Check It Out!
Dec 1, 1998 12:00 PM, Sam Berkow and Alexander Yuill-Thornton II
A common request made of sound system contractors and acoustical consultants involves the evaluation of an installed sound system. This request, often heard in a first phone call, may include such concerns as a lack of speech intelligibility or uneven coverage. Just as likely are evaluative, vague comments involving insufficient punch in the system, the sound's not being right or an uncertainty as to whether the existing system is operating at its maximum potential. Regardless of the initial customer comment, however, there are a number of questions that must be asked and answered-some technical, some financial. Chief among them involves determining the best method of establishing the problem's nature.
Assume that you have been asked to evaluate the sound system in a facility with an actual problem. The facility could be anything from a church or convention center to a studio, club or even a stadium or arena. Whatever the facility or system, one of the most perplexing technical considerations is also one of the most frequently neglected, and it requires determining how much of the problem is attributable to the sound system design (components and configuration) and how much results from the facility's acoustical environment.
The performance of an installed sound system can be evaluated in a number of ways; coverage, frequency response, power handling and configuration are all important areas to explore. Where to begin? Critical listening is certainly a primary component of any system evaluation, and it should be the goal of any measurement tool to provide data that helps a user understand what he or she is hearing.
In any event, you arrange a site visit in response to the facility's telephone calls. Hopefully a paid visit, it may turn out to be a courtesy call. The politics and economics of this first call fall under the domain of another article. You may wonder what can actually be accomplished in a site visit or which tests are effective. I suggest that the following steps be used as a guideline, and as such, they are not intended to be followed in exact order for every case. They present an outline that can be adapted to almost any situation.
User requirements and expectations This first step can begin during the initial telephone call. Find out exactly what the user expects of the system. Next, evaluate whether or not the current requirements are even remotely similar to the performance requirements established during the system's original design. These are both critical steps in explaining the need for changes to a system. A clear example would be a church sound system that had been originally optimized exclusively for speech use and was currently used for modern music reinforcement.
Systems configuration The next step typically begins with a review of every available system block diagram. If such documentation exists, it can be extremely useful (and maybe profitable) for the consultant/contractor to offer to help the facility develop even simple one-line drawings. A lack of documentation for an existing system may also be an indication that the facility is neglecting proper system upkeep and maintenance. If possible, review the system configuration prior to arriving at the site. Once on site, verify that the system is actually built as drawn. It is not uncommon to find that a system's documentation and the actual installed system represent two very different realities.
Critical listening Step three has several parts. To begin, I suggest listening to the room with the sound system off. It is often a good idea to have the HVAC system turned on and off to identify its contribution to the facility's ambient noise. The role of ambient noise in reducing speech intelligibility is often overlooked, and in facilities where speech intelligibility or critical listening is important, HVAC noise can have a considerably negative impact.
Next, spend a moment listening with the sound system powered on and the systems gain turned up to typical operating levels but without input. A system with hiss, hum, buzz or noise indicates problems with gain structure or possibly faulty components. Once these symptoms have been identified, steps can be taken toward isolating their causes, which, in turn, may lead to identifying or even resolving the facility's original problems.
Critical listening of the actual sound system performance should begin with playback of material familiar to the listener. Music with clearly defined parts (snare drums, kick drums and vocals) is particularly useful because the perceived relative levels of each part can be easily compared by ear.
Walk the room while activating each part of the system individually and in groups. A common concern is listener areas in overlap zones, areas where the listener receives sound from two or more loudspeakers. In these cases, you must determine the relative role of each sound source and its contribution to what a listener in the overlap area hears.
Critical listening should also be done at levels typical for system operation and at levels low enough to allow evaluation of coverage without fatiguing the ears or excessively exciting the room's acoustics. When excited, highly reverberant spaces have a tendency to mask coverage problems. Careful listening can identify areas where the relative amounts of direct and reverberant energy reaching the listener may be a concern.
At this point, listening to a live talker would also prove helpful. Keep in mind, however, that several variables (such as the mic in use and the quality of the talker's voice) can introduce considerable variation into this type of test.
