NOT TO BE FORGOTTEN
Jul 1, 1999 12:00 PM, Glen Ballou
We have all heard about the great newly available measuring instruments that are used to measure audio circuits, instruments like FFT (Fast Fourier Transform) and TEF (Time-Energy-Frequency) machines, which measure audio systems more accurately and quickly than those instruments offered a few years ago. We should not, however, forget many of the those older instruments; they were good then and are still good today. For instance, how do we measure distortion, frequency response, resonance, loudspeaker rattles, phase differences and impedance, for example? Much of this can be tested with state-of-the-art test gear, but they can also be tested with the simple, old-fashioned gear.
How can we measure an amp's characteristics? To measure frequency response, we can insert a sine-wave signal into the input and measure the output signal. If the input signal is the same level at all frequencies, the frequency response will be the variation of the output level. A sine-wave generator works well for the input signal. Most sine-wave generators today have low impedance output, so the input impedance of the device-under-test (DUT) does not affect output. If we sweep the input of the DUT with a sine wave from the lowest to the highest frequency desired, we can measure the output of the DUT with a voltmeter, wattmeter or dB meter. If the internal output impedance of the DUT is low compared to its specified output impedance and the impedance of the test instrument, the DUT will not require loading. If we do not know the internal impedance of the DUT, we should load the output with the recommended impedance.
If we are measuring the frequency response of a power amp rated at 8 V, and we do not know the internal impedance of the amp, we should load it with an 8 V load. Remember, however, a 100 W amp requires a 100 W resistor if we are measuring it at full load. We might want to measure the amp frequency response at more than one power level (at 1 W, one-half power and full power). Although measuring at 1 W or half power can usually be done without monitoring the output with a scope, when measuring at full power, we must monitor the waveform. If the amp has a rising response at some frequency other than the reference frequency, we may not see the true frequency response on the meter because the amp will clip. Under this condition, the response will appear better than it really is. If we want to determine the power frequency response in dB referenced to our reference frequency when we measured the output in volts, we can use the equation:
dBvariation = 10log10[(Vout)[superscript]2/(Vref)[superscript]2]
To determine the voltage response in dB, use the equation:
dBvariation = 20log10[(Vout)/(Vref)]
Output in negative numbers means that the output is less than the reference value.
With a distortion analyzer, we can check distortion of the DUT using the sine-wave generator as the signal device. Most harmonic distortion analyzers tune out the fundamental frequency and measure whatever is remaining. This turns out to be odd and even harmonics and noise. It is important that the sine-wave generator has low distortion and low noise; otherwise, the readings will include the source and will consequently be meaningless.
If we have a two-channel oscilloscope, we can put the input to the DUT on channel 1 and the output on channel 2. If we superimpose the input on the output and set the levels so they are the same height, we can see the distortion but cannot give it a value (see Figure 1). Still, this can be useful. The waveform in Figure 1 shows clipping on the positive half of the signal and crossover distortion.
We can also see phase shift and polarity reversal with the two-channel scope and sine-wave generator (see Figure 2). This can be useful if we want to keep the input wave and the output wave the same polarity, which is important when micing and amplifying live music.
Sine-wave generators are also useful for finding resonance, standing waves and loudspeaker rattles. If you suspect a circuit is unstable and sometimes goes into oscillation, apply a sine-wave generator to the input and an oscilloscope to the output, being sure to load the output with the load you will be applying in the final installation. Sweep the DUT from the lowest frequency to the highest frequency you feel is appropriate and monitor the output. Remember that the system may go into oscillation or start ringing at a frequency well above the audio spectrum, so run the sine-wave generator as high as possible, even into the RF bands, if possible. Shocking the system by hitting the input with an instant signal is also a good idea. We can accomplish this by setting the sine-wave generator at the proper output level and removing and reinserting the input connector. If the DUT is stable and properly designed, this will not hurt it, but can find an unstable condition. If, however, the DUT is unstable or not designed to withstand transients, we could destroy it. If we do destroy it while testing it, we should be satisfied; it is always better to find problems before the devices is installed.
Standing waves in rooms occur because of the room's dimensions. All rooms have standing waves, but some are more obvious than others. If the room is highly damped, that is, it has a lot of absorption in it, standing waves may not be obvious. Standing waves, however, are most annoying at low frequencies where absorption is more difficult to achieve. A room with two sets of opposing walls with heavy absorption and one set of opposing walls with little absorption exhibits a definite echo. The echo or standing wave is always there, but in the above case, it is obvious because the waves that normally help to mask it from the other two parallel surfaces are absorbed. The unwanted echo has not increased; it has always been there-it was just masked. This is why a highly reverberant room sounds rather smooth and echo free; each reflection masks the other reflections. Fuzz one set of parallel walls, and we are apt to hear two distinct echoes. Fuzz two sets of parallel walls, and we will hear one distinct echo.
Even in a relatively dead room, we can find spots where a particular low-frequency wave is strong. This is not the place we want to place a mic in our sound system unless we want guaranteed feedback. How do we know it is there? The sine-wave generator comes to the rescue again. Sweep the system with a sine-wave generator and either stand at the spot where the mic is to be placed and listen, or better, use a sound level meter (SLM) in that spot and monitor the output. If you do not have an SLM, you can place a mic in the spot (preferably the mic you will be using in the final installation) and look at the output with an oscilloscope or a voltmeter. Move the meter just a foot or two (305 m or 710 m) and notice that the standing wave goes away. Do not, however, get overconfident because a new wave may appear at that new spot. It will be a frequency that has a wavelength related to the new distance. The frequency can be calculated using the equation:
Frequency = 1,130 ft/s(wavelength)
where: 1,130 ft/s (344 m/s) is the speed of sound and wavelength is the length of one cycle of the frequency in question.
