Napisal/-a kuzma2 » Pe Mar 09, 2007 7:50 am
Splošni članek Scot Markwella o tem , kako dodati sub k glavnim zvočnikom.( objava z dovoljenjem avtorja): Žal se boste morali "pomatrat z amerikanščino."
How to Succeed in Subwoofing Without Really Trying
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Last month, during the Stereophile Show in New York City, I had a lovely breakfast conversation with our beloved and esteemed Editor wherein, in the discussion of future topics for my column, he disclosed that he was under attack from readers who did not want to dwell on the exotica that is the mainstay of Audiophilia and HiFi in general, but on more rudimentary fare, more down-to-earth information. Basic stuff, as it were. They have just purchased some new equipment and they are perhaps a little confused and intimidated with it all. Where do the bits go and what are the best ways of hooking them all together so that they operate together with some sense of continuity and synergy? In short, it is apparent that enough of our readership is troubled by stumbling blocks in the most basic areas of system assembly and set up that Mr. Gregory felt that it might be a good idea for me to put together a short series of audio equipment and system primers.
Great stuff, I thought. Now’s my chance to address some questions and problems that routinely turn up when working with Hi-Fi kit that, if one is unsure how to proceed or even how to consider the situation, can quickly turn an otherwise fine and musical system into a pile of goods that simply does not deliver the music correctly. In some instances, such things may be the result of bad advice from well-meaning people. Friends who like music but listen to computers or boom boxes are, I submit, not the ones to look to when you are assembling your first system. Likewise, I have seen many a dealer set up a system so poorly in a customer’s home that I have had to on occasion totally rearrange things and tune the system from scratch so that the customer could hear what they really had.
Of course, where to start is the key, so I suppose I will return to a mainstay that has been with me for over 30 years, and that is proper integration of a subwoofer (or two) into an existing, well-adjusted system. I fully believe that deep, quality bass response is essential for a Hi-Fi to really sound convincing across a full spectrum of music. Some do not like the experience of deep bass, but I find them to be in a minority. And usually they do not know what real music sounds like anyhow. For the purposes of this discussion, I am going to assume that your system is at a point where you are pretty happy with things. You can play a wide variety of music and the sonic results are pretty even across the board. There is nothing in your system that is actively pissing you off. No horrible resonances, no tizzy upper midband, no plastic coloration on female vocals. The point I am trying to make is that you do not want to try and integrate a subwoofer into a system that already has problems. This should be something of a last step on the road to true full-range response, not something used as a band-aid to give effects that are meant to hide deficiencies elsewhere. We will leave that to Home Cinema.
So. You have purchased your subwoofer and you are wondering what to do with it. To make this as direct and straight-forward as possible, let us assume for this discussion that you have managed this addition on your own; you searched out a likely piece on Ebay or the like, you paid, and now you have taken delivery of what seems to be a rather large and bulky box, typically these days with an amplifier built into it that will be lurking on the chassis somewhere, probably on the underside or in the back. You have no sympathetic dealer looking over your shoulder or perhaps doing the job for you. Might as well start here.
The first thing that you are going to want to do is to determine, in your system, how best to get signal to the sub. On most modern powered subs, there are typically two ways to do this: you can either use an additional set of RCA interconnects to connect to the line-level inputs of the sub’s amplifier, or, in most instances, you can hook up a set of parallel-feed speaker cables to the speaker-level input. Both of these sets of inputs will be grouped together and well-labeled. I am assuming here that you are using a sub that has connections for a stereo input; most times in a single-driver system this input is either summed for output or the woofer may have two voice coils, which in the end amounts to pretty much the same thing. You can use two separate subs for more convincing stereo bass, but that opens up a whole ‘nother can of worms so we will concentrate today just on the one.
