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Chapter III - A Lay Introduction to Room Acoustics

Untreated room = Bose 901®:
No card-carrying audiophile would be caught dead with Bose speakers. Especially the 901s. There's irony then. The very Bose design element that drives most audiophiles crazy is their use of reflections. For those who were not alive, conscious or just skipped over the 70s, Bose introduced a speaker which theoretically mimicked a concert hall’s direct/reflected sound ratios of 11/89% (forget about their full-range equalized individual drivers and stay with me on the concept.) As critics have amply pointed out, the recording process already captures the direct and reflected sound of the original musical event. Take such a natural recording, add additional reflections bouncing around the room and end up with horrible tone, zero soundstage and zero imaging. The reflected sounds arrive later than the direct sounds and partially out of phase. This obscures much subtle detail. While those speakers would play loud, that’s not exactly our top goal. This is not intended as a 901 critique. But, an untreated listening room turns your megabuck speakers into nothing more than Bose variants.

To get the right solution for my room, I needed a crash course in acoustics. I will share the basics and begin with a clear disclaimer. All this is purely from a lay perspective. I am neither a scientist nor an engineer. With that caveat, let’s look at acoustics as applied to our listening rooms. As a group, we crazy audiophiles spend countless hours tweaking, cryogenically treating our tubes, lifting cables off the floor, demagnetizing our CDs and records, selecting the best wood for our racks, properly orienting the wood grain (of course) while endlessly debating the ideal metal composition for our cables. Everything matters and we know it. Yet our crazed community often overlooks the most important element in the system, the effect of room acoustics.

I sure did. Maybe that was because it’s not a new shiny box. Or maybe we accept the room as fait accompli. Whatever the reason, there is one fact I now clearly understand. The ignored room is the by far weakest link in the audio chain. You will never maximize the sound of your system without attention to acoustics. The sound in your room is influenced by everything it hits and the sound hits everything. The entire room and every object in it are part of the overall sonic signature. Art Noxon contends that as the electronic noise floor of our equipment has been reduced to inaudibility, what's left is the generated noise of the listening room – harmful delayed and reverberant sounds that are out of phase and frequency to mask and distort the musical event.

The room is the acoustic coupler which connects speakers and listener. We take it for granted because in most cases, the room just exists. We don’t question it. It’s not viewed as an active part of our system unless there's some perceived anomaly. We take the room for granted while we change our equipment annually. We rarely take the position that the room is more important than the speakers or source component.  But the acoustics of the room are more important than any component in it. I was as guilty as others by totally overlooking the effects my room had on the sound.

Today I silently shake my head as people talk about the sound of their system without correlating it to their room. My epiphany started about two years ago when I stepped through the progression of an untreated, then partially treated and finally a fully treated room. People who deride their system as thin, hard, unemotional, lean or thick are quick to blame the speaker or ancillary equipment. As a long-term owner of Wilson speakers, I’ve read many of these online complaints. I love the look of shock on the faces of people hearing a system in a properly treated room. Untreated, the divergence figures from linearity are staggering.

Resonance peaks of 10dB add ten times the acoustic energy to a frequency. Likewise a 10dB suck-out means that a certain note is reduced in power by a factor of ten. But it’s not just single notes. The time and phase of one note actually converts into another note that mixes with the original. These are not just subtle adjustments to the musical cues, these are dynamic distortions that swamp and overwhelm the foundation of the music. For several reasons I shall focus on the low end. The actions and treatment of bass issues are the least understood and therefore often misdiagnosed.

