Introduction:
This month, we have invited a guest writer for our column on acoustics. I would like to welcome Ethan Winer who has been a professional musician and audio engineer for over 30 years. He is now the president of RealTraps in New Milford, CT. RealTraps is a manufacturer of high-performance acoustic treatments and bass traps. Over the last few years, I have gotten to know Ethan. He is knowledgeable and professional but most importantly, interested in educating people about acoustics. He has written for many audio engineering magazines and can be regularly found on the web, spending countless unselfish hours to educate others. Ethan was kind enough to write this column on bass traps, their different types, how they work and when to use them. Enjoy - and if you have the chance, please visit Ethan's site, or better yet, meet him in person at the HE 2004 show in New York this May.

Richard Rives Bird, Rives Audio, Inc.


Bass Traps: Not Just for Fishermen!
By Ethan Winer
If you ask most audiophiles to describe the main acoustic problem in their listening rooms, they'll probably tell you about too much ambience and echoes. Or perhaps they'll report that stereo imaging is poor, most likely due to early reflections off the side walls and ceilings. Indeed, everyone "knows" that to test the acoustics of a room, you simply walk around and clap your hands while listening for reverb and echoes. But to my way of thinking, a far more important problem occurs at low frequencies which you'll never identify with hand claps. Let's first take a step back and consider the bigger picture.


All room acoustic problems are caused by reflections from the walls, floor and ceiling. In a large room like an auditorium or gymnasium, the large distances between walls -- and between the floor and ceiling -- create very long reverb times. So if you clap your hands in a large space, it can take many seconds for the sound to completely fade away. These rooms also have many reflective surfaces, hence sound waves can bounce back and forth quite a few times before they fade away. However, small rooms like you find in a house have very different properties. In a normal living room or home theater, the room boundaries are too close together to create true reverb. Rather, such rooms are dominated mainly by echoes at mid and high frequencies, and modal (dimensionally related) resonances at low frequencies. Plus, the couches, bookshelves and other furnishings tend to absorb and diffuse a lot of the reflected sound so it doesn't linger.


Echoes in a small room often arrive too quickly to be perceived as distinct echoes but they're still echoes more than they are reverb for a variety of technical reasons. More to the point, audible echoes and ambience are mainly an issue at mid and high frequencies. They are also easily tamed with thin materials like rigid fiberglass and acoustic foam panels. Even something as simple as carpet, throw rugs or heavy drapes can make meaningful improvements to a room's acoustic properties at the mid and high frequencies.


How Low Can You Go?
Much more difficult to control -- and far less obvious to the untrained ear -- are problems caused by reflections at low frequencies, which for these purposes we shall consider to be below 300Hz. Low frequency reflections are responsible for two main problems: One is modal ringing which is a result of the room's natural resonances. For a given spacing between a pair of parallel walls, there will be a series of low frequencies that tend to sound louder and sustain longer than others. This ringing makes music sound boomy. It also makes it difficult to distinguish which notes are being played by bass instruments. You can hear that low frequencies are present - but you cannot discern individual notes. This effect is sometimes referred to as "one-note" bass because all of the notes sound the same - muddy!


Another even more important problem caused by low frequency reflections is the severely skewed low frequency response that exists in all small rooms. No matter how much you paid for your loudspeakers -- and regardless of their published frequency response -- as soon as you put them in a typical home-sized room, they will exhibit a series of peaks and deep nulls that extends throughout the entire bass range. The figure below shows the terrible but very typical response in a 16' x 10' x 7.5' foot room without acoustic treatment.
This horribly skewed low frequency response is the result of bass waves reflecting off the walls, floor, and ceiling, then combining with the direct sound waves still leaving the loudspeakers. At some frequencies and locations in the room, the reflections combine with the direct waves more or less in phase to create a peak in the response. At other frequencies and locations, the waves may be out of phase to cause a null. Peaks are usually less than 6 dB but nulls can be extremely severe depending mainly on how rigid and thus reflective the room surfaces are. As you can see, nulls can easily reach 30 dB suckouts or more, especially in the middle and upper bass range above around 70 or 80Hz. At very low frequencies, sound waves tend to pass right through most walls, which is why you hear mainly thumping in an adjacent room. Standard sheet rock walls also tend to absorb very low frequencies by vibrating in sympathy. However, the higher bass frequencies are reflected instead of being passed and absorbed. This is the cause for both modal ringing and a skewed low frequency response in the mid to high bass range.



I consider nulls to be more damaging than peaks because they can completely obliterate certain bass notes. In the above graph, the very large dip at 82Hz corresponds exactly with the E tone on a bass instrument. If an electric bass is playing that note, you will hear no fundamental at all, merely the second harmonic an octave above. However, the second harmonic at 164 Hz too resides in the middle of another deep null. To my way of thinking, the complete and sudden absence of specific bass notes is a serious problem and far more significant than a boost of a few dB here and there that merely adds a little fullness. Further, most rooms have a large dip somewhere between 80 and 120Hz right at the listening position. Many people first notice this as a severe lack of bass that equalizes a bit when they walk away from the couch.


Okay, now what?
Now that we've identified the major acoustic problems at low frequencies, let's look at the best ways to solve them. One thing that can help a little is experimenting with different speaker placements. Since all low-frequency problems are caused by reflections, changing the distance between the speakers and the walls (and floor) can affect the peak and null frequencies. Likewise, changing where you sit affects these frequencies too. You certainly won't be able to flatten the response completely by placement alone, but you may be able to shift the peak and null frequencies up or down to move them to a less objectionable range. However, this won't do much to reduce the muddiness caused by modal ringing. Still, it's at least worth a try as a first step.