Keep notes during the critical-listening process to aid in the identification of potentially interesting areas for measurements, especially those areas with listener positions that seem to have problems of one sort or another. Such problems are typically characterized by a dramatic change in level or tonal response when compared to adjacent areas. Once such a location has been identified, establish whether the problem is due to the sound system, the room or the means by which they both interact. An example of this distinction would be a location in which the loudspeaker sounds distorted, possibly due to a faulty component, and another position where a reflection from a balcony front (or other surface) causes cancellations in a given frequency range.
Measurement and quantitative evaluation Step four is where measurement tools begin to play a role. Three types of measurements are commonly used in the evaluation of an installed system-frequency response, impulse response and ambient and/or system noise levels. Each offers valuable insight into the system's perceived performance.
The frequency response of a system has traditionally been measured using a real-time analyzer (RTA). Because an RTA normally reads only the signal originating from the system for determination of frequency response, a signal whose frequency content is known is best for a source or stimulus signal. The most commonly used source signal is pink noise. Having equal energy in each octave band by definition, pink noise looks flat on fractional octave band analyzers. Typically 1/1-, 1/3- or 1/6-octave resolution is used.
Although an RTA is useful in many ways, particularly for ear training, the RTA's value for understanding a room or optimizing a sound system is limited. A major deficiency in simple RTAs reduces its effectiveness in evaluating sound system performance and renders it completely unable to make distinctions among problems arising from the sound system, acoustical problems in the room and interactions between the two. These limitations are attributable to RTA's time blindness. In other words, RTAs can register only the result of the sound system's performance and its interaction with the room's reverberant energy. It cannot separate these factors. To address this situation and provide more insight into the room's or system's frequency response, transfer function-based measurements are necessary.
Transfer function The transfer function of a system is a comparison of the system's input and output, which can be calculated in either the time or frequency domain. Transfer functions are usually measured with a two-channel measurement system in which the input signal and output of the system are sent to the measurement device. The comparison of these two signals will include some sort of time windowing, which allows the system to read the response of the sound system itself and limit the amount of room response included in the measured data.
This ability to exclude or at least reduce the room's contribution to the frequency response measurement is especially useful when setting crossovers, delays and EQs. It is important to note that these devices affect only the signal going through the sound system and not the response of the room itself. This distinction provides some insight into the topic of system optimization where the goal is enhancing a sound system for the best possible response given its interaction with itself and the acoustical environment.
To improve this ability further, some modern transfer-function measurement devices provide a frequency-dependent time-windowing feature. The use of several time windows (typically longer windows at low frequencies and increasingly shorter windows at higher frequencies) correlates well with the nature of human hearing and provides useful frequency response information for the evaluation of system performance.
To use the transfer function measurement for evaluation, begin by positioning an on-axis measurement mic in a loudspeaker's direct field (normally in the main cluster). Send the same test signal to the sound system and the measurement system. The output of the sound system, as picked up by the measurement mic, is sent to the measurement system on a separate channel. This dual-channel approach allows a precise comparison of the input signal (also called the reference signal) and the system's output.
Before making the transfer-function frequency-response measurement, ascertain the total delay through the entire system with considerable precision, including any throughput delay or latency in the sound system itself along with the propagation delay from the loudspeaker through the air to the measurement mic. The reference signal must be delayed by an equal length of time at the measurement system to make an apples-to-apples comparison of the two signals.
Fortunately, this delay can also be measured easily by many modern measurement systems. One technique used to find the delay through a system is to measure the system's impulse response. An advantage to this technique is that it can give some other important pieces of information about the room and the system, including polarity, direct-to-reverberant levels, the presence of reflections and the reverberation time.
Once the delay has been found and compensated for, the frequency response of the system may be measured accurately with the transfer function. Frequency response is usually represented as a graph with frequency on the horizontal axis and magnitude on the vertical axis. The vertical axis is normally centered on 0 dB, where 0 dB represents equal relative energy between the input signal and output signal. Values above 0 dB represent gain or resonance, and values below 0 dB represent either cancellation or attenuation.
