Q&A # 114 - May 4, 1999

Staff

Q What is the definition of a full range speaker? Down to 40 Hz . . . 30 Hz? I have an EAD Ovation which I absolutely love and four Carver ALIII+ ribbons for my main and rears, obviously all full range. My center channel is the Von Schweikert Research LCR-31 (which you recently reviewed). On my Ovation I am able to mark each speaker as full range or "small speaker" in which case sound below the crossover frequency (which I also determine) is routed away from the "small speaker" and fed into the sub. The LCR-31 is rated at 47 Hz (regardless of what the Von web site says). If I feed the full range to this center is there a chance of possible damage as it is rated down to "only" 47 Hz? I currently have it marked small with the crossover/roll-over at 50 Hz. I did run it at full range and played some dramatic scenes with no notice of distortion.

A Strictly speaking, full range means a flat response at 20 Hz - 20 kHz, but that pretty much rules out just about every speaker. Practically speaking, I would say that if a speaker is no more than 3 dB down at 40 Hz (all the tweeters in speakers will go to 20 kHz), then it would be considered full range. It is generally the last low octave (20 Hz - 40 Hz) that is difficult for speakers. Your LCR-31 is pretty close to full range by this definition. It won't hurt your LCR to run it as full range. What will happen is that frequencies below about 45 Hz or so will have such a low volume that you won't be able to hear them. By routing the 50 Hz and lower frequencies to your subwoofer, you ensure that any of this low frequency signal that heads for the center channel will be heard (in the subwoofer).

Q I recently bought a Klipsch sub (KSW-12) which is a 12 inch bottom firing, ported sub with a class A/B 100 watt power supply for $500. I bought it to go along with the Klipsch Legend series floor standing speakers I have. It sounded a little boomy which isn't bad for movies but when listening to music, I would have preferred a little less boominess. I asked the dealer if they had a sub that was a little tighter. They told me the Mirage BPS-150i was much tighter. It's bi-polar, uses two 8" woofers (although the cone by itself is closer to 6"), has a class A/B 150 watt power supply and costs about $600. Figuring an 8" woofer is easier to control than a 12" woofer, I can see how the response could be 'tighter'. I was told two 8" woofers are equivalent to one 12" woofer as far as subs go (the dimensions of the first woofer plus 1/2 the dimension of the second woofer). So I just traded the Klipsch in for the Mirage.

My questions: 1) Are two 8" woofers (all other things being equal) generally tighter and less boomy than one 12" woofer? 2) Can an 8" woofer produce bass as 'punchy' and 'rumbly' as a 12" woofer? (With the 12" woofer you have an extra 4" of air being pushed - doesn't this help with low frequencies?) 3) Is there any benefit to using the speaker outputs from the amp (I hava a Yamaha RX-V795) to plug into the sub, as opposed to the subwoofer output from the amp, when listening to music? Is the signal identical or is it different? I know with Dolby Digital or DTS, the subwoofer output is a completely separate signal, but with regular music CDs, is there any difference? 4) Is it OK to run the sub at 90% to 100% of it's volume setting or will that shorten the life of the sub?

I know you strongly recommend Velodyne subs but I was looking to spend $600 to $800 which is still $400 to $600 less than the cheapest Velodyne I saw. I also read your review on the Mirage sub with two 10" woofers that requires an external crossover. I've only had the Mirage sub a week or so,
but it does seem less boomy than the Klipsch. It seems the volume needs to be adjusted differently for each CD I play though. Also, unlike your review of the 10" Mirage sub, if I turn the volume all the way up, it does sound boomy to me on most CDs. And, while I like the tightness of the Mirage sub when listening to music, I found I liked the little extra 'boominess' of the Klipsch better when watching movies. Would I be OK adding the Klipsch later and having two subs and only turning the Klipsch on when watching movies? Or is there a better strategy? Klipsch also makes a 15" sub for about $750, but I assume this would be even 'boomier' than the 12" model. I have another month or two to trade in the Mirage if I decide to go with a different sub (the dealer I bought from doesn't sell Velodyne).

If I were to run two subs, would I be better off connecting the Mirage to the speaker outputs and the Klipsch to the subwoofer output on my receiver? Does connecting two subs to the subwoofer output via a 'Y' connector reduce the signal strength to each sub?