Now, of the two connection methods, I have found that I prefer the line level way most of time, as in my setup I take the line outs from my preamp and run them to another linestage, itself remote-controlled for level and muting. This means that any time I want to adjust the level of the sub or simply mute it for any reason I can do so easily from the chair. By no means is this a needed part of the setup and in many instances it may no even be prefereable, as it does involve another box, cables, and interconnects, which many folks will be loathe to splurge upon. For myself, I just happened to have one around that worked so years ago I rigged my system up so that I could be the ultimate couch potato. If you do not want this complexity than it is perfectly fine to use a speaker level input; many folks like it better this way because the sub is fed from the same amp that feeds the speakers and the timbre and dynamic structure of the main amp is carried to a degree over to the sub, thus making for a bit easier integration with the main speakers.
OK, then. You have a powered sub. It is in your room and installed at least somewhere in the vicinity of the back of the speakers, and it is hooked up to the mains and has a set of inputs attached from either your preamp or your amp. This important: DO NOT make a mistake and hook your subwoofer feeds to a set of tape outputs. This I did once upon a time with results that were, shall we say, quite loud. Impressive bass, as I recall, but because the tape outs on preamps deliver a fixed-level signal, that bass was LOUD and never seemed to vary, even as I turned the volume knob furiously. Quite a shock, I have to tell you, and not a little embarrassing in front of the wife and several observers. A good laugh was had by all including me. As soon as I pulled the plug and started over.
Once the connection bits are sorted, you need to think on just exactly where to put the darn thing. This step has caused so much furor over the years that a bit of focus and calm is called for here. Many of you may have heard that the only place for a sub is right back smack in the corner, as close to the walls and as out-of-the-way as possible. This does work in some instances, but more often the result achieved with this placement is a thick, slow, overly-resonant character to the low and midbass that tends to obscure a lot of detail and makes the lower registers of the system ill-defined and ponderous. In the world of subwoofing, placement is the money shot. You can have all manner of hardware, giant drivers, heavy-duty Class-G switching amplifiers, the lot. But if you place the driver box injudiciously, you will be faced with a quagmire instead of the deep and clean foundation you were looking for.
The secret, I have found, to getting the placement issue just right is, surprise, experimentation. You may want to try that corner, just to see if it might work, but I predict that you will find the effect not entirely what you were after. I find that in a given setup, if you can manage to keep the sub essentially on the same plane as the main speakers, between and perhaps slightly behind them, you stand a really good chance on not mucking up the midband too much. Another great location for your subwoofer is pretty much anywhere to the (either) side and rear of the main speakers, so long as it is not fully crammed in to a corner. You see, because the lowest frequencies thrive on boundary reinforcement, if you put a powerful driver fully in a corner, it is going to tend to play loud and boomy, as its response is reinforced with essentially horn loading, in a manner similar to using a megaphone to shout over a crowd of people. This is not all a bad thing; it just needs to be managed correctly to obtain the smoothest and most extended performance.
I can see the next question coming: How can I readily ascertain what is smooth and what is lumpy and what is good and what is bad? It all makes a bloody big thump and isn’t that the idea? Well, not quite. To be really right, a good sub installation will never overtly show itself. Kind of like the art of not being seen. You should be able to sit down and listen to a cut or familiar music and not be aware that there is an added presence banging away in the background. The idea is that you subwoofer remain as invisible (sonically) as possible till it is contribution is needed. To that end, once you are reasonably satisfied that you have found a spot that you feel is not to boxed into a corner, you need to start the thing making some noise so that you hear what is what. The way that I generally start is to play test tones off a CD from 20 Hz-250 Hz through the whole system, which is about as high a frequency as you would ever want to hear anything coming from a sub (really lower, but the thing you are checking for above 100 Hz is to make sure that there is little or no overlap or colouration past about 100 Hz; otherwise you will be constantly aware of its operation, and that is not acceptable).So. The idea here is to see if you can adjust the sub (by moving it around here and there bit by bit) so that you can hear these tones clearly and distinctly, in an even and smoother manner. Ideally, you should be able to hear each change in frequency as a distinct entity. If they all sound alike or are mushy or boomy, you need to keep futzing with the location. At some point I pretty much guarantee you that you will find a spot that sounds better than the others. That is where you want to be.
