Lesson 5: Causes of Horn Burning and Preventive Measures

When we use the sound system, it is inevitable that the speaker unit in the sound box in the system will be damaged due to improper use.

Generally speaking, full-range speakers have treble and bass, and bass only has bass, which may be damaged.

If the speaker is broken, the speaker will not work normally, which will interrupt the activity and affect the use.

It may even affect the confidence of customers and have a negative impact on later settlement and business development.

If the speaker unit is broken, no matter what the reason, just remove the unit and find someone to repair or replace it, so that the speaker can resume its function. However, if you simply repair or replace it without knowing the cause of the damage, it may still be damaged in the future use process.

Horn damage is a phenomenon, and any phenomenon has a certain reason.

If we know the cause of the horn damage and pay attention to avoid it in use, then the phenomenon of the horn damage can be completely avoided.

Next, I will explain to you the reasons for the damage of tweeters and woofers and the preventive measures.

First of all, talk about the reasons for the damage of the tweeter.

The tweeter in the speaker gives people a fragile feeling and is relatively easy to be damaged, which is a kind of speaker that is easier to burn out as the saying goes.

However, apart from the quality problem of the product itself, there is only one common reason for the damage of the tweeter, and that is: the signal input by the sound source equipment into the mixer is too large!

Why?

Because the input signal of sound source equipment is too large, that is, the input level is too high, it is easy to cause high-frequency harmonic distortion of the internal circuit of the mixer, which directly threatens the safety of the tweeter.

Here is a technical term: level. What does this level mean?

Level: It is an index describing the strength of electric signal, and the unit is decibel (dB). The greater the level value, the stronger the electrical signal; The smaller the value, the weaker the signal.

On the panel of our audio equipment, a level indicator is usually equipped to show the size of input/output signals.

The scale number next to the liquid level gauge indicates the liquid level value. For example, the scale corresponding to the highest point indicator light displayed by the level meter is +9, indicating that the current signal level value is +9dB.

So, what is this high-frequency harmonic distortion?

Below I will show you the commonly used Smaart measurement software for audio.

Play a test signal of 1000Hz with test software, and you can see its frequency spectrum. Some show the background noise of the equipment, so leave it alone.

By increasing the output signal of the sound card, we can see that the signal spectrum on the software is also increasing synchronously.

When I increased the output signal of the sound card to a certain extent, everyone looked at the software screen. What happened?

Is there a large number of dense signals in the high frequency area on the right side of the screen?

These signals are harmonic distortion signals generated by the internal circuit of the mixer. Because these signals are only generated in the high frequency region, they are called high frequency harmonic distortion.

This distorted signal is generated in the high frequency part, not the low frequency part.

Therefore, the distorted signal output by the mixer will go to the tweeter after being amplified by the power amplifier and entering the speaker. If the distortion is serious, it will burn out the tweeter.

Distortion, usually everyone listens with their ears. I saw it today.

The so-called distortion is that it has become different from the original.

We have seen distortion so far, so next, I will show you the distorted sound.

Listen, it's a broken sound. Once you hear this kind of voice in the future, you should pay attention. The danger is approaching.

This paper introduces the reasons for burning the tweeter. If you want to avoid burning the tweeter, you should avoid the excessive input signal, that is, avoid the high input level.

Then the question comes, how do I know the signal level of the sound source equipment, and whether it is high or not?

On an analog mixer or a digital mixer, each input channel has a key switch, which is generally marked as "PFL" or "solo". This switch is called a monitor switch. It has two functions:

Press this switch first, then plug in the earphone, and you can hear the input signal in the earphone without pushing up the fader.

Second, after pressing this switch, you can see the level of the input signal on the level meter without pushing up the fader. If we want to check which input signal level, we just need to press the monitor switch on this path.

But when checking, don't press the monitor switch of other roads at the same time. Because, if you press the monitoring switch on several roads at the same time, if there are signals on these roads, then the liquid level meter shows the mixed level of these roads, and you can't see it correctly.

