Is there a better way to set limiters?: Using some methods for setting peak limiters is like playing Russian roulette with your equipment.
Jul 1, 1997 12:00 PM, Tom Walker
"I just know how to do it." Set ting limiters, it seems, is just one of those things you know how to do. It is hard to explain to someone else, and heaven only knows how you learn to do it in the first place. The real question is, should it be this way?
The purpose of peak limiters is to let the system operate at maximum output but not go over. In other words, we want to protect the loudspeakers from amplifier clipping and other damage so it is reliable. Yet at the same time, we want the maximum sound-pressure level possible so we do not have a lot of excess equipment and its associated cost. It's a fine line. Depending upon the circumstances, such as renting out equipment to be used by others, you may want to err on one side or the other.
So how do you set limiters for your systems? How do other people set limiters? In asking these questions of a great number of people, we received some interesting answers a few years ago. Responses that set us thinking.
Method 1: Play the system slightly above normal This is the method sometimes recommended in the manuals of limiters. It goes like this. Play the system at a level slightly above normal. Now, adjust the limiter so that the threshold LED just lights during peaks in the program material. In many ways, this is a sensible method if your system is well designed and has been reliable. Also, it should be the same system every time - you and you alone are the operator. The final condition is that you are not concerned about toting around too much equipment. Not only does this method ignore the limits of the equipment, but these are a great many conditions to meet. In the real world, few systems fulfill all of these conditions in even a small percentage of the performances. If you can not meet the requirements, then using this method is playing Russian roulette with your equipment.
Method 2: Use amplifier clipping In brief, this method can be described as no amplifier clipping, ever. Here you adjust the limiter based upon the gain of the amplifier and its maximum wattage output. You can find the value of that adjustment by calculating what the maximum amplifier signal input should be. Here is an example of how to calculate what signal level would cause amplifier clipping.
Loudspeaker nominal impedance: 4 V
Amplifier maximum power: 400 W into 4 V
Amplifier gain: 26
To find the amplifier maximum voltage, take the square root of the amplifier maximum power multiplied by the nominal impedance:
sqrt(400 Wx4 V) = 40 V
To find the input voltage limit, divide the amplifier maximum voltage by gain:
40 V/26=1.5 V RMS
Therefore, the dBu of the signal into the amplifier is:
20 log (1.5 V/0.775 V) = +5.7 dBu for amp clipping
If you do not want to do the calculations or all the numbers are not available, you can believe the clip lights on the amplifier. Simply run up the signal level until those clipping lights come on and adjust the limiter so that the threshold LED is just lighting about the same time.
There are some good reasons for using this method, chief among them being keeping any clipping distortion out of the music. However, with this method, you are letting the loudspeakers fend for themselves. Back some years ago (and it was not that long ago), when a 50 W amplifier was state of the art, this no-clipping method was much safer for the loudspeaker. Today, when amplifiers are so powerful, this procedure is without a doubt dangerous and sure to shorten the life of even rugged modern loudspeakers.
Here, we should mention why we are concerned about putting too much power into the loudspeaker. Or to put it another way, what is power compression (also sometimes called thermal compression), and why is it a problem?
You want a loudspeaker to convert electrical energy into acoustic energy. At low power levels, most loudspeakers do a fairly good job of energy conversion. As power increases however, they do not do as good a job and the corresponding rate of increase of acoustic output increases more slowly. At some point, there is a significant difference between the electrical power into the loudspeaker and the acoustic power out since the efficiency is not as good as at lower power levels. That is, at higher levels, the loudspeaker is less efficient and into power compression. The difference in power in (electrical) vs. power out (sound) must go somewhere. Most of it is turned into heat, which is bad enough, but unfortunately that heat is in the loudspeaker voice coil. Anyone who has caught a loudspeaker on fire knows about this problem and never forgets the experience.
Consequently, the reason power compression is a significant problem is the heating of the loudspeaker voice coil and the wasting of power. Modern loudspeakers are very tough. The loudspeaker engineer and consultants have done a great job with new glues, formers and methods. In fact, loudspeaker engineers and manufacturers have done a remarkable job, in general, over the last 10 years. However, we do not want to test their work or waste power into something (heat) different than what we are trying to do (sound).
Back to the just-short-of-clipping method of setting limiters. Now that we understand that this method can be fatal for the loudspeaker and its voice coil, our natural tendency is to adjust the limiter very conservatively. Conservative means more reliable. But conservative also means not using those modern, incredibly powerful amplifiers to their full potential. If set conservatively, you have more expense in extra equipment and the increased expenditure of moving that equipment. It would cost much less if you could operate safely and reliably closer to the limit of every piece of equipment in the system.
Method 3: Use published specifications of the loudspeaker The previous method centered on the amplifier. This one focuses on the loudspeaker. Here we use the published specifications of the loudspeaker to calculate the safe level that we can pump into it. This is, in reality, pretty hard to do.
