broadband traps aim to absorb all bass frequencies. Depending on their design, broadband bass traps may absorb less at mid and high frequencies.
In practice, broadband absorbers are effective down to a given low "crossover" frequency that depends on their thickness. The blue line in Figure 1 shows absorption versus frequency for a typical broadband absorber two inches thick. Above 400 Hz it absorbs fully, tapering off at lower frequencies. Better broadband bass traps absorb as much as possible at low frequencies, but intentionally less at mid and high frequencies as shown in red. This lets you put enough of them in a room to truly fix the bass problems, without making the room dead sounding. This is a very important feature that distinguishes "good" bass traps from typical acoustic foam and mineral wool. Finally, the green line shows the amount of absorption from a tuned bass trap that targets 80 Hz. Which leads us to the first myth:
Myth #1: Tuned bass traps are best because they can target lower frequencies than broadband traps.
It's true that broadband bass traps are less effective below around 80 Hz, and tuned traps can be highly effective to lower frequencies with less depth. But audio rooms need substantial absorption at all low frequencies. Once you have enough tuned traps to sufficiently tame the room modes, there's not enough space for broadband traps that are also needed to absorb the remaining low frequencies. Further, rooms have three primary modes, one each for length, width, and height. So that's three frequencies to absorb. But that's just for the fundamental modes. There are also higher-order modes at multiples of each fundamental frequency. So if the main length mode is 30 Hz (corresponding to 19 feet), there are also modes at 60 Hz, 90 Hz, and 120 Hz. Now we have four frequencies to deal with just for the length dimension!
Another problem with tuned bass traps is they're more difficult to design and build, especially for a room under construction whose response can't yet be measured. Software such as my Graphical Mode Calculator can predict mode frequencies from dimensions and suggest good room proportions, but these calculations can be off by 5 percent or even more due to differences in construction. The formula that calculates mode frequencies based on dimensions assumes the room is a perfect rectangle. It further assumes that all six boundaries are infinitely rigid and thus reflect 100 percent of the sound. But most rooms are built with drywall and wood floors that flex slightly and thus absorb, and also have windows and doors that further confound mode predictions. When the room construction absorbs some of the bass, the resonant frequencies are lowered. So building a trap tuned to 60 Hz will be less efficient if the resonance ends up at 56 Hz once the room is completed.
Therefore, broadband bass traps are the most useful and common type for recording studios and hi-fi listening rooms. Peaks and nulls can occur at all frequencies, not only frequencies related to the room's dimensions. Even though loudspeakers generally face away from a wall, they become omnidirectional at low frequencies. Therefore, a loudspeaker 34 inches away from a wall has less output at 100 Hz because 1/4 wavelength (90 degrees) for 100 Hz is 34 inches. Then one octave higher at 200 Hz that same distance creates a peak as large as 6 dB. The distance and frequency are related because it takes time for the sound to reach the wall and return. As you can see in Figure 2, a distance of 1/4 wavelength is the same as 90 degrees of phase shift. The sound travels 90 degrees to the wall, and another 90 degrees back, arriving 180 degrees out of phase. This causes the waves to cancel and create a null. At 200 Hz the distance is 1/2 wavelength so the round trip is 360 degrees (in phase again) which creates a peak. Additional peaks and nulls occur at related frequencies with another null at 300 Hz, another peak at 400 Hz, and so forth at 100 Hz multiples. Nearby frequencies are also affected, though by a lesser amount.
This phenomenon is called Speaker / Boundary Interference Response (SBIR) and is explained more fully in the article describing my Frequency/Distance Calculator, and also the article Bass Waves in the Control Room by studio designer Wes Lachot. Again, SBIR peaks and nulls are due entirely to distance, and have nothing to do with room dimensions and modes. Since loudspeakers are often moved for various reasons, trying to reduce SBIR using a bass trap tuned to a specific frequency would be pointless. Further, SBIR occurs with all sound sources positioned near a boundary, not just loudspeakers. It also occurs at predictable distances from sound receivers (your ears or a microphone). This is sometimes called "LBIR," for Listener / Boundary Interference Response. Depending on how many sound sources and receivers are in the room, and how far each is from every room surface, there are likely dozens of individual problem frequencies.
The rear wall behind the listeners is another place broadband bass trapping is needed. If the seating is closer than about ten feet from that wall the reflections will be early, and will create peaks and nulls related to that distance. So absorption on that wall is always useful. Let's say a listener's head is six feet in front of the wall. That yields a deep null at 47 Hz, a peak up to 6 dB at 94 Hz, another deep null at 141 Hz, another peak at 188 Hz, and so forth at 47 Hz multiples. Now, if you move the couch back one foot closer to the wall the frequencies instead start at 56.5 Hz, and continue at 56.5 Hz multiples. Since people sometimes like to move their furniture, this again shows why broadband bass trapping (and broadband absorption generally) is best for most rooms.
