Q&A # 211 - December 21, 2000

Staff

Q I recently purchased a Vantas DPA-P87 Dolby Digital processor/amplifier to integrate with my Denon AVR-3000. I have all Klipsch speakers with Quartets for my mains, KV2 center, and KG5 for rears. The Vantas drives the center and rear channels while the Denon drives the mains. In DD mode, while playing a DVD from my Pioneer DV-434 at any reasonable volume, and when there are dynamics in a scene, I get a horrible crackling sound from center and or rear speakers. I dont have another DD source to try and the problem never happens in Pro Logic mode. Is the problem in my speakers that they cannot handle the dynamics of DD or is the Vantas being overdriven? Please help. I found out Vantas is out of business and there is no knowledgeable tech support.

A The problem is that your Vantas amplifiers are being overdriven. The crackling noise you hear is the amplifier clipping. I am surprised that this is happening with your Klipsch, which are more sensitive than most. First, try just using the Vantas to drive the center, and then try it with just driving the rear speakers. If the clipping stops with one of these procedures, then you only need to get one or two channels of amplification. If it continues even when just driving one or two speakers instead of three, then you may have to junk the Vantas and get something else. Dolby Digital requires lots of power. It is full range in all channels, and with any deep bass coming from the center or rear channels, amplifiers can be overdriven more easily than with Pro Logic, which is more limited in the extent of frequencies in the center and rear (especially the rear).

Q I have tried for several days to log on to Maple Audio Works web site http://www.mapleaudio.ab.ca, but it doesn't seem to be active. I have also tried to log on through several of the audio link sites with similar luck. After reading your interconnect review posted last week, I am interested in their interconnects. Is the company still around? Am I using the right address?

A We received e-mail from them this morning. They are working to restore their website (what a heck of a time for it to go down!). In the meantime, you can contact them by e-mail. The contact person is Gilles Charette. His e-mail address is [email protected].

Q I have my rear surround speakers set up high and behind the viewing sofa. I notice that, except for real loud action movies, I get almost nothing out of those speakers when watching DVD in 5.1. I have increased the volume on the rears but without much difference. Is it the positioning or is there not much programming for those speakers on the DVD?

A You are right about the action movies. They seem to use the rears more than some other types of films. I turn the rear channels up higher than default, as I like to have more from the rears too. You can try pointing the speakers down and toward your listening position. That will give you more noticeable sound from the rear.

Q I've been waiting for the HDTVs to become more readily available (good bang for the buck). Having started to look around the web on news groups and different manufacturers web sites, the conclusion I have been forced to come to is that marketing hype does not match the performance of the available (here in Canada) sets. Are you guys going to do a comparison of direct view 30 - 37 inch HDTV sets, similar to the one you did on DD receivers a couple of years ago? I found the DD one informative enough to finally go out and buy one that I have been very happy with (TEAC 8900).

A The problem as I see it is that some of the HDTVs on the market now are mediocre. They do a terrible job of line doubling an NTSC program, whether it is broadcast or DVD. In fact, I compared a direct view HDTV side by side with its NTSC counterpart. The NTSC picture was sharper (!!) even with the scanning lines visible. What's happening is that good HDTV is expensive to manufacture right now, so companies are making junky ones to get some HDTV sales. You should be very careful when shopping for one, whether it is direct view or projection. At this point, the HDTVs below about $3,500 US don't look very good to me. If you buy one, make sure it can show 1080i native with full resolution. This means 9" CRTs in rear projection TVs. 720p programming may never come about, but the ability to show it in native format (not converting it to 1080i), will mean it is a good product (and undoubtedly pretty expensive too). To me, there is no point in spending several thousand dollars on an HDTV that does not quite show HDTV in its full resolution.

Q I have found through trial and error that in many instances, a sealed type subwoofer delivers a more accurate bass reproduction. The bipolar subs such as the Mirage BPS series (bipolar) have the added advantage of a completely dead box which in turn adds no floor or cabinet resonance. My question is, why are there not more examples of this type of sub on the market?

A Sealed enclosures for bass drivers (they used to be called acoustic suspension) require that the driver have a lower free air resonant frequency (fs). This type of driver is more expensive to manufacture. A bass driver in a sealed enclosure needs more amplifier power too. So, with price point being so important these days in a mass market, ported subs are the norm.

Q My wife just purchased the Monitor Audio Silver 9i for our home theater and occasional stereo use. I have the top of the line Marantz THX certified components (SR18, DVD18A, 700MA amps ). I read your review on the Silver 9i. I am not familiar with speaker break in . Please tell me the hows and whys.

A Because speakers have moving parts, those parts loosen up over time. So, manufacturers build them tight, knowing that they will loosen after you use them for awhile. This means that, out of the box, they do not sound optimal. That is why they have to be broken in. It will happen by itself as you play music and watch movies, but if you want it to happen more quickly, just pick out a dynamic CD and let it play on constant repeat at modest volume all day long for about a week.

