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Why Loudness Control |
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Why Loudness Control? By Norman H. Crowhurst A
fresh approach to the need for a true Loudness control and information
on its proper adjustment. Any subject that depends upon appreciation by the human senses is bound to be one that comes up for perennial discussion. This is one of them. There are two extreme schools of thought: one of these suggests that correct reproduction requires an accurate re-creation of the original sound wave pattern; this means that all of the components in the original program must be reproduced in exact proportion and at the same level; this philosophy does not allow of anything so intangible as a loudness control. At the other extreme we have the people who believe that human hearing has been "scientifically" established by such data as the Fletcher-Munson curves; therefore, to give a true subjective impression of any reproduced program material, a loudness control must be incorporated that introduces compensation according to the accepted and proven data published first by Fletcher and Munson. In between these two admittedly extremist views a number of subsidiary questions arise and confuse the novitiate still further. Hence the perennial topic. Let's start out from the classic point of argument that faithful reproduction should be an exact re-creating of the original sound wave pattern. Pursuing this ideal to its logical conclusion one would assume the loudspeaker should generate a sound intensity in the listening room identical to that received by the microphone in the recording studio. This leads us to question number one. When most people say this, they subconsciously think of listening to an orchestral performance in a typical auditorium, from a typical auditor's seat. But, if a microphone were placed in the position of the typical auditor's seat and a recording made, no one with any musical appreciation would want the disc. The reproduced sound would be quite unlike the impression conveyed when sitting in that seat personally. Why should this be? Whether we use a stereophonic system for reproducing our sound or not, our hearing is always binaural, whereas the microphone is not, placed in the position of the typical auditor's seat or wherever you will. Some readers of this magazine have the misfortune to be deaf in one ear. To them, the remarks which follow will seem incomprehensible, except as pure theory. A few years ago the author was deaf in one ear for a period, an experience which enabled him to appreciate how different everything sounds this way. This experience also confirmed the following explanation. When we take up our seat in the auditorium and listen to the program, we are primarily conscious of the orchestra; secondarily we hear the reverberation coming from other parts of the building, as a kind of echo, although the time interval is usually too small for it to be noticeable as a separate entity. But our binaural capacity for differentiating directions enables us clearly to separate, in our subconscious perception, the original sound from the orchestra and that due to the same program reverberated around the auditorium. The difference in intensity between these two components of sound may not be more than 10 or 20 db but, because of our subconscious interest in the program itself, attention is focused on the orchestra and the reverberant sound is heard merely as a background that lends character. Because of this, our subconscious subjectively increases the apparent difference in level to much more than the actual 10 or 20 db. The microphone, however, being a much more non-subjective device than a pair of human ears, would pick up the whole sound received as a conglomerate, with only the actual 10 to 20 db differential between the direct sound from the orchestra and the reverberant sound from various parts of the auditorium. When this sound is reproduced over any system whatever, even in a thoroughly acoustically damped room, it will sound extremely reverberant and "echoy." This is because now the direct and reverberant sound both come from the same source, and our binaural faculty of hearing no longer can go to work on it in the same way and direct attention to the orchestra exclusively. Recording studio personnel, of course are thoroughly conversant with this fact. They don't advertise it, because listeners naturally prefer to think the recording transports them to a position in a typical listener's seat in some auditorium. But, to get an effect that puts the direct sound and the reverberant sound more in the correct proportion, as the listener thinks he would hear it, the microphone must be placed very much nearer to the orchestra, or, what means practically the same thing (for this purpose), a highly directional microphone must be used to favor sound from the orchestra more than the reverberation. Modern practice usually places the microphone quite close to the orchestra, or uses an array of microphones distributed among the orchestra so as first to get a pickup that represents the original program material, exactly as played by the orchestra, practically without any reverberation at all. Then, if some reverberation effect is required, the studio uses an echo chamber to add this artificially. This procedure enables the effect to be easily controlled so the resulting sound will have just the right amount of reverberation to give the desired effect. Certainly this technique produces very fine sounding records, judging by recent releases from most of the record companies. Even the sticklers for accurate reproduction of the original sound will have to admit this although, if they knew it was made this way, they would call it "phony." But let's ask them, which original sound do they really want reproduced accurately, the sound of the orchestra, as heard by, say the conductor, or one of the microphones scattered throughout the orchestra, or the sound as it might be heard in the echo chamber, somewhere between the loudspeaker that produces the sound and the microphone used to pick up the "echo" ? It is quite obvious that neither of these sounds will be the same as that we hope to hear in our living room. When you sit in the average seat in an auditorium and listen to an orchestral concert, the sound you hear is quite different from that heard by the conductor, or by any of the instrumentalists in the orchestra. All of these performers work to give the best impression to you, the paying audience. The same is true making recordings, only here we have a few more "performers", like the engineers who operate the various controls associated with the microphone levels, echo chamber, etc. The important thing is that the intended impression is put across, whether by the individual performers working collectively in the orchestra in an actual auditorium, or whether by all those who work together to make a satisfactory record.