Myth #2: Using two or more subwoofers reduces or even eliminates the need for bass traps.
There's no question that using two or more subwoofers (subs) can improve the low frequency response in a room. When placed properly they not only improve both peaks and nulls, they also reduce response variations around the room. So I never argue that using multiple subs is a bad idea. But multiple subs can't replace bass traps for several reasons:
First, subs generally operate below 80 Hz, which is the standard crossover frequency as shown in Figure 3. But that's only half the bass range! The other half between 80 and 300 Hz is arguably more important because it's the "speaking" range for bass instruments where clarity and articulation are needed most. (Some people use even lower crossover frequencies, and those subs handle an even smaller portion of the bass range.) So while using more than one sub can indeed help to flatten the response below 80 Hz, it does nothing for bass instrument clarity, only fullness. But even frequencies as high as 150-200 Hz are perceived mainly as fullness, and nothing you do with subwoofers can improve problems there either.
It's also a myth that multiple subs can reduce modal ringing as much as bass traps. As explained earlier, modal ringing is similar to reverb in that certain notes continue after the source sound stops. Modal ringing is often shown using a waterfall plot as in Figure 5. In this type of graph the response peak "mountains" come forward as they decay in volume over time. Multi-sub proponents claim that placing subs away from peak locations in a room avoids "energizing the modes" and thus avoids ringing. But unless a sub is placed in a null location for a mode frequency - which then gives so little output it's a useless location - frequencies that align with that mode will still ring. You can reduce the level of peaks, which is good, but not the time components. Further, as acoustician Nyal Mellor points out, "If all your subs are on the floor then there is no height mode cancellation going on, so you will still see height mode related peaks in the frequency response." I'll add that height-mode nulls and ringing will also remain.
While we're talking about room modes and extended decay times at select frequencies, here's an interesting fact that many people don't realize: The same resonance that extends decay times also extends rise times the same amount. The top trace in Figure 4 shows a 100 Hz sine wave sound source, and the lower wave shows the result you'll hear in a room having a mode at 100 Hz. The source starts suddenly, but in the room it grows over time, typically some fraction of a second. Then when the source stops, that frequency continues fading out over the same amount of time. So when people talk about having "fast bass," they might be referring to a room or loudspeaker that has minimal resonance.
Myth #3: Using an equalizer (EQ) with your subwoofer reduces or even eliminates the need for bass traps.
Figure 5: Peaks, nulls, and ringing are just as damaging above 80 Hz as they are below 80 Hz.
Most equalizer proponents agree that EQ works best at subwoofer frequencies, and it's best not to change the response at higher bass frequencies. However, some believe that EQ is useful up to 300 Hz or even higher. But that's just not practical because at those frequencies the peaks and nulls are highly localized. Peaks at the lowest fundamental mode frequencies exist around a room, but above the first-order (fundamental) modes the response varies greatly over very short distances. The article A common-sense explanation of audiophile beliefs shows how much the response varies at two locations only four inches apart. EQ can't even achieve the same response at both of your ears! Versus bass traps that improve the response and ringing at all locations in a room. So as with Myth #2, the problem again is that subwoofers work on only half the bass range below 80 Hz, or occasionally 100 Hz.
Further, EQ is useful only for reducing peaks, not raising nulls. Nulls are not only very deep, but even more localized than peaks. So trying to counter a null that's 20 dB deep, which is not unusual, will make that frequency much too loud elsewhere in the room. That much boost will also make your sub work harder increasing distortion. Compared to peaks, nulls affect a very narrow range of frequencies, so to counter that with EQ requires an equally narrow (high Q) boost. But applying a high-Q boost with an equalizer actually adds ringing at that frequency!
To be fair, in rooms that are square or cube shaped, low frequency modal peaks are often severe, so reducing those peaks with EQ can indeed help. For my 2007 Audyssey Report I tested this room correction system in a virtually cube shaped room 16 by 15 by 8 feet high. In my conclusion I noted, "In a nearly square room like Kal's, reducing the large modal peak via EQ removed the boominess that was apparent in all of the music tracks we auditioned. Improving the bass also increased clarity in the low midrange by contrast, since the low mids were no longer masked by the excess bass."
