Q&A # 34 - October 21, 1997
Staff
Q Kindly explain the term "tuned" as used in the following audio context:
1. this speaker enclosure is "tuned" at 30Hz
2. this room is "tuned" to 30Hz
A When a speaker is "tuned", for example to 30 Hz, this indicates that the
speaker enclosure, usually a bass reflex or ported system, is adjusted to
have that frequency as its resonant frequency. This is accomplished by
adjusting the port length (for a given port diameter) of the speaker. The
desired tuning frequency is achieved by first generating an impedance
curve for the speaker system. The tuning frequency (fB) is measured by
locating the trough between the twin impedance peaks on the impedance curve.
The length of the port is then made longer or shorter, depending on whether
fB is lower or higher than the targeted value for that given design.
In room "tuning", every room has its own resonant frequency. This
frequency depends on the room size and geometry, as well as the fixtures
associated with the room (e.g., furniture). If you stand in the shower stall and sing, you can find the resonant frequency by moving up and down the scale until the stall suddenly rumbles like a drum. There's not much one can do to "tune" a room, except to reduce reflections in corners and from walls by the addition of room "treatments" such as absorbent hangings and stuffed pillows. I suppose you could close all the doors and windows except one, and put a big cardboard tube in the window so that it sticks into the center of the room ;-) The problem with rooms that are "tuned" too highly is that
when a note is played at the tuning frequency, the note tends to resonate,
causing undesirable sound. For this reason, ported speakers are usually
tuned below their expected output, to avoid these types of noises.
Q I've read your article about N.E.A.R's 15M model bookshelf speakers, and I'm considering buying them for my speaker systems. Unfortunately the N.E.A.R's 15M model doesn't seem have a specially designed center speaker for the 15Ms - something that goes horizontally above or below the TV. Instead I could use another 15M as a
center speaker. I don't think this speaker will look good if placed above or
below the TV. So I contacted N.E.A.R for some advice. They said that
I should go for their 20M-II as this model has a matching center speaker
(the 20MV-II), and the most important thing is that this model can be placed
horizontally.
1. Does the 20M-II sound better compared to 15M since they have a
different mid/bass drivers?
2. N.E.A.R gave me advice about the speaker configuration. Two 20M-IIs
as fronts, One 20MV-II as center, Two 10M-IIs as rears and PS2 as sub.
What do you think about this configuration?
A We have the new versions of the 15M, the 10M, and the 20MV-II and are testing them now. The 15M is one of the best bookshelf speakers I have ever listened to, and I don't think you would have any problems turning it on its side for a center channel speaker if you wanted to. On its side, the 15M would not be as wide as the 20-MVII, but it is not unusually high or deep for a center channel. We have had some center channel speakers that are built with the tweeter along one side and the woofer along the other, so it does not necessarily have to be two woofers with a tweeter in the middle. Also, the 15M has a bigger bass driver, and center channel sound has lots of bass. On the other hand, if you like the idea of a long, thin center channel speaker, then the 20MV-II is fine. One caveat though, in using regular speakers for the center channel, they may not be magnetically shielded, and they can cause strange colors to appear on the TV screen. Placing the speaker on a riser above the TV (separating the speaker from the TV so it is not sitting right on top of it), or placing a piece of steel sheet metal under the speaker, can reduce this problem. We will be publishing the full review of the N.E.A.R. speakers soon. We have not had the chance to listen to the PS2 Subwoofer, but N.E.A.R. makes very good equipment, so, more than likely, it would do the job for you. However, tonality is not an issue with subwoofers, so if you want one of the servo-feedback, push-pull, or other types, you should certainly give them an audition.
Q I have 12 gauge original Monster Cable for my system. My receiver and speakers only have the spring clips so I cannot use banana plugs (or can I?) I purchased the full size crimp type angled speaker pins from Monster. My
questions are: What tool do you use to crimp the pin? Do you use a
standard wire stripping tool for solderless connectors that has the crimper
on top? More importantly, how hard do you crimp it? Do you crimp it in
more than one place? Two, three crimps? Is solder applicable with these
pins? And why don't they include some basic instructions with these pins?!
I wish I had spent a few more dollars and gotten the twist-on pins, but I
bet they don't include instructions either. I really want to make sure I
don't have a weak link in my system at these speaker and receiver points.
Thanks in advance for your feedback. Yours is the best forum I have seen on
the net and I always look forward to your updates.
A Depending on the size of the holes in the spring clips, sometimes a banana plug will fit. But let's assume they don't fit in your case, and you use the pins. The crimper on a wire stripper will work, but I use a pair of needle-nose pliers. Put the end of the 12 Gauge wire into the crimping portion of the pin, then crimp it from top-to-bottom. Bend the edges of the crimp up a little, so it is slightly "U" shaped instead of flat. After that, crimp it sideways. This should fold the sides of the second crimp up and over the first crimp, double-sealing the bond. You might add solder too, but don't use too much, and make sure it is a good solder joint. Heat the crimp and flow the solder onto the wire. Let it cool without moving it. A good solder joint is very shiny, while a bad solder joint looks dull.
Q This is the first time I have logged onto your site. Great place!! I can't help but browse through the whole Q&A section.
