Useful tips. Power: how many watts does the speaker need?

There are many different types of sound emitters, but the most common are electromagnetic type emitters, or as they are also called, speakers.

Speakers are the main structural elements of acoustic systems (AS). Unfortunately, one speaker is not capable of reproducing the entire audible frequency range. Therefore, for full-range reproduction in acoustic systems, several speakers are used, where each is designed to reproduce its own frequency band. The operating principles of low-frequency (LF) and high-frequency (HF) speakers are the same; the differences lie in the implementation of individual structural elements.

The operating principle of the speaker is based on the interaction of an alternating magnetic field created by a current flowing through the wire of a magnetic coil with the magnetic field of a permanent magnet.

Despite the comparative simplicity of the design, speakers intended for use in high-quality acoustic systems have a large number of important parameters on which the final sound of the acoustic system depends.

The most important indicator characterizing a speaker is the reproduced frequency band. It can be indicated as a pair of values (lower limit and upper limit frequency), or given in the form of an amplitude-frequency response (AFC). The second option is more informative. The frequency response is a graphical dependence of the sound pressure level created by a speaker at a distance of 1 meter along the working axis on frequency. The frequency response allows you to evaluate the frequency distortions introduced by the speaker into the original signal, and also, in the case of using the speaker as part of a multi-band system, to identify the optimal value of the crossover filter frequency. It is the frequency response that allows a speaker to be classified as low-frequency, mid-frequency or high-frequency.

Selecting a subwoofer

For LF speakers, in addition to the frequency response, an essential group of indicators are the so-called Thiel-Small parameters. Based on them, the acoustic design parameters for the speaker (speaker system housing) are calculated. The minimum set of parameters is resonant frequency - fs, total quality factor - Qts, equivalent volume - Vas.

The Thiel-Small parameters describe the behavior of the speaker in the piston-action region (below 500Hz), considering it as an oscillating system. Together with the acoustic design (AO), the speaker is a high-pass filter (HPF), which allows the use of mathematical tools borrowed from filter theory in calculations.

An assessment of the Thiel-Small values of the speaker parameters, and first of all, the total quality factor Qts, allows us to judge the advisability of using the speaker in acoustic systems with one or another type of acoustic design (AO). For speakers with phase-inverted acoustic design, speakers with a total quality factor of up to 0.4 are mainly used. It is worth noting that phase-inverted systems are the most demanding, from a design point of view, compared to speakers that have a closed and open AO. This design is sensitive to errors made in calculations and in the manufacture of the housing, as well as when using unreliable values for the parameters of the woofer.

When choosing a woofer, the Xmax parameter plays an important role. Xmax shows the maximum permissible displacement of the cone, at which a constant number of turns of voice coil wire is maintained in the gap of the speaker magnetic circuit (see figure below).

For satellite speaker systems, speakers with Xmax = 2-4mm are suitable. For subwoofers, speakers with Xmax=5-9mm should be used. At the same time, the linearity of the conversion of electrical vibrations into acoustic ones at high powers (and, accordingly, large vibration amplitudes) is maintained, which manifests itself in more efficient low-frequency radiation.

If you have decided to make a speaker system with your own hands, you will inevitably be faced with the question of choosing branded components, including the frequency of the speakers. Without experience in using products from different manufacturers, it is sometimes difficult to make the best choice. You have to be guided by many factors and compare according to many parameters, not only those related to passport characteristics. ACTON speakers will successfully complement your speaker system because, in addition to high quality, they have a number of advantages:

have an optimal price/quality ratio in their segment;
the speakers are specially designed for professional speakers used for dubbing social and cultural events;
documentation for the manufacture of housings has been developed for speakers;
interaction between the consumer and the manufacturer is carried out directly without intermediaries, which avoids problems with the availability of any spare parts and components;
information support on the design of speakers;
high reliability of ACTON speakers.

You can familiarize yourself with the model range of ACTON speakers.

Selecting a tweeter

When choosing a tweeter, the frequency response determines the lower frequency of the range it reproduces. It is necessary that the frequency band of the tweeter somewhat overlaps the frequency band of the woofer.

Some tweeters are designed to work in conjunction with a horn. Unlike direct-radiation tweeters (or tweeters, as they are called), horn tweeters, due to the properties of the horn, have a lower cutoff frequency of the reproduced audio range. The lower limiting frequency of such a high-frequency speaker can be approximately 2000-3000 Hz, which makes it possible in many cases to abandon the midrange speaker in the speaker system.

