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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING About power, watts, decibels. Explanation of terms. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur "I have 200W speakers and a 4 x 50 radio. Will they play together?" Yes, they will, don't worry. But it will be even better if you still understand what is meant by both power and watts. "Power", according to the school definition - the work produced per unit of time, for our purposes the definition is almost useless. We are more comfortable in a different, albeit unusual way: power is the amount of energy converted into the form we need in the same unit of time. It's always about transformation, the energy doesn't go anywhere, it's such a habit. The amplifier (albeit in the radio) receives electrical energy ready for use in the form of direct current from the vehicle's on-board network and converts it into electrical, but in the form of alternating current, representing a sound signal. All? No, about half, the rest goes into the heat given off to the air by small radiators on the back of the radio or large ones, "throughout the body" of a separate, external amplifier. The speaker (albeit pretending to be a "speaker") receives electrical energy in the form of alternating current and converts it into mechanical energy, now in the form of long-awaited sound vibrations. All? How can I say ... Not really. The efficiency of the speaker (since we went the school way: the ratio of the produced sound power to the received electrical power) almost never exceeds 0,5%. Where do the other 99,5% go? And there, in the heat, in general, any device created by the human mind (as well as the will of the Almighty) produces heat plus something else. In terms of energy conversion, the speaker is 99 percent identical to a soldering iron with a penny. And in the remaining half of a percent - everything: bass, highs, detail, and brilliant musicians. It's a shame? Yes, but nothing better somehow came up with. And here it is, the main difference between the power of the amplifier and the power of the speaker: the amplifier can be considered to produce it. And the speaker - consumes, not producing in exchange, as we just found out, almost nothing. Here is the amplifier. Let the one in the radio do not give a damn, then you will feel the difference. What is his power? Yes, whatever, it all depends on what level of the signal is at the input, roughly speaking - in what position the volume control. The output might be 1 watt, maybe 10, maybe 50, maybe... Wait, there must be a limit. Of course, but we did not ask what is the MAXIMUM power. And everyone has their own maximum. It is determined by what the highest AC voltage the amplifier can produce at its output when a load is connected to the output, in the form of a speaker with some kind of resistance. The output power is simply determined: as the value of this voltage squared and divided by the load resistance. We connected a voltmeter and a load to the output, an alternating voltage was applied to the input, for the convenience of measuring power - at any one frequency, and we look. The output is 2V when a 4 ohm load is connected to it. With such measurements, of course, not acoustics are connected to the output, but its equivalent in the form of a resistor, otherwise the ears will wilt. They raised it, divided it and got: the output power is exactly 1 watt. There is a small ambush here, due to the fact that we are talking about alternating voltage, the value of which can be measured in different ways. The most commonly used scale is RMS. In Russian, this word is long, so the English abbreviation RMS (root mean square) has taken root, meaning the same. In order not to go into details, it is enough to remember: for a sinusoid, the RMS voltage value is 1,41 times less than the amplitude value, that is, the root of two. The power indicated in watts RMS is the one that was obtained when the voltage was taken in RMS during the calculation, which is logical. And if we take the voltage amplitude, then the power, firstly, will be called peak, and secondly, it will become exactly twice as much as RMS. We return to the amplifier. One watt is not serious, we add it at the input. How long will the output voltage rise and what will stop it? Will stop its signal limiting. The amplifier is powered by a constant voltage, and what appears as a variable at its output cannot be greater than the supply voltage in amplitude. There is no more. And if we observe the signal at the output, then at some point the tops of the previously graceful wave will be cut off, there the half-wave wanted to go through the upper limit, the supply voltage. And broke off. We roll back the signal at the entrance until the restriction disappears, and look at the signal range. It is slightly less than the full supply voltage, because something is lost in the output stages of the amplifier. If the amplifier is powered (as in the notorious "radio tape recorder") from the car's on-board network, then the lower half-wave will come close to zero, and the upper one - to the level of 12 V. What happens? The amplitude, we will assume, is 6 V in each direction, we raise-divide and get a fabulous figure of 4,5 watts. Check if you're not lazy. It turns out that in