Then there is the issue of relative phase with the main speakers. This is the step that gives many folks pause, as they do not think that they will be able to hear the difference. But that is rubbish. If a sub is out-of-phase with the main speakers, you can easily hear it as a suck out at some frequencies and a booming, vague, and sloppy response at others. Here is the key: play just those tones between 40-150 Hz. If things are wrong, you will not get a smooth transition through the frequencies at the listening position and you will notice that you have a big boom at one or two frequencies, generally in the 60-80 Hz region. If things are right, you will get a relatively smoother and more even response that allows you to easily hear the differences between the low frequencies, and not just a dull thud coming from the back of the room. The next step, if you have access to a Real Time Analyzer, like the great portable units made by Goldline or others, is to play full range pink noise through the system at a healthy level. As you are in the listening chair, you want to see if the RTA shows you what should be at least a reasonably smooth response from near-subsonics up through a couple hundred Hz. Play with phase here, as well. Generally manufacturers will provide you with a 2-position switch that allows you go back and forth between “phase” and “180 degrees out-of-phase. Flip this back and forth and one position will yield both better quality and more powerful, punchy bass. There are also a number of constantly-adjustable phase switches on the market, and they can be a little confusing. Generally he best thing to do with them is to find the best position at either end of the dial, and then GENTLY rotate it back a bit to where the bass quality is best at the listening position.
Now you are at the point that you should start to play music. Assuming you have taken care to do the basic installation as well as you could, you should at this point be pleasantly surprised to hear a deep, weighty, foundation to your favorite discs. I find that using a really really good classical selection that has bass drum, bass fiddles, and/or an organ is generally the best for seeing about overall coherence and integration of the sub with the main speakers. One of my favorites is the EMI LP set from Sir Charles Groves and the Royal Liverpool Philharmonic Orchestra (SLS 998). The combination of organ, drum, and full orchestra makes for quite a ride, and gives you all manner of bumps, footfalls, huge, swelling organ crescendos and bass drum whacks to listen to. This should be a thrilling experience. Play with the phase again. Move the sub around just a bit to see if you can knock out any bumps in response. Another great LP for this portion of the test is the (either the original if you can find one in good shape at a decent price or the Classic Records reissue) Royal Ballet Gala (LDS 6065 Soria Series) on RCA. There are stage bumps and subway rumblings galore, not to mention a lot of full-range music. And then there is the Gladiator soundtrack on CD, which is a great test disc itself. Again, play around with slight positioning changes and phase. If you have done your job well, you should be able to tell that you have expended your soundstage, that the “lid” on the top of the soundstage that you may have felt more than actually heard before you installed the sub is gone or lifted ‘way up, and that you now have a bigger cushion of “air” around all of the instruments of the orchestra. I use primarily orchestral music, by the by, for this part of the test, because I have found that if you get it right with such material, then the odds are heavily with you that things will sound sorted and right on all other types of music. Of course, if you are not a classics lover than this is easily enough done with music of your favored type.
In the end, after you have managed to get a reasonable sound from your new subwoofer, remember the cardinal rule: do not platy the damn thing too loudly! This alone has spoiled many an otherwise fine sub setup. There is a natural human tendency to pump up the bass, and this is fine if you are doing it for playing rap, heavy bebop, reggae, or the like. But if you are playing more conventional fare, you will want to make sure that your sub gives you augmentation on the bottom, but that it does not overwhelm the rest of the music.
There. You are set. Wasn’t that bad, eh? Bollocks. You are probably cursing me and thinking that you will never get it right. Relax. We have all had that feeling. Some of us for many years. But let me assure you that with a little practice you will be a budding expert. Soon enough you will be going ‘round to your mates’ houses and adjusting their subwoofers, too. This will win you many party favors and * pints. You see, most everyone loves a good sub installation, but at the same time they really dislike a poor one. Get it wrong and you will be made fun of. But get it right and you can be the new superhero for your group of music-listening buddies.
Scot Markwell ( HIFI + magazine)
FK