Due to their design, tweeters tend to have higher sensitivity than woofers. Therefore, at the filter design stage, an attenuator (suppressor) circuit is provided in it, which is necessary to reduce excess radiation, which brings the sensitivity values of the high-frequency and low-frequency speakers to the same level.

When choosing a tweeter, it is important to consider its power, which is selected based on the power of the woofer. In this case, the power of the HF speaker is taken lower than the power of the LF speaker, which follows from the analysis of the spectral density of the audio signal, corresponding to pink noise (which has a decline towards high frequencies). For a practical calculation of the power dissipated by the high-frequency dynamics in speakers with a crossover frequency of 3-5 kHz, you can use the calculator on our website.

Let us remind you that HF speakers cannot be used without a high-pass filter (HPF), which limits the penetration of the low-frequency part of the spectrum.

Speaker Damage Factors

In the event of abnormal operating conditions, mechanical and electrical damage to the speakers is possible. Mechanical damage occurs when the amplitude of vibrations of the diffuser exceeds the permissible amplitude, which depends on the mechanical properties of the elements of the moving system. The most critical frequency zone for such damage is near and below the mechanical resonance frequency of the speaker, i.e. where the amplitude of oscillations is maximum. Electrical damage occurs as a result of irreversible overheating of the voice coil. The most critical frequency band for damage of this kind corresponds to the band located near the electro-mechanical resonance of the speaker. Both types of damage occur as a result of exceeding the maximum permissible electrical power supplied to the speaker. In order to avoid such consequences, the maximum power value is standardized.

There are several standards, using which manufacturers normalize the power of their products. The closest from the point of view of real conditions in the case of using an acoustic system for sounding public events is the AES standard. Power according to this standard is defined as the square of the rms voltage in a certain pink noise band that the speaker can withstand for at least 2 hours, divided by the minimum impedance value Zmin. The standard regulates the presence of the speaker in “free air” without a housing. When testing, some manufacturers place the speaker in a housing, thus bringing its operating conditions closer to real conditions, which, from their point of view, leads to more objective results. The known power value of the speaker serves as a guide when choosing an amplifier whose power should correspond to the power value of the AES speaker.

It is worth noting that the real value of the power supplied to the speaker is difficult to estimate without special measurements and can vary widely even with the same setting of the volume control on sound path devices.

This can be influenced by many factors, such as:

Spectrum of the reproduced signal (musical genre, frequency and dynamic range of the musical work, predominant musical instruments);
Characteristics of passive filter circuits and active crossovers that limit the spectrum of the original signal entering the speakers;
Using an equalizer and other frequency correction devices in the audio path;
Amplifier operating mode (appearance of nonlinear distortion and clipping);
Acoustic system housing design;
Amplifier malfunction (the appearance of a constant component in the spectrum of the amplified signal)

The following measures increase the reliability of operation of speaker systems:

Reducing the upper limit frequency of the woofer speaker using a low-pass filter (LPF). In this case, the part of the signal spectrum that makes a significant contribution to heating the coil is limited;
Limits the frequency band below the bass reflex tuning frequency using LOW-PASS (high-pass filter) circuits. This measure limits the amplitude of vibrations of the diffuser outside the operating range of the speakers on the low-frequency side, preventing mechanical damage to the woofer;
Adjusting the high-frequency high-frequency speaker to a higher frequency;
Design of speaker enclosures that provide the best conditions for natural convection of speakers;
Elimination of operation of speakers with an amplifier operating in nonlinear distortion and clipping mode;
Preventing the occurrence of loud switching clicks, “winding up” of the microphone;
Using a limiter in the audio path.

Note that speaker systems that are used for professional sound recording (especially in discotheques) are often forced to operate at high power. During operation, the heating of the speaker voice coil can reach 200 degrees, and the elements of the magnetic circuit - 70 degrees. Long-term operation at extreme conditions leads to the fact that the speakers “burn”. This may be caused by exceeding the permissible electrical power supplied to the speaker, or by a faulty amplifier. In many ways, the safety of the set depends on the qualifications of the DJ. Because of this, no matter which speaker you choose, you need to consider the availability of repair kits. At the same time, the situation is further complicated by the fact that, as a rule, not one speaker burns out at the same time, but several, which disables the entire set. Considering all of the above, we conclude that the question of the timing and cost of delivery of repair kits is also extremely important at the stage of selecting speakers for speakers.

The designs of high-frequency (HF) speakers are the most diverse. They can be ordinary, horn or dome. The main problem in their creation is the expansion of the direction of the emitted oscillations. In this regard, dome speakers have certain advantages. The diameter of the diffuser or radiating membrane of HF tweeters ranges from 10 to 50 mm. Often the tweeters are tightly closed at the back, which eliminates the possibility of modulation of their radiation by the radiation of low-frequency and mid-frequency emitters.

