Q&A - #13 - May 21, 1997

Q Would you please recommend a rear-projection TV with a picture quality that rivals a direct-view television, as I would like to replace my 32" Sony XBR with a bigger screen?

A Unfortunately, no projection TV rivals that of a good direct view TV. A projection TV uses three CRTs, one each for red, green, and blue. Then, the image has to go through at least one lens, and most use three lenses. Finally, the image is focussed onto a screen which scatters the light. So, you have image degradation from (1) composite image being formed by three CRTs whose individual images cannot be perfectly aligned with each other; (2) image passing through lenses, many of which are plastic; and (3) image scattering from the projection screen. Several years ago, I saw HDTV being demonstrated in Las Vegas at the CES. There was a direct view HDTV and a rear projection HDTV as well. The image quality was much less sharp on the projection TV. We now use a good direct view 35" TV here in the lab. It has a fantastic image compared to projection TVs, but of course, it does not have the nice large picture that is so enjoyable with movies. So, if you really want a big picture for movies, or watching football, etc., by all means get a projection TV, but for sharpness and color saturation, it will not be as good as the direct view TV.

Q For subwoofers, I have four 12"s, two per box, crossed over at 150 hz and two 15"s, one per box, crossed over at 50 hz. I currently have the 12s in a left-right orientation with the midrange on top with one 15 under the TV facing the viewer and the other on the rear wall facing the screen. I have 1/3 octave eq's and a sine wave generator which I have used to smooth the respones at the listening position. I understand that moving the subs while listening/testing can improve response and standing waves. Does an asymmetrical positioning of the subs produce a better overall reponse as opposed to a symmetrical placement. I am also currious if there is any information on how to build a bass trap. I would really appreciate your views on this subject.

A Multiple subs make a very big difference, partly because each one can be turned down so it is not doing all the low frequency work, with the result being less distortion. Asymmetrical placement is usually the best. Try putting a sub where you normally would be listening (the TV viewing position) and moving around the room until the bass sounds the way you want. Then reverse the position, i.e., put the sub where you were listening. With multiple subs, it becomes more complicated, so you might want to have all the subs being fed a sine wave when you are adjusting the position of each sub. Probably best to optimize for a region where there is going to be lots of bass, rather than, say 20 Hz. The 30 Hz - 40 Hz area is a region that gets plenty of action. Bass traps are more complex than traps for the higher frequencies, since low frequencies are harder to absorb. Most of them appear to by large stuffed cylinders placed in the corners. You might try putting some extra pillows (bed pillows, seat cushions, etc.) in the corner, and measuring the SPL of 31.5 Hz at the listening position as you add pillows. Then you will have an idea of how big a cushion to make. You can buy polyfil at material stores and craft stores. It costs a couple bucks for several pounds. You might also stuff it with the paper pulp material that is used for attic insulation.

Q In the 5.1 recording on a DVD, either music or movie, is audio information present below 100 Hz in the left, center, and right channels? If this information is there, is it going to be removed by either a DTS or AC-3 processor? It seems logical that a processor would remove this information since there is a sub channel and the small LCR speakers need protection. However, my left and right speakers are large and have 12" woofers that I would like hear producing low sounds. I also have two 15" subs to produce the really low signals. Thus, my problem is clear. I will obviously use my 15s on the sub channels, but I have gotten used to hearing four 12s and two 15s in my Pro Logic system. Will this continue to work in 5.1?

A You can program the processors to channel the bass from one channel to another, depending on the size of your speakers. However, full spectrum sound will go to all channels, unless you program the processor otherwise. The Low Frequency Effects (LFE) channel with AC-3 and DTS has its own signal. You can send low frequencies from other channels into it, or you can send the LFE signal into other channels, such as the front left/right (if you didn't have a subwoofer, and had large front left/right speakers). As long as your processor is not programmed to re-route signal from one channel into another, you should be fine.