all science this is the maximum value of the output power of a radio tape recorder powered by 12 V? And that was twenty years ago. And so it is now, if you buy some Vitek or Eurotec for 400 rubles, our colleagues from the Kyiv magazine "FOR Z" conducted a series of such experiments not so long ago, saving us from this sad need. Fortunately, a solution has recently been found that made it possible, if not to reach the formidable 4 x 50, then in any case, to get away from the mournful 2 x 4,5. This is the bridging of amplifiers, which is now used in all car head units, with the exception of the sound sediment like the one mentioned (for example, nothing personal). When bridged, two amplifiers operate on one load, connected so that the amplitude of the sinusoid at the output doubles. According to the method already reported to you to calculate the output power, this will be four times more than 4,5 W, because the voltage is squared, therefore - 18. Approximately this value has the maximum output power of all the head units we have ever tested (in each of the four channels, of course). Where do the famous 4 x 40 W come from, then turned into 4 x 45, 4 x 50 and so on? What is this, pure lies? Somehow it does not fit with the image of eminent and more than respectable manufacturers of equipment, but these numbers adorn the front panels of all brands: Alpine, Blaupunkt, Clarion and so on in Latin alphabetical order. After all, when it comes to individual amplifiers of the same companies, everything becomes honest and correct, there were enough opportunities to make sure over the years. There are two tricks here, the first is technical, and only the second is marketing. The technical trick is that in modern "heads" amplifiers of the so-called "class H" are used, there is a special circuit that can give the output stages an increased supply voltage for a short time. There is a capacitor and, while everything is quiet, it is charging. And at loudness peaks, it turns out to be connected in series with the power supply of the output stage, and the peak jumps without distortion, without touching the top of the 12 V ceiling. But this is if the peak of the signal level is very short, for example, the first moment of hitting the drum. Further, of course, the energy supply dries up, but the deed has already been done, even two things: indeed, for a brief moment, the maximum power became much more possible during continuous operation, and secondly, it became possible to mention this. Without focusing too much on the conditions under which the maximum output power became such. To the credit of respectable companies (see the alphabetical list above), it must be said: often in the table of technical characteristics on the last page of the instructions, continuous power is also given, indicating that this is in RMS watts, and indicating what the supply voltage was, as a rule , 14,4 V, because in this case the "ceiling" for the output sinusoid rises, and then exactly 18 - 20 W per channel appear in this line, the cases of entering the third ten are single. Why aren't they written on the front panel? Consider it a tradition, like the price of oil in dollars per barrel, and for gold - per troy ounce. Moreover, as we found out, formally they have the right. Now quickly answer the security question: when was the last time you saw car speakers that were labeled LESS than 18 watts? Therefore, any talk about "selecting" acoustics for a radio tape recorder or a CD receiver in terms of power by comparing what is written on the boxes with one and the other does not make any sense. "But how," you may ask, "did my garage neighbor's 100-watt mugs wheeze and burn out?" And this, my dears, happened not because there was a lot of power of the head unit, but because there was little MAXIMUM power. Everyone sees, but few pay attention: where, seriously, and not for beauty, in troy ounces, the maximum output power is indicated (for example, on the last page of the instructions), there is also a value of the coefficient of non-linear distortion corresponding to the given value. With us, this is abbreviated in kn, and in the English-language instructions it will look like THD and some number with a percent sign. We recall (or find out) what non-linear distortions are. They are sometimes called harmonic (THD and stands for Total Harmonic Distortion - total harmonic distortion), which is more correct. The bottom line: when the amplifier works perfectly, the output signal will differ from the input signal only in amplitude, and in direct proportion. The output voltage increased by a little more than a volt, when suddenly a whole fence of harmonics grew on the spectrogram, which means that the output signal came dangerously close to the maximum possible amplitude. The amplitude of the harmonics seems to be small (the upper scale is strongly stretched vertically), and in total they add up to a small total: less than half a percent. But: this fence was not in the sound before, but now it is. Let's add more - and now, they have sailed: on the sinusoid, the distortions of the form became clearly visible, exactly those that we expected - you cannot jump higher than the power supply. And the output signal has become monstrous, in real life we will hear, in addition to the pure tone of 250 Hz, a lot