However, it's also a myth that EQ reduces ringing as much as bass traps do. The argument is that equalizers are "minimum phase" and so are rooms. But this is only partly true for most rooms. In theory, one minimum phase device can counter ringing in another, but only if both parameters are precisely opposite. My AES Audio Myths video at 55:43 shows two EQ plug-ins countering each other completely. One EQ is set to cut three frequencies, and the other boosts the same frequencies by the same amount with the same Q. But EQ plug-ins are precision digital devices, not rooms. I've seen EQ reduce modal ringing a little, but not nearly as much as bass traps, nor all around the room as do bass traps. Indeed, if proponents claim EQ reduces ringing, it must do so everywhere in the room, not just the one place where it was calibrated. This is another problem with the "minimum phase" argument: As soon as your ears are even a few inches away from where the microphone was placed while adjusting the equalizer, the critical balance required to cancel ringing is lost.
Finally, EQ is useful only for mode frequencies. EQ can't counter peaks and nulls due to SBIR or LBIR (listener position) because those are caused by simple wave interference, not resonance, and thus are not minimum phase. In fairness, EQ could be useful in Figure 5 to tame the ringing peak at 50 Hz. As mentioned above, peaks tend to occupy a larger physical area than nulls, especially at the lowest frequencies. So reducing a peak with EQ is less likely to make the response worse elsewhere in the room as happens when equalizing nulls, or peaks at higher frequencies. I have a killer SVS subwoofer in my living room home theater that includes a one-band cut-only parametric EQ. I use that EQ to reduce a modal peak at 44 Hz by 3 dB. The EQ doesn't reduce the ringing but, as acoustician Floyd Toole notes, what we hear most is the raw response. Ringing is a secondary effect, so merely reducing a peak to flatness is a big improvement. As mentioned earlier, broadband bass traps absorb less below 80 Hz unless they're very thick. So anything you can do with subwoofers is useful. However, good commercial bass traps are effective down to 40 Hz and even lower if you have enough of them. Which leads us to Myth #4:
Myth #4: Bass traps are ineffective below 100 Hz.
Figure 6: High quality bass traps are effective down to 40 Hz or even lower if you have enough of them.
This myth may have started because most performance data published for absorber products goes no lower than 100 Hz. This is due to limitations of the acoustic labs where such products are tested, but people see that and assume bass traps don't work at all below 100 Hz. In fact, good broadband bass traps are effective down to 40 Hz and even lower. It's true their absorption is less than above 80-100 Hz, but you make up for that by having more of them. As a bonus that improves higher bass frequencies even more. In my company's video Hearing is Believing we measured the response and ringing with and without a full array of acoustic treatment in a room 16 by 11.5 by 8 feet high. Figure 6 shows the response and ringing on the same graph before (red) and after (blue) adding bass traps. As you can see, the peak at 40 Hz was reduced about 4 dB, and the ringing was reduced substantially. At higher bass frequencies the response and ringing were improved even more.
Myth #5: Bass traps are inefficient because they remove important energy from the room requiring much more amplifier power to compensate.
Some people believe that adding bass traps to a room removes energy making low frequencies softer and less impactful, which in turn requires more power from the amplifiers and loudspeakers. This is totally the wrong way to consider what bass traps do! It's true that all absorbers remove acoustic "energy" because they work by converting sound waves into heat. But they absorb only the reflected energy, not the main sound coming out of the loudspeakers. When you place an absorber on a wall or ceiling, or mount a bass trap straddling a corner, all of the direct sound from the speakers still reaches your ears unchanged.
In most home-size rooms the low frequency response is riddled with deep nulls as shown in Figure 7. The horizontal gray line approximates where "flat" is. One of the most common problems in home recording studios is mixes that seem great in your room sound boomy and bassy elsewhere. When too little bass reaches your ears due to deep nulls, the tendency is to crank the bass in the mix to compensate for what you hear. But then there's too much bass and you don't realize it.
Bass traps reduce the strength of reflections that create these nulls, raising the volume at those frequencies. So adding bass traps to a room usually gives the perception of more bass, not less. Now, in some rooms the peaks are more prominent than the nulls, especially rooms that are square or "virtually square" such as 10 by 20 feet, or nearly cube shaped like the 16 by 15 by 8 foot room mentioned earlier where the Audyssey EQ was tested. In rooms like these, multiple resonances at the same frequency combine to create peaks larger than 6 dB. The room measured for Figures 6 and 7 is 16 feet long and 8 feet high. So those dimensions create double resonances at 70 Hz and its multiples (140, 270, etc). But square rooms like these have deep nulls too, so bass traps still restore at least as much energy as they remove.
Figure 8: Adding bass traps restores more energy lost due to nulls than energy removed from peaks.
