Q&A # 190 - July 20, 2000
Q My question has to deal with dipole speaker construction. I am trying to build my own, and the question I have is: with drivers on either end of the speaker, is the air space between the drivers all shared air space? Is there nothing more than some bracing, "Acousta Stuff", and glue?
A It could be, though it depends what you're using it for. If you're doing dipole mid-ranges which may reach as high as 3 kHz, then you might want to put the drivers in separate enclosures so that the rear waves of one don't interfere with the other. If you have lots of absorption, then it may not matter at all, except that the dipolar arrangement in the same cabinet will trick each of the drivers into thinking that they're operating in a larger cabinet, due to the lack of back pressure. This may or may not be appropriate, depending on what you're trying to do. It will certainly decrease the low frequency output. If you're talking about woofers, you can certainly share the same air space, and if that air space is small enough, you'll gain the distortion-lowering benefits of push-pull operation, essentially having an isobaric configuration without the enclosure. But since they're operating acoustically out of phase in terms of their relationship to the outside world, unless you have a really wide baffle, you won't get much in terms of low frequency output. Something you might want to consider are dipolar mid-ranges and tweeter arrays, with bipolar woofers. The crossover between them might be tricky, but it can be done well. To refresh our readers as to how the drivers move in dipolar speakers vs. bipolar speakers, here are some animated gifs. Click here to see dipolar. Click here to see bipolar. Electrostatic, planar magnetic, and ribbon speakers are dipolar in their output of air on the front vs. the rear of the enclosure.
Q I'm interested in getting an HDTV-ready RPTV, but I'm confused about the CRTs. Most RPTVs have 7 inch CRTs, but I've read that to get the full resolution of HDTV, you need at least 9" CRTs. Is this the case or does this apply to 60" and larger displays?
A There are a number of things to consider when choosing your HDTV. One is the size of the CRTs as you mention. It takes 9" CRTs to resolve the 1080i format. Secondly, the RPTV may not even use CRTs. It may use LCDs, in which case, it won't fully resolve 1080i. LCDs down the road will do it, but not right now. Third, even if it will do 1080i, it probably won't handle 720p, because that requires a higher scanning rate, which most projection TVs won't do. Lastly, projection TVs often scale the input signal to some predetermined format, such as 1080i, without giving you a choice, and many times, they don't do a very good job of it.
Q I am in the process of purchasing a front LCD projection TV for my media room. In evaluating the different projection units, I have noticed that some projectors do a better job displaying scenes with various black backgrounds. I narrowed my selection to unlikely home theater projection units, the Epson Powerlight 7250 and 7350. The main difference is that the 7350 has 300 more lumens (1600 vs. 1300) and a 300:1 contrast ratio vs 200:1. Since I have no windows in my media room, I do not feel that the extra lumens make enough difference for me to pay the extra $1,500. But, I am concerned that 200:1 will not handle the dark backgrounds in some movies. Any input? Is there a bigger item that I should be looking for in selecting a projection unit?
A You have hit on one of the main problems with LCD projectors, namely, the ability to do blacks well. I would suggest the unit with the better contrast ratio. Even though you might not need the extra brightness, the blacks will be better.
Q Could you please explain the proper use of subwoofer crossovers and the best way to hook up a sub? I am not asking about a home theater system but rather a 2-channel system with a separate amp and preamp. I am confused about the function of the low-pass crossover and the high-pass crossover. How do the low-pass and the high-pass crossovers interact and when each is used or not used? In addition, how do you determine the initial crossover settings, just by taking into account the frequency response of the main speakers? I realize the crossover setting depends upon the slope of the crossover so could you explain a little about the slope and integrate this explanation in the crossover frequency setting explanation? By the best hook up, I am referring to using either the speaker level or line level? I am referring to a 2-channel system with no sub pre-out.
A I think part of the confusion stems from the fact that the subwoofer low-pass crossover is adjustable, and the high-pass crossover is fixed. In other words, if you send a signal to the subwoofer, you can adjust the low-pass frequency to, say 50 Hz, which means everything below 50 Hz goes to the subwoofer and everything above 50 Hz is filtered out (removed). The high-pass might be 90 Hz, and this means everything above 90 Hz is sent out from the subwoofer back to your satellite speakers and everything below 90 Hz is filtered out. The high-pass stays at 90 Hz no matter what you set the low-pass to. There is a low pass frequency at the subwoofer pre-out jack in the receiver too, and it is fixed. There is no high-pass associated with the subwoofer pre-out. With your setup, you need to use a Y adapter between the preamplifier and the power amplifier. One leg goes to the preamplifier output jack, the second to your power amplifier input jack, and the third goes to the line-level input on your subwoofer. Do this for both channels, using both input jacks on your subwoofer. The slope is usually a multiple of 6 dB per octave, i.e., 6 dB, 12 dB, 18 dB, 24 dB, and means that above the low-pass setting, the signal decreases at a rate of 6 dB per octave or its multiple. So, if the low-pass were set to 50 Hz, the signal would be 6 dB lower at 100 Hz, then decreased 6 dB more at 200 Hz, 6 dB more at 400 Hz, and so on. Most powered subs these days have slopes of 18 dB or more, so that you don't hear voices coming from the subwoofer. Subwoofers work best when set to 50 Hz, because above 50 Hz, you can begin to tell where the subwoofer sound is coming from. For the really small modular speakers, you don't have much choice, and you have to set the low-pass to nearly 100 Hz, but for bookshelf speakers and floor standing speakers, start by using a 50 Hz low-pass setting on the subwoofer.