of new things: both 500 and (especially) 750 Hz, and further to the most impossible frequencies, the consolation that they are all multiples of 250 Hz is rather weak, for hearing, this is either a creak or a wheeze, depending on the fundamental frequency. Now a question from an ancient anecdote: where will we make the waist? What is the maximum output power? If there was still very little distortion, then it will be 13,5 watts. RMS, as you now understand by seeing what the output voltage is. If there, where under half a percent, then it will be almost 19 watts. And if we agree with 10%, then we get a fabulous value for such amplifiers of 23 watts. But it’s better not to agree: do you see what lies behind this inconspicuous figure? The result of our analysis is at first glance paradoxical: on the one hand, the amplifier has only one maximum achievable output power, depending on the supply voltage and load resistance. But at the same time, you can specify it as you like, the question is what level of distortion is considered acceptable. Traditionally, for really powerful, external amplifiers, the maximum power value is indicated at a k.n.i. equal to 1%. For head units, manufacturers prefer 10%, for reasons that no longer need comment. Why, with such, in general, miserable values of the maximum power of the head unit amplifiers, the "200-watt" 6 x 9 attached to them begin to wheeze, or even burn? Why wheeze, you have already seen: wheezing is the harmonics that appeared at the output of the amplifier when it was overloaded. A person thinks that his mighty radio tape recorder has overloaded the speaker, but in fact he plays what they gave him, thinking with his burdock brains that this is how it should be. And why do they burn if they have such power - like a pellet for an elephant? And let's take another look at the results of previous experiments with distortions, and then even continue them. I added something there: conditional curves showing which part of the frequency spectrum falls on the low-frequency head (actually "burdock"), and which part - on the block of high-frequency heads in its center. Naturally, this fully applies to any multi-band acoustics, and we have no other. Something is playing, and there is a powerful component with a frequency of 250 Hz. The tweeter is still on vacation: on the blue field depicting its operating range, there is almost no signal, and rightly so, this is not its frequency. When the distortion becomes half a percent, something already appears, but so far it's okay, the amplitudes are small, and most of them fall into the area where the tweeter filter is already starting to cut off the unnecessary. At 10% it's already not good: the tweeter is put to rest, and a lot of harmonics fall on it, and even with a level higher than the content of the high frequencies in a normal phonogram. Let's go further, to the limit: we unscrew the input signal so that after cutting off the tops of the half-waves, the quiet sinusoid turns into an almost rectangular signal, in which the harmonics are forty percent of the main signal. Here is the squeaker, most likely, Khan. But we have the same amplifier, and the frequency is still "non-beeping". With some natural gift, such a signal can also spoil the midbass. Rectangular pulses carry much more energy at the output than a sine wave, and the electrical power that is supplied to the speaker in this case will be more than 50 watts. Imagine a 50-watt soldering iron, then remember that the speaker is a 99,5% soldering iron, and the fate of the voice coil, which, unlike the soldering iron winding, is made not from nichrome, mica and asbestos, but from much more delicate materials, will cease look cloudless. Does all this mean that you can not look at the power of acoustics at all? Not really. You just have to look a little differently. This view of power "from the other end" will be the theme of the second issue of Total Recall. Dealing with power in the end It seems to me that those who started methodically "remembering everything" together with us in the last issue had enough time to grieve that "4 x 45 W" inscribed on the radio tape recorder is not the most direct road to happiness. It's time to choose a path, and at the same time dispel some of the myths that have taken root in our environment cleaner than that of saxaul. And at the same time, remember some properties of our body (with some parts of which we listen to music) and how these properties influenced the system of measures in acoustics. Clearly, without an amplifier - not life. We begin to choose and, of course, the first thing we look at is the maximum (we already know what it is) power, what are we fighting for? About how to choose it, the conversation will be separate and unexpectedly short for you. But first, let's define where this power comes from. What makes a separate amplifier such a qualitatively different device compared to the inherited amplifier channels built into the head unit? From the previous issue of "V.V." It became clear that it was all about nutrition. The amplifier creates an alternating voltage at the output with a swing, from top to bottom, no more than the supply voltage of the output stages. For the radio amplifier, this is the voltage on board, 12 V on a muffled car, about 14 V - on the go. The main