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Acoustics Facts and Fiction

by Ethan Winer August 30, 2004

In 1979 TEAC introduced the 144 Portastudio, and the recording industry hasn't been the same since! In those days a decent limiter cost nearly $1,000 and a good spring reverb unit would set you back more than $2,000. We can all be grateful that very high quality audio gear is now available for even the most modest budget. But one important feature still distinguishes a state of the art recording facility from most semi-pro and project studios: real acoustic treatment, especially bass traps.

Properly designed bass traps will transform a muddy sounding room, having poorly defined bass, into one that sounds clear and tight, and is a pleasure to work in. Without effective acoustic treatment, it is difficult to hear what you're doing, making you work much harder to create a good mix. Likewise, adding bass traps also improves larger recording rooms by reducing their reverberation time at low frequencies, and by eliminating standing waves that cause peaks and severe nulls in the room's low frequency response.

What's the point in buying gear that's ruler flat from DC to microwaves when the acoustics in your room create peaks and dips as large as 35 dB throughout the entire bass range? How important really are jitter artifacts 110 dB below the music when standing waves in your studio cause a huge hole at 80 Hz exactly where you placed a microphone for the acoustic bass? Clearly, frequency response errors of this magnitude are an enormous problem, yet most studios and listening rooms suffer from this defect. Worse, many studio owners and audiophiles have no idea their rooms have such a skewed response! Without knowing what your music really sounds like, it is difficult to produce a quality product, and even more difficult to create mixes that sound the same outside your control room.

Before exploring the specifics of bass traps, however, let's consider the larger picture. There are four primary goals of acoustic treatment: 1) To prevent standing waves in control rooms and small studios, 2) to reduce reverb time in larger studios, churches, and auditoriums, 3) to diffuse sound in the room to avoid ringing and flutter echoes, and 4) to keep sound from leaking into or out of a room. That is, stopping your music from disturbing the neighbors, and keeping the sound of passing trucks from getting into your microphones. This article addresses treatment designed to control the sound quality within a room by eliminating standing waves and flutter echoes, and by reducing the reverb time. As opposed to isolation that prevents sound propagation between rooms. Sound transmission and leakage are reduced by isolating the building structures, generally by floating the walls and floors, and hanging the ceilings with shock mounts.

Many studio owners and audiophiles install acoustic foam all over their walls, mistakenly believing that is sufficient. After all, if you clap your hands in a room treated with foam (or fiberglass, blankets, or egg crates), you won't hear any reverb or echoes. But thin treatments do nothing to control low frequency reflections, and hand claps won't reveal that. Basement studios and rooms having walls made of brick or concrete are especially prone to this problem - the more rigid the walls, the more reflective they are at low frequencies. Indeed, simply building a new sheet rock wall a few inches inside an outer cement wall (filling the space with fiberglass) helps to reduce reflections at the lowest frequencies.

Some recording engineers may ask why acoustic treatment is needed at all, since few people listening to their music will be in a treated room. The reason is simple: All rooms sound different, both in their amount of liveness and their frequency response. If you create a mix that sounds good in your room, which has its own particular frequency response, it is likely to sound very different in other rooms. For example, if your room has a severe lack of deep bass, your mixes will probably contain too much bass as you incorrectly compensate based on what you are hearing. And if someone plays your music in a room that has too much deep bass, the error will be exaggerated, and they will hear way too much deep bass. Therefore, the only practical solution is to make your room as accurate as possible so any variation others experience is due solely to the qualities of their room.

Professional Studios Use Real Bass Traps

The most common application of bass traps in recording studios and control rooms is to minimize standing waves. (See the sidebar Why They're Called Standing Waves below.) As you can see in Figure 1, standing waves develop inside a room when sounds bounce off the floor, walls, and ceiling, and collide with each other in the air. Left untreated, this creates severe peaks and dips in the frequency response that change as you move around in the room. At the mix position, there might be near-total cancellation centered at, say, 100 Hz, while near the back of the room 100 Hz is boosted by 2 dB but 70 Hz is partially canceled. Although it seems counter-intuitive, a device that traps low frequencies will in fact increase the amount of bass a room can produce. When the cancellations caused by standing waves are eliminated, or at least reduced, the most noticeable effect is improving the bass level, quality, and clarity from your loudspeakers.


Figure 1: Standing waves in a room combine in the air to create peaks and dips in the frequency response.

