The Ear is a Variable Element
Having gone through all the problems of the loudspeaker, its enclosure, and the environment in which the sound is to be heard, we come to the last element in the acoustic chain. This is the all-important ear, which does the listening. The ear is a variable device. This is not to say that we can vary our acuity of hearing at will; it is not variable in that sense. Perhaps it would be more accurate to call the ear the component with the most variants built into it.
No two people have the same hearing characteristic, any more than they have the same fingerprints. An individual's hearing capabilities may even differ from one ear to the other. There are many factors that affect one's sense of hearing. These factors are the subject of the present section, along with means that may be utilized to adjust the performance of a system to an individual's ear so that he may obtain optimum satisfaction from it.
The ear is not a linear device. What it hears, and how well it hears it, depends upon not only the ear itself but in a great measure upon the sound. One of the most important characteristics of hearing is the change in sensitivity of the ear for different frequencies as the sound grows softer or louder. When music is loud, the ear has one response characteristic. When the music is soft, the ear has an entirely different hearing pattern. Therefore, a system that may be balanced for the ear under one condition of loudness may be entirely out of balance for another condition. This is shown in Fig. 18-1. This particular characteristic is called the Fletcher-Munson curve, after the two researchers who formulated it. This family of curves so well known to the art is indicative of the average ear. Any individual's ear may, and probably does, differ considerably from it.
To what practical application may we put this piece of clinical information? Certainly, it shows that what the ear hears under one condition it may not hear under other conditions, and that a system adjusted for one condition mg), not be properly adjusted for another condition. Let us explore this in a little more detail.
Fig. 18-1. The Fletcher-Munson contour lines of equal loudness for nor
mal ears. The curves show how much louder than the 1000 hz note other
frequencies have to be produced to be heard equally as loud as the 1000
hz note.
Sensitivity to Frequency Varies with Loudness
Examination of these hearing acuity curves tells us that for very loud sounds the ear hears all frequencies at almost the same intensity; for loud program material, the ear may be considered fairly "flat." It hears the sound exactly as reproduced. As the loudness of the sound decreases, the ear begins, to lose sensitivity to the lowest frequencies and the highest frequencies. Therefore, when the level of the music is very low, the ear does not hear the program exactly as reproduced.
It hears the middle frequencies fairly well, and loses both ends of the spectrum. Putting it more bluntly, the ear decides to concentrate on a small portion of the sound, as it were, so it may "understand" it better. As lightly put as this thought is, perhaps there may be considerable evolutionary truth to the statement. We do know that most of the intelligibility of sound exists in the middle spectrum area of the frequency curve. We receive perfectly good communicative intelligence from the telephone with a reproduction of frequencies ranging from about 400 hz to 5000 hz. The lower and higher ends are not at all necessary for the ear to make sense out of what it hears. Thus perhaps the ear has learned to adapt itself to very low level sounds in such a fashion that it closes its "consciousness" to those parts of the spectrum that it does not need, and concentrates entirely on the small middle band, from which it may extract the desired message.
Whatever the reason, the fact exists that the ear does have this differential frequency sensitivity with a change in signal level. In high fidelity practice. however, we are concerned with more than just communicative intelligence. We want the lowest throb from the bass violin and the highest tweet of the piccolo. These low and high limits belong to the musical intelligibility of the sound, and they must be present in the individual's hearing mechanism for the full program to be received by his consciousness.
Adjusting the Volume Changes Frequency Sensitivity of Ear
Suppose we are listening to our high-fidelity system at a good comfortable concert hall listening level (one at which our ears are "flat"). Let us assume that the amplifier response is flat too, with the bass and treble controls set at midpoint. We now hear the program to our ear's psychoacoustical satisfaction. Suppose now that the hour is late, the children are in bed, the neighbors do not like our selection of music, and we want light background music by which to relax. We turn down the volume control to where the music can barely be heard.
What do we actually hear? By simply turning down the volume control of the amplifier, we have destroyed the wide range high-fidelity characteristic of the chain of the system. The ear no longer hears the even rhythmic beat of the bass at the low end, or the tinkling of the triangle at the high end. It hears only the middle frequencies of the music.