If we measure the distance from the source to the mic, we can use that as the wavelength of the frequency in question. This does not have to be a straight line; it may be a reflection off a wall, floor or ceiling, or all three.
We just looked for a feedback type of frequency. What if we are installing a home theater or stereo system? The predominant problem is now the lost frequency. Low frequencies have a tendency to go through house walls and windows rather than reflect back into the room to fill it with beautiful bass. A signal lost is a signal lost forever; no amount of EQ will ever bring it back.
So what are we to do? Never place the best seat in the house at the spot where the signal is at a minimum. Sweep the system with a sine-wave generator to be sure there is not a big hole where the master's ears will be. If there is, move the chair or reposition the loudspeakers until a sweep is as smooth as possible. Do not try to boost the system with EQ; it will not bring the lost signal back, but it will apply two bumps, one on each side of the lost frequency.
If we want to measure the frequency response of our complete system with a sine-wave generator and an SLM, we will have to warble the tone to average out the standing waves and other anomalies. Warbling can be accomplished by moving the frequency dial back and forth while sweeping the generator. This is the way measurements were made before pink noise and RTAs. This may not be as accurate as today's method, but if all we have is a sine-wave generator and an SLM or voltmeter, it will give us a reasonable frequency response of the room and the system.
How many times have we had a loudspeaker that rattles or an object in the room that rattles? With a sine-wave generator, it is easy to locate. All we need to do is sweep the system and listen for the rattle. If it is an object in the room, maybe we can tighten it or move it to where it is not affected by the loudspeaker output. If it is in the loudspeaker, it may be due to poor construction, a loose driver, a loose or resonant grill, or possibly the loudspeaker cone is going into resonance because we are trying to use the loudspeaker below its cutoff or at too high a level. At any rate, it gives us information we can use to correct the problem. As we can see, the sine-wave generator, which has probably been sitting on the top shelf collecting dust, is still a useful tool.
Most of us today have a pink-noise generator (PNG) and a real time analyzer (RTA), which we probably use to measure and EQ rooms, but there are many other things for which we can use these devices. We can measure the frequency response of our electronics by inserting the pink noise into the input of a DUT and measuring the output with an RTA. This is a quick way to check out our electronics and test our filter sets-do they combine, or are they 24 independent filters?. Also, are they narrow or wide band, 12 dB or 24 dB? Do they cause distortion, or are they impedance sensitive?
The PNG and RTA are also useful for measuring impedance. With, for example, a Gold Line DSP-30 with the impedance program, we can measure the impedance of loudspeakers or anything else directly. If we do not have a DSP-30, we can make a black box to work with our RTA (see Figure 3). If we feed the DUT with our PNG and put the black box between the DUT and the RTA, we can see the impedance of the DUT over the entire audio spectrum on the RTA. The device is calibrated for 4 V, 8 V, 16 V, 600 V and a 70.7 V system. All we need to do is set the calibrate switch to the impedance we want to measure, insert the pink noise, set the level to any reference point on the screen that we want, switch to read and look at the response on the RTA (see Figure 4). Each +-6 dB represents doubling or halving the impedance. Simple and useful, be sure to take it with you when installing a 70.7 V system; it can save time and amps.
Another handy gadget to carry in the toolbox is a polarity checker. With this simple device, we can measure the polarity between the high-frequency driver and the low-frequency driver of a loudspeaker, or we can check each loudspeaker in a 70 V system or cluster to assure ourselves they are all in polarity. Remember, one out-of-polarity driver can ruin a good system.
Every so often, we find ourselves with a signal generator that has too much signal for our input or an output from our DUT that is too high or improperly matched to our test instrument. This is the time for the in-line devices made by Shure, Sescom and others .
These devices can do many things. Having the same male or female connectors on each end, they can change gender. They can also reverse polarity. The hot pin and common are reversed; for instance, pin 2 and pin 3 are reversed on an XLR connector. Lifting grounds is possible; pin three floats on an XLR connector. The device may be a line-level to mic level attenuator, a 10 dB, 20 dB, or 40 dB attenuator, allowing it to change level. If we have to reduce our signal by more than what one device can do, we can connect the units in series. Be sure to observe impedance were applicable. The unit may go from low to high impedance or visa versa, but impedance changing always attenuates the signal. Usually accomplished with a built-in transformer, the device can also go from unbalanced to balanced or visa versa. Some units, equipped with battery-powered single-frequency sine-wave generators, can insert a signal into a circuit. Lastly, they can change connector type. If the test equipment uses one type of connector, and the DUT has one of the many audio connectors that are out in the field-RCA male or female, 1/4 inch plug or jack-we can use an adapter that will correct our interface problem. Some come internally prewired, but just as many come without through connections so we can wire them to satisfy our needs. Of course, we can make our own adapters using connectors and wire, but if we calculate the cost of our labor, we will probably determine it is cheaper to purchase them rather than try to make them and store them.
Always have an ample supply of these useful gadgets in your toolbox. Whatever the job, whatever the test, there is always more than one way to do it, so never get hung up on using only the latest test gear. Much of the old standby gear of yesteryear will not only do the job, it will do it better.
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