A typical miniature cone tweeter produces high-frequency sounds well, but has a very narrow radiation pattern—usually within an angle of 15 to 30 degrees (relative to the central axis). This angle is set when the speaker's output is typically reduced by -2 dB. The angle of deviation from both the horizontal and vertical axis is indicated. Abroad, this angle is called the angle of dispersion or dispersion of sound.

To increase the dispersion angle, diffusers or attachments for them are made in various shapes (spherical, horn-shaped, etc.). Much depends on the material of the diffuser. However, conventional tweeters are unable to emit sounds with frequencies noticeably higher than 20 kHz. Placing special reflectors in front of the tweeter (most often in the form of a plastic grid) allows you to significantly expand the directivity pattern. Such a grille is often an element of the acoustic frame of a tweeter or other emitter.

An eternal topic of debate is the question of whether it is necessary to emit frequencies above 20 kHz at all, since our ear cannot hear them, and even studio equipment often limits the effective range of sound signals at a level from 10 to 15-18 kHz. However, the fact that we do not hear such sinusoidal signals does not mean that they do not exist and do not affect the shape of the time dependences of real and rather complex audio signals with much lower repetition rates.

There is much convincing evidence that this shape is greatly distorted when the frequency range is artificially limited. One of the reasons for this is the phase shifts of various components of a complex signal. It is curious that our ear does not sense phase shifts themselves, but is able to distinguish signals with different forms of time dependence, even if they contain the same set of harmonics with the same amplitudes (but different phases). Of great importance is the nature of the frequency response decay and the linearity of the phase response even outside the effectively reproduced frequency range.

Generally speaking, if we want to have uniform frequency response and phase response throughout the entire audio range, then the frequency range actually emitted by the acoustics should be noticeably wider than the audio one. All this fully justifies the development of broadband emitters by many leading companies in the field of electroacoustics.

Placement of HF emitters There is a problem - the result largely depends on where the heads are placed and how they are oriented. Let's talk about the HF head, or tweeter.

Features of HF heads From the theory of sound wave propagation it is known that with increasing frequency, the radiation pattern of the emitter narrows, and this leads to a narrowing of the optimal listening zone. That is, it is possible to obtain a uniform tonal balance and the correct scene only in a small area of space. Therefore, expanding the radiation pattern of the HF emitter is the main task of all loudspeaker designers. The weakest dependence of the radiation pattern on frequency is observed in dome tweeters. It is this type of HF emitters that is the most common in automobile and household speakers. Other advantages of dome radiators are their small size and the absence of the need to create an acoustic volume, while the disadvantages include the low lower limit frequency, which lies in the range of 2.5-7 kHz. All these features are taken into account when installing a tweeter. The installation location is influenced by everything: the operating range of the tweeter, its directivity characteristics, the number of components installed (2- or 3-component systems), and even your personal taste. Let’s immediately make a reservation that there are no universal recommendations on this issue, so we cannot point the finger at you - they say, put it here and everything will be OK! However, today there are many standard solutions that are useful to familiarize yourself with. All of the following applies to non-processor circuits, but this is also true when using a processor; its presence simply provides much more opportunities to compensate for the negative impact of a non-optimal location.

Practical considerations. First, let us recall some canons. Ideally, the distance to the left and right tweeters should be the same, and the tweeters should be installed at the height of the listener's eyes (or ears). In particular, it is always best to move the tweeter heads as far forward as possible, since the further they are from the ears, there is less difference in the distances to the left and right drivers. The second aspect: the tweeter should not be far from the midrange or bass/midrange head, otherwise you will not get good tonal balance and phase matching (usually guided by the length or width of the palm). However, if the tweeter is set low, the sound stage falls down, and you seem to be above the sound. If the setting is too high, due to the large distance between the tweeters and midrange speakers, the integrity of the tonal balance and phase matching are lost. For example, when listening to a track with a recording of a piano piece, on low notes the same instrument will sound low, and on high notes it will soar sharply upward.

Directivity of the HF head. When you have figured out where to install the HF head, you should decide on its direction. As practice shows, to obtain the correct timbral balance, it is better to direct the tweeter towards the listener, and to obtain good depth of the sound stage, use reflection. The choice is determined by your personal feelings about the music you listen to. The main thing here is to remember that there can only be one optimal listening location.
It is advisable to orient the tweeter in space so that its central axis is directed towards the listener’s chin, that is, set a different angle of rotation for the left and right tweeters. There are two things to keep in mind when orienting a reflective tweeter. Firstly, the angle of incidence of the sound wave is equal to the angle of reflection, and secondly, by lengthening the path of the sound, we take the sound stage further, and if you get carried away, you can get the so-called tunnel effect, when the sound stage is far from the listener, as if at the end of a narrow corridor.

