White Paper Measuring and Optimizing Sound Systems: An introduction to JBL Smaart™ by Sam Berkow & Alexander Yuill-Thornton II

JBL Smaart is a general purpose not intended as an inflexible procedure, acoustic measurement and sound system rather as the starting point for you to optimization software tool, designed for modify as you like and assist you in use by audio professionals. Running on a understanding and optimizing sound Windows® computer and utilizing system performance. almost any standard Windows compatible (MCI compliant) sound card, 1) What am I trying to measure and JBL Smaart offers an accurate, easy-to- why? use and affordable solution to many of the measurement problems encountered Before making measurements of a sound by sound system contractors, acoustical system it is critical to ask yourself, “What consultants and other audio am I trying to measure and why?” professionals. The performance of a sound system, JBL Smaart was designed by a team whether it is a permanently installed consisting of acoustical consultants, system, touring sound system, or some sound system designers, mixers, and hybrid of the two, is determined a installers. The goal of the project was to number of ways, both qualitatively and create a tool which would provide easy quantitatively. The following is a list of access to information which will help some of the most important questions to systems sound better, by identifying ask when determining the level of potential problems and quantifying performance of a given system. system performance. To meet this goal both a real-time module and disk-based • : Does the analysis module were developed. Perhaps system have the ability to deliver sound most importantly, the features within over the intended frequency range, within JBL Smaart suggest a methodology for the expected deviations? optimizing sound systems. This methodology is developed and explained in this paper. The method described is • Power Handling: Can the system and whether it is an existing system, a handle the desired amount of power touring system, or a new system to be without excessive distortion or failure? optimized.

• Coverage: Does the system provide However, there are several steps we feel sufficient coverage of required areas at are necessary to any successful exercise all frequencies? in sound system measurement and optimization. The order in which they • Subjective Quality: This is always the would be followed might differ according most important criteria. Does the system to the tuner’s personal preference and the meet the task at hand. The procedure outlined audience/owners/performers/operators here is based on our own experience and expectations for perceived sound quality? assumes a system that is already in place.

• Stability: Does the system feed back The Transfer Function & System with (s) open and the gain set Configuration to a useful level? (This is obviously not important if the system is playback only.) At the heart of JBL Smaart’s Real-Time Module is the transfer function • Noise: Is the system noisy? Are hums, calculation. The transfer function (stated buzzes and other unwanted noise present simply) is a comparison between the in the system? input and output of a system. The transfer function of a system can be • Configuration: Do you understand displayed in either the time or frequency the system configuration? Some sound domain. To make the results of this systems have groups of speakers driven calculation easy to understand, JBL from a single source. Others are divided Smaart displays the result in the into several sections, each controlled by a frequency domain. To accomplish this different set of control circuitry (such as calculation JBL Smaart compares two equalizers, delays, crossovers etc.). signals, each of which are input to your computer’s sound card. • Operation: Are all components of the system working?

No piece of hardware or software (even JBL Smaart) can accurately answer all of these questions by itself. Tuning a sound system requires an understanding Fig. 1 of the hardware, a discerning ear, accurate measurements, and a disciplined Figure #1 displays a typical setup for and systematic approach. We doubt that using JBL Smaart to make transfer any two system tuners approach the function calculations. The use of two problem exactly the same way. Also, the signals, measured at the input and output process necessarily differs, depending on of a system, allows JBL Smaart to the complexity of the system in question display a comparison of the input and output as a function of frequency. This configuration allows the test signal to be A Step-by-Step Approach noise, music or almost any broadband signal. The Coherence function feature of STEP 1: Evaluation Listening JBL Smaart is used to check the validity Before you begin measuring a sound of the data, thus helping users to system, we strongly recommend listening understand whether that data is reliable. to it! You should attempt to qualitatively For the transfer function calculation to be answer the questions listed on the valid the two input signals must be previous page. This will require you to aligned in time. This can be accomplished move around and listen to each section using the Locator and Internal or subsection of the system. Explore the Signal Delay features edges of the coverage pattern to see included within JBL Smaart. where the various elements are covering and where they are not. It may also be After making a measurement in the helpful to turn off parts of the system in configuration displayed in Figure #1, the order to make a more detailed evaluation results may be stored and the system of various subsystems and components. reconfigured as shown in Figure #2: Practical Note: Unless you designed the system, take some time to try and understand what the system designer had in mind and how the various elements relate to each other.