component of the external amplifier is the power supply. It receives a constant voltage from the on-board network, turns it into an alternating one of a rather high frequency (tens of kilohertz), the variable can already be increased using a transformer, which the power supply of the amplifier does, and then, already increased, is rectified again and fed to the amplifier itself. Up to how many volts the voltage was inflated during this activity, at such a height the ceiling of the output voltage swing will pass. What follows is simple arithmetic. Let's assume that out of 12 V on-board power supply created 50. In reality, it will be two voltages of different polarity, 25 V each, it's more convenient. This means that the output voltage range will be (in each direction) no more than 25 V minus some pennies lost in transistors. The maximum output power is 25 squared divided by the load impedance. This is according to Ohm's law, he is relentless. It comes out a little over 150 watts. Only this is the peak value, on the RMS scale - exactly half as much, about 75 watts. The figures are quite real, such amplifiers - in bulk. Can you get more out of this amp? The first stage of "afterburner" for many models will happen by itself, it is worth starting the engine. For many amplifiers, the output voltage of the power supply is not stabilized and is in proportion to the input. And when, with the engine running and the generator running, the voltage on board becomes not 12, but, on a well-adjusted machine, 14,4 V, the voltage at the output of the power supply will increase from 50 to 60 V, and the "ceiling" for the output voltage of the amplifier will also rise , and the maximum power will increase to 108 watts. Wow increase, right? Just don't get too excited just yet. Will this make the amp play louder? And what is it from, exactly? The overall gain, from the signal source to the output, remained the same, it does not depend on the power supply (and if it suddenly depended, the component responsible for this would urgently request permanent registration in the trash can), which means that it will be as it played. It’s another matter that if distortions appeared before at some volume, this is when the output voltage tried to jump over the bar set by the power source at the peaks of the signal, now this moment will move to the area of \uXNUMXb\uXNUMXbhigher volume. How far will it move? Let's guess. One and a half decibels. One click of the volume, or even none, it depends on the step of the regulator. And what have we gained in comparison with the "past life", when there was no amplifier at all? There seems to be a lot in watts. And in decibels, the maximum undistorted volume, again, seems to be not very: 5,4 dB. But this is only "seemingly", as we will see later, happiness is not in just clicks of the volume control. We still need to organize some kind of harmony between the capacities. See, for example, what power the acoustics have, and choose an amplifier based on it, right? Wrong! I did it on purpose, for the purpose of provocation. About how you can ruin the acoustics with insufficient power, it was in the last issue, now let's try to do it with the help of excess. It will be much more difficult, I warn you. Let's return once again to the phrase that I uttered (and wrote) on various occasions so many times, the last time - in the last issue. Here it is: "And when we talk about the power of the amplifier, then we are talking about what HE GIVES. And when we talk about the power of the speaker, it is about what HE TAKES." The maximum power of the amplifier is the one that it cannot give more than, because it starts to distort the signal, and we did not buy it for this. The maximum power of acoustics, therefore, is the one that it cannot take more than, because WHAT? Also starts to distort the signal? And she starts to do it immediately and little by little, not at all like an amplifier, acoustics do not have a hard limit bar. In ancient times, there was a Soviet standard, according to which the so-called nominal power of speakers was normalized. Special conditions were stipulated there, the frequency band, and so on, in general, the power was considered such that the non-linear distortion did not exceed 10%. The best bass speaker of that time was called 6GD2, the first digit is just the rated power. There were 4 more HDs, 3 HDs, and so on, then they adopted the definition of nameplate power, which no longer depends on distortion, but on survivability, and all these HDs at once grew fat to 10, 20, 75 and the like. These GOSTs ordered us all to live long, and now power is defined differently, and it is very important to understand this in order to experience the attitude that it deserves towards this indicator. I'll ask you to type it in red, if I forget - you yourself then in pencil, okay? The power indicated on the acoustics is not the one at which it should work, but the one that destroys it. Of course, there must be a relationship between the capabilities of acoustics and the resources of the source of this probable destruction, but this is a relationship, not an identity. Imagine: you bought a car with a top speed of 200 km/h. And you turned up tires with a speed index T (190 km / h). What can't you drive? At 191 km / h, all four wheels are in tatters? Or