Ok, now you can see the input level, and then the problem comes again.

What's the problem? Just now I said that the input level is too high, which will lead to the distortion of the mixer circuit. So, how high is the level?

In fact, in our daily work, there are often some vague descriptions or statements, such as too big, too strong, too high and so on. It's okay to say that it's not important, but some security-related things are not so vague. We have to find out.

So, how high is the level? Let's see through experiments.

The left picture and the right picture are actually projections of two classes.

At present, the left picture shows the input level meter of the mixer, and the right picture shows the output signal state of the mixer. We can compare and see how much the input level of the mixer reaches and the output signal of the mixer is distorted.

Now I gradually increase the input signal of the mixer, and the output signal of the mixer is also increasing synchronously.

When I increase the input signal to a certain extent, the distortion appears. Look, what is the current input level display? + 16。

Then I'll see if the input signal is interrupted. If you go down to this position, is there no distortion, and then see what the current horizontal display is? +3, right?

A little more, +6, is there any distortion? not have

Increase it again, it exceeds +6,+10, and it is distorted.

Then reduce it to +6, and the distortion will be gone.

Well, as you can see, when the input signal level exceeds +6dB, the distortion begins to appear, and the more it exceeds, the more serious the distortion becomes.

Ok, so you are saying that the input level is too high? Yes, +6dB! It's clear now.

In addition: for X32 or M32 digital mixer with the highest electrical square meter scale of 0dBFS, if it exceeds-12, the corresponding input level indication will be distorted.

Then, in our actual work, what should we do if we find that the input level of a certain channel, such as the host microphone, is too high, exceeding +6?

You tell the host, hello, please keep your voice down? I can't.

So in what way can we reduce the excessive investment level?

It reduces the excessive level by adjusting the input Gain knob (gain) on each input channel of the mixer. You see, after I turned down the input gain knob, the volume dropped.

So, what is this input gain knob?

Let's talk about gain first. Gain: It is an index describing the degree of signal amplification or attenuation by the internal amplification circuit of the equipment, and the unit is decibel (dB).

The greater the gain, the greater the signal amplification, the smaller the gain, and the smaller or even attenuated the signal amplification.

If 0 is the middle value and the gain is 0, the signal will not increase or decrease, and the size will not change; The gain is positive and the signal is amplified; The gain is negative and the signal is weakened.

The input gain knob of the mixer is used to adjust the gain.

When rotating clockwise, the gain increases and the signal becomes larger;

Turn counterclockwise, the gain decreases and the signal weakens.

The fader of the mixer is also used to adjust the gain, but only the output gain of the channel or the overall output gain of the mixer is adjusted.

When we find that the signal level is too high, we will adjust this input gain knob to reduce the signal to a safe range.

Then there is another problem. As mentioned above, when the input level exceeds +6dB, it will start to distort, which is unsafe for the system, while when the input level is lower than +6dB, it will be safe. So isn't it safer for me to adjust the input level to-10 or even -20?

Of course there is no problem with safety, but what's wrong with turning the input level so low? Yes, the voice is low.

When we do sound reinforcement, we must first amplify the sound. We can't say that the voice is vague for the sake of safety, and we can't meet the requirements of customers.

So it involves the question of how much the input level should be adjusted.

Because the audio signal is fluctuating, the control of the input signal cannot be limited to a certain value, but given a safe range.

This safe range is to control the maximum level of the input signal between 0 and +6dB.

A digital mixer with the highest level of 0, such as X32 or M32, can be adjusted to the input level of-18 to-12dB.

When adjusting this level, if you are Andhadhun, don't make a hullabaloo about, use the microphone to adjust. It's no use tuning it, because everyone's voice is different from yours.

Whoever uses this microphone will be called to audition and then adjust.

This work must be done, especially the performance rental activities. If it's too much trouble, try to save trouble. Let the actors play without adjustment. Will the actor compensate you for the big signal and the burnt horn?

