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Fig. 111. Construction of a tuner-amplifier compartment.

cut in the control panels are supplied with each component. For whichever unit you have decided to put on the bottom, use the bottom of the compartment as the mounting level. Then, a couple of inches above that unit, so that there will be air around it, fix an internal shelf inside the compartment and behind the control panel. This gives you your mounting level for the upper unit. Cut the proper holes for both units in the control panel, as designated by the templates supplied. Place your components in position, plug them together and you're in business.

To mark the holes from the templates and make your cutouts, first mark the level of the top of the internal shelf on the inside face of the control panel. Then take the control panel out of the cabinet. Next, using the bottom and the shelf level as references, mark the cutouts from the templates and cut as required. In general, the control panels in cabinets designed to house high-fidelity equipment are made removable so that this can be done.

If you design your own cabinet, be sure to include provision for making this panel removable. As far as that goes, while you're at it, make the mounting board for the record changer or the turntable removable also.

One simple way to arrange a removable control panel is to fix a frame of thin molding in the front of the compartment, then push the control panel in from behind, and screw it down to the frame from inside (Fig. 111). Be sure that you can get that internal shelf out, too. If the control panel is removable but the shelf is fixed, you'll obviously never get the control panel out. This also is very easy to arrange. Merely fix the shelf to a pair of cleats, which in turn screw to the sides of the cabinet but are not glued. The whole business can then be easily removed and replaced by taking out a few screws.

Instruct your cabinetmaker specifically on these points, otherwise he will follow normal furniture-making procedure and fix these parts in place permanently when he assembles the cabinet.

There is still one other consideration. People have been known to buy hi-fi equipment and then find later that they want to change it. If you change your equipment, you can be absolutely sure that the new units will not fit into the old cutouts. You've got to take the old control panel out and replace it with another. If your control panel is permanently fixed in place in the cabinet, instead of being removable, you'll have a rough time getting it out and you may well have to do considerable damage to the cabinet in the process.

In the case of the larger amplifiers consisting of a preamplifier and a separate power amplifier, the only part that has to come through the control panel is the preamp. These preamps are generally, if anything, smaller than the flat amplifiers discussed earlier. To install one, follow the same procedure as for the flat amplifier, except that you have a good deal less worry about ventilation of a preamp only, since it generates less heat than the combined preamp and amplifier. Once the preamp and tuner are taken care of, the only thing left to worry about is what to do with the possibly quite bulky power amplifier. Sometimes it will fit at the rear of the compartment in which you already have the other two parts. This is an excellent place for it, because it is likely to be the hottest unit you will have and, at the back of the compartment, it will be in a position to get the most air. If it hangs out the back an inch or so, that doesn't do any harm. In fact it does a little good as far as ventilation is concerned.

Don't push cabinets containing electronic equipment up tight against the wall. Set them out an inch or two so that air can readily get in and out of the compartments where the most heat is generated.

If your power amplifier is too big to fit in the same compartment with the other units, you will have to give it a separate compartment of its own somewhere else in the cabinet. But again, be sure that it is adequately ventilated. The more air it gets, the bet ter. Leave the back off such a compartment and, if you find that you have to cut it down in size to where the amplifier is a very close fit, cut intake holes in the bottom of the compartment to improve air circulation.

Speaker enclosures

At first glance, the subject of speaker enclosures seems like a gigantic and impenetrable wall comprised entirely of utter confusion. Acoustical engineers are in unanimous and happy agreement on one point and one only: to operate properly, a high-fidelity loudspeaker system must be enclosed or placed behind a baffle. From here on, the disagreements range from mild to positively vitriolic, as to what the size, type, form and composition should

be. To avoid having my head chopped off at the ankles, I shall evade any evaluation of the various principles in current use. We will discuss types of cabinets utilizing different acoustical principles, but primarily from the point of view of the resulting external appearance.

Decide intelligently what kind of speaker cabinet you want, listen to a number of speakers in a number of different types of enclosures and determine which one suits your ears the best. After all, your like or dislike of the various tonal qualities produced by different speakers enclosed in different ways is a highly personal matter. True, there is perfectly good sense, from the point of view of the engineers, in discussions of comparative specifications and frequency response curves. But, for the nontechnical person, this type of data provides, albeit precise, a fairly meaningless description of a result that either feels good or doesn't feel good to our ears. We each hear with a pair of ears that is a little bit different from the next fellow's.

