Q&A # 135 - August 27, 1999

Staff

Q I have a problem with echo, but I wonder if a room can be made too quiet by using carpets?

A There are two main ways of getting rid of echoes. Break the waves up randomly with uneven surfaces, or absorb them.

The first option is to reflect the sound randomly. Bookshelves, lined with books of different sizes, do a good job of this. Generally speaking, it's good to have variety of objects in the room in order to "randomize" and kill reflections that would otherwise loiter, so long as those objects aren't between you and the speakers.

To absorb sound, carpets, draperies, sofas, pillows, and even people work very well. A thing to remember about absorption is that most materials do not absorb evenly across the frequency spectrum. For instance, thin drapes may absorb treble well, but do almost nothing for mid-range. Even heavy drapes absorb almost no bass content. Hanging plants at a 45⁰ angle from the speakers to the wall where the sound reflects towards the listener can help a little. Michael Green Room Tunes are a good professional product. Over-absorption may result in a very dull, lifeless sound. In other words, you want a little reflection, just not too much of it.

 

Q I wanted to know whether there are limitations on plugging equipment into an amplifier's built-in outlets. I own a Yamaha DSP-A2, and I use two of the outlets for my DVD player and my sub (B&W AS6).

A There is definitely a limitation to these outlets. In the case of the Yamaha, it is 120 watts for the two switched outlets and 180 watts for the unswitched outlet. Sources like DVD players require very little power (18-38 watts), but your powered sub would very likely be too much. Depending on its amp, subs can need from 250 to 800 watts plus when they are cranked up. Although it is convenient to have sources come on with the Yamaha, many Home Theaters are located in humid basements. If yours is in such an area, manufacturers usually recommend leaving sources like DVD players plugged in to a live outlet to prevent condensation on the lens. So, go ahead and use the unswitched outlet on the Yamaha for your DVD player.

Q A question regarding the Titan sub project. I am building a very similar design using Celestion AD12L drivers. After dumping all the specs into a few of the software packages they recommend, I came up with a 71 liter enclosure with a single 4" port. I particularly liked the downward firing design of the Titan (as it protects the driver and gives a clean outward appearance) and wanted to use it, but I am unsure of how to add the port. I have a preference for the port to be downward firing also, but does this have negative effects? The other option is to have it at the back of the enclosure.

A If the port will not fit on the same side of the downward-firing driver, it's good to put a bit of distance between the port, the driver, and the adjacent walls, on the order of six inches or so. So, the rear-firing port will probably be the way to go if the goal is to hide both the port and the driver. Firing a port directly into a room barrier may limit turbulence and possibly enhance room loading. Keep in mind, though, that either the floor or a rear wall near the port will change the tuned frequency of the port slightly, so that you will probably need to measure the tuning frequency with the enclosure placed as desired, or in a similar location, and retune the port as necessary by changing the length. With any bass reflex design, it's often a good idea to run through a couple iterations to arrive at satisfactory results, as they're complex and harder to predict than comparatively simple sealed designs, so don't jump the gun and end the project before it's begun.

Q On the issue of stuffing material into speaker ports to reduce boominess, here are some alternative suggestions.

A Oftentimes a predominance of bass in a listening room is caused by resonance in the room itself, not your speakers. Of the overall sound your system makes, you are listening to about 80% room and 20% speakers. I know that seems strange, but it is true. Thus, adjusting your room, or speaker placement, can often have a much larger effect than modifying your speakers by plugging the port. Here is what I recommend:

1) Try re-positioning the speakers if possible. Move them out from the back wall, move them away from the side walls, angle them differently, etc. Try a few inches at a time until the sound balances out. Take your time and listen during the process.

2) If #1 doesn't solve the problem, try dampening your room in corners, where bass frequencies build up and overload a room. Use plants, or wall-hangings, or anything sound-absorbent. The idea is to break up the sound pressure building in those corners.

 3) Try putting the speakers on spikes or cones to de-couple them from the floor. Often the speakers sitting directly on the floor excite room resonance in the bass region. Spikes or cones will help prevent this. If none of that works, you can try plugging the port, which will not damage your speakers, but they will also not be performing as they were designed. If all else fails, you should consider other speakers.

Q I currently have a Denon AVR-2700 with M&K LCR55 and V75MKII. The system sounds fine to me for home theater, but music seems a little bright and at times fatiguing. I've contemplated looking into separate power amplifiers such as entry level multichannel amps from Rotel, Parasound, and Harman Kardon. My question is will I be able to notice a difference for the amount of money I'd consider spending? Can you compare the amplifier section in the Denon AVR-2700 to separates?

A The LCR-55s are a moderately mean load with receiver amplifiers, eliciting a very sharp character under such circumstances. They chewed both the Yamaha RX-V990 and 992 to whimpering shreds. They did work fairly well with the larger (and now discontinued) Denon AVR-3200. I don't know if the preamplifier section is better than the 2700, but it sounded very nice fed though the digital input, bypassing the A/D section. I suppose that an A/D conversion, though an anti-aliasing filter, could conceivably smooth the top-end some more by rolling it off, but I certainly wouldn't consider them a mismatch for a moderately-priced $1,000 - $2,000 5-channel amplifier, such as an ATI 1505 or comparable product. It would smooth out the top a bit, and let the midrange come through the way it was meant to be, clear but juicy. Obviously, a bigger amp section won't do all that much for the bass, since the speakers deliberately roll-off at 80 Hz or so.

