Technical & Editorial
- Written by Colin Miller
- Published on 30 November 2007
By knowing that speakers we were listening to met certain standards for on-axis frequency response, we knew what flat really meant. It did not ensure musical satisfaction, but did provide a reference for a world of audio in which factions often eschewed references of foreign standards, and baked up their own to taste, with justifications akin to "It just sounds right," or "It sounds more like music." Those statements hold water when made in the context of personal opinion, but is tantamount to, "We think it sounds good." Many audiophiles cued into loudspeakers with flat frequency response, and some found that they could indeed be musically rewarding, in some cases perhaps even more so than those altered to pander to taste.
Similarly, even though low frequency reproduction did exist before the cinematic experience came home, the demand for bass extension and output capability surged forward with the popularity of home theater, not as a matter of standardization, but as a matter of spectacle. Sound systems reproducing high-budget special effects had to keep up with thundering explosions, quaking rumbles, and even the subsonic grunt dubbed into every smack of a fight scene. And so the consumer demanded more. More extension, more output. Television speakers would no longer do, nor would mid-sized bookshelf or floor-standing speakers. Man wanted to move the earth, or at least his house, and the audio industry complied. Enter the "Home Theater" subwoofer, a mid/high-priced accessory necessary to get the most out of any home experience. Again music reproduction benefited. Although some shied away because of negative experiences with overdriven, boomy bass bombs, many enthusiasts realized that when they left the better performing subs online after the theater closed and music resumed, there were some notes on their relatively new digital medium that they hadn't noticed before. More fun! More to listen to, and more to FEEL!
But with every step of progress, there are difficulties. Deep bass is hardly an exception. Room acoustics probably get less attention in the popular consumer press than any other area of audio reproduction, other than perhaps the recording process itself. Ironically, it's also one of the most prevalent variables in the outcome of what you hear. Upgrading equipment doesn't do you a darn bit of good if you don't have the right place to put it. For example, would you buy a 911 C-4 Porsche and then drive it in the mud? I'd hope not.
Unless you're listening in an anechoic chamber (and you probably wouldn't want to) you're going to receive a significant amount of sound after it has interacted with the room by the time it gets to you. Because of their relatively long wavelengths compared to the average listening room, low frequencies can interfere with themselves dramatically.
While living with my parents, I encountered my first serious bass dilemma. In college I had become very fond of punchy, tight, deep bass. I owned a variety of the larger Klipsch floor-standing models, all of which honestly played lower than 35 Hz. I had also acquired some Audiovector Vs later in my scholastic career with similar bandwidth. On returning home during the summers, and eventually after graduation, I realized, much to my frustration, that however I toed in, moved out, pulled in, pushed back, or launched forward my speakers, the lowest frequencies, at my listening position (the middle of the room), tended to disappear. From that perspective, the bass was there, meaning all around me, but not here, meaning most importantly, where I could hear it.
The entire house shuddered, knick knacks fell off the shelves of the adjacent bathroom, and my parents wondered if the financing of my education had merely produced a jarhead who liked to listen to music with ridiculous amounts of low frequency content. I DID (DO) like to listen to "ridiculous" amounts of low frequency content. I had a Yamaha equalizer with the 30 Hz and 60 Hz bands shoved to the ceiling, but I simply wasn't getting any. It appeared a hopeless trend, one that I hoped wouldn't bleed over into the rest of my life. I had to nip its bud, so to speak.
Many had advised me that corner placement leads to greater bass output, so I moved the speakers toward the corners. At the time of that particular stage of the experiment, I was using Klipsch Forte IIs with horn-loaded mid-ranges and tweeters, so with the controlled dispersion of mid-range and treble frequencies, increases in primary reflections and subsequent loss of depth due to nearer walls were not an issue. Did I get more bass?
There was more bass, but I still didn't get any. In fact, it was even WORSE at my listening position. When I turned to Audiovector Vs, I tried every position for the speakers which I thought might be practical. Same result. In fact, I even tried loading the rear ports into the closet on the other side of the room, which simply created a some kind of Helmholtz resonating machine, making some bass notes horrendously amplified while others a fraction of an octave down remained silent.