You will hear a lot of talk about flatness of frequency response in the system, from some unusably low frequency at the bottom to some other equally unusable high frequency at the top. If you're technically minded—or just plain curious—take a look at the response curve of a decent amplifier (Fig. 112). It will be pretty flat. But then take a look at the response curve of a pretty good speaker (Fig. 113). By comparison, it looks like the path of an alcoholic trying to get home from the corner saloon—it wobbles all over the place. What you may not know is that the frequency response of your ears is likely to be pretty wobbly, too. Now, if you happen to come across a speaker that dips in its response where your ears peak, and peaks where your ears dip, it's going to sound pretty flat to you—but it might sound pretty frightful to your next-door neigbor, whose hearing response is likely to be quite different from yours.

To complicate matters further, take the same speaker and house it in several different ways. You'll change its response each time you change the housing. On top of that, take the same speaker, in the same enclosure, and put it in three or four different rooms. You'll change its response every time you change rooms.

Actually, if you want to delve into the technical aspects of speakers and their cabinets, there's a considerable amount of literature available on the subject in the form of books and articles. Also, there's a huge amount of material published by speaker manufacturers.

The infinite baffle

Now, as I have already said, engineers are agreed on one point —your speaker system should be in a housing. Without going into the whys, the reason for this is to improve the bass response of the speakers. There are a large number of different speaker enclosures and speaker systems on the market, but most of them will fall into one of three main categories:

  1. Infinite baffle
  2. Reflex baffle

3. Horn

Of these, the infinite baffle is probably the simplest and technically the most foolproof. In theory, an infinite baffle would consist of a plane surface of infinite height and infinite width with a speaker mounted through a hole in this surface. In such a situation the front radiation of the speaker would be completely isolated from the back radiation, and the air pressure or loading on the cone would be the same on both front and back. These conditions can be approximated by mounting the speaker through a hole in a wall between two rooms of roughly the same volume. In such a case the room that has the back of the speaker facing into it is, in its entirety, effectively a speaker enclosure.

This kind of thing has been done on occasion, but obviously it

Fig. 113. Speaker response curves are quite different from those of amplifiers. The hill-and-valley nature of a speaker's response is shown in this typical curve.

is not a very practical approach. Therefore, the conventional infinite baffle, so called, is quite another thing. It consists simply of a box, fully enclosed on all sides, made as airtight as possible, with one hole in the front for the speaker. The only thing that's critical

Fig. 114. The conventional infinite baffle is primarily a big closed box.

about it is that it be large enough in cubic volume for the size speaker that you're using (Fig. 114). The relative proportions of height to width to depth seem to have very little effect as long as you somehow get the necessary cubic volume inside the enclosure. But there's a rub. The infinite baffle, for a speaker of given diameter, is larger and more cumbersome than enclosures based on the other widely used principles.2

2 There are other shortcomings inherent in infinite baffles. One of the most striking is the development of a single note bass resonance. When not properly designed the resultant bass-boom can be annoying. However, some ears find this juke-box-effect quite pleasant.

20'V

FREQ

Fig. 114. The conventional infinite baffle is primarily a big closed box.

about it is that it be large enough in cubic volume for the size speaker that you're using (Fig. 114). The relative proportions of height to width to depth seem to have very little effect as long as you somehow get the necessary cubic volume inside the enclosure. But there's a rub. The infinite baffle, for a speaker of given diameter, is larger and more cumbersome than enclosures based on the other widely used principles.2

2 There are other shortcomings inherent in infinite baffles. One of the most striking is the development of a single note bass resonance. When not properly designed the resultant bass-boom can be annoying. However, some ears find this juke-box-effect quite pleasant.

The table shows the recommended cubic volumes of conventional infinite baffles for speakers of various diameters.

Remember, these are recommended averages. Speakers vary quite considerably in response curves, range and cone resonance. With some speakers, you can get away with a good deal less cubic volume than the recommended figure, particularly if it's a speaker with a very low cone resonance. In general, if you've got room enough to stick to the recommended sizes, do so. If you have space sufficient to allow you to exceed the volumes recommended, this is all to the good. If, on the other hand, you find you haven't got the space necessary for the recommended volume and have to pare it, be careful of what speaker you use. You could conceivably cheat the speaker. The speaker won't mind—but you might.

