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Recordings, Speakers and Compatibility: Part II.

The following continues the thread begun in Skeptimania 72, and deals with the assorted variables and interactions that will impact the sound of any home audio system. Unfortunately, a production snafu in printing Issue 72 resulted in a figure being left out, leading to confusion amongst many readers. In this column, then, we are printing Figures 2A and 2B, the figures left out of Issue 72, illustrations of microphone patterns. Figure 2A illustrates how three omnis might be used in a spaced array. In this case, the dotted lines highlight the horizontal axis of their pick-up patterns. Figure 2B illustrates an example of a of "coincident" microphone array: the double figure-eight, sometimes called "XY" or "Blumlein." For an explanation of the significance of these microphone patterns, please refer back to my column in Issue 72. Better late than never ...

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Hi-Fi Sound: The Playback End: In the last installment, we discussed hi-fi sound at the recording end and how it might affect what we hear at our end. In this installment, we will go over what is at the listener's end, and try to determine how our hardware will interact with various kinds of recordings.

During home playback of any two-channel recording, the "sound" of the microphones employed and the placement technique involved in using them will be altered in important ways that are beyond what was recorded, mixed down, and transferred to disc or tape at the studio. Aside from what can be accomplished by a surround-sound, DSP-type, ambience-synthesizing system, such as those offered by the Yamaha and Lexicon, the most important ingredient at the playback end will be the loudspeaker/room combination.

There are a number of factors (such as room/power response, bandwidth, crossover design, and distortion levels) that determine the quality of a pair of loudspeakers. However, one prime characteristic that determines how any of them will interact with differing microphone techniques will be the polar radiation patterns of a pair of them (or of a larger group of speakers if a surround-sound installation is involved) and the way those patterns interact with room boundaries.

For example, speakers that are designed to be somewhat directional throughout the midrange and treble, and/or which are operated in acoustically "treated" rooms that have sound-absorbing material at the speaker end, may be able to form tight central images if the listener sticks fairly close to the centralized sweet spot. Note that here we are not necessarily talking of a studio-style, fully Live-End/Dead-End arrangement (LEDE room), but rather dealing with an environment with at least a nominal amount of absorbing material on the front wall, as well as the walls adjacent to and towards the outer flanks of the speaker systems.

Narrow or "controlled" dispersion speakers have been in vogue for some time, particularly among high-end enthusiasts. A few, like the excellent Dunlavy SC-II reviewed by me in Issue 70, are electro-acoustically quite elaborate (the SC-II crossover is very complex), with wide-band frequency response, a very flat first-arrival signal, and surprisingly smooth power response. However, a number of other, often exorbitantly priced brands are utterly basic, with not much more than a rudimentary, first-order crossover, an OEM dome tweeter, and a 6" woofer installed in a small, minimum-diffraction box. Many of these systems are not "engineered" at all, and while some of them deliver decent sound, often by virtue of the high-quality drivers employed, more than a few offer surprisingly little bang for the buck.

By way of contrast, under some conditions, a widely spaced pair of speakers that have a broader and more uniform dispersion pattern, and which are operating in a room more like what most of us have at home (with somewhat more reflective side and front walls and irregular furnishings), may be able to present a better blended, and for some listeners at least, a more realistic soundstage than many narrow-dispersion systems. This may be the case even if the imaging lacks rock-steady focus, due to the direct signal being so heavily embedded within the strong reverberant field such systems may generate. In actuality, the very diffuseness of the sound may be able to synthesize a passably decent, live-concert-hall effect, even if only two speakers are employed. What's more, if the system involves more than the usual two channels, and includes a center speaker that is similar in design to the mains, the diffuseness that ordinarily exists when employing two wide-dispersion speakers may be considerably reduced.

With two-channel playback of conventional recordings, wide-dispersion speakers, like those shown in Figure 1, tend to spread the sound around a typical listening room. They have the potential to add the same kind of phase colorations and diffuse center imaging to the blend during playback as a spaced-array microphone technique may have added to it as it was being recorded, although wide-dispersion speakers with very uniform dispersion at all frequencies should not generate vague half-left and half-right images like the space-microphone technique would be inclined to do. The result may be a kind of homogenized, spacious effect that many advocates say better mimics what is found at live performances.

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Omni-directional or "bipole" speakers that are pulled well out into the listening room can possibly amplify this effect still further, although "dipole" models, with their reduced radiation in the horizontal plane at extreme off-axis angles, will not ordinarily generate strong sidewall reflections. Dipole speakers tend to enhance front-to-back depth more than lateral spread, and their lack of strong sidewall reflections (a result of the out-of-phase null between the back and front) often makes them subjectively more effective at this than bipolar models, which are more omnidirectional.

Two-channel recordings that were made with a spaced-array configuration may sound somewhat diffuse on wider-dispersion speakers in normal rooms. More directional speakers may benefit such recordings by attenuating room-generated, comb-filtering reflections and tightening up the focus in the center. However, as I noted previously, speakers in that category nearly always also reduce soundstage width, which may not work to good effect with some musical material.

While they may have problems with some space-array-produced recordings, wide-dispersion systems may get along well with transcriptions that were done with quasi-coincident (ORTF and NOS) and coincident microphone (Blumlein, for instance) setups. They do this by generating a certain amount of subjectively realistic, reflective diffuseness themselves, while at the same time only moderately impacting the coherent, image-forming direct sound. The degree of impact will vary, of course, and some musical styles may benefit from the combination better than others. Taste is always going to be a factor, needless to say, and some individuals will prefer the focused characteristic of narrow-dispersion speakers over wider-dispersion models, no matter what kind of microphone array was used to make a recording.