Fig. 2 STEP 2: Identify Potential Problems Examining the list of questions above, Measuring the transfer function of the are there any obvious problems that need equalizer (Figure #2) is particularly to be addressed? For example, unwanted useful when the resulting transfer noise, such as hums and buzzes function is inverted and overlaid on top associated with ground loops and “dirty” of the previously stored power, can degrade system performance measurement. The need to switch and should be addressed before JBL between measurement points within a Smaart testing is begun. Loose and system suggests that the use of an intermittent connections should be fixed. external mixer, as configured in Figure A gain structure that leaves the system #3, may prove extremely useful in the hissing should be explored and corrected. field: Practical Note: The system must be stable before trying to make measurements. Systems that seem to be changing gain or have noises that come and go are not good candidates for critical measurement. Spend some time sorting things out first. Fig. 3 STEP 3: Select Measurement Points & Positions Practical Note: Acoustical reflections of This is one of the most important steps in sound energy from large (and some not the process. You need to select so large) surfaces may generate “comb measurement points that will show you filters” in the measured signal. The what you need to see. There are two result is a system of dips in the kinds of measurement points: electrical frequency response that are evenly and acoustic. spaced in frequency. They are easiest to see when the frequency axis is set to Electrical measurement points are used linear, as they appear to be a set of to sample the input or output of a valleys evenly spaced across the plot. particular piece of equipment. If you want to measure a piece of equipment, or STEP 4: Compare Positions the result of a string of series-connected In making acoustic measurements of any pieces of equipment, make the system, it is important to make a number connections at the input and the output. of measurements to make sure that you are not being fooled by something Acoustic measurements are made with a affecting the measurement (such as microphone. When making transfer reflections). Move the microphone function measurements, a reference around and look at what happens to the signal is also required. The connection measured frequency response. Also, you for the reference signal should be made may wish to put sound absorbing at the input of the speaker system, the materials on the floor between the source input of the processor if it is a processed and microphone to reduce the effect of system, or at the input of the system’s the reflected sound energy from the floor equalizer(s). (this is another good place to think “mirror”)... Microphone selection and placement are very critical. The microphone must be a STEP 5: Set Equalizers and Delay known quantity. A measurement Settings microphone should be of high-quality Setting equalizers and delays can be very construction, having the flattest time consuming. There seem to be two frequency response characteristics that distinct stages to the process: coarse you can reasonably afford. When adjustment and fine tuning. In the first selecting the microphone position, ask stage, large adjustments to EQ and delay yourself two questions: “Is this a useful settings are used to make a system place to make a measurement?”, and roughly correct. Sometimes, the sheer “What other things will the microphone size of these adjustments may seem a pick up in this location that might affect little daunting, but if things are sounding the measurement?” Reflections into the right, you are probably OK. The next side or back of a measurement stage takes place when the system is microphone can seriously reduce the getting close to right. At this point, accuracy of a measurement. Think changes of a few dB can make the “mirror”, and look around for surfaces difference between a good-sounding that might catch you unaware! system and a great-sounding system. choice might be something you have Learn to recognize this transition. heard so many times you don’t even like it anymore. Only when you are very After making a number of changes to familiar with (several) program selections equalizer settings, it is important to go will you be able to use them as a basis to out and listen to what is happening to the quickly and accurately evaluate a system system. Make sure it is moving in the by listening. right direction. Just because it looks good on an analyzer screen doesn’t mean STEP 7: Stability Testing that it is right. Remember, you are It is very important to explore the working for the ears, not the stability of any critical sound system instruments! before it goes into service. Otherwise, you may find yourself in the Important Notes: uncomfortable position of trying to find and equalize feedback frequencies during • Always make delay adjustments a performance or other event. Obviously before trying to make fine adjustments in this is not a concern for playback only equalizer settings. A combination of systems. small delay and changes can completely change the character of a Unstable sound systems are those that delay system. have, at one or more frequencies, an overall gain, including the acoustic path, • Setting delays and equalizers can of more than one; in other words, help make some poorly designed sound feedback. It follows that a stable system systems sound better. However, only in has a comfortable margin of gain before extreme and very rare cases is it possible feedback (GBF) at its intended operating to correct poor loudspeaker coverage level while delivering the intelligibility with these types of devices. and frequency response characteristics required for its purpose. STEP 6: Critical Listening This is what it is all about. Here is where Important Note: you take off the measurement hat and put on the listening hat. Put on a CD (or • Feedback can damage audio other program source) and “walk the components. Exercise caution when system”. Listen in the front rows and testing system stability. Feedback is back in the cheap seats. Try it at low particularly dangerous when it builds up levels. Try it at high levels. Run it though very quickly and overdrives the system, its paces. Turn the source off and listen causing overloads or clipping. It might be to everything in silence. Make sure that a prudent safety precaution to use a the noise floor is low enough not to limiter or compressor during stability affect the dynamic range of the system. testing to help protect system components. Remember that non-linear Use material that is familiar to you. devices such as limiters or compressors Don’t be afraid to listen to things others CANNOT be used during transfer may not like. For this purpose, the best function measurements. involves reducing the gain of the system Typical Causes of Instability at the problem frequencies. Given the nature of the problem, the most obvious Instability, or feedback, is often the result solution is to apply equalization. of interaction between the off-axis Although equalization is not a panacea or response of the speaker system and the a substitute for good system design, it is off-axis response of . The one of the most powerful tools you can biggest problems usually arise when bring to the task of stabilizing an existing narrow peaks in the off-axis responses of sound system. both loudspeaker and microphone coincide. These types of interactions can JBL Smaart can help you to quickly be very troublesome, and they are not as identify problem frequencies and apply easy to control as the on-axis responses. equalization with great precision. But before you start turning knobs, consider Other possible causes or contributors to that equalization affects the overall stability problems include acoustical frequency response of the system. There characteristics of the room and/or signal are other strategies that might be equally processing equipment; particularly effective, or more effective, and could reverberation units used in music afford you greater freedom to make the reinforcement systems. system sound good.