From the point of view of cabinet design, the infinite baffle, outside of its bulk, is a rather nice type of enclosure to work with because its relative proportions of height, width and depth are not critical3. Depending on the space you have available to allot to it, you can make it either long and low or narrow and high. You can put it on the wall, or you can put it in a corner. You can use a closet or you can build it into a skew-shaped space—as under a stairway—or some other odd-shaped niche for which you have no other use. It is entirely conceivable that, in your particular residence, you might have no place where you could put a speaker enclosure of even quite modest size if it has to be a regular parallelepiped shape, but yet you have a much larger area of irregular shape that could be used for the purpose. Some speaker manufacturers specifically recommend that their speakers be used in infinite baffles. In such a case, you would be wise to find some way of following the manufacturer's recommendations, since otherwise you will not get the sound from the speaker that the manufacturer intended and which was probably the sound that you heard demonstrated when you decided on it in the first place.

3 The air-loaded enclosure, which can be considered a variety of the infinite baffle, is another matter entirely. This type of unit is designed for specific make and model speakers and its dimensions are critical. The enclosure and speaker are frequently sold as a unit. They are available in a utility finish for custom installation. The enclosure is quite small—2.5 cubic feet being a typical internal volume.

Speaker

(inches)

Volume

(cubic feet)

8 10 12 15

5Vi 63A 8 10

A few words of caution regarding infinite baffles are in order. Because the infinite baffle is quite flexible as to shape and proportion, this sometimes leads people to put the speakers either too low or too high from the floor, particularly speakers that handle high frequencies. Low-frequency tones tend to disperse through the room reasonably readily, but high-frequency tones are directional, and dispersion becomes more and more difficult the higher

the frequency. Wherever possible, try to get whatever speaker or speakers are handling the high end at about the same height off the floor as your ear will be when you are seated to listen. The overall result will be much more satisfactory. Due to the fact that conventional infinite baffles are pretty big boxes, the individual panels of which they are composed are pretty good sized. It is a good idea to stiffen them internally by adding battens, approximately s/4 x 11/2 inches to 2 inches, edge-glued and screwed on the inside of the panels to aid in damping panel resonance (Fig. 115).

In the larger cabinets housing 15-inch systems or multiple 12-inch systems, it is often desirable to add cross-braces from top to bottom, side to side and front to back, made of stock as heavy as 2x4 inches, for the same purpose—damping panel resonances. Lining the inside faces, top, bottom, sides and back of an infinite baffle with acoustical absorptive material such as 2-inch Fiberglas damps standing waves which will otherwise cause some quite annoying sound effects that have nothing to do with the program material you want to listen to.

Bass reflex enclosures

The next important type of speaker enclosure is known as a phase inverter or bass reflex. This type of unit is almost a sealed enclosure like the infinite baffle, except that it has an opening or port in the front, in addition to the speaker opening (Fig. 116). The purpose of this port is to utilize cabinet resonance along with speaker resonance, to extend the bass response downward and at the same time equalize it over a wider range. An advantage inherent in the phase-inverter or bass-reflex cabinet is that it allows you to use smaller dimensions for a speaker of given diameter than would be possible with an infinite baffle. A 12-inch speaker can be reflexed in a cabinet as small as 4 cubic feet, and a 15-incher can be put in one as small as 6 or 6l/£ cubic feet.

In the case of the reflex baffle (as for the infinite baffle) some manufacturers specifically recommend that their speakers be used in this type of enclosure. Among the advantages claimed by those who advocate the reflex baffle are that it will damp the peak that occurs at the speaker's resonant frequency, at the same time extending the bass response of the speaker below the point that would be achieved with an infinite baffle. In addition, the speaker's efficiency is increased since the back wave is utilized as well as the front wave at low frequencies.

To get back to our primary concern, which is the external aspects of the cabinet, a reflex enclosure, like an infinite baffle, is quite flexible as to shape. It can be a regular parallelepiped, a

Fig. 116. The reflex baffle is a popular enclosure. The port actually tunes the enclosure.

Fig. 116. The reflex baffle is a popular enclosure. The port actually tunes the enclosure.

corner cabinet or an irregular shape. If you use an irregular shape, you're sort of looking for trouble because you're liable to have an awful time computing the cubic volume to get the port size right. But, if you have no other choice, go ahead and do it that way but be very careful with your volume computations, or the whole thing is liable to come out sounding a bit off. Considering

Fig. 117. Speaker enclosures have been known to come apart from vibration and air pressure caused by a speaker working at high levels. Brace the joints with glue blocks.

the necessity for accurate volume computations, the regular parallelepiped is by far the easiest shape to work with. However, it's not critical whether the shape is made long and low or high and narrow. And it doesn't make much difference whether the port is alongside the speaker hole or below, as long as the area is right and it is not too close to the speaker opening. So actually you do have a high degree of flexibility in working out the lines and proportions of the reflex enclosure.