Setting method. Having outlined, in accordance with the recommendations given, the location of the RF heads, it is worth starting experiments. The fact is that no one will ever say in advance where exactly a 100% “hit” with your components will be ensured. The most optimal location will allow you to determine the experiment, which is quite simple to set up. Take any sticky material, for example, plasticine, double-sided tape, Velcro or model hot glue, put on your favorite music or test disc and, taking into account all of the above, start experimenting. Try different locations and orientation options in each. Before finally installing the high-frequency driver, it is better to listen a little more and correct it on plasticine.to nowhere.

Creativity. Setting up and choosing the location of the tweeter has its own nuances for 2- and 3-component systems. In particular, in the first case, it is difficult to ensure close proximity of the high-frequency driver and the low-frequency/mid-range emitter. But in any case, you shouldn’t be afraid to experiment - we’ve come across installations where HF heads ended up in the most unexpected places. Is there any point in having an additional pair of tweeters? For example, the American company Boston Acoustics produces sets of component speakers, where the crossover already has space for connecting a second pair of HF heads. As the developers themselves explain, the second pair is necessary to raise the level of the sound stage. In test conditions, we listened to them as an addition to the main pair of tweeters and were surprised how significantly the space of the sound stage expanded and the elaboration of nuances improved

An amplifier and a loudspeaker are links in the same chain; one simply cannot work without the other. In the last issue we examined in some detail the question: “What power should the amplifier have?” and now let’s try to answer the second: “What power should the speaker be?” Partially the answer to this question was given in the previous material, since, as mentioned above, it is impossible to consider one without the other, but a number of details remained untouched and, as we promised, this time we will analyze them in more detail.

TYPES OF POWER

Many manufacturers of automobile speakers use non-standard methods for measuring power, which, by the way, are not always more attractive than those generally accepted for household equipment - it’s just more convenient for them. However, most use standardized parameters, among which we are usually interested in three: rated (RMS), maximum and peak power. The main one of these parameters is the rated power, and this is what we will mean in the future when we simply say “power”. The numerical ratio is as follows: the maximum is usually 2 times higher than the rated power, and the peak is 3-4 times higher. This rule cannot be called strict: there are some models whose maximum power is only slightly higher than the rated one.

Be that as it may, since the rated power is the smallest of the above, a number of manufacturers use a little trick: on the packaging and the first page of the instructions, unreasonably large power figures are given in large numbers without indicating its type, and the truth can only be established by finding the technical parameters in the document , or by looking at the back of the speaker, or by looking for some inconspicuous inscription on the packaging. Don't fall for this trick.

So, the rated power is precisely the one within which you can listen to music on these speakers for a long time without fear of nonlinear distortion and, even more so, of speaker failure.

WHAT IS MORE IMPORTANT – POWER OR SENSITIVITY?

In the last article we noted that doubling the power raises the sound pressure level by 3 dB. That is, a speaker with low power but high sensitivity is capable of developing the same sound pressure (the same sound volume) as a more powerful but less sensitive head. Therefore, if you have to choose between two speakers of equal sound quality, one of which is more sensitive, but less powerful than the second, then it is better to choose the first. Why overpay for the power of the amplifier, if even with a low-power one you will get the same volume?

By the way, due to certain circumstances (for example, the characteristics of transistor amplifiers), truly highly sensitive speakers for the automotive sector are practically not produced. But within each class, significant discrepancies in sensitivity can be found, and this is the source of all sorts of speculation: our tests extremely rarely confirm the correspondence between the declared values and the real ones, so we advise you to pay attention to our “special prizes”, and not to the given figures.

Sometimes you come across speakers with low sensitivity, but really high rated power, which at low power play not only quietly, but also with worse quality, but if you “twist the knob” well, the sound becomes optimal. This option can be recommended for those who listen only to loud music most of the time and are ready to purchase an amplifier with a power of at least a hundred watts per channel.

Significantly increases the sound volume and reduces the speaker resistance to 3, and even to 2 ohms - recently more and more such models are appearing. The only circumstance. What must be taken into account is that the amplifier must cope well with such a load. We categorically do not recommend connecting 2-3 ohm speakers directly to the built-in amplifier of a car radio or CD receiver - even if this works, it will be a severe test for the head unit and, most likely, it will eventually fail.

