Part 1: THE LOUDSPEAKER
SECTION 1: The Sound Circuit The Loudspeaker: the Start of the Acoustic Circuit
Although the loudspeaker is the prime source of sound in any reproducing system, the sound we actually hear when we listen to a radio or phonograph is not entirely the result of the loudspeaker performance.
We hear the result of many interacting factors, which constitute the subject matter of applied and practical acoustics. Not until the electrical signal (which is the counterpart of the original sound) acts upon the loudspeaker mechanism is the signal transformed into sound waves.
Contrary to popular belief, this transformation is not the end of the road to fidelity. The loudspeaker is only the beginning of one chain among others in the high fidelity circuit, but this chain, consisting of the acoustic circuit extending from the vibrating system of the speaker diaphragm to the nerve endings in the listener's brain, is obviously of more than passing interest. This is in fact the circuit most intimately and critically involved in the subjective or personal factor in the listener's high fidelity equation. The personal factor ultimately sets the standards that make a system sound good or bad in his opinion. This factor is an important element; it has made an art of the science of high fidelity.
We have jumped the gun somewhat in discussing one of the more personal elements of the acoustic circuit in order to emphasize the fact that there is more to the sound than just the original vibrations of the loudspeaker. We shall discuss these more personal factors in greater detail after we have examined the entire acoustic chain in orderly sequence. An understanding of this acoustic chain will be the first step toward a full appreciation of the scope that high fidelity sound reproduction must cover; this must be understood before an honest and practical approach can be taken to the personal high fidelity system problem.
Sound to "Signal" Transformation
The loudspeaker is the device that produces sound from a pure electrical signal. This signal has itself gone through many subsidiary chains before reaching the terminals of the loudspeaker. In these subsidiary chains there have been many different kinds of link (acoustical and mechanical, electrical, electronic, and electromagnetic, as illustrated in Fig. 1-1). In even the simplest of broadcast linkages between the original performer in the broadcast studio and your radio antenna, there are involved at least these five distinctly different transformations of the original program. The voice of the singer has vibrated the air particles, setting up a wave motion in the air in the studio; this wave motion in air is acoustical in nature. The traveling acoustic wave impinges on the sensitive microphone, whose ribbon or diaphragm in turn is forced to vibrate in synchronism with the acoustic wave flowing past it. The delicately suspended diaphragm will move back and forth, riding the sound wave just as a cork will bob up and down in a water wave. Here is the first transformation. Acoustical energy has been changed to mechanical energy. The vibrating diaphragm with a small coil attached to it, vibrating in a magnetic field, generates an electrical current in its coil, just as the rotating armature in an automobile generator produces a charging current. This then is the second transformation, the change from vibrating mechanical energy to electrical "vibrations." The electrical current or voltage thus generated (depending upon the type of microphone being used) operates upon a whole chain of amplifiers utilizing vacuum tubes, which convert the electrical signal to an electronic signal (transformation number three). Other vacuum tubes in the transmitter impress these signals onto high-frequency electric (radio) currents (transformation number four). At the final stages of the radio transmitter the fifth transformation takes place: the electronic signal is converted to an even different type of signal, an electro-magnetic wave motion, which then travels to your antenna with the speed of light.
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PERFORMER GENERATES SOUND WAVES IN AIR
SOUND WAVES HIT MICROPHONE DIAPHRAGM MAKING IT VIBRATE COIL ATTACHED TO DIAPHRAGM MOVES THROUGH MAGNETIC FIELD GENERATING ELECTRICAL CURRENTS TRANSMITTER
-- VACUUM TUBES TRANS FORM ELECTRIC CURRENTS TO ELECTRONIC CURRENTS
-- ELECTRONIC CURRENTS ARE TRANSFORMED TO HIGH FREQUENCY ELECTRIC CURRENTS
-- HIGH FREQUENCY ELECTRIC CURRENTS ARE TRANS FORMED INTO RADIO WAVES
Fig. 1-1. The chain of transformations from sound waves to radio waves.