Detecting Instability Mechanical Solutions and Acoustical Solutions The simplest way to expose a stability problem in a sound system is to turn up The physical position of microphones the gain, slowly and carefully, until the and in relation to each system feeds back. Not a particularly other can affect the feedback frequency elegant approach, but it almost always (or frequencies). Reducing the gain at a works. If feedback does not occur in the problem frequency can sometimes be as system until the gain is increased well simple as using a different microphone, beyond the intended operating level, and or reorienting one already in use. This the system is free of any noticeable strategy is best employed when the “ringing” at normal levels, it’s pretty microphone in question is intended to stable. If not, you will need to find ways remain stationary. Moving or reorienting to improve it. Depending on the loudspeakers may also be a possibility. situation, the best solution could be Mechanical solutions are most attractive electronic, mechanical, acoustic, even when they can be applied without giving educational, or some combination of the up any of the sound system’s design four. goals.

Some Approaches to Stabilizing a Sound Stability problems often arise when System loudspeakers are placed close to (or behind) microphones. In such cases it Stabilizing an unstable system, or giving may be possible to add some sound a system more “margin” (GBF) primarily absorbing material or a baffle that reduces the speaker’s field at the microphone’s position or simply reduce the operating level(s) of the loudspeaker(s) in question. Important Note: of the output of the system, through some system of delays, to the input. • Moving microphones are moving targets. When speakers or performers With a simple enough system, polarity or move around with microphones, phase changes could solve a feedback feedback frequencies may shift. Always problem immediately. When the polarity try to perform stability testing in a is inverted, instead of positive feedback manner that resembles how the system (something we don’t like) we should get will actually be used. negative feedback (something that may be beneficial). However, in large, complex systems with multiple return Educational Solutions paths many wavelengths long, phase or polarity changes might just tend to shift An otherwise stable system may lose the feedback frequency, without stability when a number of microphones increasing stability. are open at one time. In this case, the best solution might be to train the The most common solution to the operator to keep microphones open only problem of feedback is to use when they are actually in use. equalization to take out offending peaks. By peaks, we mean places in the Educating users in microphone technique spectrum where there is significantly can also be beneficial. Many people tend more gain (or energy build-up) than to grab microphones or lean very close others. The procedure involves: when they speak. Both of these actions can cause problems. Grabbing a cardioid • Carefully, running the system into microphone can increase its physical gain feedback at certain frequencies when the user’s • Identifying problem frequencies, and hand closes off the rear ports to the • Setting up filters (i.e., EQ stages) to microphone element, making a stable compensate system suddenly unstable. When people lean too close to a microphone, they can Important Note: reflect some energy at problem frequencies back into the microphone • We strongly recommend the use of themselves, possibly causing feedback. parametric equalizers for this type of application. Electronic Solutions How Much Equalization is Enough? Some reverberation units can cause an otherwise stable system to become As you equalize a system to increase unstable. If this seems to be the case, try stability, keep in mind you are reducing experimenting with other settings and/or gain, even though you are reducing it reducing the overall level of electronic only at specific frequencies. In many reverberation. Keep in mind that cases, the frequencies in question have reverberation generators do what they do proportionately too much gain anyway. (very simply put) by feeding back some You may actually improve the system’s frequency response at operating levels overlay stored measurements allows while increasing stability. both equalizers and delay units to be accurately set. JBL Smaart provides If there are two or more feedback the capabilities to make these frequencies, equalization tends to work measurements, and benefits from the best when the frequencies are fairly close provided together. You may find, after you have under the Windows . applied a number of filters at widely spaced frequencies, that all you have really accomplished has been to reduce the overall gain of the system – without really increasing its stability or GBF. It may be necessary to explore other solutions. (In some extreme cases it may be necessary to alter the system design to correct instability.)

STEP 8: More Critical Listening If you, and everybody else concerned, are satisfied with the system, you’re done. More likely, you will need to repeat some combination of Steps 2 through 7 to obtain the best possible performance. Optimizing a sound system is usually a gradual, cut-and-try, give- and-take process (which often takes more time than one would like or expect). We hope that JBL Smaart will help to make this process much easier for you.

CONCLUSIONS:

The ability to measure the transfer function between two signals (which have been aligned in time) allows sound system performance to be measured using a variety of test signals, including noise and music. This measurement technique provides an easily understood measurement which can be used to optimize sound system performance. The ability to