Q I know you have dealt with ohms and wattages so much that you're probably sick of it (but thanks to you guys many beginners are much wiser in this area). But just one more, what do speaker manufacturers mean when they say 4/8 ohms or 4 to 8 ohms impedance? If it means 6 ohms why don't they just say so? They can't switch from 4 to 8 ohms and vice versa or could they?

A When the spec sheet of an amplifier says that it will handle 4/8 Ohms, it means that it can deliver power to speakers with an impedance as low as 4 Ohms. All of them will handle 8 Ohms. With lower impedances, more current is demanded, and some amps can't handle it without overheating. Actually, the impedance of the speaker is "nominal" which means that it "averages" around 8 Ohms or 4 Ohms. The actual impedance varies with the frequency of the music signal, from perhaps 5 Ohms to 12 Ohms for an 8 Ohm (nominal) speaker. Speakers will state their nominal impedance, with the standards being 4, 6, and 8 Ohms. If your speakers state a nominal impedance of 8 Ohms, no problem. If they are 4 Ohm speakers, then you just have to make sure you have an amplifier with a good power supply. Generally speaking, the mass market receivers don't do very well with 4 Ohm speakers when cranking it up for a movie with a powerful sound track. Best to stick with 8 Ohm speakers that have good sensitivity (> 88 dB/w/m) with these receivers. If you have a receiver with pre-out jacks, then get as much power as you can afford in an outboard multi channel amp, and the speakers that you really want, regardless of their impedance and sensitivity.

Q I have read about "passive radiators" in the KEF Q-series and the B&W 600-series line. What is the difference between a "passive radiator" and a woofer? Can it act as a replacement or is it just a supplement to a woofer?

A Passive radiators are very efficient means of getting more bass. They are usually (but not always) in sealed enclosures. One driver is active, meaning that it has a voice coil, connected to the amplifier. The passive radiator can just be another speaker driver that is not connected to the amplifier, or it can be a flat membrane. Often, the passive radiator is weighted (called mass loading). As the frequency drops, the active driver and passive radiator both move, and start to become in phase with one another. At a certain "tuning" frequency, the two are both in phase, but the active driver is hardly moving at all, and the passive radiator is really flapping away. Then, below this frequency, they begin to go out of phase with one another again, and the passive radiator is doing most of the work. You probably know about the little trick with a crystal goblet. Wet your finger and slowly rub the tip of it around the rim of the goblet. Soon, the goblet will ring with a high pitched sound, and you will only have to barely move your finger. This is the tuning frequency of sorts. Little energy is needed to keep the goblet singing. At the tuning frequency of the passive radiator design, the active driver is not moving very much, but the passive radiator is moving quite a bit. Some subwoofers use the passive radiator design to get a lot of bass, so in that sense, it is a supplement.

Q I have listened to Theta Casablanca, Lexicon DC-1 and Meridian 565. The Meridian was conncted to Meridian speakers, while the others were not. My question is this. The Meridian sounded very open and articulate in the highs. Do you think the other 2 processors have this ability? I will not be able to afford the Meridian speakers anyway. I must use my current speakers. With this in mind, I have heard all 3, but they did not sound as clean. However, this could be due to the Meridian speakers. Any input would help me. The stores I must go to are 500 miles away.

A All three processor are top of the line. If possible ask and see if the stores will let you borrow the processors to test in home with your existing speakers. I know that most Meridian dealers only demo the 565 with the digital loudspeakers, and this is not really fair for people who only want the processor. The speakers do improve the overall sound. Believe me, the 565 sounds great without their digital speakers!

You will also want to look at future upgradablility. The Theta offers two different levels of D/A conversion, and they will also be offering a digital out card to use with other D/A converters. It also has Dolby Digital and DTS, with circle surround coming soon. The Lexicon only offers the internal D/A conversion. There are no digital outputs. They currently only offer Dolby Digital. I do not know their DTS plans. The Meridian has only one kind of internal DAC, but they also offer digital outputs. They currently support Dolby Digital and DTS w/ MPEG on the way. Both the Lexicon and Meridian also offer THX enhancements.

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