Here, a positive wave front from the loudspeaker (left) is reflected off the wall on the right, and the reflection collides with other waves that continue to emanate from the loudspeaker. Depending on the wavelength (frequency) of the tones and how far you are from a room boundary, the reflected waves either add to or subtract from the waves coming from the speaker. Worse, different locations in the room respond differently, with a boost at some frequencies and a reduction at others. The action of sound waves colliding and combining in the air is called acoustic interference. Note that this phenomenon is not limited to only those frequencies related to a room's dimensions - all low frequencies combine in the air to either increase or reduce amplitude, depending on the size of the room and where you're standing or sitting.

For recording engineers, problems caused by standing waves and acoustic interference are often first noticed when you realize your mixes are not "portable" or do not "translate" well. That is, songs you have equalized and balanced to sound good in your control room do not sound the same in other rooms. Of course, variations from different rooms and loudspeakers are a factor too. But bass frequencies are the most difficult to judge when mixing because acoustic interference affects them more than higher frequencies.

Another problem is that the level and tone quality of bass instruments vary as you walk around the room. The sound is thin here, too bassy over there, yet not accurate anywhere. Indeed, even if you own all the latest and most expensive recording gear, your mixes will still suffer if you can't hear what's really happening in the low end. Aside from portability concerns, it's very difficult to get the bass instrument and kick drum balance right when acoustic interference and excess low frequency ringing combine to reduce clarity. And when every location in the room has a different low-end response, there's no way to know how the music really sounds.

Many people wrongly believe that using near-field monitor speakers avoids the need for acoustic treatment. In truth, even with small loudspeakers playing softly, acoustic interference still causes standing waves - the imperfect frequency balance is exactly the same but at a lower level. Although higher frequency reflections and echoes are proportionately reduced as you get closer to the loudspeaker, the change in frequency response caused by low frequency reflections remains more or less the same. Likewise, adding a subwoofer will not fix problems that are due to poor room acoustics. While a subwoofer can compensate for inadequate loudspeakers, it will not solve the problem of an irregular response caused by acoustic interference. In fact, a subwoofer often makes matters worse by compounding and hiding the real problem.

Another common misconception is that equalization can be used to counter the effects of room acoustics. But since every location in the room responds differently, no single EQ curve can give a flat response everywhere. Even if you aim to correct the response only at the listening position, there's a bigger problem: It's impossible to counter very large cancellations. For example, if acoustic interference causes a 15 dB dip at 50 Hz, adding that much boost with an equalizer to compensate will reduce the available volume (headroom) by the same amount. Such an extreme boost will surely increase low frequency distortion in the loudspeakers too. And at other room locations where 50 Hz is already too loud, applying EQ boost will make the problem much worse. (Also, the size of the null zones can be extremely tiny - I've measured changes of 15 dB over a distance of only four inches!) EQ can sometimes help at higher frequencies, but not always. If a room has ringing tones that continue after the sound source stops, EQ might appear to make the ringing a little softer but it will still be present.

Yet another common misconception is that small rooms cannot reproduce very low frequencies, so they're not worth treating at all. A popular (but incorrect) theory is that very low frequencies require a certain minimum room dimension to "develop," and so cannot be present at all in smaller rooms. The truth is any room can reproduce very low frequencies, as long as the reflections that cause acoustic cancellations are avoided. When you add bass trapping, you are making the walls less reflective at low frequencies, so sound that hits a wall or ceiling will be absorbed instead of reflected. The net result is exactly the same as if the wall was not there at all - or as if the wall was very far away so reflections are attenuated by the distance and thus too soft to cancel as much. See the sidebar Big Waves, Small Rooms for more on this.

Some people mix using headphones in an attempt to avoid the effects of their room. The problem with headphones is that everything sounds too clear and present, making it difficult to find the ideal volume for some tracks. When listening through headphones, a lead vocal or solo instrument can be heard very clearly, even if it is quiet, so you'll tend to make it lower in the mix than it should be. Listening with headphones also misses the physical impact of bass instruments, the part you feel rather than hear.

Note that standing waves and acoustic interference also occur at higher frequencies, such as sustained clarinet or flute tones. You can hear the effect and identify the problem frequencies and locations fairly easily by playing sine waves (not too loudly!) through your loudspeakers. This is also a good way to assess how important bass traps are for your particular studio and control rooms. If you have SoundForge, WaveLab, or a similar audio editor program, it's simple to create sine wave files at different low frequencies for testing. Special CDs that contain various tones and pink noise suitable for room testing and analysis are also commonly available. To determine the severity of low frequency problems, play different sine waves one at a time through your monitors and slowly walk around the room. It will be very obvious at which frequencies the peaks and valleys occur, and where they cause the most harm. There's no point in playing frequencies below what your speakers can produce cleanly - we suggest 50 Hz, 80 Hz, 100 Hz, and so forth through 300 Hz. If you have a computer connected to your loudspeakers, you can download the NTI Minirator program which generates a variety of useful audio test signals.

