Read!!! Part 1 of my Thesis.

[PoorManQ45]

I am writing this for my own personaly use. I am posting it on here to help you guys/girls choose your audio equipment easily. Note, this is specifically about home stereos', but the principles are all the same.

Feel free to correct anything that you think/know is wrong. Please post a link stating otherwise.

This is the first chapter:

1

Speaker Enclosures

Speaker Enclosures Defined

One of the most common fallacies about the design and construction of a speaker enclosure is that the enclosure is some sort of musical instrument-that it should resonate or have tone. Various types of woods are thought to be more suitable because they are used in musical instruments; or various shapes are chosen because they are shaped like a timpani or a piano sounding board, etc.

The first clear distinction that must be made with regard to speaker enclosures, then, is that they are not musical instruments and that they do not generate musical tones. They are more accurately defined as precision reproducers of musical tones. This means that ideally they will add no tonal coloration of their own, but will remain passive, responsive only to the controlling input signal. The speaker and enclosure are as impersonal as a mirror. Any startling effects emanating from them must be only those produced by the original recording artist.

Two Factors To Consider

In order to construct a speaker enclosure, two factors must be considered in conjunction with each other: (1) the speaker or driver and (2) the baffle or house in which the driver is mounted. The approach can be from either of two directions: (1) the speaker chosen will determine the type of enclosure to be constructed, or (2) the choice of enclosure desired will dictate the speaker required.

Where decor, size, and limited available placements in a room are the ruling factor, good design consists of selecting the enclosure that best answers the problem at hand and of carefully matching optimum speaker to it.

If performance is the main objective, the speaker should be chosen first, and then an enclosure designed which will provide maximum utilization of the potentials inherent in the driver selected.

Today's designer is afforded a marvelous variety of acoustical devices from which to choose. Low-frequency speakers range from 8 inches to over 30 inches in diameter, with every degree of hard to soft suspension available in each size. The most commonly used sizes are 8 inches, 12 inches, and 15 inches. High-frequency drivers can be conic, ionic, and electrostatic or compression types.

Five basic Enclosure Types

A speaker by itself causes interference with its own output. This is primarily due to the fact that the surface which moves the air (called a diaphragm or cone) causes the air to move both in front and in back of it. These simultaneous impulses generated by the motion of the cone are, unfortunately, out-of-phase. This means that if they are allowed to meet, they will cancel each other. Therefore, if the front of the diaphragm/cone is not separated from the back of the diaphragm/cone, a large percentage of the energy fed into the speaker will be wasted in a fight between the air moved on either side of the diaphragm/cone surface.

There are five basic forms that this separation can take:

1.Finite Baffle-flat baffles, open-backed cabinets.2.Infinite Baffle-walls between rooms, large enclosures totally sealed.3.Bass Reflex-where the radiation from the rear of the cone is usefully added to the radiation off the front of the cone by means of phase inversion.4.Horn projectors.5.Combinations of any of the above.

Finite Baffle

Finite Baffles are most commonly encountered in furniture-type console radios and consist of a board about 2 feet square, with the speaker mounted in the center. This forces the back radiation from the diaphragm/cone to travel a longer path to reach the other side. The larger the board used, the lower the cutoff frequency (frequency at which cancellation starts to occur). The wavelength of the cutoff frequency can be calculated as follows:

W = V/Fwhere,

W = wavelength in feet, V = velocity of sound in air (about 1140 ft/sec), F = frequency of sound in Hertz (Hz).

It can be seen that to prevent cancellation of bass frequencies at 30 Hz, the board would have to be 38 feet square, or 19 feet each side of center.

Infinite Baffles

If the speaker were mounted on a board of infinite length and width, the back radiation would never meet the front radiation, and no cancellation could take place. Then the only determining factor would be the ability of the speaker to move enough air at very low frequencies to allow it to be audible.

An infinite baffle can be constructed by mounting a speaker in a wall so that the front of the diaphragm/cone is in an entirely different room. The area into which the back of the diaphragm/cone exhausts may be as small as 15 to 20 cubic feet if the surfaces are made non reflective to sound waves with fuzzy soft material such as glass wool, Kimsul, etc.

Closet doors, when the closet has a substantial amount of clothes hung in it, can be used as a mounting surface for an infinite baffle. When the door is closed and gasketed, it becomes an infinite baffle. Of course, service presents no problem because of the easy access.

A special technique used in many of today's smaller speaker systems compresses the infinite-baffle idea into a 2-cubic-foot box. In this case, the air that the back of the cone compresses, also serves as an additional spring in conjunction with the one mechanically designed into most speakers. The drawback to this method is very low efficiency as a result of having to substantially increase the cone's mass part of the design process.

Speaker enclosures, in conformance with other facts of life, simply don't allow something for nothing.

Bass-Reflex Enclosures

Another approach is to add the radiation from the rear of the diaphragm/cone to the radiation from the front. This is done by using the volume of air in the enclosure which acts in conjunction with the mass of air entrapped in a tuned port hole to create an in-phase, additive relationship.

This combination of the rear radiation being added in phase to the radiation off the front of the cone results in almost twice the output for a given excursion of the cone than would be expected if the speaker were mounted in an infinite baffle.

These phase-inversion or bass-reflex enclosures offer greatly increased bass response with a minimum of structural and tuning work.