As with the infinite baffle, internal battening of the panels is a good idea. In this case, you're utilizing the cabinet resonance which you don't want mixed up with spurious panel resonances, if you can help it. In the reflex cabinet you don't use as much absorptive material as in the infinite baffle. Generally, absorptive material is applied to the inside of the back and then to either the top or the bottom and the right or the left side, but not both.

Both infinite and reflex baffles should be made of good, solid 34-inch plywood, preferably with the joints reinforced internally with glue blocks (Fig. 117). Both baffleboards and backs must be firmly screwed in place with screws spaced not more than 6 inches apart all the way around. Grille cloth should be as acoustically transparent as possible, otherwise you will attenuate the highs. The best materials for this purpose are woven of monofilament plastic yarn. Several companies specialize in the manufacture of cloths specifically for this purpose. Such cloth should be readily available from your components dealer. One rule-of-thumb test for the acoustical transparency of a piece of proposed cloth is to hold it up to the light in an otherwise darkened room and see how transparent it is (Fig. 118). The reason for darkening the room is so that the color of the cloth will not affect your judgment.

Horns

The other type of cabinet of major importance, the horns, tend to get rather complicated. In fact, before even starting to describe them, I feel that I cannot caution you too strongly against attempting to do the acoustical design of a horn yourself. This is not a job for an amateur. Even if you have enough acoustical engineering knowledge to design a horn, the chances are that you do not have the extensive and expensive instrumentation necessary to test what you're doing. If you want a horn and don't like the appearance of the ones you've seen, stick to the internal specifications of the one you like and change the outside appearance to suit yourself. If you take a cabinet of fixed dimensions, you can change its exterior appearance by varying external details. By adding, subtracting or changing moldings, legs, grille cloth, bases, etc., you can get immense flexibility in external appearance without changing internal specifications one bit. But stop right there. Don't try to change dimensions or proportions.

It is almost impossible to describe the design of horns in simple terms. The whole subject very soon becomes unwieldy. Horns can be classified in one way according to the type of curve upon which the flair of the horn is based. The curve most often used is an exponential one. However, another type of curve, the catenoid, has also been used fairly extensively.

A horn type speaker cabinet should not properly be called an enclosure, because a true horn operates as an extension and additional part of the speaker. Therefore, the speaker and its associated horn must be considered as a single integral unit. For this to happen the speaker must be specifically designed for the horn, or vice versa. Therefore, if you intend to use a horn type speaker system, be sure to use the right speaker in the horn you've picked or, alternately, the right horn for the speaker you have.

In addition to the three major types of speaker enclosures already mentioned, a number of other principles have been utilized for forming speaker enclosures. I shall not attempt to deal with them in any detail, but refer you to existing literature.

One device used strictly for reproducing basses and requiring separate arrangements for mid-range and treble is called an air coupler. These tend to be rather bulky and, from our point of view, rather limited as to the flexibility of proportion, making them a little difficult to design around.4

You may very well come across types of enclosures not mentioned—it is impossible in a limited space to try to cover them all. But no matter which you like and want to adopt, just be sure to

Fig. 118. The grille cloth must be acoustically transparent. One test is to hold a piece up to a light. If you can see through it, chances are that you can hear through it.

check the possible effect of changes in size and proportion before you decide to make any such alterations. It is no great effort to inquire and a good deal better to do so than to suffer needlessly with unsatisfactory results.

Stereo equipment considerations

Because there has been so much hoopla about stereo recently, no discussion of components in relation to furniture could be

4 Do not confuse the air coupler with the air loaded low-efficiency enclosure. The latter units are small and extremely compact.

Fig. 118. The grille cloth must be acoustically transparent. One test is to hold a piece up to a light. If you can see through it, chances are that you can hear through it.

considered complete without mentioning the questions and problems stereo can raise.

Tuners

Although methods are being developed to broadcast stereo by FM multiplexing, as of this writing the only generally available stereo broadcasts are AM-FM; that is one channel is on AM, the other on FM. To receive and reproduce such broadcasts in stereo you need either an AM-FM tuner that can tune each band separately and has separate outputs for each band, or you'll need two entirely separate tuners, one for each band.

Stereo AM-FM tuners are available, and, while they are sometimes slightly larger than their nonstereo counterparts, if you have allowed adequate space for a monophonic AM-FM tuner, you won't have trouble getting a stereo unit to fit the same space.