vice versa, tires have a Z speed index (240 or more), and you get knocked off your feet, choosing a suitable car for such rubber. Unreal. Nevertheless, quite often one has to hear (and even read) how the acoustics to the amplifier (and vice versa) are selected, looking first of all at the power, and then at everything else. So let's go for the last time, so as not to return to the question. The numbers on the acoustics, accompanied by the words Power, without indicating what is meant by this, do not mean anything, this is part of a modern, but rooted tradition. If the acoustics manufacturer is at least relatively correct in the numbers he gives, then he can indicate the long-term power, and this is the maximum non-destructive (or minimally destructive, do not forget about this) power supplied to the speaker for half an hour according to the scheme: a minute works - two resting. At the same time, a noise signal is supplied, passed through a filter that cuts off everything below 40 Hz and everything above 4 kHz, so this is almost irrelevant to the tweeter. Now, if the acoustics survived these most difficult half an hour in their life, the used power value is recorded. If it died, it is taken from previous experience with less power. Short-term power is one that will not destroy the speaker (or destroy it, but at the last moment) after 60 cycles of "a second orem - a minute we rest." All the described procedures involve bringing the tested sample of acoustics as close as possible to the edge of the grave, so it’s somehow not very reasonable to focus on them as a normative indicator for someone who paid for the acoustics out of their own pocket. The only type of indicator that even slightly resembles the possible real use of its legitimate property is rated noise power according to the IEC 268-5 standard, when the acoustics must remain alive after 8 hours of continuous operation on the already mentioned noise signal. It is almost never indicated. Landmarks here should be different, you should not look for them on boxes with acoustics. Our in-house specialists in acoustic tests repeatedly recommended (when the manufacturers completely lost their shame and it was unthinkable to remain silent) to be equal to indicators that at least roughly indicate the range of possible values. For 6-inch component acoustics, the boundaries of reasonable risk lie somewhere between 40 and 90 W (this is wide, inside you already have to look at the design features), for a 5-inch speaker it is naturally lower, 30 - 70 W. We consider these values to be rated noise power. You can disagree, but refuting experiments - at your own expense, please. The numbers, in principle, resemble the common values of the maximum output power of widespread amplifiers, so the simplest, on the verge of primitivism, answer to the question of matching the power of an amplifier with the power of acoustics is already ready: a typical amplifier is suitable for working with typical acoustics. Anyone - with anyone. In principle, if you do not want to bathe, you can take it into service. But the answer is too simple to somehow claim to be exhaustive, that's clear. A slightly more common answer can be found in the experience of the superbison of the world of acoustics. The undoubted superbison is JBL, which has equally succeeded in acoustics for home, stage, automotive and designed for sounding rooms and open spaces. The company's technical circular contains the following recommendation: in the case when the volume level is under control (it explains: it means a house or a studio, but not a word about a car), the maximum amplifier power (RMS) can be twice the rated noise power. In the case when the control is not perfect (this is about sound systems), parity must be observed. Next, you need to look at the realities of life. In real life, I have reason to believe, both the amplifier and acoustics will be used to reproduce music, and not test signals that only very approximately resemble music. A musical signal is not a sine or even noise, it is a signal with a large difference between the average value and the peak. Short-term signal peaks, with rare exceptions, do not threaten the health of acoustics, which mainly have to resist thermal stress, and the heat generated on the voice coil is a function of the average level of the supplied signal. I had to see in the documentation of the most serious manufacturers of acoustics, how, next to quite real (and indicating all the normative data) figures of long-term power, the values \u10b\uXNUMXbof withstanding power at short (say, XNUMX ms) peaks were given. The numbers sometimes reached hundreds of watts, and this is no longer marketing, this is a fact, even a very powerful one, but a very short burst of signal will not destroy the speaker. And the amplifier has a fundamentally different view of the level peaks. At least for a millisecond it will exceed the signal level of the maximum power bar - and it will be mercilessly decapitated, that is, it will go further along the wires to the acoustics already in a distorted form compared to the original source. This cannot be allowed. And here it already makes sense to look at your musical tastes. Why so? You can also try. I passed a number of musical fragments through the computer and chose quite significant in