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ELECTRICAL SIGNAL CD LOUDSPEAKER THE VIBRATING DIAPHRAGM SETS AIR PARTICLES INTO MOTION CAUSING A SOUND WAVE TO BE PRODUCED
-- ROOM; ROOM ACOUSTICS WILL DETERMINE LIVENESS OF ROOM BY CONTROLLING AMOUNT OF REFLECTED SOUND DIRECT SOUND BAFFLE OR ENCLOSURE DIRECTS THE SOUND WAVE, DETERMINES ITS FREQUENCY RESPONSE, AND DETERMINES HOW THE SOUND BECOMES SPREAD OUT THROUGH THE ROOM REFLECTE SOUND (i) BRAIN THE BRAIN, THROUGH TRAINING, RECOGNIZES THE ELECTRICAL IMPULSES AS SOUND
-- EAR SOUND WAVES ENTERING EAR VIBRATE EAR DRUM CAUSING MECHANICAL MOTION OF INNER PARTS OF EAR WHICH IN TURN CAUSES NEUROLOGICAL ELECTRICAL ACTIVITY IN NERVE CABLE FROM EAR TO BRAIN
Fig. 1-2. The acoustic circuit from loudspeaker to "hearing."
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As brief as this resume has been, it should be apparent that between the performer and the antenna of the home radio there are many technological bridges. In any event this is the chain that must exist before your radio "hears" anything. This same chain of events must be completed in reverse after your radio has received the space-borne signal before you finally recognize the signal as sound.
"Signal" to Sound Transformation
Rather than belaboring the point by describing the manner of reversal of this chain in your home apparatus, we shall proceed on the premise that an electrical signal finally finds its way to the terminal of your loudspeaker, and that this signal is in some general way representative of the original sound in the studio. The signal the loudspeaker sees is purely electrical in nature. This signal, "vibrating" electrically in accordance with the original signal, in turn causes the loudspeaker "motor" to vibrate in synchronism with it.
The Loudspeaker: Prime Source of Sound
Thus the loudspeaker mechanism, being energized to provide mechanical vibrations of its own, vibrates the air in contact with it and becomes the prime source of sound in the reproducing system. The degree of fidelity of the reconversion of these electrical impulses to sound depends not only upon the loudspeaker itself and the preceding electronic circuits, however, but also upon the rest of the sound circuit, the other links in the chain, which impose other restrictive operating conditions upon the speaker. This acoustic chain is illustrated in Fig. 1-2.
Nothing in nature works by itself without affecting something else; action and reaction are synonymous in the world of physical reality.
The loudspeaker is a real physical object and it reproduces very real airborne sound waves. You cannot feel these sound waves under ordinary conditions, but you can feel the source that gives them life. One can readily feel the vibrations of the diaphragm (the paper cone of a loudspeaker) by placing the fingers lightly upon the diaphragm while it is reproducing sound. This is factual evidence that the diaphragm is mechanically in motion while it is making sound. It is often very easy to see the loudspeaker vibrations as well as feel them. When the loud speaker is reproducing low frequencies of moderate intensities, the diaphragm may be clearly seen to be moving back and forth in its housing.
However, whether the eye can see or the hands feel the diaphragm of the loudspeaker move, move it does, and as it moves it reacts with something else. You will notice that when you put your finger even lightly upon the vibrating diaphragm of a loudspeaker the sound changes; your touch has modified the sound. The loudspeaker has reacted to your touch to change its mode of vibration. Suppose your finger does not touch the vibrating diaphragm. What is there for the diaphragm to react with? Quite simply, the air. It is always in contact with the loud speaker. It always presents a definite physical load, as real as the weight of your finger, which the diaphragm must cause to vibrate.
Loudspeaker Reacts with Surrounding Air
Air has weight, as well as other acoustic properties (which we will discuss later) that affect the operation of a loudspeaker. The very fact that the loudspeaker makes the air in contact with it vibrate introduces an air reaction upon itself. The vibrating air acts upon the diaphragm to modify its original motion, and there perhaps is the key to the entire acoustic circuit concept - modification of one element of the chain by another, and modification of that element by the next, until in the end there is a complete chain. This is the acoustic circuit, in which the series of modifications finally comes into a state of equilibrium.
Let us take one more brief look at our chain before narrowing our focus to examine the links more closely. The vibrating diaphragm, because it is contact with the air around it, makes that air vibrate more or less in accordance with the original vibration of the loudspeaker diaphragm. But how well does this air vibrate? In an ordinary sense we might, with reasonable justice, say that if the sound is loud the air has been vibrated rather well; if the sound is weak, the air has been vibrated inefficiently. As general as this statement seems to be, it is nevertheless quite true.
"Baffling" Controls the Sound Intensity and Distribution
Is there anything that can be done to the speaker or the air to make an originally weak sound appear louder without altering the loudspeaker itself? Can it pull itself up by its bootstraps, as it were? The answer is an unequivocal yes. Without increasing the power input, a mere whisper (especially a low frequency whisper) may be transformed into a roar if we modify the surrounding elements with which the loudspeaker has to react. For example, if you are playing baseball, and you want to make yourself heard in left field, you do a very natural thing, something you learned long before you knew anything about acoustics. You cup your hands around your mouth and shout into a "megaphone" made up of your two hands, which shape the sound as it comes out of your mouth.