A good studio is built with all kinds of sound insulation to keep out extraneous and undesired noises. But all such devices only attenuate the undesired sounds, they don't completely eliminate them. Consequently, it is still advantageous to have a fairly high level of sound at the microphone, so as to give the biggest possible margin over unwanted background sounds. This also makes it easier on the recording system, because it gets the program material that much farther above background noise from the microphone itself and the amplifier system. But picking up sound for the record at this high level and recording it through a system with standard equalization characteristics, means that the original sound pattern can only hope to be truly re-created when played back through the correct compensating playback equalizer characteristic and reproduced over a loudspeaker system at the original sound intensity. This will be too loud for comfort - much louder than one would receive sound in the average seat of an auditorium. The intensity of sound in the living room, reproduced this way, would be comparable to the level of sound in the area occupied by the orchestra itself. To get the true perspective of the music. one needs to be a little farther away or to hear the sound at somewhat lower level. Consequently it is usually desirable to reproduce the sound at a somewhat lower intensity than it is picked up at the microphone for recording. Let us now consider another angle. Live musicians can, and do, provide program music for any of the purposes for which we use reproduced music. As well as playing to an audience in an auditorium, musicians, upon occasion, play to entertain people in a living room, or to provide background music in a restaurant or club, while the occupants talk. A group of musicians playing under any of these conditions will naturally adjust their performance to suit the purpose in hand. They will play louder in an auditorium and much quieter in a restaurant or club. How do musicians themselves make this loudness adjustment? This does not mean that each instrumentalist will reduce his volume by a precise number of decibels. Rather, each reduces his own loudness so that the same sense of balance is achieved at the lower loudness level. This, naturally enough, is done according to the judgment of the musicians' ears. While musicians' hearing certainly is differently conditioned from that of most of us (which is what we infer by crediting a person with a "musical ear"), there is no intrinsic difference between the ears of musicians and those of the rest of the population. The famous Fletcher-Munson loudness contours were based on the hearing of musicians as well as of other groups of people. So one would expect an average musician to have a sense of comparative loudness similar to an average person of any other group. As this is the case, each instrumentalist will automatically adjust the loudness of his playing so as to obtain a perspective of over-all balance that agrees with the general pattern of human loudness sensation.
While the bass player will undoubtedly reduce the apparent loudness of his instrument by the same amount as other players in the orchestra, the actual intensity difference is much less. The higher instruments reduce by 30 decibels, while the bass player only reduces by 12. As we have seen in the preceding discussion, music is never recorded at such a low level. If you want to use recorded music for this purpose, you will have to turn the gain or volume control down by 30 decibels or so, and if you want to play at a level suitable for the average living room you will still need to turn it down, by possibly 12 or 15 decibels, from the intensity at which the music was originally recorded. But to retain the balance at which the group of musicians would naturally play, the bass frequencies must be turned down to a lesser extent than the middle and higher frequencies. This is our argument for using a loudness control. If an ordinary volume control is used (which should more accurately be termed a gain control), to produce this effect, the balance is disturbed. The effect produced is not that of the orchestra playing more quietly but of its being farther away. This is because all frequencies are attenuated in proportion, which is exactly what happens when you listen to a musical program from a distance, unless of course there is exceptional transmission through floors and ceiling, such as may occur if the "distance" is from one floor to another of an apartment house! This has about sated the esthetic factors that form the basis for discussion as to whether the loudness control is needed at all and, if so, what kind of control should be provided. Having settled that we do need control facilities to make this adjustment, the next point is if a special loudness control is needed, or whether an ordinary volume (or gain) control can be used satisfactorily in conjunction with the regular tone controls? The alternatives in this stage of the argument are: whether we have a loudness control that automatically increases the relative amount of bass, measured by intensity, as the volume goes down, or whether we use a "straight" gain or volume control and then use the bass knob of the tone controls to get more bass when working at a lower loudness level, if necessary also giving a little boost to the treble end for the same reason. Don't both of these methods really achieve the same thing? And, using just a gain control with tone controls, surely, is simpler than having to "bother with" an additional loudness control? Time was when it was argued that equalization facilities were not required for all of the different recording characteristics - the tone control would serve for this purpose as well. Going back still further, there was a time when tone control consisted of just a variable resistance in series with a capacitor, conveniently placed somewhere in the circuit, which produced only a variable treble cut. There was no true bass control. All too often, this control was operated with the treble fully cut, because this removed a maximum of unwanted background noise, scratches, and plops, either due to scratch on the record or due to static coming in on the radio. The average listener of those times couldn't tell the difference between the maximum and minimum position of the control, as far as the quality of the program itself was concerned. The only effect noticed was the apparent reduction of background noise. But, since then, listeners have become educated to high fidelity. And the fidelity, both of equipment and of recordings, has improved tremendously. Nowadays, the comparative novice in high fidelity can tell the difference between bass boost and treble cut, or vice versa. It requires a little more education to discern the difference between records played with the correct equalization characteristic and with the wrong one, but it certainly is possible to tell the difference. But, a decade or so ago, strong arguments were put forward that the tone control should serve this function in addition to any other tone compensation that may be desired. Nowadays, any self-respecting preamplifier provides equalization characteristics for RIAA, and usually one or two other recording characteristics that may be encountered; and the average high-fidelity listener appreciates this position and reserves the use of tone controls for their proper function - adjusting for differences in balance that may become apparent, due to differences in the acoustic characteristics of the studio and the listening room, or due to any other variations in the balance of musical composition of the program. Now the extremist comes along with the loudness control and says this also is a must, in addition to tone controls and equalization. Are we going to accept this further stage as a necessity, along with equalization, or is this really a luxury that is completely unnecessary? This would seem to depend upon the accuracy of your perceptive powers. This can be illustrated at Fig. 2, which shows a comparison. between the range of control given by the average tone control and the kind of change in response needed in a loudness control, to follow the variation predicted by the Fletcher-Munson curves. For the tone control to get enough boost in level at the low-frequency end, gives too much boost at frequencies higher up. The boost has to start in the region of 500 cycles. whereas the loudness contour shows not very much difference at 500 cycles based on true loudness contours.