That this need not be so we will soon show.
First, however, it will be of interest to the reader to try a little experiment for himself. Select a vocal composition, preferably a female vocal rendition backed by a full orchestra. Play the recording at a fairly loud level and listen carefully to the low notes and the high notes that "surround" the soloist. Play one section over several times so that you are familiar with the musical accompaniment as well as the vocal part. Now turn down the volume so that you can barely hear the vocalist. Invariably, the ear will completely miss the accompanying low notes and high notes that it heard before. It hears only the voice and the notes immediately surrounding it. The fidelity of the system has been watered down by the poor frequency response of the ear at low listening levels. This situation does not, fortunately, present us with an insoluble dilemma. We can adjust the system either manually or automatically to take care of this characteristic of the ear.
Loudness Controls on Amplifiers Compensate for Ear Deficiencies
Many amplifiers are equipped with "loudness controls" in addition to or in place of ordinary volume controls. There is a definite difference between these two types of controls. The loudness control, sometimes called a contour control, changes the characteristic of the amplifier so that as it is turned down to lower levels of loudness, the low end response and the high end response are automatically boosted in relation to the middle frequencies by an amount necessary to compensate the ear for its deficiencies at the ends of the band. Once the loudness control has been set, the volume control may then override it varying the overall level of the intentionally deformed curve of the amplifier as affected by the contour or loudness control.
Amplifiers that do not have such loudness controls may be adjusted manually through the bass and treble controls. When the volume control is turned way down, the bass and treble controls may be turned up to boost the ends of the spectrum to compensate for the hearing loss at these points.
Ear Compensation in Multi-Speaker Systems
There is still another method for compensating the ear for its deficiency and that is through the use of controls on the loudspeaker systems themselves. For instance, in the case of a three-way system, with its usual crossover network and volume controls, aural compensation may be made without touching the amplifier, thus avoiding certain disadvantages inherent in the other method. If the amplifier has been turned way down in level and the bass control turned way up, intermodulation distortion may be severely increased within the amplifier. Intermodulation distortion exists between low frequencies and high frequencies if there are nonlinearities in the circuit. This is true acoustically, mechanically, and electrically. Thus, if we greatly increase the level of the low frequency signal in the amplifier in relation to the middle frequencies, the probability of intermodulation distortion will be increased. This particular situation may be especially aggravated under conditions of extreme bass boost. The amplifier may be forced to work near the limits of its power handling capacity in this lowest part of the frequency spectrum, which will introduce non linearity into the amplifier.
Obviously, there is a definite advantage to control of the loud speaker characteristic at the speaker system itself without touching the amplifier. The control usually found in the midrange speaker of the three-way system may be used to turn down the level of the middle range of the reproducible spectrum, so that by comparison the low end response and the high end response are boosted far above the midrange response. Then the volume control of the amplifier may be turned down to produce the necessary listening level, and the total sound emanating from the system will be properly compensated for the listening deficiencies of the ear at the low levels.
How Much Adjustment Should be Made to the System?
The degree of compensation that will be necessary will depend on the individual's particular ear characteristics and upon average level of the program material. The number of controls on today's high fidelity equipment is indicative of the realization that high fidelity is a personal thing as well as a technical art. The many controls on pre amplifiers, amplifiers, record compensating devices, and loudspeakers are all means by which the listener may alter the colors in his musical picture. It is a moot question whether the listener should have the privilege of editing (or shall we say "auditing") the music presented to him. According to one school of thought, the listener should not corrupt the conductor's interpretation of the work; he should leave the controls flat. This is the concept of realism. Then there are the knob and gadget fanciers, who prefer to "set the stage" before listening.
The "music lovers" and the "gadgeteers" are not necessarily on opposite sides of the fence. Together they constitute the portion of the listening public that appreciates good reproduction.
Perhaps we should change the word "reproduced" to "recreated." To the purist, reproduction means an exact duplication of the original.
On the other hand, the word "recreated" implies something basically different. To recreate is to make over. We have all at many times recreated the music to which we have listened. For example, when Beethoven wrote the thunder into his Pastorale Symphony, he intended it to sound near at hand. Yet in hearing his work at a subdued level we have moved the storm away to some far hilltop. We did not reproduce" the music; rather we recreated it to conform to standards different from those Beethoven had in mind.