The ideal passive solution for low frequency problems in all small rooms is bass traps. These are mechanical devices that absorb bass. There are several different types. I'll describe some of the common designs and explain the pros and cons of each. But first allow me to debunk a common myth about bass traps. Often someone will ask for my advice about a lack of bass in their system. But when I suggest they need bass traps, I often hear "I don't want to trap the bass because I already have too little. I need more bass!"


In truth, adding bass traps to a room generally increases the perceived level of bass. The damaging nulls are caused by reflections from room boundaries while bass traps are designed to absorb rather than reflect. When reflections are reduced, low frequency response becomes flatter. At locations where certain bass frequencies were cancelled, adding bass traps increases their level. Of course, other places in the room experienced peaks in the response. In that case, bass traps do reduce the level. But remember that peaks tend to be lower in amplitude than suckouts. The real point is that bass traps tend to flatten the response by increasing and decreasing the level of various frequencies.


Types of bass trap
There are three common types of bass traps with a few variations within each category. One popular type is the Helmholtz resonator, named for Herman von Helmholtz (1821-1894) who was a leading scientist of his era. The principle is very simple and based on a large box with a small opening that resonates at a particular frequency. Think of a soda bottle that sounds a tone when you blow across its mouth. That's the general idea. Any sound waves at the trap's resonant frequency cause the air inside to move back and forth. Since it takes energy to move air, the excess energy at that particular frequency is removed from the room. That's an oversimplification to be sure but close enough for our purposes. Helmholtz resonators can be designed for narrow bandwidth (to remove one problem frequency) or broad band applications to operate over a larger range. However, these are necessarily rigid and heavy structures. They are not easily built and usually take up considerable space.

(The wood panel bass traps to the right are in my home recording studio and built directly onto the walls in the front of the room. Additional traps, not visible in this photo, are installed on the side and rear walls.) Another more popular type of resonant bass trap is the membrane absorber, often called a panel trap because it uses a front panel made of thin plywood or a similar material. Panel traps also employ fiberglass to broaden their absorptive range. When designed well, they are effective over a range of about one octave. This is still not ideal but broader than a narrow-band Helmholtz absorbers. When panel traps are used to treat a room, two different types are usually built - one with a center frequency of ca. 100Hz, another centered at around 200Hz. This yields an acceptable amount of absorption in the important bass range of 80 to 300Hz. Although not as cumbersome and difficult to build as Helmholtz resonators, panel traps are still large and bulky, relatively expensive and require a fair amount of labor to build.

In my opinion, the best type of bass trap for most rooms is a broadband absorber based on rigid fiberglass. Rigid fiberglass is similar to the fluffy type used for home insulation but compressed to about one quarter its original thickness and formed into panels 1 to 4 inches thick. A 3" thick panel of rigid fiberglass is therefore about as absorbent as uncompressed fiberglass one full foot thick. The overwhelming advantage of rigid fiberglass traps is that they're not based on resonance. They operate over a much wider range of frequencies. Indeed, rigid fiberglass panels are commonly used to absorb mid and high frequencies as well. Even higher performance can be achieved when rigid fiberglass is combined with a membrane. Such a hybrid design can achieve very high absorption over a wide range of frequencies and often be operational down to 80Hz and even lower.


It's difficult to get substantial absorption below ca. 80Hz from using fiberglass-based traps. Fortunately and as explained earlier, most rooms don't need much absorption below 80Hz because sheet-rock walls pass and absorb those very low frequencies. Unless you have walls made of cement or block or use two layers of sheet-rock for increased isolation between rooms, fiberglass bass traps are an ideal choice. Further, a parametric equalizer can be used to minimize resonance or modal problems at the low frequencies. You should always try to minimize bass problems passively first, using a combination of speaker and listener position adjustments and appropriate bass trap augmentation before using a parametric equalizer. However, there may be practical limitations in the room whereby you cannot install as many bass traps as you would like or you may not be able to move the speakers into their optimal position. In cases like these, parametric equalizers are ideal.

The example to the left shows commercial bass traps that combine rigid fiberglass with a membrane for enhanced absorption at low frequencies. This particular design is effective to about 50Hz. Absorption panels meant to tame mid and high frequencies are generally fairly thin and can be placed flat on walls and ceiling to intrude very little. However, bass traps by definition need to be fairly large or at least thick and are most effective when placed in or near the room corners. You can definitely have too much absorption at mid and high frequencies to result in a room that sounds lifeless and sterile. But it's probably not possible to have too much absorption at low frequencies. As more bass traps are added, the low end continues to become tighter-sounding and the response becomes more linear.


Conclusion
It amazes me to see listening rooms or home theaters filled with the most expensive gear
available, yet there's not a single bass trap or any other acoustic treatment in sight. There is no question that acoustic treatment and especially bass traps are more important to achieving truly high fidelity than pretty much anything else. It's a real ear-opener when, for the first time, you hear music reproduced with the same detail, fullness and low-end clarity the original engineers heard when they produced the recording. Indeed, it is truly awesome to be able to hear each individual note the bass player elicits form his instrument, with all notes sounding equally full and powerful.


For more information on Ethan and Real Traps please visit his site.
For more information on Rives Audio, please visit their site.

Note: Both Rives Audio and RealTraps will be at the HE 2004 show in New York City, May 20-23rd. Please drop by and meet the folks behind both companies.


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