Using the multiple time-windowing techniques mentioned earlier, the results of a transfer-function measurement can be displayed with equal resolution across the audible spectrum. This constant-resolution display is a major improvement in modern measurement techniques because it presents data in a way that is easier to read and understand than traditional displays where low-frequency resolution was typically much lower than higher-frequency resolution for a given measurement.
When measuring clusters, it is often necessary to examine each element of each loudspeaker system individually, a practice with several purposes. First, it verifies that each component is, in fact, working. Second, the transfer function of each component will reveal whether the crossovers and amps are providing proper frequency and gain control. In areas covered by more than one loudspeaker or cluster, inspecting the frequency response of each cluster or loudspeaker individually and in combination will offer additional insight into the system's performance.
Another advantage of transfer-function-based measurement systems is their ability to provide phase information about the system. The phase display shows the relative delay in the output signal for each frequency. This information is essential for correctly setting precision delays within individual loudspeaker systems and between various systems making up a cluster.
Although the time-windowing inherent in transfer-function measurements helps isolate the system factors from room factors, frequency-response measurement will still include some of the system's interaction with the acoustical environment. This is one reason why most (if not all) transfer-function-based systems include some kind of coherence function, which can be used to judge the quality of the transfer function data, indicating noise contamination or other nonlinear events.
The impulse response As mentioned earlier, the delay through the system under test must be known with considerable accuracy before a valid transfer-function measurement can be assessed. Although there are several useful techniques for measuring this delay, a given system's impulse response provides a great deal of important additional information.
An impulse response is simply the response of a linear system to an impulse. A linear system is one whose output is not dependent upon the input signal, such as a loudspeaker or room. An example of a nonlinear system would be a compressor; the level of the output signal is a typically nonlinear function of the input-signal level. Measuring the impulse response of a system is actually a little more difficult than it may sound, which is primarily due to the practical difficulties involved in generating a perfect impulse. There are, however, several systems available that use various mathematical techniques to calculate the impulse response of a system and make it a fairly simple matter for users to perform these measurements.
The information afforded by measuring impulse response is essential. Aside from the delay through the system, the polarity of the system is also part of the impulse response when plotted in the time domain with a linear-amplitude scale. When measuring a sound system in a room, including the direct-to-reverberant level and the presence and arrival times of discrete reflections, the rate of broadband decay and the overall S/N ratio of the system can also be determined. Recently, a number of papers have been published showing speech-intelligibility calculations based upon impulse response measurements. The direct-to-reverberant level is certainly an important factor when evaluating a sound system's speech intelligibility.
Although the impulse response may be displayed in the time domain, it is also common to transform the impulse response of a system into the frequency domain. This transformation uses a series of FFT (fast Fourier transform) calculations and allows exploration of the system's response as a function of frequency vs. time. Such displays as the 3-D color spectrograph or more traditional waterfall plot can be used to display time, frequency, and energy characteristics of the system simultaneously and meaningfully. When looking at the impulse response of a sound system in a room, its response is often represented only in the earliest part of the measurement. The response of the acoustical environment-reflections and diffuse reverberant decay-tend to dominate the remainder of the impulse response measurement.
Ambient noise measurements Additionally, taking the time to measure the ambient noise level of a room can provide insight into HVAC noise problems and other noise sources that have an adverse effect on speech intelligibility. Traditional octave-band measurements, reported as NC or PNC values (noise criteria or preferred noise criteria), are helpful in their own right, but modern FFT-based analyzers allow high-resolution measurements that can display the spectral components of the ambient noise sources more completely.
When evaluating a sound system, looking at the spectral content of the system's self-noise can offer insight into problems with gain structure and even mains power connections and ground loops. Using high-resolution FFT measurements allows the impact of changes to the system to be easily evaluated as problems are sorted through.
In conclusion, the ability of modern measurement systems to provide transfer function-based frequency response measurements, impulse-response measurements and high-resolution noise-spectrum measurements provides a powerful set of tools for evaluating both the performance of a sound system and the system's interaction with its acoustical environment. The availability of fast and reliable PCs means that these measurement techniques that were beyond the practical reach of most contractors and consultants only a few years ago are fast becoming part of our standard field evaluation toolkit.
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