RATIO OF SPEAKER POWER AND AMPLIFIER POWER

In principle, there is nothing wrong if the RMS of the amplifier is less than that of the speakers, but in this case you need to handle the sensitivity control even more carefully. The paradox is that a less powerful amplifier, when it starts to overload, is more likely to burn out your speakers than a more powerful amplifier! It's all about a phenomenon called “clipping” - i.e. operation in limiting mode, when the amplifier produces a highly distorted signal with a large content of higher harmonics. It is for this reason that tweeters most often burn out in speakers. By the way, in head units there are no sensitivity regulators in principle, so you just need to once by ear determine the beginning of the appearance of distortion when the volume increases, and then never turn the regulator knob further than this level.

POWER AND FREQUENCY RANGE SPEAKERS

Another reason for the failure of speakers, especially those reproducing the low/mid ranges, is ignoring the frequency range they actually reproduce. Many manufacturers indicate an extended frequency range of their speakers to attract buyers. For example, for a coaxial speaker with a standard size of 10 cm and a power of 30 W, the frequency range is 50 - 20,000 Hz. It is not the upper value that is confusing, but the lower one. If you put a 50 Hz signal at the stated power level into this speaker, not only will you not hear 50 Hz, but you could easily destroy the speaker. This often happens when, being carried away by various schemes for raising the bass, they forget that the speaker is simply not capable of reproducing the lower register. The result is a torn cone of the woofer/midrange speaker. To prevent this from happening, the range of frequencies reproduced by the speaker should be limited using at least a second-order high-pass filter. The set filter cutoff frequency depends on the speaker size. So, practice shows that for 10 cm heads it should be about 100 Hz, for 13 cm heads - 80 Hz, and for 16 cm heads - 60 Hz. Anything below should be reproduced by the subwoofer. Moreover, by limiting the lower frequency range of the signals reproduced by the LF/MF speakers, you will immediately feel better output in the rest of the range, their more lively and loud operation. Speakers that can perform well without a low-bandwidth filter do exist, but they are in the minority.

The general rule is this: the narrower the frequency range sent to the speaker or a separate head, the more power it can withstand. For example, for many individual high-frequency speakers, several power values are given at once, depending on the high-pass filter cutoff frequency: if the speaker operates starting from 2000 Hz, this is one power, if from 5000, the power value is much higher. The same applies to midrange speakers, bass/midrange heads and subwoofers - the only difference is that they can vary two limits of the reproduced frequency range at once: upper and lower.

Typical relationships between the power of HF, MF, LF/MF and subwoofer heads are the same as for amplifiers; they were discussed in the last issue.

SUBWOOFERS AND THEIR PARAMETERS

Separately, we should consider a special class of speakers - subwoofers. This type of loudspeaker has recently become part of car audio systems, but due to the fact that it allows you to reproduce deeper bass, it has become very popular among car enthusiasts. However, a car subwoofer is very different from a home subwoofer. So, if for home equipment the power of a subwoofer of 300 W is considered “above the roof”, then for a car it is an average, normal parameter. Why such power? Let us remember that a subwoofer in a car should “shout out” road noise, but at home there is no such need. In addition, the design of car woofers has its own characteristics. To obtain deep bass in small volumes, manufacturers make a number of sacrifices, the main one of which is a reduction in sensitivity. To get sufficient volume with low sensitivity, you have to supply high sound power. Creating a powerful car amplifier is also not an easy task, so recently the design of a subwoofer with two separate voice coil windings has become popular, and some manufacturers go even further, installing as many as 4 voice coil windings. Such a solution gives greater flexibility when selecting the optimal resistance for a specific amplifier - to put it simply, it allows you to “squeeze” the maximum watts out of it. The required resistance is obtained through the appropriate connection of the windings (series, parallel, parallel-series). True, power, resistance and the number of windings do not affect the musicality of the subwoofer. Even a low-power, but properly built subwoofer can surpass its monstrous SPL counterpart in sound quality. Although to create the required sound pressure you will need at least two low-power subwoofers. Depending on the task at hand or the genre orientation of the speakers, the rated power of the subwoofer is chosen to be 2-4 times higher than the power of the full-range speakers. The greater its power, the better, because you can always make it play quieter, but louder – not. But at the same time, it is necessary to take into account the real capabilities of the on-board network of your car (and wallet, of course).