terms of the ratio of average (dangerous for acoustics) and peak (which should be feasible for the amplifier) power. The signal level was measured in decibels relative to the maximum recorded on the disk, but for clarity, I converted everything into a percentage of the maximum power. The first picture is 60 seconds of "Procession of the Dwarves" (track 6 "Let's Test!"). If the system is set up so that the largest peaks of the signal do not go beyond the output power of the amplifier, then in general for this minute the acoustics will get about one and a half percent of this power. Even in those 12 seconds, when the orchestra is completely unbelted, the thermal load will be no more than half the power. A minute of activity of Yamato drummers (remember, you came to Moscow?). The signal level is chosen so as to easily miss the peak of activity at 21 seconds. As a result, the average power of the entire fragment is less than a percent of the maximum, and its most intense part is one tenth of the maximum. Third example: "In the Pocket" (Kai Eckhardt, "NAIM Sampler", track 8). The average power is 13% of the maximum, and turning up the volume in a sincere attempt to ruin the acoustics will mean cutting off the numerous peaks caused by the skillful work of the drummer. Don't listen to audiophile delights? We will not force. Here is a fragment of the soundtrack of the punk rock band Kurban (Turkish and, by the way, quite curious). Here already - yes, the guys on the stage do not rest, and the average power for a long time is about 40, or even more percent of the maximum. But the guidelines, in principle, remain the same as those proposed by the bright heads from JBL, God bless them. It's just that rock music falls into the category of "faulty control", which is logical. An attentive reader may be puzzled here: "Wait a minute, it turns out that we listen to music on one or two, a lot - ten watts connected to the acoustics? Why then does it play loudly? You yourself have heard: loudly." I will answer: why shouldn't she play loudly? After all, you are easy to manage with decibels (even those who did not know how before). We take any acoustics from any of our past tests and look at the sensitivity indicator. Well, let's say 87 dB, that's right, average-typical value. Such sound pressure will be created by these acoustics at a distance of 1 m with a power of 1 (one and only) W supplied to it. It's not quiet anymore, by the way. In order for this acoustic to create a sound pressure level of 90 dB, which is standard for control listening in sound recordings, it only needs 2 watts. Apply 10 watts - get 97 dB. It's quite loud. Moreover, keep in mind that we have at least two such speakers, and they do not sound in a muffled room, but in the cabin, where there are much less losses, and the reflected sounds come to us. What then, you ask, will the speaker do when those very peak hundred, say, watts come to it? Exactly what it should: for a short time, within a fraction of a second, it will scream at 107 dB. Give him that 100 watts continuously, in the form of noise or, worse, a tone, and the scream will be death. And so - everything is under control, do not worry. What will be discussed next? Now about acoustics. About what characterizes it, in addition to power, this topic, I hope, we have closed. In acoustics, everything is measured differently than in the ordinary world. There are several reasons for this, explanations of others can lead to the paradise of science, we will not touch them. Others lend themselves to simple interpretations. Or they can simply be taken for granted, whichever you prefer. The human ear cannot add and subtract. Just multiply and divide. Evolution (or the Creator, choose according to your taste) arranged it in such a way, as it seems to me, guided by technical expediency. Hearing works in a huge range of volumes. The sound pressure (measurable, as you know) corresponding to the pain threshold exceeds the sound pressure of the hearing threshold by ten million times (in words, so as not to count zeros). Hearing adapted to this by becoming (by the will of evolution or the Creator) logarithmic. People came up with logarithms later, but in our head they sit by nature. The logarithmic nature of hearing is that it evaluates the difference in loudness not by how much more sound pressure is, but by how many times it has become more. So (if we remove now all the intermediate chapters of the story) a unit of measurement was invented, on which absolutely everything is based in acoustics and electroacoustics - the decibel. Whoever knows everything about this, do not read further, however, I asked about this when I opened this series of publications. I give the rest, no matter how many of them, the opportunity to master operations with decibels in five minutes and subsequently do it easily and gracefully. So: a decibel is a unit, which, if added, means "multiply", and if taken away - "divide". For example: sound pressure is 3dB higher. It means double. Another 3 dB? Double more. More than 1 dB is 1,25 times, approximately. More than 10 dB - ten times. And vice versa: subtract 3 dB from the sound pressure, and this will mean that it has halved. Authors: Andrey Elyutin, MitrAlex; Publication: cxem.net
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