And in case the horn is burned and the activity is affected, will the organizer give you money? Even if you don't lose money!

Therefore, for the safety of your own equipment and to ensure the smooth progress of customer activities, the work you should do must be done.

In addition, if several people are required to use a microphone in an activity, let everyone try the microphone, tune one, and record the position of the gain knob in the program, for example, the first person is at 1 1, the second is at 12, and the third is at 2. When playing, adjust the position of the gain knob according to the previous record. You can't be lazy about this. If you are lazy, you may burn a tweeter.

To sum up: if you want to avoid burning high notes, you must control the input level of the sound source. By adjusting the input gain, the input level is controlled within a safe range. Let's talk about burning the bass horn.

The main reason for burning out the woofer is also the distortion of the equipment. What equipment? This is a power amplifier. If the power amplifier is overloaded and distorted, it will burn out the woofer! As mentioned earlier, the power amplifier has a bad problem. If the input signal is too large, the power amplifier will be overloaded and distorted, which is called clipping distortion.

What does this distortion look like?

The above picture is a normal signal waveform.

The following figure shows the waveform of clipping (overload) distortion.

The round head of the signal was flattened and became a square head. This waveform is called a square wave.

Once the power amplifier is overloaded, the square wave distortion signal contained in the output signal of the power amplifier will enter the voice coil of the speaker, which will lead to the rapid temperature rise of the voice coil. How high can it be?

I have done speaker design and destructive testing in a well-known speaker company in China before. When 400w clipping signal power is input, the voice coil temperature rises to 580℃ within a few seconds, and the voice coil burns out after several tens of seconds.

When you turn on the voice coil when you burn the horn, it is usually burnt black, which is caused by the overload distortion of the power amplifier.

Here I would also like to say a strange phenomenon, that is, when the power amplifier is overloaded, the woofers are generally burnt out, but the tweeters are fine. Is this confusing?

Why is this happening?

Generally speaking, high notes give people a fragile feeling, while low notes give people a strong feeling. Why is bass easy to burn instead?

Because, for a full-range speaker, there are treble and bass in the speaker. Usually, the power of the woofer is several hundred watts, while the power of the tweeter is only tens of watts.

For example, JBI, SRX 7 15 speakers, the input power of the bass horn equipped inside is 800w, and the power of the treble Laba is 75w, which is ten times different!

Dual amplifier low frequency: 800 watts

Biampere helium: 75 watts

There is also a built-in frequency divider in the full-range speaker. The built-in frequency divider, also called passive power divider, has two functions.

Firstly, according to the design requirements, the treble signal and the bass signal are separated and transmitted to the corresponding speaker unit.

The second is to distribute power according to the power of the horn unit, and load the input electric power to the corresponding horn unit after distribution according to the power of the unit.

When the power amplifier is overloaded, the output signal is distorted. When the horn power is high, the distributed distortion signal power is high, and the temperature of the horn voice coil will rise faster and higher.

It's like boiling water with an electric kettle. The high-power electric kettle burns faster than the low-power electric kettle.

In this way, the electric power distributed by the woofer in the sound box is much higher than that of the tweeter, so the woofer is easy to burn out when the power amplifier is distorted.

When the power amplifier is distorted, the voice coil temperature of the tweeter will also rise, but because the distributed power is small, generally only a few tens of watts, the temperature will not rise too high to the extent of scorching, so it will not be damaged.

Having said so much, everyone should understand. After introducing the reasons for burning the woofer, I'll talk about preventive measures. In order to avoid burning out the woofer, we must first ensure that the power amplifier is not overloaded and distorted. Then the problem comes again. How do I know if the power amplifier is overloaded? How do we know whether the power amplifier is overloaded or distorted? It's very simple, that is, know whether there is distortion through the indicator light on the power amplifier panel.

The indicator light of the power amplifier has two functions:

1, which shows the size of the input signal.