This is because speakers with such tightly directional radiation patterns may tend to sound very clear and tight-imaged with coincident-source recordings, and they will certainly have the same effect with spaced-microphone transcriptions, as well, compared to wider-dispersing speaker designs. However, they may also sound smaller and less enveloping as a result, and restrict the perception of the best soundstage to the listener who sits in the above-noted sweet spot. On the whole, those who listen with more directional speakers may notice a much greater variation in imaging quality between recordings than those who listen on wide-dispersion speakers. If they tend to sit close to their speakers, they may also be more aware of midrange and treble recording artifacts (the sound of score pages being turned, for instance, or chairs creaking), due to the clarity-enhancing nature of direct-field listening.

Narrow dispersion speakers, like those shown in Figure 2, tend to complement the pickup characteristics of coincident-source microphones and will form tight and controlled images for listeners sitting in the "sweet spot." This is particularly true if the room is like the one illustrated: somewhat LEDE (Live-End, Dead-End) like, with absorbing material at the speaker end and possibly some hard, reflection-scattering material at the rear. As noted, when listened to from fairly close up, they will often deliver a degree of clarity that the wide-dispersion speakers pictured in Figure 1 will be hard pressed to duplicate.

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Whatever advantages each has, there is no doubt that microphone design and placement techniques will strongly effect the way each kind of speaker system (not to mention the way they are positioned in specific rooms) will be perceived by the listener.

The frequency-response flatness of a speaker will also play a big role when listening to different recordings. A disc or tape that sounds crisp and detailed on one pair of speakers in one room, may sound strident and brittle on a second pair in another room. A recording that sounds rich and robust when played on a third pair may sound bloated and boomy on a fourth. The reason for this may possibly be found in the type of speaker systems used for the original monitoring job and the acoustics of the monitoring room involved (in addition to the kind of microphones used, of course) because the resulting blend may be equalized to complement those speakers. If the speaker/room combination used to master the disc are at least remotely similar to what you have at home, the recording may sound quite good. If not, things may not be to your liking.

In any case, remember to consider the quality of the recording, particularly in terms of soundstaging and imaging, when comparing speakers for purchase or properly arranging the ones you already own in your home listening room. If possible, attend some acoustic-instrument concerts to get an idea of what proper, unamplified sound is like. When involved in critical listening during speaker-shopping or speaker-arranging sessions, try to ascertain what kind of soundstage is being produced and if that soundstage, particularly if you do not always listen from the "sweet spot," suits your needs. Pay particular attention to left-to-right spread and perceived depth, and also take care to keep the listening distance to the speakers (which will impact the direct-reverberant balance) similar to what you will encounter at home.

Also take listening height into consideration, because some speakers, particularly MTM (d'Appolito) or MTTM (THX) arrays, and certainly any electrostatic or planar or ribbon-line-source, panel-type speakers, tend to beam in the vertical direction. Do your listening from the same height as you will be at when at home. Make sure the speakers you purchase do the kind of job you want on the recordings you plan to listen to.

As an aside, I will point out that a three-speaker frontal array, working with five-channel music program material that displays a truly independent and "hard" center channel, may be able to form remarkably tight images at half-left and half-right soundstage locations in nearly any kind of decently outfitted room. What's more, they should be able to do this better than any kind of two-speaker set up -- wide or narrow dispersion.

A system employing a hard center will also be able to deliver a more focused center image, even when listening from on-axis, simply by virtue of the ears not having to deal with four events, rather than the two that a true center will deliver. That is, with a "phantom" center, the left ear hears the left speaker somewhat before the right ear hears it and the right ear hears the right speaker a bit before the left ear hears that unit. Each ear hears two events, for a total of four, and short time delays are involved. This is unnatural. With a true center image, each ear hears only one event, and they are simultaneous.

There are a number of five-channel DVD and CD audio recordings out there right now (either Dolby Digital or DTS), and most employ either a phantom center or a derived center that is taken from the L+R part of the mix. In some cases, it is a simple L+R and in others it appears to be the result of some electronic steering. What's more, in most cases the overall level of the center is greatly subdued, in order to prevent a collapse of the soundstage toward the middle.

In each five-channel musical recording I have encountered, however, the center did not display the kind of really isolated, strong, and full-bandwidth center channel that you find on many DVD movie releases. To see how effective a truly discrete center can be, I suggest that you listen to the song Tina Turner sings at the opening of the movie Goldeneye. For all of its technical limitations, that piece demonstrates what is possible with a well-handled, "hard" center channel. After one listening session, you will no longer consider the use of two speakers up front, no matter how high their quality or what kind of radiation patterns they deliver, as a way to present a proper musical soundstage. And oh, after the song, enjoy the movie.

Two final suggestions. To help evaluate the musical soundstaging of your speakers, I suggest you obtain a copy of the Delos Surround Spectacular test disc (DE3179), which has some very helpful hard-left, half-left, center, half-right, and hard-right test-tone sequences for both conventional stereo and Dolby Pro Logic surround-sound systems. I have been using this disc off and on for some time, and find it to be an essential layman's tool for system setup and testing procedures, particularly when it comes to working with a center channel, an array of surround speakers, and a surround processor.

To check out any true five-channel setup you might have, I suggest obtaining copies of either the Delos DVD Spectacular disc (DV-7001, reviewed in Issue 68) or the new DVD Music Breakthrough disc (DV-7002). Neither of these has a truly independent center channel (they are derived, L+R mixes, blended and sent to the center speaker), but the overall impact of the surround channels, working in combination with the front soundstage, will give you a taste of the future. -- HF
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Author:Ferstler, Howard
Publication:Sensible Sound
Geographic Code:1USA
Date:Jan 1, 1999
Words:2321
Previous Article:What Really Matters in CD/DVD Player Performance, Part III: Does Price Really Matter?
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