In addition, the type of acoustic design of the subwoofer is of great importance. In particular, the additional power reserve for the worst option in terms of output is especially welcome - an endless acoustic screen; the speaker plays in a large volume, for example, in the trunk. Models in a closed case have higher sensitivity, but are also low, and the best in terms of output are models with a bass reflex, especially in a bandpass type case.

WHAT HAPPENS WHEN THE NUMBER OF HEADS INCREASES

There are often installations with dual or triple LF/MF heads, and there are a great many options with two subwoofers. What does this do and why is it needed? By doubling the heads, you increase the sound pressure level by at least 3 dB, this is equivalent to doubling the power, provided that the electrical power supplied to them from the amplifier also doubles. If two heads receive the same power from the amplifier as one, then the sound pressure level will change little. In this case, we do not gain anything in terms of power, but the increased radiation area from the diffusers will give deeper bass. However, this effect depends on the distance at which the heads are separated, and will appear at frequencies for which this distance is commensurate with the wavelength or exceeds it. Those interested in details are referred to the book “Broadcasting and Electroacoustics” edited by Yu.A. Kovalgin, published by the publishing house “Radio and Communications” in 1999. There, on page 224, the problem of the efficiency of speakers, which include several heads of the same type, is discussed. In acoustics, such speakers are usually called speakers. They are used to increase directivity and increase the efficiency of speaker systems.

It is precisely because of the improvement in bass response that dual heads are used only for bass/midrange or subwoofer heads. There are also options for dual tweeters, but they are rare and have other tasks, for example, reducing the directivity of speakers at high frequencies. In many cases, using two LF heads can solve complex problems - in particular, two 12-inch heads are easier to accommodate than one 15-inch. However, it is worth considering that the cost of two heads will be clearly higher than one of the same series, but of a larger standard size.

TYPES OF POWER OF SPEAKER SYSTEMS

Nominal– root mean square value of electrical power limited by a given level of nonlinear distortion.

Maximum sine– the power of a continuous sinusoidal signal in a given frequency range, at which the speaker can operate for a long time without mechanical and thermal damage.

Maximum noise– electrical power of a special noise signal in a given frequency range, which the loudspeaker can withstand for a long time without thermal and mechanical damage.

Peak– the maximum short-term power that the speakers can withstand without damaging them when a special noise signal is applied to them for a short period of time (usually 1 s). The tests are repeated 60 times with an interval of 1 minute.

Maximum long-term – electrical power of a special noise signal in a given frequency range that the loudspeaker can withstand without irreversible mechanical damage for 1 minute. The tests are repeated 10 times with an interval of 2 minutes.

Material provided by Car&Music magazine, No. 12/2003. Rubric "Useful tips", text: Edouard Seguin

Harmonic theory

Amplitude compression

What to do?

Overloading (clipping) power amplifiers- a common occurrence. This article discusses overload caused by an increased input signal level, which results in the output signal being limited.

Having analyzed the “phenomenon” of this kind of overload, which allegedly causes damage to the speakers, we will try to prove that the true culprit is amplitude compression (compression) of the signal.

WHY DO LOUDSPEAKERS NEED PROTECTION?

All loudspeaker heads have operating power limits. Exceeding this power results in damage to the loudspeakers (LS). These damages can be divided into several types. Let's take a closer look at two of them.

The first type is excessive displacement of the GG diffuser. The GG diffuser is a radiating surface that moves as a result of an applied electrical signal. This surface can be conical, domed or flat. The vibrations of the diffuser excite vibrations in the air and emit sound. According to the laws of physics, in order to produce a louder sound or reproduce lower frequencies, the diffuser must oscillate with a larger displacement amplitude, while approaching its mechanical boundaries. If it is forced to move further it will result in excessive deflection. This most often occurs with low-frequency GGs, although it can happen with mid-frequency and even high-frequency GGs (if the low frequencies are not limited enough). Thus, excessive displacement of the diffuser most often leads to mechanical damage to the head.

The second enemy of the GG is thermal energy resulting from thermal losses in the voice coils. No device is 100% efficient. As for the GG, 1 W of input power is not converted into 1 W of acoustic power. Almost most GGs have an efficiency of less than 10%. Losses caused by low efficiency are transformed into heating of the voice coils, causing their mechanical deformation and loss of shape. Overheating of the voice coil frame causes weakening of its structure, and even complete destruction. In addition, overheating can cause the glue to foam and enter the air gap, causing the voice coil to no longer move freely. Eventually, the voice coil winding may simply blow out like a fuse link. It is absolutely obvious that this cannot be allowed.

