Response equalisation - REq-X
By Tommy Schack, Svend Gade, Ole Thorhauge and Peter Sims
24 Feb 2011
REq-X – a unique feature of Brüel & Kjær's PULSE system – can significantly improve and widen the performance of transducers. Here we take a look at exactly how the technology works.
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Technology
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REq-X technology is a unique feature of Brüel & Kjær’s PULSE platform that enhances the analysis of sound and vibration measurement by significantly optimising the performance of both microphones and accelerometers.
Using REq-X we can expand the uses of transducers beyond those which they were designed for, we can flatten the frequency response curve, we can increase the frequency range over which transducers can operate, and last but not least, we can improve measurement accuracy – by up to 10 dB.
How does it work?
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| Enlarge image | | REq-X flattens the result by 'mirroring' the response |
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The principle of REq-X is that by taking the known frequency response of a transducer, we can then inverse the graph to give us the correction spectrum. Calculating an inverse FFT analysis on the correction spectrum then gives us an impulse response of the correction spectrum. Finally the input signal is convolved with the impulse response of the correction spectrum in order to apply the correction, as illustrated in Figure 1.
The great advantage of REq-X technology is that it takes care of all of that for you, meaning that the built-in equalisation filter calculates the equalised signal in real-time, delivering a flatter response which can then be directly used for analysis.
All Brüel & Kjær transducers are delivered with an individually measured calibration chart of the transducer’s frequency response on a data disk that also contains corrections for different sound fields and accessories. For all TEDS accelerometers the frequency response is coded into the TEDS information and is automatically detected when connecting to PULSE system.
Different types of microphones
Microphones are divided into three types according to their response in a sound field. These are free-field, pressure-field and diffuse-field (or random incidence) microphones. They are specifically designed to work for one use, and shouldn’t normally be used in other situations.
When a microphone is placed in a sound field, it modifies the field. Figure 2 shows how the pressure rises in front of the microphone due to local reflections, causing the microphone to erroneously measure sound pressure as too high.
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| Enlarge image | | Figure 3 shows the significant performance variation of a free-field microphone |
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Free-field microphones have a uniform frequency response for the sound pressure that existed before the microphone was introduced into the sound field, and are designed to compensate for their own disturbing presence. However, as Figure 3 (left) shows, free-field microphones can only be used reliably in a free field with the microphone pointing towards the source. Here, the blue curve shows the frequency response of a free-field microphone when used in a diffuse field without REq-X. The green curve shows an even more pronounced frequency response when used in a free-field, but at an angle to the source.
Diffuse-field microphones and pressure-field microphones show similar discrepancies when used outside of their respective designated areas.
So what does REq-X do for microphones?
| "The great benefit of REq-X is that a free-field microphone can be used over a wide frequency range, even in a sound field it is not designed for" |
With REq-X there are typically 50 different equalisation curves to choose from for a microphone, depending on the sound field and which accessory you want to equalise for. Not only that, but you can also compensate for the purpose-specific design of the microphone, allowing you to use them in different fields. What’s more, the microphone can be corrected for angle of incidence in steps of 30 degrees, that is, for 0˚, 30˚, 60˚, 90˚, 120˚, 150˚ and 180˚ angles of incidence, and for various microphone accessories such as windscreens, microphone grids and nose cones – thereby improving the measurement accuracy by a further 5 to 10 dB.
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Of course, high-quality microphones are made with an optimised frequency response for their intended sound fields, but even though the microphones are well designed, we can effectively enhance the frequency response. Using REq-X we can flatten the frequency response and consequently improve the measurement accuracy by up to 1.5 dB over the specified frequency range.
Figure 4 shows the result of an equalisation of Free-field Microphone Type 4190 used in a diffuse field. We can see that in this case the improvement is about 3.7 dB at 10.3 kHz. Thus the great benefit of REq-X is that a free-field microphone can be used over a wide frequency range, even in a sound field it is not designed for.
What about accelerometers?
All accelerometers give a constant output signal for a constant acceleration, from very low frequencies right up to a limit that is set according to the increase in output due to the resonance of the accelerometer. Traditionally, accelerometers are not used close to their resonance frequency as this could result in a large error in the measured signal. As a rule of thumb the accelerometer can be used up to one third of its resonance frequency without response equalisation, to ensure that the error does not exceed approximately 10%, or 1 dB.
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| Enlarge image | | Figure 5 shows the advantage of REq-X when mounting on a metal clip |
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Using REq-X we can now halve the measurement inaccuracy to 5% over the same frequency range. Alternatively, if we stick instead to the 10% limit and use REq-X then we can extend an accelerometer’s frequency range. Using uniaxial accelerometers, the upper frequency can be extended by up to 100%. For other types, the upper frequency can typically be extended by up to 50%.
Figure 5 shows the measured frequency response of Accelerometer Type 4507 mounted with a metal clip. The resonance frequency was measured to be 19 kHz. Without the use of REq-X the upper frequency is 7 kHz. Using REq-X the upper frequency is raised to 12 kHz – an increase of more than 50% – and the accuracy has been improved dramatically all the way up to the resonance frequency.
Where can I find out more about it?
For further reading, see the original article in Brüel & Kjær magazine by Tommy Schack, Svend Gade and Ole Thorhauge here.
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