 

Q I'm curious if you could tell me the nominal specs for digital interconnects, and answer a question regarding them. Given the signal on these cables is digital (either there's a voltage present or not), basically, whatever equipment that's connected only needs to determine if the signal is above or below the threshold for either value. How is it that better quality cables affect sonic quality? I would think that even with poor quality cables (assuming they're of the proper impedance), the variations wouldn't be enough to cause errors in a digital signal. I don't doubt that there are audible differences between cables. I'm just curious to understand how it is that they exist.

A The impedance of a digital cable is critical in that any variation from the proper one (75 Ohms) causes internal reflections of the signal within the cable. The impedance at the connectors is important, but also, the impedance all along the cable must be controlled. This is done by spacing the conductors with the dielectric. As a result, only the most careful engineering of the cable will result in the impedance being 75 Ohms every inch along the cable. Lesser quality cables might have an overall impedance of 75 Ohms, but the impedance varies at each point in the cable. This causes the digital signal to have errors once it reaches the DAC.

At the lowest level, line drivers are basically switching the voltage on and off in the cable, sending ones and zeros. Digital audio is currently synchronous, meaning the data signal is clocked. The ones and zeros are therefore encoded in a way that the receiving end knows when to sample or 'latch' the next value. I don't remember the exact encoding scheme for PCM, probably Manchester. A characteristic impedance mismatch will result in transient reflections of the signal (the sharp transitions of a square wave). These reflections combine with the actual signal and change the level. At steady state there is no effect (the waves cancel out), but typically you want to push data through the cable fast enough that the reflections would still be around when it is time to latch the value. There are many places where this mismatch can occur. First is in the source component like your CD player (the output connector). Then there is the cable connector on the source end, then the cable itself, then the cable connector on the receiving end, then digital input jack on the receiver. Often times companies will skimp on the 75 Ohm connector or cable and use a 50 Ohm. Other cable characteristics such as nominal impedance per foot (attenuation), velocity of propagation, geometry and shielding can also help preserve the data signal. For example a very noisy RF environment could induce currents in an unshielded digital cable and alter the values. The question then is, how much is really required to preserve the integrity of each bit? That normally depends on the quality of the line drivers and receivers, the physical environment, the length of the cable and most importantly the data rate and encoding scheme.

Q I have a question about connecting a passive sub. The sub has only a pair of binding posts, so it will only accept speaker wire and it has no built-in crossover. Can I use an old stereo receiver to power the sub by going from the sub out of my processor to the CD or tape input of the receiver and have the receiver's output power the sub? Do  I need a crossover in the chain or is it only low frequencies that are sent from the sub-out (Sherwood AVP-9080R)? If I use an external crossover, would it go between the receiver and the sub and do you have any suggestions? I am very confused about this.

A It is likely that your processor has a crossover before the sub-out, so you can use this output connected to the input of your old receiver to drive the passive subwoofer. However, the crossover in the processor may be too high, and it will have a fixed value. Therefore, another crossover will be helpful. You could use an inductor between the old receiver and the sub driver, or you could use an EQ between the processor and receiver. Using an inductor requires mathematical calculations based on impedance values. Using an EQ won't require such calculations. I would use the EQ. It will let you shape the sound at several frequencies, rather than just provide a low pass crossover frequency. You can also use it to roll-off everything below, say 25 Hz, which will be better for your receiver's amplifier.

Q My television ( Pioneer Pro 700 SHD) has multiple scan component inputs and up-converts all signals to 1080i. I want to add a progressive scan DVD player to my system. Since my TV converts everything to 1080i, is there any advantage to buying a computer based DVD that outputs at 780p over one that outputs at 480p?

A From the people I have spoken to, the Pro 700 will up-convert 720p to 1080i, but it should keep 480p as is (480i gets converted to 480p). All sources that are above 480i will be locked into full mode on your TV. So, if you use an external line doubler like the iScan Plus, progressive DVD player, or Faroudja DVP-2200, then it will be displayed in the full mode much like watching an anamorphic DVD. For all anamorphic sources this is fine, but 4:3 and standard letterbox material will be stretched to fill the screen. This is great if you have the Faroudja DVP-2220 because it can scale to 4:3 and letterbox internally. But the DVP-2200 is about the same price as the TV. As far as progressive DVD goes, if you feed the TV a 780p signal, it will get converted to 1080i. You can output a 480p signal with the progressive DVD players on PCs. I assume you are referring to the RKR boxes that use the Mpact decoder chip. I only assume this because that box has been a popular discussion topic lately. Software decoding is also quite good now. I am currently looking at several products, and we will report on these shortly. Just to be clear though, passing a progressive signal from your PC or DVD player will be in full mode on your TV.

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