If you can hear this difference. which an educated high-fidelity listener certainly can, then a loudness control is definitely an advantage, in comparison with the use of a volume control with separate tone controls. The next question is, what arrangement of loudness control and gain or volume control should be used? Do we need both controls, or is one control that can serve both purposes enough? Some preamplifiers have appeared with a single control, and a loudness volume switch, which alters the function of the control from one to the other. A little thought will show that this achieves practically no advantage over an amplifier provided with only one or the other. To be effective, a loudness control must produce the right contour appropriate to the level at which the program is played. As different programs are often recorded at quite widely differing levels (as regards the input delivered to the preamplifier), there is no guarantee that setting the volume/loudness dial to a specific position (say 5) will always reproduce a program at the same loudness, regardless of what disc is being played. But this is how a loudness control should operate, otherwise the loudness contours will not come in the right place. What we need then is two separate controls: a gain or volume control, to adjust the input received from the recording, so that setting the loudness control to a particular position will give the right loudness in the room for that setting. For example, if you normally listen at a level corresponding to 50 in the loudness contour family, the loudness control should be set to a position which makes this correction from the average recording level of 70. Then the gain control should be adjusted on each particular recording, so the loudness in the room corresponds with the average to which you have become accustomed. Then, once you have set the gain control for this particular degree of loudness, you can alter the loudness control to play the music softer or louder. Of course, you do not always have to go through this routine just to get it right for a particular recording. It is much simpler to set the loudness control for the desired loudness. This you will become used to with a little practice. Then adjust the gain control so the actual loudness corresponds with the setting on the loudness control. Some loudness controls, instead of providing a continuously adjustable range, use a multi-position switch, that may have three positions for example, marked low, normal and high, referring to the degree of loudness. This control can then be set according to how you want the music to play in the room and the normal volume control can then be used to get the loudness right. This will generally give a pretty close approximation to the right loudness contour and from this point the tone control can be used to adjust for slight deviations in balance, if it does not sound to the best advantage. The switched arrangement has the advantage that the correct contour can be tailored into the circuit. Some loudness controls use just a simple tapped potentiometer, the schematic of which is shown in Fig. 3. This does not give sufficient steepness to the loudness compensation, particularly for playing at low levels. In fact, it is little better than the use of the tone control. Maybe, if you have a separate tone control and a loudness control of this type, the two can be used together to get results that come nearer to being right than just the tone control by itself. But the best arrangement, if you have a loudness control that is continuously variable (like most volume controls), is to use one which comes as a ganged potentiometer, two or more decks with associated components wired on to the potentiometers to produce the required variation of loudness contour as the control is turned. But discussion of the precise way of getting the right characteristics into a loudness control is another article altogether. It must be said that, after becoming accustomed to the correct use of a good loudness control, going back to a system that only has the regular volume and tone controls seems to lack the degree of smoothness in reproduction obtainable with the loudness control. Especially is it difficult to get satisfaction for low level listening, which is where the good loudness control really does its job well. This can easily be demonstrated on a preamplifier that has both forms of control, playing a good wide-range program that has plenty of bass and treble. Turn down, first of all, the loudness control and the effect is that the orchestra or program is being played more quietly, but still in the room with you. Now restore the loudness control to the maximum comfortable level and turn down the volume control. This time the effect is not of the program being played more quietly in the same room, but it seems to go farther away as if it is no longer in the room with you but playing from somewhere down the street. Sometimes it is nice to have both of these facilities available, but if you principally want to present music that sounds good, you certainly want to have it sound as if it is in the room with you and not being played from a distance away. So it looks as if we really need loudness controls. Just a final word, however. This decision is not going overboard on the second extreme mentioned at the beginning of this article. The Fletcher-Munson curves may be (and are) scientific, but they do not represent the exact loudness contours of your ears. In their investigation, it appeared that individual hearing is so divergent that it is extremely unlikely that anyone has average hearing. But use of these average curves assures a control that should satisfy. Although each of us has hearing that diverges one way or another from these curves, they represent the shape of almost everyone's hearing characteristic. This means that our individual loudness difference contours will be very close, although the actual contours deviate widely. So a well-designed loudness control, based on loudness differences, will sound right almost anyone. Posted November 23, 2013 |
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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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