A I define boominess as too much output in the 80 Hz region, compared to other frequency regions. Another problem area is around 120 Hz - 160 Hz, where too much output can make voices "chesty", like they have been smoking three packs a day. Too much output in the 800 Hz region causes voices to sound nasal, and too much at 6 kHz causes excessive sibilance. Woofers are responsible for boominess and chestiness. These artifacts can be caused by several things. One is harmonic distortion from the cone if it is driven hard. Second is harmonic distortion from the amplifier. A third cause is from the big box, usually empty except for the driver, that resonates. It is generally true that smaller drivers are easier to keep under control. That is the main reason the Mirage BPSS-210 uses smaller drivers (10"). However, the Mirage also uses servo-feedback, which further reduces distortion. The high-performance Velodynes use servo-feedback too.

The problem with small drivers comes from needing to move lots of air for the low frequencies, and small drivers don't do that very well. Let's assume that 8", 10", 12", and 15" drivers use all of that dimension for their piston. The 8" has a piston surface area (the flat two-dimensional area facing into the room) of 50.27 square inches, the 10" has 78.54 square inches, the 12" has 113.1 square inches, and the 15" has 176.71 square inches. Even small drivers are capable of 2" peak-to-peak motion these days, so we can just make direct comparisons between these areas. Two 8" drivers are like a single 12", as you said. However, an 8" subwoofer could easily produce boominess if it has significant harmonic distortion. Also, if the amplifier driving it is small, harmonic distortion could arise. Harmonic distortion in subwoofers is not nearly as irritating as harmonic distortion in higher frequency ranges. So, the boominess is often actually desirable, especially for movies, as you have found. But, for music, it is annoying. With a servo-feedback subwoofer, as long as you don't set the crossover any higher than about 60 Hz - 70 Hz, you can turn it up quite loud without hearing any boominess. The ideal situation for me, is to have an 18" driver (254.47 square inches piston surface area), since I hardly have to turn the volume up at all. This keeps the distortion very low. Of course, it sits in a very large box, and this is not practical in all situations. I use a 10" in the living room, also turned down very low, but it is for a music-only system that we don't ever play very loud.

The signal coming from the subwoofer output jack on receivers has passed through a crossover, with the low pass set to around 80 Hz. The signal coming from the speaker outputs is full range, and has not passed through a crossover. So, the subwoofer output jack signal has phase shift, while the signal from the speaker output does not. And, this is before it has even arrived at the subwoofer. However, generally, we prefer to use the line-level subwoofer output jacks rather than the speaker-level connections due to possible impedance matching problems between the speaker connections, the speaker cable, and the high-level inputs on the subwoofer.

I would suggest considering another Mirage as your second subwoofer rather than getting a second sub that has lots of boominess just because it does have the boominess. On the other hand, you may be able to manipulate the boominess on the Klipsch merely by adjusting the low pass crossover to a lower frequency, say, around 50 Hz, and maintaining a conservative hand on the volume control. That way, you could have both subs on even when playing CDs. Using a Y connector should not cause any problems with your bass signal. It might if you were trying to divide the signal three ways by using two Y connectors, in the case of unusual input impedances, but not by using just one. However, if it does reduce your signal a little, just turn the volume control on your subs up to compensate.

As to the cost of Velodynes, they have lots of models in the price range you mentioned, and we will be publishing a review of one of their new CT series in the coming weeks.

Q I am in the process of putting together a home theater system. I plan to build the speakers myself, so I've been trying to educate myself on good characteristics and configurations for home theater speakers. I am in the process of designing a center channel setup. The D'Appolito configuration seems to be a very popular choice judging by many commercial manufactures as well as articles I've read (such as the one by Chad Grey). Here is my question. In the subwoofer DIY page, Brian Steele talks about D'Appolito designs not having "broad horizontal dispersion". Instead, he uses a Synchron coincidental driver (KEF Reference series style). Mr.. Steele's theory is that the MTM (Mid-Tweeter-Mid) is used for its greater power handling and sacrifices the horizontal dispersion. Do you guys have any wisdom on this matter?

A Building speakers from scratch means choosing raw drivers, as well as designing the crossover and enclosure. If you're planning on modifying drivers, look into a good program that covers mechanical and electrical engineering, as well as specific courses in acoustics. That said, assuming that you're looking to design and assemble a loudspeaker from available drivers, we can probably help. As far as the D'Appolito configuration for a center channel, I'm of the opinion that it may work very well in some circumstances where the listeners are all in the central location directly in front of the speaker, but that if a large listening area is desired, the MTM general design is to be avoided. The advantages of that same configuration when stacked vertically (see diagram below) is that the midrange/midbass drivers cause cancellations above and below the speaker along the vertical access, lessening reflections off the floor and ceiling, often to the benefit of improved imaging. Also, as the article you've read stated, it raises power output. It both doubles the power handling, and doubles the sensitivity of the speaker, so that one more driver quadruples the dynamic range of the speaker on axis. Of course, it retains the same improvements in dynamic range when laid sideways, but now provides a very small listening area, and loses the benefit of limited vertical dispersion. If you and another cuddle on a couch while watching a movie, it's not really an issue, because the sweet spot is right where you both are. If you're the only one who really cares about the sound quality, and you can always sit in the middle, same deal. However, home theater is somewhat social at times, so this flaw of a sideways D'Appolito may be worth addressing. Here are a few methods.