There is no loudspeaker equivalent of the clipping light on the amplifier (unless you consider that half-second distress scream before a loudspeaker fails to be the equivalent). It is likewise somewhat difficult to verify what power the amplifier is really putting out at different input levels. Couple this with the fact that specifications are at static impedances, such as 8 V, and loudspeaker impedances vary with frequency, making the calculation more difficult. If this is not complicated enough, the enclosure of the loudspeaker acts as a filter network, which means the specifications (which are often for free air) may not apply to your particular case unless you are using a box from the loudspeaker manufacturer.
Then there is the issue of the specifications from the loudspeaker manufacturer. Often after a loudspeaker (or any other product) has been in production for a number of years, changes are made in the materials, production methods or production location. Experience shows that the newer model is usually better than the older one of the same model number. But we all know, too, that specifications are written to put the product in the most favorable light. And those tested conditions may or may not be the same as what you are experiencing.
The consequence of all this is simply that the method using calculations based upon the loudspeaker specifications is a process with a great number of variables. And the only way to make the calculation is to assume answers to some of those variables. Assuming is another name for guessing and that means we are probably not reliably operating at the limit of the equipment.
Method 4: Actually measure the onset of power compression All of the previous limiter-setting methods have the disadvantage of being a destructive test until you get it correct. They are also not precise and repeatable in their methodology, and they do not take the venue or acoustic environment into account. In this modern age, we certainly should be able to do better than what boils down to using experience for quality guessing. Unquestionably, too much money is tied up in equipment for the success of the sound system to be based on guesses, no matter how wise.
The perfect answer would appear to be testing the actual loudspeaker and amplifier system in the venue where it is going to be used, or at least testing the loudspeaker in the enclosure it will be operating in as opposed to relying upon data that could be for different conditions.
The ideal test would seem to be one that could be re-run again with the limiter operating to verify that the peak limiting is active where you think it is. Think about that last statement for a minute. For your loudspeakers, the peak limiter is equivalent to a rigger's safety harness. A rigger wouldn't suspend himself four stories above a concrete floor if the harness system were put together with guesswork, and a sound-system designer shouldn't trust limiter settings based on guesswork either. Wouldn't it be nice to know your safety equipment will function as intended before you entrust your life or your loudspeakers to it?
To take the guesswork out, AudioControl Industrial has developed an audio analyzer that actually measures the loudspeaker and the onset of thermal compression while monitoring the amplifier output. To see how this analyzer, the Iasys (pronounced like "I assist"), solves the problems of other methods just discussed, let's first understand how it uniquely tests.
The Iasys analyzer connections are depicted in Figure 1. They consist of a microphone that is placed on-axis in the coverage pattern at a typical distance and is then connected to the input of the analyzer. The output of the analyzer goes to the amplifier with the gains of the amplifier set at maximum (full power). The amplifier output goes to the loudspeaker under test with a "Y" connection to the analyzer loudspeaker level input. Pretty much the type of hookup you would expect.
With these connections, the Iasys analyzer can monitor the sound-pressure level from the loudspeaker, watch the amplifier output for clipping and control the level and other parameters of the test signal. To put this another way, with these connections, the analyzer can increase the signal to the loudspeaker and literally measure, not guess at, the onset of power compression. Since the built-in, fast microprocessor is constantly monitoring the test, the test halts if there is any loudspeaker distress or amplifier clipping prior to power compression. Measuring the loudspeaker allows the Iasys analyzer to make an exact recommendation for the limiter setting. That recommendation is in dBu of signal to the amplifier based upon actual performance, not assumptions. See Figure 1 for an example of what the test results from the analyzer will be.
This testing is automatic because the analyzer does the setup. Iasys adjusts levels, picks test frequencies and checks that connections are made properly. To do this automatic setup, the Iasys measures the background noise level, sets its microphone pre-amp gain and regulates the test signal according to the background level. During this process, the operator can go do something else, including get a cup of coffee. Then the Iasys conducts a prerequisite test of the loudspeaker's ability to reproduce different frequencies. After that test, the self-contained analyzer knows what frequency is appropriate, and safe, for the limiter recommendation part of the test. It then continues its automatic operation, while the operator is still finding that cup of coffee, and runs the test for the onset of power compression. The entire procedure takes about five minutes, less if testing multiple units of the same loudspeaker.
After receiving the recommendation for setting the limiter, it is easy to verify that the limiter unit works as intended. You add the limiter to the signal chain before the amplifier and adjust the limiter to the settings recommended by Iasys. Actually, you probably should adjust the peak limiter to settings slightly less than recommended to take into account the "knee" built into most contemporary limiter models.
This second test, with the limiter active in the signal chain, allows you to test particular loudspeakers' actual performance. It also allows you to test the real-world system performance of the limiter, amplfier and loudspeakers together. You know your system is operating at peak performance. You also know you aren't using too much or too little equipment and that the system is operating at a safe and reliable level.
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