Visibility of tuning dials may be more of a problem with stereo —you'll have two of them to tune instead of one—but, you ought to be able to see a tuning dial reasonably readily anyhow.

Ventilation is just as important to a stereo tuner as to any other. Stereo equipment won't run any cooler; on the other hand, it will not run much hotter either. Venting adequate for a straight AM-FM tuner is not likely to stifle a stereo unit.

If, for any reason, you are committed to keeping a nonstereo tuner, but still want stereo, then you may have a bit of trouble. You have to get another tuner, and the second one must be treated with just as much courtesy and consideration as the first.

Record changers

Here we find a very happy situation. There is no difference at all in the physical size of a record changer, be it mono or stereo. Be a bit more careful about the mechanics of mounting the changer, however. If the shock mounts are very springy, as they often are, put in a bit of steel wool to damp them. Be rather more careful to level the changer when it is to be used for stereo. Remember, in stereo you are getting information from both sides of the record groove so you don't want to start off with the stylus tending to lean more toward one side than the other.

Turntable and arm

Here again, from the point of view of the space required for housing in furniture, there is no difference between a mono-phonic table and arm and a stereo unit. When playing stereo records, the stylus pressure must be as well centered as possible in the record groove. All considerations previously discussed relating to turntables and arms still hold true.

Tape machines

The extent to which stereo will affect the housing of your tape machine depends largely on the type of machine. This does not mean the make, but rather what it is intended to do. Some tape machines are set up to record monophonically only, but will play back either mono or stereo. Others will perform both recording and playback functions in either mono or stereo.

The units that play stereo but will record only mono differ in physical size very little, if at all, from their strictly monophonic equivalents. They, therefore, introduce no problems. However, the machine that will both record and play back stereo will, of necessity, be somewhat bulkier than its simpler mono brother. This is primarily because of the unavoidable addition of a second complete record preamp. In machines designed so that the electronic parts are mounted in one integral unit with the mechanical parts (the tape transport), the additional space may be taken in any direction, length, width, depth or any combination of these. As a result, the only solution is to measure the make you want to use, and revise as necessary the space you have allotted to tape in your cabinet plans.

If, however, your tape equipment is designed with the electronic parts completely separate from the tape transport, the transport space will be unaffected by a stereo conversion. Bu' the preamp space will be nearly doubled, because now you must have two of them.

Preamps and Amplifiers

For stereo, as for mono, amplifiers are sometimes designed with the preamplifiers on the same chassis, and other times separately. High-powered amplifiers will generally be separate from the preamps while the ones boasting, say, 10 to 20 watts per channel, are likely to combine both functions on one chassis. This is actually quite a trick, if you will just stop to think about it. Two independent amplifiers plus two independent preamps have been piled on a single chassis, and in the process they have been kept down to remarkably manageable size. The average stereo combination preamplifier—amplifier delivering up to 20 watts per channel will measure 15 to 16 inches wide, 4i/2

to 5 inches high and 10 to 12 inches deep. This is very compact considering what is included in the package! Yes, it's bigger than

Fig. 119. The simplest stereo setup, but the least desirable acoustically, is to have both speaker systems in a compart-mented enclosure.

a monophonie amplifier of comparable power per channel, but it is nowhere near twice as big.

The preamp will approximate the size of the complete amp-preamps we've been discussing. You may be able to pare an inch

Fig. 120. Identical speaker systems in identical enclosures. or two from the width or the depth, but the height won't come down much. The main difference in size between the mono and the stereo preamp will usually show up in the depth.

Speakers

Actually you have four alternatives to choose from in setting up stereo speakers. The first, and least desirable for reasons that will be dealt with in the next chapter, is to mount two separate speaker systems at either end of a single enclosure that is partitioned down the middle to divide it into two entirely separate compartments (Fig. 119). By least desirable is meant least desirable acoustically. From the furniture point of view, this is by far the simplest method.

The second method is to use two identical speaker systems in two identical enclosures (Fig. 120). This method is generally the best because the speakers can be placed at whatever distance and angle from each other that is best. The obvious disadvantage is that, first, you've got to have a second box, and also you must find space somewhere for it.

The third method is a good compromise when there just isn't space to put two complete speaker systems in the room. For one channel set up a complete speaker system as you would

Fig. 121. Stereo setup using different speaker systems.

for mono, but cut the woofer off at 250 to 350 cycles, the lower the better. Don't let it get up into the mid-range. Use a separate speaker or speakers for mid-range and high-frequency speakers

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