One, which you've mentioned, is to use a concentric tweeter/woofer array, used by KEF, Thiel, and Tannoy. This provides a pretty even response on any axis. The downside is that you've mounted your tweeter to a baffle that constantly changes, and may or may not be the best surface to deal with diffraction artifacts (delayed arrivals due to uneven surfaces that smear performance in the time domain while more importantly causes uneven treble response). Some of these work well, others don't. If you find one you like the sound of, you've got a solution right there. Another is to do what M&K does, use relatively small mid-bass drivers with inherently wider dispersion than the common 6" woofer, and put them smack against each other, with the tweeter to one side, instead of between them. The closer the woofers are, the less the lobing effect. This maintains the benefit of increased dynamic range and lower distortion, while maintaining a fairly good horizontal dispersion. The speaker no longer looks very symmetrical, which is probably why most manufacturer's don't do it. This doesn't have any significance unless the grilles are off. We have M&K's THX select system under review right now, and I have to say that it works very well. Details on that will follow soon.

Another solution, which Atlantic Technology takes full advantage of, is to make the center channel a three-way speaker by adding small midrange drivers which can be vertically stacked. You then have a D'Appolito system in a vertical position, with woofers in a horizontal alignment. At 300 Hz and below, depending on the distance between the woofers, lobing doesn't become much of a problem, as the wavelengths are relatively long. The difference in distance from driver to listener off axis translates to less of a phase shift, and hence, there is less or no cancellation. You can also keep the D'Appolito configuration, but modify the crossover to change the system from a simple two-way to a two and a half-way crossover. Take the first woofer and tweeter, and design a system with a very gradual roll-off (low Q). Then, crossover the second woofer only when the first woofer begins to roll-off the lower octaves, so that the second woofer contributes to the extension and bass output, but not to the midrange frequencies which would cause the undesireable lobing characteristics. Getting the two woofers to sum flat would take a little trial and error in terms of cabinet design and crossover components, but if you're designing a loudspeaker from scratch without any expert help or expensive measuring equipment, I'd not count on getting it right the first time anyway. A reference which you absolutely cannot do without if you want to take this seriously is "The Loudspeaker Design Cookbook" by Vance Dickason. Basic enough to dig into from the start, but very informative, with plenty of extra references. Perhaps even enjoyable reading. At least I thought so. Good luck, and let us know how your project turns out.

Q My house has a fairly large unfinished basement that I would like to finish, at least in part, into a home theater/listening room. I can't seem to find much more than scraps written about the desirable
characteristics of a home theater room or a listening room. What proportions are desirable? How small is too small? What acoustical characteristics are desirable for the front wall, side walls, rear wall, ceiling, and floor?

A The "ideal" room ratios are supposedly 0.6 x 1 x 1.6. In other words, if you have eight foot ceilings, the smallest useful horizontal dimension would be 8 / 0.6 feet wide, and the length of the room would be 1.6 times that. That calculates out to a room that is 8' high x 13' 4" wide x 21' 4" long. The theory behind this is that the "ideal" room proportions distribute standing waves relatively evenly across the low frequency spectrum, avoiding big dips and peaks. If that's not possible, then just try to avoid room dimensions that are multiples of each other, for instance 8' x 16' x 24'. A small room isn't necessarily bad, except that it usually has closer reflective surfaces. Absorption near the speakers, along wth diffusion (such as that provided by book shelves and books of different sizes) around the rear of the room certainly help. Slighly non-parallel walls can provide a benefit of breaking up axial (between two parallel walls) modes, but also make prediction more difficult.