Figure 8 shows the response of the same room before and after adding bass traps. After adding bass traps the peaks were reduced and the nulls were raised. If you consider the shaded areas above and below the flatter blue After line, more energy was restored to nulls than was removed from peaks, especially below 200 Hz. Most peaks are around 6 to 10 dB, but nulls can be very deep. So raising a null that's 20 dB down gives a huge increase in the amount of bass that's heard. Further, as explained above, peaks, nulls, and ringing are all caused by reflections, so using bass traps to reduce the strength of reflections is a Good Thing. Any energy that's removed is stuff you want removed! Just like the improvement from absorption that reduces unwanted echoes and ambience at higher frequencies, which also "removes energy" from the room.
So it's clear that adding bass traps usually increases the perceived amount of bass, and that's not some psychoacoustic trick. The actual measured amount of bass also increases. The room measured for Figure 8 was loaded with acoustic treatment. Yet even with nearly complete coverage of every surface, you can see that more energy was restored than removed because the nulls are deeper than the peaks are high. The only time adding bass traps makes the overall bass level softer is with square and cube rooms that have severe coincident peaks as described above. But these rooms are excessively boomy so, again, the energy removed is content that should be removed.
A related myth - a misunderstanding really - is that bass traps reduce bass overall, including the direct sound from the speakers. This question is common from newbies in audio forums, questioning why bass "traps" are recommended when their problem is too little bass. So I was surprised when Gene DellaSala from the otherwise competent Audioholics web site stated in one of his YouTube videos that bass traps remove too much energy from rooms, requiring much more amplifier power. As was proven above that's just wrong. I find Gene's comment especially surprising because in another YouTube video he brags about having two 2 KW power amps (4,000 watts total) to drive his $50,000 loudspeakers. Talk about wasted energy! (And wasted money.) My entire home theater shown in Photo 1 runs off a single AC circuit, and the total power to all five speakers is 600 watts not counting my SVS PB12-Ultra/2 subwoofer. The SVS sub admittedly does the heavy lifting, yet this system plays as loud as a live rock concert without noticeable distortion.
Myth #6: Bass traps, and acoustic panels generally, are ugly and inappropriate even in a dedicated home theater or listening room.
It's impossible to disprove this myth because "ugly" is subjective. But why do many of these same people tolerate huge equipment racks, 6-foot tall loudspeakers, wires as thick as a garden hose, and 140 pound kilowatt power amps resting on enormous isolation platforms in the middle of the floor? I happen to think acoustic treatment looks nice, in a high-tech sort of way. I totally understand that not everyone likes this look! Especially when their audio room is also their living room, or if others in their home object. This is one of the great things about dedicated home theaters and listening rooms. Even if it's just a section in an unfinished basement, you can do whatever you want without criticism from others.
I know for a fact that all of the rooms in the photos above sound totally awesome. In my opinion, I think they look awesome too. But even if you (or your better half) hate the look of acoustic treatment, that doesn't mean you're destined to forever endure poor sound. With acoustic treatment you can have: Effective, Affordable, or Attractive - pick any two. High quality treatment can be disguised as art, hidden behind wall hangings or Navajo rugs etc, or even built into the walls and ceiling and hidden completely behind stretch fabric. But that costs more than hanging panels that are visible. Only you can decide what it's worth both in cost and aesthetics to achieve a given level of sound quality.
Click the images above to see them full size.
After all this talk about what multiple subwoofers and EQ can't do, I hope it's clear that I'm not opposed to using either approach. If you have space - and the budget - for two or more high quality subs, that's awesome. And no matter how many subs you have, EQ can absolutely help to reduce the volume of one or two bothersome peaks at the very lowest frequencies. As mentioned, I use one band of cut-only EQ in my own system and it definitely helps. When set up properly (not trivial), multiple subs will improve the response below 80 Hz, and judicious use of EQ will lower peaks and possibly reduce ringing slightly at the very lowest frequencies. But only bass traps improve the response everywhere in the room, and reduce ringing a significant amount, and operate over the entire range of bass frequencies. Even when a room has "good" dimensions, that merely ensures its modes are spaced more or less evenly. But all rooms resonate and have peaks and deep nulls. So I just want readers to understand that when the goal is state of the art sound, bass traps are still needed. Always. In every room.
Ethan Winer has been an audio engineer and professional musician for more than 45 years, and is co-owner of RealTraps where he designs acoustic treatment products for recording studios and home listening rooms. Ethan's Cello Rondo music video has received more than 1.8 Million views on YouTube and other web sites, and his book The Audio Expert published by Focal Press is available at amazon.com and his own web site.
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