In my specific case, the X-2s are capable of prodigious bass. Conversely, highs—shrill or brittle—are quite easily tuned by adding some strategically placed drapes or pillows to partially solve the problem. Not the bass. Bass reflects and interacts at various frequencies, either reinforcing or canceling out depending on speaker location and shape and composition of the room. Using a water analogy, a rock dropped into a pond forms waves in all directions, forward, backwards and sideways. Drop another rock near the side and you get multiple waves which invariably meet. At some points the waves will be canceled while at others, they will be made larger. When a sound wave is reinforced due to a resonance room mode, you get one-note amplification which some people incorrectly attribute to good bass. It's not. It’s just a few exaggerated notes screwing up the overall sound. The problem is that every single room has these unique interactions. The room's dimensions determine the distribution of these resonances but no matter what the dimensions, modes always exist. There’s one other absolute for every room – bass accumulates in the corners which manifest as fluctuating pressure zones.

In order to manage and treat a room’s acoustics, we need to understand how sound behaves in a room beginning with the fundamental differences between high and low-frequency behavior.  While oversimplified, from approximately 200Hz up, sound behaves analogous to a beam of light, a ray or a ball coming out of the speaker. The highs hit a reflective surface, bounce off (reduced by friction), ricochet off the next reflective surface and continue until the energy dissipates or is absorbed. Bass is different. It rolls out in every direction like a wave moving forwards and backwards while simultaneously piling up in the corners. Think of diving into a swimming pool. Your body moves forward but the wave you produce goes in all directions – forward, sideways and backwards. Highs are blocked or reflected by objects in the room but bass goes around them. This concept is often elusive even to experienced audiophiles yet this distinction between high and low-frequency behavior is essential for obtaining optimal sound.

Bass waves bouncing around the room are insidious. Every listening room on the planet has constructive and destructive standing waves which arrive either in or out of phase based upon room geometry. No golden formulae or magic short of room treatments can stop them. You have some limited control because these standing waves don’t exist to the same degree in every spot.  Your listening seat and speaker location do have an effect on the lower and midbass balance. Subtly moving your speakers and to a lesser degree your listening chair will help find the smoothest and most natural bass. 

I run a small business. My biggest fear is the unknown; not knowing what I don’t know. I’m not playing cute word games. The truth is that smart rational people take known problems and solve them. It’s the unrecognized problems that can be killers in business and audio.

Before I started my room-building journey, I did not fully understand acoustic ills. I’d heard the terminology and acquired a little knowledge but I really never paid much attention to the room itself. As the saying goes, a little knowledge is a dangerous thing. Did you ever read a medical book, then worry that every minor symptom was a potentially new disease? After learning of potential acoustic evils, I was positive that all my previous rooms were indeed infected. Before we get to the cure, let’s take a look of at some of the acoustic problems we all face.

Wide dispersion speakers and first reflections: Unless you are listening to electrostatic or other planars, you are using some type of wide dispersion speaker. Everyone ‘knows’ that narrow dispersion produces beaming with a small sweet spot while wide dispersion gives what many consider a spacious sound. One problem is that the farther off-axis you sit in a speaker’s dispersion, the worse the quality of the sound gets. It now includes harmful colorations and a skewed frequency response not even close to the original source. Our wonderful high dispersion speakers have off-axis sound bouncing off the walls, then mixing out of phase with the direct sound at the listening position. Due to arrival time differentials between direct and reflected sound, the clean transient originating from the speakers becomes a muddled mess. The time delays from these reflections are too small to be detected as echoes. Our brain combines and averages them into a colored distorted sound which impinges imaging, detail and soundstaging.  With wide-dispersion speakers, you are listening to the mid and high-frequency combination of your speaker and room reflections. The wider the dispersion, the more your room interferes.

Reverberation: Reverberation is the sound decay after the initial loud direct sounds and first reflections. Reverb is an important part of every listening room without which music would sound unnatural. The primary term is RT60 as the time necessary for room reverberation to decrease by 60dB. When people talk about live or dead rooms, they refer to RT60. Acoustic treatments affect reverb time as do furnishings. Rugs, drapes, overstuffed chairs and the like all tend to reduce a room’s RT60. As an ideal, there appears to be a general consensus of .20 to .50 seconds for small rooms. These times relate to home listening. Large rooms like churches and theaters can have reverb times in the seconds while an anechoic chamber has an RT60 of near zero. Too much reverb causes the decaying sounds to interfere with the direct sounds to create messy blurs. Yet overdamping can be just as bad.  For ideal music reproduction, the brain needs the reverb cues.