Once, after an entire afternoon of experimentation, I finally achieved good bass response in my listening position. The speakers were directly to the side, against the wall, and facing the rear. I couldn't even turn them in, or the bass would disappear again. The arrangement would have been fine had my main speakers been THX dipoles, but I couldn't even turn my ninety-two pound towers into gigantic headphones! Imagine my heart crumbling into a pile of dust.
Tears welled in my eyes, and I became so distressed that I hauled my beloved Audiovector Vs down to the hi-fi shop to sell on consignment. I replaced them with some M&K S-80 satellite speakers, figuring that my lust for the bottom octaves would have to peter out on its own, unsatisfied. I mounted them four feet off the ground, on a shelf against the rear wall, about six feet apart in the center. To my surprise, I got better bass.
Huh? Smaller speakers, better bass? It got me thinking, (dangerous habit these days), and I borrowed an M&K MX-70B, a smallish subwoofer with two 8" drivers in a push-pull configuration which could delve into 27 Hz without so much as a whimper. However, instead of locating the little beastie in a corner, or on the floor, I mounted it on top of a small table smack dab between the satellites, against the wall, so that the middle of the sub was also 4' off the ground, and in the center of the wall. Hallelujah! I listened to music until 3 a.m. that night, crawling through my CD collection. Soon afterwards, I returned the sub, moved out on my own, and bought some larger floor-standing speakers that went deeper, and weighed even more. These, for those interested in history, currently reside in my parents' memorial to my departure, "Garage Full of Colin's Junk."
Had I known then what I know now about standing waves and room acoustics, I probably would have tried the trick of putting the speaker or sub at the listening position, walked around the room listening for the ideal placement, realized that there was none, and moved out of that square cubby hole long ago. Of the demonstrators I talked to at HiFi '97, none of those who had square rooms similar to mine were at all enthused about their acoustics. So why is it that bass, especially low bass, is such a bane to easy setup? There are several reasons, most of which have to do with standing waves, phase differences, and room resonance characteristics, all of which are explicitly related.
Standing waves, essentially, are waves that stand around. The peak of the wave, and the trough, don't move across the room, but stay (stand) in one place. [Click here to see animated example of standing wave.] They occur in the listening room when sound waves bounce back and forth between parallel walls, and are caused by the interactions of the reflected waves with each other. The result of addition and subtraction of the reflected waves is the standing wave. So, if your head is in the place where the peak is "standing", you will hear the sound quite clearly, but if you move your head to where the trough is "standing", the loudness is greatly reduced. They don't diffuse much, they're not absorbed much, nor do they pass through walls and go on their merry way. Because of the room shape, construction, and furniture decoration, they hang out for a relatively long time in order to potentially cause some relatively serious problems. Compare the standing wave with normal "propagation" of a sound wave from the speaker across the room [click here to see wave moving across the room]. In this case, no matter where you are in the room, you will hear "normal" sound as the wave moves past your ears.
Phase differences I'm referring to are time arrival differences of two waves of identical frequency, as they relate to each other in terms of cycles. In this case we're not talking about the 180 degree absolute phase difference caused by switching the polarity of your wires. We're talking about a relative phase shift induced by a delayed arrival of the identical wave, the degree of which will vary by the amount of delay, and the frequency involved from a few degrees to hundreds.
Rooms resonate, amplifying certain frequencies, or kill frequencies depending on how standing waves interfere, essentially how those bouncing waves pile on top of each other. The result of that pileup depends on their relative phasing, and which standing waves are accumulated.
Since bass doesn't tend to travel in just one direction, or easily bury itself in draperies, carpet, or "Masters of the Universe" sleeping bags, by the time it gets to the listener, much of it has bounced around the room. Because low frequency wavelengths run from roughly ten to sixty feet, whatever cancellation (relative phase shifts near 180 degrees, or any multiple of 360 plus 180 degrees) or constructive interference (relative phase shifts near 0 or any multiples of 360) that does occur won't differ much within the space of a person's head. While most of us have the use of two ears, they will likely experience the same amplitude or very similar amplitude variations as a function of time, regardless of absolute delay. Very simply, even though your ears can experience a slight delay between themselves, they can't experience a significant phase shift as a result of the space between them, as the distance of about half a foot would equate to an eight degree shift at 50 Hz, and then only if the wave was coming from the side.