Q Regarding your recent review of a ribbon hybrid and mentioning that line source speakers will have less dropoff in volume at various distances compared to cone speakers, could you elaborate?

A A ribbon in itself is simply a transducer. Line sources and point sources, in reality, are merely theoretical extremes. To be truly a point source, the object would have to be a point, or at least a small, pulsing sphere. To be a true line source, the object would have to be infinitely long, and very thin, theoretically, infinitely thin. Whether or not a transducer, or for that matter, a loudspeaker, acts similar to a theoretical point source or line source depends on its own dimensions, the nature of the diaphragm, i.e., a stiffer diaphragm is in this sense acoustically larger (more directive) than one that flexes, and how that relates to the frequencies it reproduces.

To emulate a point source, the radiation pattern must be spherical, or at least somewhat hemispherical. Dynamic drivers that have wide dispersion along that frequency range generally act very much like a point source.

To emulate a line source, the radiation pattern must be cylindrical.  Long ribbons tend to emulate a line source at higher frequencies, as the length of the ribbon causes the radiation to beam so long as the wavelengths of the reproduced frequencies are small compared to the length of the ribbon.  But, even a 6 foot ribbon is not a line source at 100 Hz. By definition, if the ribbon doesn't beam along the axis of the length, then it's very much not a line source. 

The rough approximation of whether a driver beams (as I said, it depends on the nature of the diaphragm flex) is that it will begin to beam as the wavelength approaches the diaphragm diameter (or length). Even though the mid-range ribbon in that review is supposedly 20-something inches, the radiating area of each side of the tweeter is 7 inches in length. With the speed of sound at sea level at room temperature (average humidity I guess) being more or less 1070 ft/sec, that means that the approximate frequency of beaming would be 1.8 kHz. It will beam a bit less below that frequency, and probably approaches a line source at the crossover frequency of 2.5 kHz. So, it might act like a line source, in terms of dropoff for a very narrow frequency range. The tweeter definitely acts very much like a point source just above the crossover point, which would mean that the frequencies above the crossover point are falling off at a greater rate than the frequencies just below the crossover point, indicating that the frequency response would only be matched at a given distance, and that getting closer or farther would throw everything into imbalance.

However, ribbons flex a lot, especially near the edges, so I think that aside from being quite directional horizontally in the midrange, exacerbated by having two radiation areas, separated by a substantial distance, causing more directionality and comb filtering to boot, the dropoff is probably more like a point source, which in this case, is a good thing, giving the design a reasonable chance at good performance on-axis at a range of distances, with probably a narrow dip in the total power response in the upper mid-range, unless they boosted the on-axis response, neither of which is ideal, but then loudspeakers inherently are a compromise.

Q I am currently using a Sony DA30ES with a 10 year old pair of Mission 770s and am considering upgrading to improve the sound. My listening room is approximately 14' x 16' in the basement of my house.

After stumbling across the RPG and Auralex web sites, I thought maybe I should take their advice and acoustically treat the room. The listening room is a typical rec room found in many homes, and I don't believe I have any glaring negative acoustical properties in the room that would seriously degrade the sound, i.e., large picture windows or other areas of high reflectivity. In fact, there are no windows in the room. After e-mailing the room specifications to both companies and receiving their treatment recommendations, I visited a couple local audio dealers who offer acoustic treatment services.

Well, everyone has a very different opinion. One says I need a new amp and recommended a Rotel RB-991. RT. The other says I need new speakers. Neither recommended acoustic treatments. Naturally, both RPG and Auralex claimed following their advice would dramatically improve the quality of the sound. Obviously going ahead with any of the recommendations would offer an improvement. My question is how effective are acoustic treatments and what could I realistically expect by treating the room? It would cost ~$1000 - $1500 to go with one of the upgrades.

I am not a hard core audiophile, but I do appreciate good sound. My complaint with my existing set-up is that I get tired of listening after ~30 minutes ( SPL ~85dB). I enjoy the sound initially, but I guess it grows too bright after awhile. Maybe I'm just getting old. Which way would you go?

A As I grow older, I notice that I don't like to listen to high volume any more. I suspect that the connections between the three little bones in our middle ears are stiffer in later years, and perhaps the signal ultimately transmitted to the inner ear (cochlea) is more distorted when those bones are asked to move more, as in cranking the volume. Anyway, I can't handle mass market products like I used to, because they sound harsh at high volume anyway. My suggestion to you is to upgrade your preamp/power amp setup before you deal with the room treatment, especially if you don't have any serious problems with the room now. Also, you can make your own room treatments. Start by putting some blankets on the walls until you see where any improvement might be made. Then, go to a bedding store and buy an egg shell mattress cover. A king size one will be about $35, and they work very well, although they suffer in the aesthetics department.

� Copyright 2000 Secrets of Home Theater & High Fidelity
Return to Q&A Index.