Comb filter:
The comb filter effect is a type of standing wave occurring at higher frequencies.  Reflections from the walls and corners produce a delayed signal that mixes with the direct signal from the speaker. This produces a distortion called comb filtering. The out-of-phase signal mixing with the direct signal distorts the signal and produces a new signal with peaks and valleys that incidentally appear like a comb. You end up with hardness, soundstage issues and a reduction in the harmonic textures of the music. In his writings, Art Noxon states that "…there are many potential reflection points that can cause a sound launched from a source to return to that source and interfere with itself.

"There are also many potential ways for sounds to travel from one source to another and cause interference. Likewise, there are many ways for sounds launched from single or multiple sources to arrive at a central listening position at different times and interfere with one another there. All of these interfering waves cause the resulting amplitude of the sound to either increase or decrease to some degree depending upon the frequency (tone) of the wave. The resulting adjustment to the amplitudes at each frequency is called a comb filter."

Moving the speakers, listening position or rearranging the listening room will not eliminate the comb filter effect. The solution requires room treatments.

Standing waves: These are artificially boosted bass notes at specific frequencies and locations in the room. A standing wave is the stable sum of two waves traveling in opposite directions.  These waves exist between a pair of walls "whenever the distance between the surfaces is any even multiple of one-half the wavelength."
At resonant room frequencies, the note of the wave will be amplified with longer decays. These exist in every untreated room. You end up with classic muddy uneven bass lacking in clarity and definition. Rooms that are rigidly built and tightly sealed have greater problems with standing waves as the bass waves are effectively trapped with no ability to leak out or dissipate.

Slap/flutter echo: This echo occurs between parallel walls. As its name implies, it can be easily identified by a single sharp clap of the hands. In an untreated room, you will hear a series of distant mid to high echoes. Since the wavelengths of high notes are short, they are directional to continue bouncing between the walls. Rooms with excessive slap echo degrade the soundstage and contribute a ringing which is often interpreted as harshness. Fortunately this is easily treated with absorptive and/or diffusive wall treatments or non-parallel walls.
Structural vibrations: Go outside, find some nice firm dry ground and slap it with the palm of your hand. Now slap the walls in your listening room. Unless your walls produce the same dull thud as the ground, you will have structural resonance and room distortion.  In your listening room the sound from your speakers excites the walls and vibrates them at their resonant frequency. To make matters worse, walls store energy and release it at a later time to interfere with the new direct sounds. 

What starts as a wonderful pure 60Hz bass note is stored in the walls and returned to the listening position as a delayed 70Hz bass note released long after the original propagating wave. It's not just the walls either. Everything in the room will vibrate - the windows, doors, floor, furniture, pictures down to the smallest knickknacks. They all have a cumulative effect on the sound which increases with volume levels.

Resonant room modes: Resonant room modes exist in every listening room and are arguably one of the major deterrents to good sound because they often go untreated. Bass (<200Hz) radiates out from your speakers, bounces off the walls and these reflections meet and interfere with a new original signal from the speaker.

These standing waves produce peaks (anti nodes) where the two waves sum; and nodes where the sound is nulled. Room modes are resonances excited by the propagated sound energy to produce new waves not encoded in the original recording. The number of modes and their frequency of location depend on room dimensions and speaker placement. The closer you place speakers in a corner, the more modes are stimulated. Modes manifest in a variety of ways including thick muddy bass, single note boominess and bass and mid-bass suck-out. This can occur simultaneously in various parts of the room depending upon dimensions, speaker and listening location.The attack and decay that defines an instrument can be radically distorted. A piano or guitar note might have a very quick sharp attack followed by a long decay. The resonant room modes might amplify the attack and attenuate or cancel the decay.  Instinctively we blame our speakers. This could be a very erroneous conclusion.