To appreciate the difference between the perception of higher frequencies vs. lower frequencies, sit in the middle of a stereo array in a remotely symmetrical room forming that trusty equilateral triangle you learned about when you weren't passing notes in class. Now listen to something, anything. (It DOES have to be through the stereo. You can't just hear things in your head.) Unless you're listening to one of those Q-sound recordings that fakes surround sound with a pair of stereo speakers, i.e., Sting's "Soul Cages", it sounds like it's in front of you. At least we hope so. Ever wonder how Q-sound works? For those of you who haven't tried this, it's worth the ride. For those who have, why not follow along for the heck of it? Turn off the amplifier first, (remember safety first kiddies) then switch the polarity of just one of the speaker's connection terminals, positive to negative and vice-versa. Turn everything on, play it again, and sit down with the rest of the class, but make sure to sit in the middle.
If you're an equal distance from both speakers, you're now noticing that the entire soundstage got turned inside out like a snapdragon on crack. Higher frequencies that before sat anchored center stage now emanate from nowhere and everywhere at the same time. Bass however is different. Directionality isn't the issue, amplitude is. The bass is gone. If the setup isn't symmetrical in both levels, manufacturing tolerances, or acoustics, you might have a wee bit left due to uneven output or room reflections. Otherwise, you've got nada, zip, diddly doo, nothing.
We can avoid this kind of phase shift, changing the absolute polarity of one speaker (180 degrees for all frequencies) by following the owner's manuals and keeping track of our pluses, minuses, reds, and blacks. Room acoustics get a little more complicated.
Now for the imaginary field trip. For the sake of simplicity, let's consider an infinite planar source which technically can't exist but allows us to consider bass moving only in one dimension. The waves don't spread, change amplitude as a function of distance, or do anything that could make any calculations difficult. If you don't mind, let's also create an acoustically transparent diaphragm for this source which only affects the movement of air with the initial generation of the wave, and doesn't interfere thereafter. While we're at it, make it bipolar, so that it generates a wave of equal amplitude and phase backwards as well as forwards. Now that we're defying physics, let's use it.
Put this hypothetical source four feet from the rear wall. Also, for simplicity's benefit, this room will have only one wall. Side walls, floors, and ceilings wouldn't theoretically affect a wave generated by an infinite planar source launching a wave parallel to these surfaces, so let's not bother thinking of them.
So we've got this physically impossible wave-making machine. It pushes and pulls air, changing the immediate air pressure, which travels as a wave. Look in your pocket and pull out a handful of surfers. The surfers will ride the waves in different phases of their cycles. Label half of them "Compression", and half of them "Rarefaction", and don't worry that waves have to be breaking to ride them.
Simultaneously generate a wave forward and backwards, making sure that the backwave is in phase with the frontwave. We'll do this at 69 Hz, using 138 total surfers in a second. Good thing these surfers work cheap. Place the Compression surfers on the part of the wave with the highest air density, and Rarefaction surfers on the relative vacuums of the wave. Hit instant replay and watch.
Assuming you programmed your wave machine to generate the compression first, and assuming atmospheric conditions are such that sound travels at 1,100 feet per second, 4/1,100 of a second after the first Compression surfers left the source, they are both four feet away from the that source, one at the wall ready to bounce back, the other going towards infinite space. At 8/1,100 of a second, the Compression surfers riding the backwave have bounced off the rear wall and are at the source going in the same direction that the other Compression surfers riding the frontwave went initially. Slight problem, though. Rarefaction surfers are leaving the same way the compression surfers went, but the one rarefaction surfer riding the frontwave is doing so at the same time and direction as the compression surfer riding the backwave which was delayed bouncing off the rear wall and returning. The Compression surfers and Rarefaction surfers don't get along, and since they are phase-shifted 180 degrees, completely cancel each other, as do the waves. As the source continues to generate waves, the secondary arrival caused by the wall reflection, delayed 8/1,100 of a second from eight feet of extra distance, is phase shifted 180 degrees at 69 Hz, and will continue to cancel. No waves dude!