Horn Projectors

Bass response can be substantially increased if the front of the cone is coupled to a long, expanding horn. The old Edison phonographs had a diaphragm about the size of a dime. When the large "morning glory" horn was attached to the small diaphragm, each motion of the dime-sized surface was transformed from a small-area radiation into a large-area radiation. This means that horns act as acoustical transformers. Such transformers can take many shapes such as conical, hyperbolic, catenary, parabolic, and exponential.

In all types of horns, the effect is to present to the small diaphragm at the throat a very high but consistent acoustical impedance while transforming the high-pressure, low particle-velocity wave from the surface of the cone into a low-pressure, high particle-velocity wave with a low impedance, matching that of the air in the room as it reaches the mouth of the horn.

This consistent high impedance at the throat of the horn can be maintained as long as twice the square root of the mouth area times pi is greater than the wavelength required. If twice the square root of the mouth area times pi is less than the wavelength, then the horn does not properly ?load? the diaphragm at the throat to the air impedance at the mouth. From this it can be seen that to have a usable low-frequency horn, large dimensions are required. For 30 Hz, a mouth area of 45 square feet would be needed. Nine times five feet is a large mouth in any room.

Folded corner horns can materially reduce the enclosure dimensions required to achieve such a large mouth area by folding the horn back and forth in its own path before using the corner of the room as the mouth of the horn.

For all these reasons, a horn-loaded speaker mechanism is very efficient; i.e., it will provide more acoustical watts output per electrical watts input.

Combinations

Combinations of these basic forms make up most of the commercial offering in the high-fidelity market today. Economy, decor, and associated equipment limit ideal theoretical considerations. The result is an engineered system whose compromises are not detectable under actual home listening conditions, even in direct comparison with so-called theoretical perfection. This is particularly true because no speaker system has yet achieved anything close to perfection, and while some come closer than others, it is still a question of how much distortion your personal ear enjoys or rejects the most. One person may be tone deaf but appreciate dynamic range. Another may be quite sensitive to pitch (which would require an expensive turntable) and yet completely ignore distortion, compression of dynamic range and regularity of frequency response.

Various popular combinations include:

1.Acoustical labyrinth Advantages: Increased bass loading over extended range. Disadvantages: Large size; out of phase at crossover.

2.Back-loaded horn: This is really a bass-reflex device in which a horn has been placed on the port. Advantages: Good, efficient bass response. Disadvantages: Rough response; phasing at crossover difficult.

3.Front horn-loading of woofer: bass-reflex loading of back of woofer, and straight axis horn for high frequencies. Advantages: Proper phasing can be achieved; high efficiency obtained; very smooth response; exceptional polar response. Disadvantages: Unless size is quite large, bass response falls off below 40 Hz; transient response slightly less than that of a horn woofer. These samples of commercially obtainable systems illustrate that almost any conceivable combination could be tried, and a quick look at the back issues of early High Fidelity magazine will show that indeed they have been.

Design Considerations

This chapter, then, has discussed the important design considerations that should be taken into account before one decides on which of the five enclosure types to plan around. These considerations are:

1.Decor and physical size2.Efficiency3.Smoothness of tonal response4.Range of tonal response5.Distortion in its many forms? A. Harmonic B. Intermodulation or Doppler C. Transient D. Phase6.Polar characteristics

Summary

décor and physical size will be determined by use. The owner of a small apaertment may desire to reproduce only string quartets (where the small infinite baffle with a "long throw" woofer is a good compromise); the owner of a 60-foot living room may wish the Boston Symphony present at a distance equal to first row center (where a large, straight, woofer horn carefully phased at the crossover to a multicellular or sectional horn would create a hard-to-disbelieve illusion). In other words, those who listen to small folk-singing groups, string quartets, modern jazz combos, etc., can accept a degree of power inefficiency in their speakers, whereas those who listen to pipe organs, symphony orchestras, massed choirs, etc., cannot.

No one accepts roughness of tonal response except as a compromise to expense; and in the systems described in this book, every effort is made to achieve the smoothest tonal response possible at any size, price, or performance level.

Deliberate limitation of tonal range, however, is often used and can provide many benefits. In a small speaker, extended low frequency response may be at the expense of excessive distortion, since the small woofer moves nonlinearly at the low frequencies. Much better sound can be achieved by limiting the speaker's low frequency response at the frequencies where distortion starts to increase too rapidly. This can often be achieved by inexpensive high-pass filters either in the amplifier or even in the crossover network.

If both low distortion and wide dynamic range are desired, "think big." If low distortion is desired and dynamic-range compression can be tolerated, then "think small." Cost and décor limitations usually result in a compromise between distortion, efficiency, and less control of irregularities. The better the balance, the better is the combination. Much latitude is offered to the designer in the choice of such balances.

Proper phasing of the low- and high-frequency units is a little-known art, and it will be discussed in detail in the chapter on infinite baffles and also in the chapter on crossover networks. Suffice it to say that without very correct phasing of high- and low-frequency drivers, transients will never be properly reproduced. Sound travels at about 1140 ft/sec, and the ear can detect as little as three milliseconds difference in short duration sounds (castanets for example). Since sound travels 1.14 feet per millisecond, sounds having less than a three-foot path difference can be heard.

While the variety of problems to consider and the infinite number of possible answers can at first seem confusing, approaching each basic design one at a time will allow today's knowledge of the art to fall into place naturally.

There are 7 more parts to come. 8 total. I will be posting them over the next few weeks. Then I will put them all together and present them to AZ and over mods to consider.


Modified by PoorManQ45 at 12:59 AM 10/11/2004