2. Display the working state of the power amplifier.

Let's explain the meaning of the power amplifier indicator.

Generally speaking, the input signal indicator light equipped on the power amplifier panel has three colors, green, yellow and red.

Say, what's this color scheme like?

Does it look like a traffic light at an intersection?

Traffic lights take these three colors as the provisions of traffic industry standards, and our audio industry also uses these three colors, which have similar meanings.

First of all, when the green light is on, it means that the input signal is normal and the power amplifier is in normal state, so you don't need to pay attention to it or do anything.

When the yellow light is on, it means that the input signal of the power amplifier is relatively large, and the output power of the power amplifier is between 50% (the yellow light flashes) and 80% (the yellow light is always on).

At this point, it means to remind users to pay attention, cheer up, and pay attention to the power amplifier that controls the signal size.

When the signal is further enhanced, the red light will light up. When the red light starts flashing, it means that the current signal input to the power amplifier has reached the limit that the power amplifier can bear, and the output power of the power amplifier has reached 100%.

At the same time, remind users not to add signals to the power amplifier.

If you don't understand this situation or don't see the red light flashing, continue to increase the signal, and the red light of the amplifier will always be on, and then the amplifier will laugh at you there.

It said to itself: I just remind you not to increase the signal. You didn't listen, did you? Give it back to me!

Ok, I don't insist, I'm free, I'm twisted! In a few seconds, your horn will be finished!

So when the red light of the power amplifier is always on, it means danger! The power amplifier is overloaded, so remind the user to immediately reduce the input signal level of the power amplifier, otherwise the woofer in the speaker will be completely burned out for 30-60 seconds. Whether it is 30 seconds or 60 seconds depends on the strength of your horn.

After explaining the meaning of the indicator light of the power amplifier, everyone said, if you want to avoid burning out the woofer, can the red light of the power amplifier stay on? No way! Therefore, it is necessary to put an end to the situation that the red light is always on.

Will the red light flash? It is also possible to shine occasionally.

But when the red light flashes, it has reached the edge of danger, so it is recommended to avoid it as much as possible.

To sum up, in order to avoid burning out the woofer, it is necessary to put an end to the phenomenon that the red light of the power amplifier is always on, and try to avoid the phenomenon that the red light is on.

This idea is good, but we will encounter very realistic problems in practical work. What are the problems?

That is, when we use the sound system, the mixer is usually under the user's eyes, but where is the power amplifier?

Generally speaking, either push the amplifier cabinet to the edge of the stage, or put the speaker in the computer room or corner.

In this way, can the operator still see the power amplifier indicator light?

I do not see any at all. This is a problem.

Think about it, if you can't see the traffic lights while driving, is it fatal?

This is a problem we encounter in practical work, so how to solve it?

For example, the performance system, the power amplifier is on the other side of the stage. I'm Andhadhun. I can't see, so I call two brothers. One can send a walkie-talkie. Go and watch and shout when you see the red light on the loudspeaker.

Everyone said, is this realistic?

So do we have a problem now? When we don't see the power amplifier, the operating system may be overloaded with the power amplifier and burn the speaker, which will bring security risks. Moreover, after the power amplifier is overloaded, it only takes tens of seconds to burn the horn, and accidents usually occur in this short time.

When we encounter this situation, it is unrealistic to solve it by manpower, so we can only find other solutions. What solutions can we use to solve this problem?

That is, the limiter is used before the power amplifier.

Now, let me tell you the difference between using a limiter.

At present, before the digital mixer is connected to the power amplifier, I use the COMP function on the output channel bus of the digital mixer to demonstrate.

Don't connect the voltage limiter yet, push the signal up and push the red light of the power amplifier down all the time.

At this time, the voltage limiter is turned on and the red light is turned off.

Increase the signal again, and the red light is still not on.

You see, after connecting the voltage limiter, even if the signal is bigger, the power amplifier will not overload and run a red light. So, how to use the pressure limiter? We'll talk about it next time.