The fact is that you have "horn loaded" your mouth. You have produced certain modifications of the physical environment through which the sound had to propagate. In making these modifications, you have altered the degree of intensity (and directivity) of the sound.
Not only have you made it louder, but you know intuitively that you have also directed it into a location where you want it most to be heard. You have "baffled" the sound, modified it, caused it do your bidding.
Here then is the next link in the acoustic circuit. First the vibrating diaphragm and the air that is vibrated by the diaphragm, and now the baffle, which modifies the manner in which the air is vibrated, both in intensity and in directivity. The size of the baffle, its shape, and its construction will (along with other factors) determine how the air vibrations are modified. Our hands cupped around the mouth modify a shout to a degree; a cheerleader's long megaphone will modify it to a much greater extent. The baffle then is the first physical item outside of the loudspeaker itself over which we have some physical control. A good baffle is a battle half won in the struggle to perfect a high fidelity system.
The loudspeaker baffle or enclosure is the one determining factor for the final performance of the loudspeaker itself. The size of the baffle, its construction, and its actual shape will determine how well the loudspeaker will reproduce the low frequencies, how well high frequencies will be dispersed in the room, and even where in the room it should be placed for optimum performance.
The Sound is Modified by the Room
However, the sound has yet to reach our ear, and our consciousness. The diaphragm is vibrating, as is the air, with the baffle molding these vibrations in intensity and in direction. Some of the sound so transformed breaks away from the speaker-baffle combination, traveling straight to the listener's ear. More of the sound, however, travels as quickly to other parts of the room. In fact, only a small part of the total sound produced actually moves directly to the listener's ear, for his ear occupies only a tiny portion of the total physical volume of the room. The overall sound fills the room, every nook and cranny of it, although in different degrees.
It traverses the room, bouncing from wall to wall, and sooner or later some of it reaches the listener's ears. When it finally gets to the ear some of the sounds that were generated at the same time have al ready arrived.
Thus the ear receives an "echo" of the original sound. To put it more scientifically, the reverberant nature of the listening room will greatly affect the total make-up of the sound waves reaching the listener's ears. The "singer in the bathroom" knows the truth of this statement. The hard shiny walls reflect sound well, causing part of it to bounce around a good deal before it comes to the listener's ear.
Let the same singer close himself up in a well-stocked wardrobe closet, and shout though he may at the top of his voice, he will sound "dead" because the room is dead. It has no life, no "reverberation." The room condition then is the next link in the acoustic chain.
The "liveness" of the room determines not only the apparent spacious ness of the sound but also what sounds will be re-enforced and what sounds will be absorbed. The amount and texture of draperies in the room, the wall textures, the amount of large untreated plaster surfaces, the thickness of pile of the rugs, and other structural and decorative factors affect the sound before the ear hears it. The listening room modifies the sound in intensity, in tonal characteristics, and in directivity.
Different Ears "Personalize" the Sound
Now the sound has actually entered the ear, but not the consciousness. Hearing is a very personal matter, and must be dealt with on a personal basis. You may have been born with large bone structures; I with small ones. You were born with certain anatomical features in your ear. I was born with the same general features, but in different mechanical proportions. The same sound that I hear, as it strikes your ear drum, may move that sensitively stretched membrane in a slightly different manner. Why? We may be of different ages; perhaps my ear mechanism has become more ossified than yours. Perhaps you have had a sinus condition that has affected your hearing mechanism, or perhaps you are a woman and have more sensitive physiological auditory reactions. Our hearing processes grow older, just as our hair grows grayer, and our hearing processes change accordingly. All these factors combine to produce different reactions within the inner ears of different individuals.
Not only are our ears different, but we have different neurological reaction times. We react differently to different stimuli. You might shout "ouch" sooner than your neighbor in response to the same un expected pin prick, simply because your nervous system reacted faster to the stimulus. The pin jab actually caused minute nervous electrical currents to flow between your wounded finger and your brain. In a factually similar manner, the sound waves acting upon the auditory mechanism cause a transformation to be made from sound waves back to minute electrical (neural) impulses, which the brain then recognizes as "sound." Even if the actual ears of different listeners were identical, the listener's neurological activity would in the end deter mine what the brain heard; and that is what counts. It is the brain, the mind, the consciousness that determines whether one likes what his ears "hear." What a remarkable process this is! We started with an electrical signal in the loudspeaker and we end with an electrical signal in the nervous system.