Generally speaking, most rooms need as many bass traps as you can fit and afford. Although it is definitely possible to make a room too dead at midrange and high frequencies, you probably can't have too much low frequency absorption. The effectiveness of bass traps is directly related to how much of the room's total surface area you treat, which includes the walls, floor, and ceiling. That is, covering thirty percent of a room's surfaces with bass traps reduces low frequency reflections far more than covering only five percent. It would be great to invent a magical acoustic vacuum cleaner that sucks the bass waves out from the air. But, alas, the laws of physics do not work that way. At the minimum I recommend placing bass traps in all of the corners. For even better results, put additional traps in the ceiling corners as well as the regular wall to wall corners.

Al Fine

Although this article focused on acoustic treatment and bass traps for recording studios, the same principles apply to audiophile listening rooms and home theaters. To my way of thinking, the main difference between a recording studio control room and a home theater or home listening room is the amount of treatment, not the type. For example, home theaters are often intentionally less live sounding because reflections from the additional loudspeakers must also be absorbed.

Acoustic treatment, and especially bass traps, are fundamental to music reproduction in any room, and should be considered an integral component rather than an optional accessory. Indeed, no matter what loudspeakers you own or how much they cost, you'll never realize their full potential until you also address the contribution of the room's acoustics.

Sidebar: Why They're Called 'Standing Waves'

If you've ever used an ultrasonic cleaner to clean jewelry or small electronic components, you've probably seen standing waves in action. When you drop a pebble into a pond, a series of waves is created that extends outward from the point of impact. Since a pond is large, the waves dissipate before they reach the shore and are reflected back to the place of origin. But in a contained area like the tub of an ultrasonic cleaner, the waves bounce off the surrounding walls and create a pressure front that makes them "stand up" and remain static within the cleaning solution.

The exact same thing happens in the control room when your loudspeakers play a sustained bass tone. Static nodes develop at different places in the room depending on the loudspeaker and listener position, the room's dimensions, and the frequency of the tone. At every point where a null occurs in the room, the waves are literally standing still.

Sidebar: Big Waves, Small Rooms

There is a common myth that small rooms cannot reproduce low frequencies because they are not large enough for the waves to "develop" properly. While it's true that low frequencies have very long wavelengths - for example, a 30 Hz wave is nearly 38 feet long - there is no physical reason such long waves cannot exist within a room that is much smaller than that. What defines the dimensions of a room are the wall spacing and floor-to-ceiling height. Sound waves generated within a room either pass through the room boundaries, bounce off them, or are absorbed. In fact, all three of these often apply. That is, when a sound wave strikes a wall some of its energy is reflected, some is absorbed, and some passes through to the other side.

When low frequencies are attenuated by the acoustics of a room, the cause is always canceling reflections. All that is needed to allow low frequency waves to sound properly and with a uniform frequency response is to remove or at least reduce the reflections. A popular argument is that low frequencies need the presence of a room mode that's low enough to "support" a given frequency. However, modes are not necessary for a wave to exist. As proof, any low frequency can be produced outdoors - and of course there are no room modes outdoors!

Here's a good way to understand this: Imagine you set up a high quality loudspeaker outdoors, play some low frequency tones, and then measure the frequency response five feet in front of the speaker. In this case the measured frequency response outdoors will be exactly as flat as the loudspeaker is capable of. Now wall in a small area, say 10x10x10 feet, using very thin paper, and measure the response again. The low frequencies are still present in this "room" because the thin paper is transparent at low frequencies and they pass right through. Now, make the walls progressively heavier using thick paper, then thin wood, then thicker wood, then sheet rock, and finally brick or cement. With each increase in wall density, reflections will cause cancellations within the room at ever-lower frequencies as the walls become massive enough to reflect the waves.

Therefore, it is reflections that cause acoustic interference and standing waves, and those are what affect the level of low frequencies produced in a room. When the reflections are reduced by applying bass traps, the frequency response within the room improves. And if all reflections could be removed, the response would be exactly as flat as if the walls did not exist at all.

Ethan Winer is head of RealTraps, where he designs and manufactures acoustic treatment and bass traps. You can read about Ethan's products at the RealTraps web site www.realtraps.com .


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