Q I purchased a Rotel 965 receiver - it's the second unit I have now - and when the volume is set to minimum, I can hear a buzz from the tweeters. It's the same when I go over 65 (a very loud hum and noise from the tweeters). I've tried to disconnect everything and leave the Rotel only with speaker cables, and I still hear a hum. I suspect it's something with grounding, but how do I fix grounding?
A If you have any nearby lights on rheostats, that can cause the noise. If so, try turning them all the way off or all the way on. Fluorescent lights can also cause a problem. Other than that, you can ground the chassis using a wire connected to one of the screws that hold the chassis cover on and connect the other end of the wire to the ground post on a grounded AC receptacle. You have to be very careful with this so as not to short anything out. If one of your other hi-fi components is a three wire grounded system, you can ground the Rotel to the chassis on that component. Make sure your AC outlet has an actual ground connection first. Many old homes are lacking ground wires in the receptacles, even if they have grounded AC sockets. I am sure you realize I must say that any electrical wiring changes you make are your own responsibility.
Q I'm interested in getting an outboard DAC hooked up to my CAL Icon MKII as the transport. My friends told me to get a DAC that has HDCD decoding, and although I do not have HDCD discs, because the DAC has HDCD, my normal CDs will sound and play better than before. Is this true! If it is, in what way will my normal CDs sound better?
A The idea behind HDCD is that a 16 bit CD is decoded in such a way that it reproduces sound more as if it were 20 bit. So, the DACs must have 20 bit resolution. As such, non HDCD discs are given a boost too. However, now that we have 24 bit upsampling, the idea that non-HDCD discs will sound better with HDCD DACs is not so important. If you don't plan to buy any HDCD CDs, then get a DAC that has upsampling as one of its features (it might have HDCD too). Also, you can play HDCD discs with a DAC that does not have HDCD decoding.
Q I have a question about setting levels of the subwoofer and channel balance. When running through the test tones and a few other Test CDs, I find that I am turning down the gain of my subwoofer control, even though I have a THX multi-channel amp and a THX Processor. The dealer helped in the set up and arrangement of the speakers and he too turned down the level of the sub to better balance all the channels. So why do I need lots of amp/watts for the subwoofer? I read a few weeks ago and many times in the past from your Q&A that the more power for the LFE, the better. When the gain/level is turned down to 1/2 or 2/3 position, am I not reducing the power output of my amplifier? Thanks for your help and assistance. I'd like an impartial opinion before I purchase another sub or more powerful amp to drive the LFE.
A There are several reasons to get a sub with lots of amplifier power. One is that the subs with high power amplifiers also have much better drivers that will go to very low frequencies. Secondly, even though you are turning the volume control down, there are still peaks in the sound tracks that will demand plenty of power from the amplifier. Low frequencies are extremely demanding on the amplifier, especially when you go down to 20 Hz and below. Let's put it this way. I have the ButtKicker that I reviewed recently, and it is driven with a 2,000 watt power amplifier. I have it configured so that it gets frequencies down to 15 Hz or so. When I am watching an intense movie, that 2,000 watt amplifier starts clipping. Also, even if you never really crank the subwoofer up, it will still sound better if it is a high power unit, because at low volume, there is almost no distortion with the big subwoofers.
Q I need help with proper surround speaker type and placement. Here is the setup: Denon AVR-4800 THX/Ultra receiver with Surround EX. New speakers to be purchased. I am looking at Paradigm Monitor 9s with dipole surrounds or the THX Select M&K 750THX system (great review from your magazine!) My room is 21' wide x 16' deep. Due to room design, I must have my HT setup with the sides being the wide dimensions of the room (21 feet) Also, there is no way to mount surround speakers on the wall to the sides in a classic THX dipole design. (One side is a brick fireplace and the other is partially open to the kitchen. Bummer.) The back wall is completely available for mounting in any configuration. I want to buy an extra set of matching surrounds for the EX surround feature. What is the best setup in this scenario? Can dipoles be used effectively on stands to the left and right of the viewing area, where I don't have walls to mount them on, or would direct radiators serve better? If I go with direct radiators, what would be the best placement and orientation for them including a set for the 'Back EX Surrounds'? Are direct radiators effective if mounted to the sides like a dipole would be, or should they be kept more behind the listening position? Can all four surrounds, LS, BS, BS, RS be mounted in a line on the back wall? Also, what would be the optimal height? Sorry for the barrage. I have not been able to get good unbiased (read non-salesman) responses at my local dealers. I am leaning toward the M&K arrangement as I like how they sound and the reviews. I can't audition both the M&Ks and Paradigms side by side as they are at different locations.
A You could use dipoles or direct radiators in the front and on the sides. If you have that much space side-to-side, then put the speakers close to the couch and aimed toward the center of the room if they are direct radiating, and more towards the couch if they are dipoles. With your setup, I would suggest having direct radiating or dipoles but not both. For the rear EX/ES, the speakers should go on the wall up high, pointed down towards the couch. Our review of the Yamaha RX-V1 receiver next week will include extensive diagrams of the recommended speaker placement for EX/ES in ideal situations.
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