Determining the permissible power of multi-band speakers has always been a serious problem for users and developers. Users who replace damaged tweeters most often

They are convinced that what happened is not their fault. It would seem that the output power of the amplifier is 50 W, and the power of the speaker is 200 W, and, nevertheless, the high-frequency speaker fails after some time. This problem forced engineers to figure out why this was happening. Many theories have been put forward. Some of them have been scientifically confirmed, others remain as theories.

Let's consider several views on the situation.

HARMONICS THEORY

Studies of the energy distribution across the signal spectrum have shown that, regardless of the type of music, the level of high-frequency energy in the sound signal is much lower than the level of low-frequency energy. This fact makes it even more difficult to figure out why tweeters are damaged. It would seem that if the amplitude of high frequencies is lower, then the low-frequency speakers should be damaged first, and not the high-frequency speakers.

Speaker manufacturers also use this information when developing their products. Understanding the energy spectrum of music allows them to significantly improve the sound of tweeters by using lighter moving systems, as well as using thinner wire in the voice coils. In speakers, the power of high-frequency speakers usually does not exceed 1/10 of the total power of the speaker itself.

But because in the low-frequency (LF) range there is more musical energy than in the high-frequency (HF) range, which means, due to its low power, high-frequency energy cannot cause damage to the high-frequency speakers. Therefore, the source of high frequencies powerful enough to damage tweeters is somewhere else. So, where is he located?

It has been suggested that if there are enough low-frequency components in the audio signal to overload the amplifier, it is likely that, as a result of limiting the output signal, high-frequency distortion will be sufficiently powerful to damage the tweeter.

Table 1. Harmonic amplitudes 100 Hz square wave, 0 dB = 100 W

Harmonic	Amplitude	Level in dV	Level in W	Frequency
1	1	0	100	100 Hz
2	0	-T	0	200 Hz
3	1/3	-9.54	11.12	300 Hz
4	0	-T	0	400 Hz
5	1/5	-13.98	4	500 Hz
6	0	-T	0	600 Hz
7	1/7	-16.9	2.04	700 Hz
8	0	-T	0	800 Hz
9	1/9	-19.1	1.23	900 Hz
10	0	-T	0	1000 Hz
11	1/11	-20.8	0.83	1100 Hz
12	0	-T	0	1200 Hz
13	1/13	-22.3	0.589	1300 Hz

This theory became quite widespread in the early 70s and gradually began to be perceived as “dogma”. However, as a result of studies of the reliability and security of power amplifiers under typical conditions, as well as the practice of operating amplifiers and speakers by typical users, it turned out that overload is a common occurrence and it is not as noticeable to the ear as most people think. The response of amplifier overload indicators is usually delayed and does not always accurately indicate the real overload. In addition, many amplifier manufacturers deliberately slow down their response based on their own ideas about how much distortion must occur for the indicator to light up.

More advanced and better sounding amplifiers, incl. Amplifiers with soft clipping will also damage tweeters. However, more powerful amplifiers cause less damage to tweeters. These facts further strengthen the theory that the source of damage to high-frequency speakers is still amplifier overload (clipping). It would seem that there is only one conclusion - clipping is the main reason for damage to high-frequency speakers.

But let's continue to explore this phenomenon.

AMPLITUDE COMPRESSION

When the amplitude of a sinusoidal signal is limited, the amplifier introduces large distortions into the original signal, and the shape of the resulting signal resembles the shape of a rectangle. In this case, an ideal rectangle (meander) has the highest level of higher harmonics. (see Figure 1). A less clipped sine wave has harmonics of the same frequencies but at a lower level.

Take a look at the spectral composition of a square wave signal with a frequency of 100 Hz and a power of 100 W presented in Table 1.

As you can see, the power reaching the tweeter after passing this signal through an ideal crossover with a cutoff frequency of 1 kHz is less than 2 W (0.83 + 0.589 = 1.419 W). That's not a lot. And do not forget that in this case a severe, ideal overload of a 100-watt amplifier is simulated, capable of turning a sine into a meander. A further increase in overload will no longer increase harmonics.

Rice. 1. Harmonic components of a 100 Hz square wave relative to a 100 Hz sine wave

The results of this analysis indicate that even if a weak high-frequency speaker with a power of 5-10 W is used in a 100W speaker, harmonic damage to it is impossible, even if the signal takes the form of a meander. However, the speakers are still damaged.

This means that we need to find something else that could cause such failures. So what's the deal?

The reason is the amplitude compression of the signal.