A benefit/detriment of small rooms is that, below a frequency whose wavelength is 1/4 the longest dimension of the room, waves no longer exist as waves. The room then becomes a pressure cooker of sorts. For instance, if the room's longest dimension is 11 feet, that corresponds to a wavelength of roughly 100 Hz, so that below 25 Hz, where the wavelength is 44 feet, efficiency of the bass driver increases at 12 dB/octave. It then becomes feasible for a speaker that rolls off around 30 Hz anechoically to yield response well below 20 Hz. In addition, at 25 Hz, where cancellation becomes impossible, since standing waves cannot really exist in a small room with a maximum axial distance of 11 Hz, a room with a maximum dimension of 22 feet could have real problems producing even bass across the listening area at 25 Hz and 50 Hz, especiallty in the middle of the room. On the other hand, if you put a subwoofer which truly went down to under 20 Hz without any room effects, the low frequency response would be absurdly exaggerated. Generally speaking, to accommodate our usual listening position of being seated, floors and the wall behind the front speakers should be absorbtive, while the ceiling and the rear wall should be reflective, but not uniformly flat. It's just a general rule, so don't assume that you have to carpet the wall behind the TV. If none of these formulas will fit in your room, then just grab a beer, crank your system up, and don't worry about it.

Q I will be purchasing an entire home theater system, which will include a Yamaha RX-V995 receiver with DTS and DD. I would like to know which DVD player to get. Obviously, I want something that is compatible with DTS. I was looking at the Yamaha and Sony players. I also want something that is set up for future technology.

A All of the newest DVD players will output DTS bitstreams, but as far as future technology, it appears that music is in for a change late this year or early next year. Panasonic's 24/192 has been approved for music DVDs, and Sony/Philips' SACD is also in the wings. Both of these are on the DVD format, not CDs, and none of the current DVD players can play them. However, it will be a couple of years before there are significant numbers of music discs out there for consumer use, so I would suggest getting whichever DVD player suits you (make sure it has the DTS digital out stated on the front of the player or in its spec sheet), and worrying about 24/192 and SACD later.

Q How would one know if a DVD widescreen version is of the real widescreen (using all the 720x480 resolution space of the DVD format) and not just a letterbox onto a 640x480 resolution space? Will it be labeled on the box differently ? How does one read the "fine-line" of the label? Also, I found that on my DVD player (Pioneer DV-414), when watching a "widescreen" movie, I can't set it to Pan & Scan unless the disc has the P&S track (usually on side 2 of the disc).

A If the movie has been transferred to the DVD as "16:9 enhanced", this means it is taking advantage of the mode on 16:9 TVs that allows you to "stretch" a 4:3 image sideways so that it fills the screen without any (or without so much) blank area at the top and bottom. This gives you a better picture than simply enlarging the entire widescreen image. The DVD box will state that it is 16:9 enhanced. Of course, you need a 16:9 TV to take advantage of this feature. Originally, the DVD player was supposed to be able to play the movie in widescreen or P&S from the same track. That did not work out, so the manufacturers just put the widescreen version on one side, or one layer, and the P&S version on the other side, or the second layer. We may eventually have players that will play both sides of the disc without having to turn the disc over, but in the meantime, multi-disc players are coming out now, and long movies ("The Ten Commandments", "Dances with Wolves") are being released on two discs rather than two sides of one disc.

Q Is it possible to record DVDs on VHS tape by taking the output from the television and sending it to the VCR? I was accustomed to making high quality copies from laserdiscs that I rented, but now I have not been able to make any copies from the DVD rentals. Am I hooking up something wrong, or is the copy guard preventing this?

A Yes, it is copyguard that is preventing you from copying DVD movies to tape. I think the manufacturers are not really concerned about consumers making copies onto VHS tape because VHS is analog and the copies are terrible compared to the original. Even S-Video does not approach the quality of a DVD. Also, analog tape can't take advantage of the 5.1 surround sound tracks. It is the future digital tape and recordable DVDs that scare them, because perfect copies can be made and sold as pirated movies. Probably 99.9% of consumers don't ever even remotely consider pirating movies or music. The 0.1% of consumers who make up the pirates, will find a way to get around the copyguard and sell pirated movies, and perhaps even sell the technology to consumers who want to make their own copies for private use.

Q I have a Pioneer 46" projection TV and was wondering if I will realize any improvement in resolution with a DVD player over a VCR with this type of TV? The DVD sounds like the way to go, but will the TV be my weak link now?

A You are in for a real treat. DVD looks far better than a VCR tape with any TV. Although it will be very nice even using the composite video connection (RCA) out of the DVD player to the TV, you should use the S-Video connection from the DVD player if your TV has an S-Video input jack. No, your TV is not the weak link. Your old VCR is. Once you see how fantastic DVDs are, your VCR will probably gather dust, as mine has for the past several years.

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