If an equipment or speaker manufacturer made a product that distorted the signal like our rooms do, they’d be out of business in a second. Yet that’s exactly what occurs in any untreated listening environment. If you want to test this, play a test disk with a slow low frequency sweep or sustained low frequency notes and systematically walk around your untreated listening room. You will find a series of loud areas and quiet spots. Both suffer inaccurate responses. Go to any forum and watch the verbal jousting about speaker X's weak anemic bass while another poster counters with the opposite experience. I guarantee that in a vast number of cases, the issues are not the speaker’s but the room’s.  

Untreated rooms produce resonant modes with a high Q to indicate a sharp steep resonance. With the energy reflecting in the room, these oscillations die out more slowly. This is of course bad. While there are curtains, carpets and furniture, to be effective absorptive material has to be approximately a quarter of the wavelength thick it’s supposed to affect. With sound traveling at approximately one foot per millisecond, a 20kHz wavelength is about a half inch long. 1/6th-inch fabric absorbs it nicely. Curtains and carpets might work well for high frequencies 8kHz and up then.  But the real problems are lower. As you descend in frequency, the required absorption thickness goes off the scale. Want to absorb a 20Hz wave from that subwoofer? Just make your curtains 14 feet thick. But it’s not just the thickness. These hypothetical curtains would have to have the right density too. See the problem?

You can mitigate the trouble somewhat with speaker placement. Moving a speaker away from corners and walls will reduce some resonant modes but not all. Some listeners do the opposite by intentionally putting speakers or subwoofers in the corner to stimulate apparent bass. Some notes will be emphasized but it won’t be accurate or real.

Most readers do not have the luxury of choosing their listening room dimensions of course. Even building a house from scratch, I had restrictions. Let’s first dispel the myth that golden ratio dimensions eliminate room modes. Not true. What good ratios do is spread the modes to prevent clumping and to reduce the spacing between modes. As Art Noxon explains in his essays, these golden ratios are based on speakers placed in the corners. Unless you run Klipschorns, that’s rarely the case.

All things being equal, I set out to design a room with dimensions as favorable as possible given the overall floor plan limitations. Below are three examples of room modes. The worst possible is a 10’ x 10’x 10’ cube.

Then follows a golden room ratio at 1.6 x 2.6 x 1.

Finally my room is 29 x 19 x 9.  Don’t get misled by the 10 x 10 chart. The few lines are actually multiple modes piling up at the exact same frequencies. Also, the large distances between modes produce a very uneven sound.

Non-Parallel walls?
Some very knowledgeable people dispute the advantages of non-parallel walls for home listening rooms. Frankly, I just wasn’t sure. With a fairly large footprint to work with, I did explore this option. It’s agreed that non-parallel walls will reduce slap echo. However, the more I researched, the more I became concerned that the cure could end up worse than the disease. Parallel walls in rectangular rooms have well-known acoustic properties. Mathematical equations can very accurately predict the acoustic response in these types of rooms. While non-parallel walls might solve one set of problems, other areas could become completely unpredictable. In traditional rectangular rooms, you can almost always find a seating location with a relatively smooth response patter. Start mucking around with wall angles however and there's a chance you’d never find a smooth bass response. According to ASC, even with non-parallel walls the room modes will exist due to the small room dimensions relative to the sound wavelengths. But now they will be far harder to analyze, detect and treat due to their irregular distribution. The goal is optimum distribution of room mode frequencies. Uncontrolled room modes will deliver non-linear bass response. That's a risk I did not want to take. On top of that, slap echo is easily treated. I thus decided in favor of a predictable rectangular room with parallel walls. With the problems defined, the next chapter will begin to look at the ASC solution.
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