Currently, I have a pressing problem. Due to a rare chance (lucky!!), I will
be able to get a pair of Definitive BP2000s for about half-price and a
Sunfire CinemaGrand at about 20% off. But I am not quiet sure what preamplifier
to get for them. Could you give me some advice? The setup will be used
approximately 70% movie and 30% music. Also, how does the BP2000 compare to the
Mirage OM-6, as they have the same retail price?
Thanks a million!!
A If you can get the BP2000s for half price, that is a great deal! They have 15" woofers in them, self-powered by 300 w rms. Regardless of the comparison with the Mirage OM-6s, it is too good to pass up. As to preamplification, I assume you need an AV processor to be part of it, since you are mostly into movies. You have a couple of options. One is to get a receiver with pre-ins and pre-outs, and connect the Sunfire CinemaGrand to the pre-outs (pre-ins, specifically a set of 5.1 inputs, can be used for other outboard digital surround processors). This is the least expensive route, and gives you the most features. Secondly, stand-alone surround sound processors, with no amplifiers, are more expensive because not as many of them are sold, but they generally have slightly better sound. For the first option, there are several new Yamaha receivers, as well as Onkyo Receivers. As to the second option, although we don't have a review unit yet, the Acurus ACT 3 might be worth looking at if you get the chance. It has Dolby Digital and DTS built-in.
Q Does the on-screen display menuing degrade the picture quality of video sources? Why or why not?
A Good question. It is very possible to have the image degraded, not actually from the on-screen display but from the path through the product providing the display, e.g., an A/V receiver. If the product does not have the bandwidth, passing the signal through will degrade it. I like the displays for set up, but once I have something tweaked, I
usually route my signal straight from the source to the TV.
If you have either the LD or DVD version of Video Essentials, you can
put up the sweep pattern or resolution chart and compare with and
without the on-screen to see if there is any visible loss. The loss
will usually appear on the right half of the sweep pattern in the higher
frequency information. You might not be able to see the lines as well
defined, and they might appear to be somewhat softer.
I know that seeing the volume of your surround processor is nice and
easy to read in the dark, but when you have a group of friends over, or a
significant other, they do not enjoy the extra stuff on the screen during
the movie. I usually make minor adjustments during a film because
I am compulsive, but others just want to watch the movie.
Q I have a technical question for you. If you have an amp rated at 100w into 8 ohms and then substitute the 8 ohm speakers with 16 ohm
speakers, you still have a 100w amp do you not? It seems to me, according to
Ohm's law, that your amp would still try to supply 100w but the amount of
current would be less, per the rule of power. 8 ohms @100w would
allow 3.535 amps, while 16 ohms @100w would allow 2.5amps. Is my theory
correct?
A Amplifier power output is generally rated into 8 Ohms, i.e., 100 w/ch into 8 Ohms. This is based on the rail voltage, which is the fixed value DC voltage delivered by the power supply. Your calculation is correct for a voltage of 28.288 Volts and 8 Ohms, but since the rail voltage is fixed at this value, 1.768 Amperes would be drawn at 16 Ohms. At a voltage of E, and a DC resistance of 8 Ohms, a current of E divided by 8 gives the current. At 4 Ohms and the same rail voltage, the speakers will attempt to pass (allow the passage of) twice the current, and thus, according to Power = Voltage x Current, twice the power from the amplifier. Unfortunately, most amplifiers can't deliver twice the current at 4 Ohms as they can at 8 Ohms, and so, they "clip" at a less-than-rail voltage. At 16 Ohms and rail voltage, half the current, and thus, half the power, would be delivered to the speakers. In essence then, the amplifier is RATED for 8 Ohm loads, but will deliver other power values depending on the impedance. The 100 watt amplifier is 100 watts because of the accepted value to be listed at that impedance. Some high-end amplifiers are rated into 2 Ohms, and a few even into 1 Ohm. The majority of speakers don't ever dip that low (1-2 Ohms) anywhere in the audible spectrum, but what that (1-2 Ohm rating) specification does tell us is that the power supply is heavy duty, and that is important to know. Interestingly, tube amplifiers seem to "love" 16 Ohm speakers!
Q I have a problem with both Home Theatre
surround sound and music played from the CD on my computer. In both
situations, I get an awful headache listening to the audio. It has
happened many times. So, I don't bother with listening to either anymore. Whereas the music on the computer situation is a non-problem because I
don't listen to music while I work, I would like to be able to take
advantage of SURROUND SOUND. SOOO, any ideas as to what my particular
problem might be and how I can identify a surround sound system that will
not make me sick? I do not have this problem in the movie theatre. I do
not have this problem on my Audiophile quality stereo ($6K - 10K) system.
A What might be bothering you is bad digital. Computer-based CD players (CD ROM Drives) often sound terrible because of the low quality DACs in them. I am currently testing a product that helps out with any digital component, called the Z-Man Audio Enhancer. It smoothes out the brightness of CD
players, LD players, computers, etc. It is a low cost upgrade for mass
market equipment. Look for the review in the coming month.
� Copyright 1997
Secrets of Home Theater & High Fidelity
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