Loudspeakers: When Is Good Enough, Enough? Part 3 - page 2
Setting Up Your Own Sound Stage or Surround Environment
Our mission with the loudspeakers, which will hopefully grace our home for many years, is to determine "when is good enough, enough? Let's see if we can pin down a checklist of what we're trying to accomplish.
- The dynamic range capability of a speaker system is intimately related to the room's cubic volume and acoustics. It is probably the hardest parameter to know if you've got "enough of" beforehand. You must first get a specific system with all its loudspeakers, into a specific room. A system's dynamic range is directly related to the combined piston area of the drivers, the loudspeaker system's efficiency and the room's size and acoustical properties. There are two similarly related goals here; to play music as loudly as you'll ever want to hear it for a sustained period; or to play a movie as loudly as you want without the system showing any signs of strain or distress. I'm going to tell you up front, that is a very tall order.
Let's take the example of my current home system. Hopefully, the description of my struggle to attain dynamic range with a solid foundation of bass will illustrate at least one example of the obstacles that may be encountered.
Our small home was built in 1948. It has a fairly open floor plan with the living room/home theater system connected to the dining room on the right and a stairwell in the rear which leads down to the bedrooms. To the left of the listening position, a leather loveseat-size sofa, is the front door. The system is set up diagonally with my beloved 40" Mitsubishi (CRT) TV in a corner atop which are mounted the left, center and right channel speakers of a modified Infinity Modulus system. ($1800 SRP for four satellites, a center and a 300-watt 12" subwoofer). The Modulus left and right satellites consist of a single 4" mid-woofer and a ¾" hard dome tweeter and hang outboard of the Mitsubishi monitor on a special bracket that Infinity engineers designed for the Modulus. The center channel consists of two 4" mid-woofers and a ¾" hard dome tweeter.
The six side and rear surround speakers are a different story. The Modulus left, center and right speakers at the front of the system are just barely acceptable to my significant other looks and size-wise so I elected to use three pairs of JBL SoundPoint SP5, 5 ¼" two-way in- walls ($150/pair) for the left and right side surrounds and left and right rear surrounds.
The two left-side two-way systems are wired in series, in a corner, over our front door and facing at 90° angles toward each other. Similarly, the two right-side systems are wired in series, on a three-sided wall column, over our dining room table and facing at 90° angles away from each other. The rear surrounds are 180° from the main left, center, right system and feature the two final 5 ¼" in-wall systems. These two in-walls are wired separately to channels six and seven of my Harman Kardon AVR7200 receiver. These two systems face at 90° angles toward each other. (Refer to the pictures to get a better idea of my layout.)
I use two subwoofers to handle the low frequency chores. Up front, behind the Mitsu TV and equipment rack, there is an Infinity Intermezzo 1.2, a 12" sub sporting 850 watts in a sealed enclosure ($1800 SRP). To the rear, behind a leather chair but 3 feet from either wall because of the stairway is a ported Infinity Interlude IL100S 10", 250-watt subwoofer in maple ($500 SRP). I would have liked to use the much more potent, also ported, 300 watt, 12" Modulus subwoofer in the back but it again did not pass the WAF test. Too big to fit comfortably and black is a definite mismatch to the furniture in that part of the living room.
With our open floor plan I'd say that my system is working into 3000 cubic feet + of space. That figure represents is a rather small 1500 cubic foot living room. The additional 1500 cubic feet is a sum of the stairwell in the rear which goes downstairs, the dining room, a small hallway and my office. Also, there are continuous, single-pane windows which almost completely cover the right-front sidewall of the system and left-rear side windows which cover over 60% of that wall.
I suspected all these single pane windows would absorb lots of bass energy. And they do. When I performed the separate RABOS set-ups for each of these subs I found that the room was so "loose", so absorptive of low frequencies, that the 10" Interlude IL100S sub required no RABOS equalization ! If you think about it and look at the pictures showing where this sub is mounted it may start to make sense. The sub sits sort of out on a precipice, away from most surfaces, much like it would "see" if it were being tested in a full sized, 4pi anechoic chamber. So the three fully clockwise potentiometer positions (out of a possible 21 positions for each control) are: for Frequency; 80Hz, for Attenuation; 0 dB , and for % Width; 49.5%. That last parameter, % Width, says this is a very wide-band "Q" correction. My final point-by-point frequency response for the IL100S indicates 40Hz - 85 Hz, -1dB or 37 Hz - 94 Hz, -3 dB.