Results with other frequencies will vary. At 138 Hz, 8/1,100 of a second equates to a phase shift of 360 degrees, and the waves will constructively interfere and get bigger. At 34.5 Hz, 8/1,100 of a second equates to a 90 degree phase shift, and nothing happens, as the Surfers meet neither friend nor foe. Trends like this can be problematic.
The wavelength of 69 Hz is approximately 16 feet, and one quarter of this = 4 feet. One fourth seems like a bad ratio for room placement. If you did this experiment with a parallel wall behind the listening position, creating standing waves in our theoretical room, you would have infinite cancellation at the frequency whose wavelength was four times the distance between the walls. On the other hand, you would have a large gain in sound pressure near the walls themselves at the frequency whose wavelength was half the distance between the walls, but have infinite cancellation exactly between them. A 16 foot distance between walls would create resonance not only at 69 Hz, but also at about 34.5 Hz, causing high SPLs near the walls, and cancellation halfway between them. At about 17 Hz, any SPL would theoretically not be possible. Kind of a pain isn't it?
Back to the real world. We have more than two walls. At low frequencies, bass tend to travel in a more spherical pattern. In addition, most of us have furniture, and irregular rooms. Although these real world factors make predicting room response more difficult, these variations are saviors as they tend to break up standing waves, and absorb, to some extent, low frequencies.
If you wish, invest in some sophisticated software to calculate all of your variables. The collection of data itself would be a chore, figuring out absorption and reflection ratios as a function of frequency for all the different materials, factoring in the contents of your sock drawer, etc. We still have hope, though, in retaining control of our environment by using these complicating real world factors to mold it. Even without fancy software or a team of interns to take measurements, we understand the fundamentals (pun intended) of low frequency reproduction in the home. We are creatures of thought who ask questions in order to solve problems. So, the question remains, "Aside from designing a room with dimensions that comfortably exceed the largest wavelength, complete with 60 foot ceilings, how do I fit Big Bass in my listening room?"
Well, to start things off, three things can happen to sound when it hits a surface. It can bounce (be reflected), it can pass through, or it can be absorbed. For our purposes, passing through isn't something you can control, unless you're going to change the construction of your walls, so we'll ignore it. These various methods center around how bass frequencies reflect or are absorbed by the room as a whole. All or none may apply. As they say, mileage may vary.
A. Diffusion. Because of the long wavelengths of bass frequencies, trying to break them up with small objects is hopeless. Try putting large objects like dressers, coffee tables, cabinets, and bookcases around the room. Putting a large dresser in a spare corner has generally reaped good effect for me. Essentially, the more complex you make the sum of reflective surfaces of the room, the better your chances that all of this evens out. You just have to love that law of large numbers. Hopefully, the placement of these additional surfaces will work well in diffusing higher frequency reflections also.
B. If you have a choice, select a slightly irregular room to begin with. Avoid squares like the plague! Cubes are almost a guaranteed failure. A spherical room would be a death trap if you put the subwoofer in the center. Good luck.
C. Experiment with speaker position. Subwoofer-based systems obviously have more flexibility with this. The trick with putting the subwoofer in the listening position, moving around the room to find the best bass, then putting the sub in that place, works very well. Probably your quickest fix if the majority of the bass comes from the sub. You can fine tune the position of the loudspeakers to room boundaries knowing the ¼ wavelength rule. (Distances to room boundaries will tend to cancel at ¼ of a frequencies wavelength.) Manipulate the distance and use the cancellation or boost to yield the best overall response. Even with the scientific calculator blazing, it usually involves a little trial and error.
D. Absorption. You can always go out and buy some bass traps. If you feel like a DIY project and don't eschew a little sweat, you can make them yourself for substantially less money. There's a fantastic article on room treatment in the June 1995 issue of Electronic Musician. If you don't have it around, check it out at your local library. If they don't have it, make them get it, or order it yourself as a back issue.
As optimistic as I like to be, it's honestly quite a headache if you happen to have one of those rooms problematic for bass. So far, of all the eight different locations I've tried to plant an audio system, seven of them needed differing amounts of rearranging. Of those seven, all of them were worth the effort. The other was hopeless. If you happen to have one of those (hopeless), aside from seeking professional help, your only option may be moving. Those U-Hauls do have a pretty good ride. Too bad their stereos suck.