Ariel2

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The Ariel

By Lynn Olson

Balancing the Speaker System

Since speakers are electroacoustic transducers and antennas at the same time, they have complex behaviour that cannot be reduced to simple graphs or curves. Even the most sophisticated instrumentation only hints at what's really going on, so careful listening is still an essential part of designing a speaker system. I use a combination of listening to music and pink-noise, and measuring the speaker with several different MLSSA test modes, such as impulse response, group delay vs. frequency, frequency response at different angles, and the cumulative decay spectra.

Careful listening to pink-noise is a very sensitive way to discover resonant colorations (more so than music, and much more repeatable), but it can fail to detect notches in the spectrum unless they are very sharp and deep. So you have to be careful when you tune a system so you don't inadvertently create broad depressions. In addition, pink-noise testing tells you nothing about dynamic qualities, so you can end up with speakers that are smooth and inoffensive but don't sparkle and sing on real music. Still, pink-noise testing lets you quickly detect and remove resonant colorations; just alter the crossover and add or remove cabinet damping until the speaker really begins to sound like an actual waterfall.

MLSSA, FFT, LMS, or 1/3 octave measurements provide an essential cross-check to make sure that you're not subtly skewing the spectrum as your ear gradually adapts to the sound of pink-noise. (Adaptation is a serious problem with pink-noise testing. Listening and tweaking sessions should be kept under 10 minutes so your mental reference point doesn't begin to shift.)

All tests have their blind spots, so cross-checking is very important, especially when you listen to music. You may have to choose between a sense of verve and directness and a tuning that is relaxed and neutral; this is your call. The drivers in the Ariel are exceptionally flat, which makes the system tuning easier. If the drivers had large peaks like metal-dome tweeters, Kevlar, or carbon-fiber drivers, the tuning process becomes far more difficult, and requires a lot of experience in knowing what to equalize and what to leave alone.

In practice, subjective tuning results in a 3-way round of testing, using pink-noise, measurements, and music listening. This is the sequence I use:

Aim for the intended acoustic target slope (4th-order Gaussian or whatever) by using computer optimization with LEAP or XOPT, or use old-fashioned cut-and-try with many repeated measurements made at 2 meters distance at 0 degrees, 15 degrees, 30 degrees, and so on.
(WARNING: Do not make system measurements at a 1 meter distance. For all multiway speakers, not just the Ariels, the crossover radiation pattern doesn't fully "gel" until you get at least 2 meters away. If you fine-tune the system at a 1-meter distance it will almost certainly be wrong at 2 meters. Fortunately, the measurements at 2, 3, and 4 meters are very similar, so the 2-meter measurment is valid for greater distances.)
Fine-tune the new crossover in half-dB steps by ear and repeated MLSSA, LMS, or IMP measurements. Keep doing this until you are satisfied with the overall technical and subjective performance.
Record the measurements, crossover description and topology, and the version number.
Have music-listening sessions using several amplifiers and a wide variety of sources. Instead of listening critically, ask yourself if you're enjoying what you hear. Do you feel the music? Does it move you? Focus on the emotional qualities, not just the usual audiophile sonics. In other words, what does it do well?

Go back to Step 2 until you feel genuinely satisfied that the whole system has reached its full potential. For example, crossover on this Web page is actually the 15th go-round using this multi-step procedure above. So don't expect perfection the first time around, and don't expect music-listening or measurement sessions to do it all. It takes both.

Speaker Location and Image Quality

The best imaging and spatial qualities result with 50 to 55 degree spacing between the left and right speakers, the tweeters placed towards the inside, and both speakers aimed at a point about 1 to 2 feet in front of the listener. In effect, this forms an equilateral triangle that ends about a foot short of your nose. When the speakers are correctly aimed, you will see the large-radius edge appear slightly closer to you than the small-radius edge, with an inch or two of large-radius sidewall showing.

Visualize a sphere with a 1 metre radius extending in all directions outward from the tweeter; keep this imaginary sphere free of obstructions of any kind. If your room is big and uncluttered enough to provide a free radius of 1.5 metres, so much the better. The less junk there is in the immediate vicinity of the speaker, the better it will sound. (No, various room-damping tricks won't help if the free-space criteria can't be met. You can't fool Mother Nature!)

If you can set them up like this, and have a pretty good system, you'll hear a smooth and even halo of sound extending about 2-3 feet outside the speaker pair and about 5-10 feet in height. If you're lucky enough to have top-quality directly-heated triode amps, these figures double, providing spatial qualities approaching the best and most natural multichannel systems.

By contrast, if the speakers are too far apart, or not toed-in enough, the center-fill will be weak and "phasey," and the intermediate center-left and center-right localizations will tend to jump towards the speaker. If the speakers are too close together, or toed-in excessively, the stereo image will compress, lose its natural quality of airiness, and off-axis images will disappear.

When the toe-in and spacing are correct, you will hear a perfectly even distribution of sound and a well-proportioned impression of space on nearly all stereo recordings. You should be able to move left-to-right over several feet and not have the image shimmer, wander, or collapse back into the speakers. If it does, you'll experience listening fatigue just from muscular tension trying to keep your head in the "sweet spot."

Unfortunately, most systems I see in homes, dealerships, and the CES have the speakers aimed with zero toe-in and spaced too close together. This gives the worst of both worlds: narrow, unstable images that collapse off-axis, intermittent depth perception, and a very limited impression of the spatial texture of the original recording.

Since many people, including audiophiles and reviewers, have never heard stereo as it's intended to be heard, try a simple experiment with a boom-box that has detachable speakers. Go ahead, put the thing on a kitchen table, space the speakers 60 degrees apart with yourself at the tip of an equilateral triangle, and aim the speakers at a point about a foot in front of you. For once, don't listen to the awful sound quality; just pay attention to the stereo impression.

The point of this simple experiment is to demonstrate that good, in fact, excellent stereo can be delivered by the crudest and most basic systems. There's no reason you can't have this same experience with much larger systems in your living room.

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mercoledì 02 luglio 2014

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