For the Infinity Intermezzo 1.2 sub mounted up in the front corner, behind the TV and equipment rack, the RABOS CD test disc ends up indicating that there is only a single, very broadband room peak, like the top half of a sign wave, for the entire 2 ¼ octave bandwidth of 20Hz-100Hz. To tame this very broadband peak the RABOS settings ended up being: for Frequency; 52 Hz ; for Attenuation, -14.1 dB ; for %Width, 49.5%.
These settings yield ±1dB
20 Hz - 74 Hz or ±1 dB @ 20 Hz to -3 dB @100 Hz.
From the very "loose room" equalization settings I'm using (above) I'd characterize the room another way by saying that it seems almost impossible to generate a standing wave. You sure can't feel or hear standing waves anywhere in the living space. And, acoustically anyway, that's not the blessing you might think. What is happening now is that I'm getting only marginal corner bass reinforcement from my front 850-watt Intermezzo sub and virtually no boundary reinforcement for the 250-watt Interlude which is operating out from both walls and in a 90° corner, hanging out above the descending stairwell. This means that my system is depending almost entirely on the piston area of the two subs for all bass information . The room is more bass trap (the loose single pane windows) than bass augmenter (solid walls).
When pushed really hard I have just barely enough bass quantity to match the SPLs of the rest of the system and the quality of the rear mounted, under-powered (and ported ) Interlude 10" sub noticeably deteriorates, sounding uncontrolled and tubby, as volume levels increase.
Fortunately, at the front, for most all but the very loudest soundtrack crescendos, the 850 watt, sealed Intermezzo sub stays composed, sounding quick and always tuneful on both music and effects. This is one heck of a subwoofer!
Now to the front Modulus trio. The left and right satellites as well as the center measure -3dB at around 140 Hz. Styling-dictated, small, sealed cabinets with (relatively) large 4" drivers in sealed enclosures are the culprits here. Match them with subs which do not get above 100Hz (regardless of what the back panel silkscreen says) and I've got a big hole in the response similar to most of the HTIBs on the market.
Play a Dolby Digital DVD which has an 80 Hz rolloff in the software and the hole becomes even bigger (80Hz to 140Hz; almost a full octave!). Is it any wonder then, even backed up with 100 RMS watts/channel from the H-K 7200, that I don't think the front of the system plays loudly enough? I'm missing an incredible amount of primary and second harmonic bass frequencies from 80Hz to 140Hz. It is these frequencies which add a substantive "foundation" of upper bass to satellites. Plus, it's very difficult to blend the satellites of a system such as this with the subwoofers. A frequency gap this large will pinpoint and highlight the subs' location relative to the satellites fairly easily.
The final element of my current system is the six JBL 5 ¼" two-way inwalls which I use as surrounds. Given the 140Hz low end limitations of the three Modulus' and the compromised positioning, low power and lack of control of my rear mounted sub, I'd have to say that the surround speakers and the front 850 watt Intermezzo sub are the only adequate performers of this loudspeaker menagerie.
So, as far as dynamic range is concerned I'm fighting an uphill battle. My first task then is to find a trio of left, center, rights that can get down to 80Hz (or at least 100 Hz) -3dB. Task two is find a small, yet very powerful sub (for the rear of the room) that can actually produce flat linear response from 27Hz -3dB up to 120-140 Hz. Both of these tasks are difficult to accomplish in small enclosures. But neither is impossible.
Perhaps someday I'll construct a Part 4 wherein we'll discuss frequency response, distortion and the materials which go into a speaker. All, either positively or negatively, contribute to the final sound quality. It's just a matter of understanding what particular measurable parameter is affected by which part. The human ear is an incredibly complex yet perceptive instrument. And we continue to understand more each year about the ear-brain relationship with the real world.