Compared to older amplifier models, today's high-quality amplifiers have greater dynamic range and sound better when driven. Therefore, users are more tempted to overdrive amplifiers and clip them on low-frequency dynamic peaks, because in this case, large audible distortions do not occur. This results in compression of the dynamic characteristics of the music. The volume of high frequencies increases, but the volume of low frequencies does not. This is perceived by ear as an improvement in sound brightness. Some may interpret this as an increase in volume without an accompanying change in sound balance.

For example, we will increase the signal level at the input of a 100-watt amplifier. Low frequency components will be limited to 100 W due to overload. As the input level is further increased, the high-frequency components will rise until they also reach the 100 W clipping point.

Look at fig. 2, 3 and 4. The graphs are graduated in volts. At an 8-ohm load, 100 W corresponds to a voltage of 40 V. Before the limitation, the low-frequency components have a power of 100 W (40 V), and the high-frequency components only have a power of 5-10 W (9-13 V).

Let's assume that a music signal with low-frequency and high-frequency components is fed to a 100-watt amplifier (8 ohms). We use a mixture of a low-level HF sinusoidal signal with a high-level LF signal (see Fig. 2). The level of the high-frequency components supplied to the tweeter is at least 10 dB lower than the level of the low-frequency components. Now let’s increase the volume until the signal is limited (+3 dB overload, see Fig. 3).

Rice. 2. A low-level, high-frequency sine wave mixed with a burst of high-level, low-frequency sine wave

Rice. 3. Output of a 100-watt amplifier with 3 dB of overload

Rice. 4. Output of a 100-watt amplifier with 10 dB of overload

Please note that, judging by the waveform, only the low-frequency components were limited, and the level of the high-frequency components simply increased. Of course, clipping generates harmonics, but their level is significantly lower than that of the meander we discussed earlier. The amplitude of the HF components increased by 3 dB in relation to the LF (this is equivalent to the amplitude compression of the signal by 3 dB).

When the amplifier is overloaded by 10 dB, the amplitude of the HF components will increase by 10 dB. Thus, every 1 dB increase in volume causes an increase in the amplitude of the HF components by 1 dB. The growth will continue until the power of the RF components reaches 100W. Meanwhile, the peak level of low-frequency components cannot exceed 100 W (see Fig. 4). This graph corresponds to almost 100% compression, because... there is almost no difference between the HF and LF components.

Now it’s easy to see how much the power of the RF signal exceeds the power of a 5-10-watt tweeter. It is true that overloading will generate additional harmonics, but they will never reach the level of the amplified original high-frequency signals.

You might think that the signal distortion would be unbearable. Don't fool yourself. You will be amazed to learn how high the overload limit is, above which it will no longer be possible to listen to anything. Just turn off the overload indicator on the amplifier and see to what level you turn the amplifier's volume control. If you measure the level of the amplifier's output signal with an oscilloscope, the level of overload will surprise you. An overload level of 10 dB on low-frequency components is common.

WHAT TO DO?

If we can protect amplifiers from overloading (clipping), we can use speakers more efficiently. To prevent overload and resulting amplitude compression, any modern amplifier must use the so-called. slip limiters. They prevent the aforementioned amplitude compression because When the threshold value is reached at any frequency, the level of all frequencies decreases by the same amount.

In external limiters, the response threshold (threshold) is set by the user. Fine tune

This threshold for limiting amplifiers is quite difficult. In addition, the clipping level of amplifiers is not a constant value. It changes depending on the supply voltage, AC resistance, and even the nature of the signal. The limiter threshold must continuously monitor these factors. The most correct solution would be to tie the threshold to the amplifier overload signal.

It is quite logical to build a limiter inside the amplifier. In modern amplifiers, it is easy to determine the moment when an overload occurs with great accuracy. It is to this that the so-called built-in amplifiers react. slip limiters. As soon as the amplifier's output signal reaches the overload level, the control circuit turns on the limiter control element.

The second parameter, after the response threshold, inherent in any limiter is the response and release times. More important is the recovery time after overload (release time).

There are two options for using amplifiers:

work as part of a multi-band amplifier complex,
work on broadband speakers.

In the first case, either only the low-frequency band, or the mid-frequency and high-frequency bands can be supplied to the amplifier. When setting a long release time and operating the amplifier in the mid-high frequency bands, the “tails” of the limiter recovery can be noticeable by ear. And, conversely, with a short release time and operation in the low frequency band, signal shape distortions may occur.

When operating an amplifier on a wideband speaker, you have to look for some compromise value for the recovery time.

In this regard, amplifier manufacturers take two paths - either a compromise release time is selected, or a release time switch (SLOW-FAST) is introduced.

CONCLUSIONS: