Q&A # 151 - December 7, 1999

Staff

Q I am in the middle of renovating my basement for a home theatre and would like your idea on paint color. What is the most import aspect of the color of the room (reduce glare, reflection, etc.), or does it matter at all?

A The color and reflectivity will matter a great deal. I have seen home theater rooms with all kinds of colors, but deep red curtains seem to be the most popular. Of course, this is for aesthetic reasons, and I feel that the best color will be a neutral one, such as a medium gray (flat, not gloss or semi-gloss). The TV screen should be surrounded by a black rim, perhaps a few inches in width, depending on the size of the screen. This way, the image will be framed, regardless of how much ambient lighting is around it. Use indirect lighting on the walls. This could be a set of track lights on the ceiling that shine onto the side walls. Put another set of lights above and to the side of the screen. The ones above should be deeply recessed into the ceiling, and several feet out from the screen. The deep recess will allow the light to shine onto the floor and not put any light onto the screen. Lights to the side of the screen might be soft illumination behind side curtains (curtains should be a neutral color). All lights should be on a rheostat, and the set near the screen should be on a separate rheostat from the ones that shine on the side walls. The reason for the lights is to give a balanced illumination. Otherwise, with a completely darkened room, your eyes will dilate, and the bright image on the screen will result in fatigue.

Q (1) For the past 5 or 6 years I have been dabbling  in specialized enclosures, specifically, transmission line type enclosures. I  have had great success with the ones I have built. I realize that they can only be good for a selective bandwidth of frequencies, but in car audio, there is an awful lot of people who aren't concerned  with anything over 80 Hz.

(2)  In SPL competitions, it doesn't matter what kind of range you have as long as 40 to 50 Hz is really really loud. In the enclosures I am speaking of, they basically consist of a single sub that has an immediate enclosure which is vented thought several semi-dividers until it is finally vented into a enclosure about half the size of the immediate enclosure. The final enclosure is then vented directly to the outside of the enclosure on the same plane as the subwoofer in hopes that IF the rear wave emerges from the final enclosure, it will be on the same plane so not to cause any cancellation.

(3) However, I don't feel like it is right. I am thinking that there are standing waves just sitting in each of those semi-dividers and the enclosures for that matter. I was wondering  if I would make a 45⁰ angle at the end of the passages so as to direct the wave, would this work?

(4)  I looked at the design of the Bose wave radio, and it appears that they have rounded the passages on the bends. Is that to prevent reflection?

(5)  If you direct a wave, does this case an extra phase shift? Say the 60 Hz sine wave is 16 feet long. If you wanted the rear phase to come out in phase with the front you would have to delay it 8 feet to shift it 180 degrees. Now if you wanted to vent it out of the same side as the speaker, you would have to make a 180 degree turn inside the enclosure. Would the reflection of the wave take any time?

(6)  Also do you know any formulas for porting enclosures?

 

A (1) A transmission line system can do well over a wide range of frequencies. A pair of my speakers has a ribbon midrange mounted in a transmission line, and a pair under review have a transmission line behind the tweeters. Both work very well. Transmission lines are nice for drivers with low mass (such as a ribbon, or a tweeter diaphragm) because they allow the rear wave to die out instead of reflecting off the back of the enclosure or tweeter chamber and interfering with the motion of the diaphragm. Mid-bass cone drivers and woofer cones don't usually benefit as much from this arrangement, as their relatively heavy diaphragms are less affected by a rear wave reflection. As wavelengths get longer (e.g., under 80 Hz), the rear waves become more of a suspension characteristic than a wave, as all but the largest enclosures will only reflect changes in internal pressure, as the waves no longer "fit." That's not to say that the low frequency pressure changes don't happen, but just that they don't exist as, or behave much like waves.

Transmission line subwoofers aren't very common in homes because, as you've probably come to realize, to support deep bass reproduction, they have to be long, and by that result, large, even when folded, in order to get extension satisfactory for full-range home reproduction. While the high-frequency limit is a function of the driver, the low frequency limit is a function of the driver and the transmission line. In an undamped transmission line, the rear wave, once expelled from the enclosure, is free to combine with the front wave. The relative phase shift is a function of the distance difference between the paths. Phase shift at a given frequency equals distance divided by wavelength of that frequency, multiplied by 360 [degrees], or 2 pi [radians]. Or, Delta Theta=(Distance/Lambda)*(units of measurement/cycle).

The resulting phase shift between the delayed rear wave and front wave will cause cancellations (when the resulting phase relationships are closer to increments of 180, including the initial polarity swap) and reinforcements (when phase relationships are closer to increments of 360, again including the polarity difference), causing comb filtering, and ultimately rolling-off the low frequency extension regardless of the driver's natural low-end limit. For a straight, undamped transmission line, the ultimate roll-off begins at the frequency whose wavelength is 4 times the length of the line. At this frequency, the rear wave, which initially is phase shifted 180 degrees, will be delayed half a cycle relative to the front wave (must travel back [1/4 lambda] and then around to the front [another 1/4 lambda], resulting in a total 360 degree phase shift.) This is the lowest reinforcement. As the frequency decreases, so does the additional phase shift, so that as frequency approaches 0, the resultant phase relationship approaches 180. Most transmission lines for home I've read about by DIY'rs are stuffed, or at least damped along the walls. This does three things. One, it can slow down the sound, to make the transmission line effectively longer, increasing the low frequency extension limit.  Two, it attenuates the rear wave, so that resulting cancellation and reinforcement are lessened, as are the filtering effects. Three, it dampens possible standing waves within the transmission line, which would otherwise degrade reproduction.

(2)  If you have a particular frequency you want to go after maximum SPL, you WANT the interaction as opposed to trying to avoid it. So, start with a folded transmission line, as you describe, with the rear opening near the driver, and make the total length equivalent to 1/2 of the wavelength of that frequency (you don't have the additional  trip back from the opening as you do with a straight transmission line). You may want to still use some damping material to line the insides of the transmission line to avoid standing waves at higher frequencies which would detract from what you were trying to accomplish, and which may require that you slightly shorten the transmission line to achieve maximum output at the desired frequency. Obviously, it will take a little experimentation.

(3)  It would make a difference for higher frequencies, like 300 Hz or so, but so would applying damping material to the inside of the enclosure, if you haven't already done that.

(4)  Perhaps. I don't pretend to know the motives of Bose products. To their credit, they are perhaps the most commercially successful speaker manufacturer the the world in terms of total sales.

(5)  It shouldn't make a significant difference at 60 Hz, especially if the turns happen in less than 4 feet, as the wave cannot then "stretch out" enough to turn around, but rather creates a back pressure that the position of the woofer, and the air behind it, would already be in the process of countering.

(6)  It's far easier, and quicker, to obtain software for this purpose. I've run across shareware in the past, but for about $100 you can get software that will allow you to design bass-reflex systems based on Thiel-Small parameters of particular drivers. If you don't have a means to obtain those parameters (better to measure yourself, but manufacturers often provide approximate values), forget trying to really nail a ported design. You can do well in terms of output by a little trial and error, but to combine extension, a smooth frequency response, and a reasonably good phase response, it takes a lot of effort, and diagnostic hardware to go with it. The difficulty in properly aligning a reflex system is probably a reason why I'm such a fan of sealed systems for home projects. I'm currently debating the implementation of a very large push-pull sealed system, or a relatively manageable isobaric sealed configuration. One would require roughly 8 cubic feet per enclosure, the other 2. Perhaps more on that later.

 

Q I have Bose AM 7 for my front speakers and Bose AM 5 for my rear speakers. I have a Pioneer VSX D503S Audio/Video Receiver. I want to buy a DVD player and upgrade to a digital ready receiver. I have been told that my speaker system will not work with a digital receiver. I have been told I need a power sub. Are they right? Is there any way I can use my speakers with a new digital ready receiver? I want good sound but can't afford another speaker system. What do you suggest? What receiver do you suggest in the $300 -- $400 range.

A Your Bose should be fine. "Digital receivers" just means that the receiver will decode Dolby Digital (DD) and DTS, or just DD if it is last year's model. All the new ones will handle DD and DTS. But they all have digital and analog inputs for various sources, and all have analog voltage out through their speaker binding posts to speakers. They may process the signal internally as digital 1s and 0s, but they connect to all conventional sources for inputs. The Sony STR-DB830 that we just reviewed is in your price range. You will need a good subwoofer though. The Velodyne CT-150 is a superb one for $799.

Q I am writing in the hope that you can help me sort out my bass problems. I have a small room 8 feet high, 10 feet wide, and 14 feet long. I am using two Boston Acoustics 595 THX subs.  They are a sealed box sub . I have tried moving them around the room to try and get a good sound but have been unable to do so. I get plenty of thump but not enough body if you know what I mean. What I would like to do is build two new boxes for those 12 inch drivers but make them ported. I think this may be the way to go in such a small room. What do you think? If you think as I do can you help me with a good compact design to do so? I don't mind how complex it gets. I also need to know the speaker specs. Boston won't give them out. I was hoping you may have some idea on that.

A Building a vented enclosure for your 12" drivers may not give you what you want. As you mention, first you will need the T/S parameters of the drivers to ensure that a vented enclosure even will be suited for this driver. Not all drivers work well in this configuration. The 595 THX is most likely a sub that uses some sort of EQ to give it the extended bass in a reasonably small enclosure. These electronics are usually contoured specifically for a given design, and changing the enclosure may actually make the sound of the design worse, and potentially damage the driver. With two subs at 250 watts each, you should be getting some tactile effect. I would suggest experimenting a bit more with placement, as well as the sub's crossover point (if it has a variable crossover setting) and how you currently have your bass management setup for your system. If you still are not satisfied, you may want to think about purchasing different subs. Using two subs in the wrong manner can actually give you 6 dB LESS than using one sub. Try putting one sub on top of the other, making sure that you set the phase switch the same for both, and the low-pass frequency the same for both.

Q I have a Toshiba SD-3109 DVD player. I run the Video and Audio cables to my Symphonic VCR. Then the signal goes to my TV through the coax cable from the VCR. I would run the DVD player directly to the TV, except it's an old console model RCA that only has a coax and antenna inputs. I am having trouble with this setup. The picture quality seems to fade in and out. I know the DVD player works fine, and the TV can handle the picture. I was just wondering if there is some sort of adapter I can get so the video out goes directly to the TV.

A The problem of fading in and out you are seeing is a result of Macrovision. The VCR is causing this. I have seen such adapters in the past when LD was more popular. You should go to your local Radio Shack and see if they still carry the part. I do not remember the part # as it has been a few years since I have seen one. At the moment you are not experiencing the highest quality of DVD.

Q In the near future I will be purchasing a new computer. I was wondering what I should be considering with respect to speakers, sound cards, and possibly DVD. If I wanted to use non-powered speakers such as Paradigm Atoms, how could this be done? Would it be possible to somehow interface my old Pioneer receiver with the computer? Since this purchase is a few months away, what other issues should I be researching at this point?

A Sound Blaster Live is my current favorite PC card. It has an S/PDIF output (digital output RCA jack) that can go straight into your receiver (assuming your receiver has a digital input). The SB Live also has analog outputs. Do you want to run DVD off of your PC? Software decoders, like WinDVD, do a great job. You will need a quality video card like a Matrox G400Max that does scaling. Software decoders also require a beefy PC like a PIII 450 or greater.

Q I, like many others, am ready to make the transition from my two-channel stereo system to surround sound. I currently have the Magnepan 2.5r for my main speakers, run by the Carver C-1 preamp and Carver M-1.0t two channel power amp. I am considering replacing the preamp with the Vantas DPA-S50 (I'm willing to forego DTS), adding the Magnepan MGCC1 center and MGSS1 surround speakers, and the HSU VTF-2 subwoofer. I'm assuming using Maggies for center and surround is preferable, however, I haven't found many reviews of these speakers. Do you feel the Vantas will be able to handle the 4 Ohm load of the center and surrounds? Also, with dipole main speakers, will placement of the subwoofer be more difficult? Finally, should I set the crossover on the sub in the 65 - 85 Hz range?

A The Maggies being planar speakers, have only modest sensitivity, and those nasty 4 Ohm loads. Such speakers won't do well with any mass market receiver or decoder/amplifier that I know of. If you use Maggies all the way around, you will be best off having a preamp/processor and a good outboard five-channel power amplifier. Subwoofers are not simple to place, regardless of whether you are using dipoles or not. Start with placing it in the front right corner, about 18" out from the front wall and 12" away from the right side wall.

Q I have two questions: The first one involves the passive bi-amplification of home theater speakers. I own a pair of Paradigm Reference Studio 100 speakers, and it states in the owner's manual that bi-amping the speaker is the only way to get the full potential out of the speaker. I am wondering if bi-amping actually adds to the bass performance, soundstage, and sound detail, or does it simply add more power? Would I be better off bi-amping for instance a Sunfire Cinema Grand amp for my mains and using another three-channel amp for the surrounds/center or spending that money on a high-end amp in line with a Proceed or in particular the new Sonic Frontiers Channel 5 amp ($4999) that I am very interested in and was mentioned in your review of the Anthem MCA 5. Would 300 watts of high-quality amplification be enough, or would simply using bi-amping get me a better sound for my main speakers? Question 2: Is the buying of a high quality amplifier now a future-proof investment? Will all digital amps replace the amps of today and do they actually sound any better?

A Passive bi-amplification with a top notch amplifier probably won't net you any improvement. Active bi-amping probably would though. If you are going to use a nice amp like the Sunfire or Proceed, don't worry about bi-amping. Your sound will be terrific as is. I have been using the Cinema Grand Signature here for some time now, and those 400 watts per channel make a huge difference when playing high-impact action films. At low (sane) volume, 100 watts per channel are fine. Digital amplifiers (not digital switching amplifiers) are down the road a few years, and they show great promise because the signal will be digital all the way from the source (the DVD) to the speakers. This will allow an unaltered signal reaching the speakers, and even the speaker may be digital some day. As to being better, yes, I suspect that they will. But for now, a high quality power amplifier is a good investment

� Copyright 1999 Secrets of Home Theater & High Fidelity
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