Moreover, we have seen that the mere act of turning down the volume may entirely change the tonal value to the ear, changing completely the effect the composer wanted to produce upon us. Thus, both in dynamic range and in tonal nuances, we have "recreated" the music.
Whether we like it or not, we become our own impressarios simply by turning down the volume control.
Nature also takes a hand in this. As we grow older our sight grows weak and our ears grow insensitive. The aging process that turns our hair gray causes changes to our ear drums and auditory processes as well. These changes destroy to varying extents the tonal ranges of our hearing and there is nothing (physiologically) that we can do about it. Nature limits not only the sensitivity but also the frequency response of our hearing. Of course, the deterioration of hearing with age is very gradual. Constant changes are produced in our ears, and in our auditory processes. Also, childhood maladies may leave us with ear impairment, and even though they may be small they may cause one to hear things differently.
Even sex enters into the picture insofar as women as a rule are more sensitive to high frequencies than men. Often the woman of the household hears the high pitched scanning frequency from the tele vision set (15,750 hz), while the mere man is totally oblivious to this spurious sound. When the high-fidelity system is playing, her ears, being more sensitive to highs, find piercing and shrill the sounds that to the man of the house are rich and brilliant. Because of these differences in the hearing characteristics of individuals, there can be no best balance, no best range, no optimum loudness. These are matters of personal physiological inheritance; health; and musical, educational, and listening environment. In order to enable the average music enthusiast to recreate the music so that it is satisfying to his ear, he is given the many controls on the pre-amplifier, amplifier, and speaker.
For that matter, long before the music reaches the listener, whether it be over the radio or on disc or tape, many different "editors" have had a hand in altering the music to a far greater extent than the com poser ever thought possible. Today, all music reproduction is mechanical. It is mechanical if we hear it through any means other than the original composer-performer. We do not hear the Philharmonic at home in the same way the audience in Carnegie Hall hears it. Be tween us and the conductor is that long acoustic and electronic chain discussed in the first section of the book. The number of microphones and their placement, the "hearing" abilities of these microphones, the technical excellence (or lack of it) in the long series of speech input amplifiers, the technical manipulations of the telephone lines that bring the program to the various broadcast stations, the quality of the broad cast transmitter itself, and the characteristic performance of the tuner, amplifier, and loudspeaker all intervene between the orchestra and the listener.
At each of these bridges the human factor is introduced. An individual or group of individuals is constantly at watch at these crucial points to see that everything is "right." But "right" for whom and for what? For the technological equipment, of course. Is there a tremendous crescendo coming? Quick, crank down the gain or we'll blow the transmitter off the air. Is there a long sustained pianissimo passage coming? Crank up the gain, or we won't modulate the transmitter properly. It is, of course, true that all these precautions are taken to insure that the listening audience gets optimum service from the technical apparatus involved, but in the accomplishment of this deed, it is the technical apparatus that gets the babying, and the technical apparatus that determines what we are going to hear.
In the days before the invention of mechanical reproducing systems, there was only one person between the performer and the listener, and that was the conductor. Today there are dozens of people and dozens of technical processes, through which the music has to run the gauntlet. These are the editors of the music that we hear. These processes and the people who supervise them are not always perfect.
In fact, we cannot even be sure that the conductor of the orchestra is perfect. We need only read the various concert reviews and record reviews in the daily newspapers, the music magazines, and the high fidelity publications to learn that critics often express diametrically opposite views of a particular conductor's efforts. They discuss at length whether he interpreted a phrase right, whether his dynamics were good, whether he lacked lyricism here or forcefulness there. Certainly, the conductor thinks he is doing a good job, yet here are critics whose privilege and duty it is to criticize the work. It all boils down to the fact that in art there is no right or wrong. It is largely a matter of taste. To satisfy this taste, high fidelity brings to the listener all the necessary ingredients to flavor the music to suit his own artistic palate, and perhaps most vital to these individual recipes for musical recreation at home are the important acoustic phenomena associated with loud speakers, their enclosures, and the rooms in which they perform.