Event, Meeting > ‘Can we make quasi-anechoic measurements in normal rooms?’
Title: ‘Can we make quasi-anechoic measurements in normal rooms?’
Location: Royal Academy of Engineering, London
Description: Lecture by John Vanderkooy, Audio Research Group, University of Waterloo, Canada, with Steyning Research Establishment, B&W Group Ltd, UK
Start Time: 18:30 for 19:00
Date: Tuesday 10th March, 2009
Lecture Report
John Vanderkooy presented research into methods to improve loudspeaker measurements made in non-anechoic rooms. The lecture began with a discussion of the motivation for the research:
- Not everyone has access to an anechoic chamber
- Anechoic chambers may not be effective below 100Hz due to inadequate LF absorption
- Low frequency calibration of anechoic chambers may be ineffective
- Low frequency noise from air conditioning, industry and the environment can easily contaminate the measurements.
Impulse response measurements made in an echoic room or an imperfect anechoic chamber will have reflections that contaminate the results and will also often have significant levels of added noise. John presented measurements from a 110mm driver in a small sealed cabinet to illustrate the algorithm developed to overcome these limitations.
The algorithm comprises the following steps:
1) Measure an impulse response, typically 5–6ms of which is reflection-free following the initial response of the loudspeaker, and obtain the frequency response..
2) Apply a minimum phase filter to the impulse data such that the frequency response becomes flat to DC and, optionally, a high-pass filter with a corner frequency significantly above that of the loudspeaker.
3) Truncate the impulse response such that all room reflections are removed. The resulting frequency response will have high-pass characteristic at a higher corner frequency.
4) Apply an inverse filter to that of step 2.
Now the impulse response has the low frequency persistent decaying oscillation extending cleanly beyond the first reflection arrival time.
There are several impulse response windowing methods and filter types that can be used. John explained that a rectangular window introduces ripples into the frequency response, while other types cause data to be lost towards the end of the truncated impulse response.
Methods of shortening the impulse response discussed were the Backman method and the Fincham method. The Backman method of flattening the frequency response to DC causes the impulse response to have a very long but zero-valued tail, making it suitable for truncation. The Fincham method, which raises the apparent corner frequency of the loudspeaker’s LF roll-off, shortens the impulse response, again allowing truncation to be applied without significant loss of data in the tail. As originally described, the Fincham method seemed to apply the step 2 filter to the test signal, which results (when the inverse filter is applied) in increased contamination of the acoustic measurement by low frequency noise. This can be avoided by applying the step 2 filter to the measured impulse response instead, and apparently this was the method actually employed.
Results obtained from a mid-size test speaker measured in a reverberant space were presented to show that reflections contaminate the measured frequency response if not windowed out. If they are windowed out conventionally, however, the frequency response at low frequencies is inaccurate because the impulse response is truncated prematurely. Whereas if the impulse response is processed using a 5ms rectangular window and Fincham filtering the result is a much more accurate frequency response below 200Hz.
Design of the Fincham filter requires knowledge of the loudspeaker’s bass alignment, which can be obtained either from analysis of its impedance versus frequency behaviour or from a near-field acoustic measurement. Accuracy of the frequency response obtained from the processed impulse response is not too dependent on the alignment parameters used..
John explained that the resulting low frequency response has a strong imprint of the model applied but argued that the result is still useful because we have good knowledge of the behaviour of loudspeakers at low frequencies. He also demonstrated that cabinet diffraction does not compromise the method, whereas it does provide difficulties for Prony Method modelling of the impulse response because diffraction cannot be modelled as an exponentially decaying oscillation.
John concluded the lecture by showing that conventionally gated impulse responses have validity at mid and high frequencies, so that obtaining the low frequency response using the method described gives a final measurement result which is in large part free of imperfections caused by room reflections across the entire audible frequency range. John ended the lecture by encouraging all present to try this methodology for themselves.
Report by Matthew Neighbour and Keith Howard
Can we make quasi-anechoic measurements in normal rooms?
John Vanderkooy
Audio Research Group, University of Waterloo, Canada
Steyning Research Establishment, B&W Group Ltd, UK
John Vanderkooy presented research into methods to improve loudspeaker measurements made in non-anechoic rooms. The lecture began with a discussion of the motivation for the research:
- Not everyone has access to an anechoic chamber
- Anechoic chambers may not be effective below 100Hz due to inadequate LF absorption
- Low frequency calibration of anechoic chambers may be ineffective
- Low frequency noise from air conditioning, industry and the environment can easily contaminate the measurements.
Impulse response measurements made in an echoic room or an imperfect anechoic chamber will have reflections that contaminate the results and will also often have significant levels of added noise. John presented measurements from a 110mm driver in a small sealed cabinet to illustrate the algorithm developed to overcome these limitations.
The algorithm comprises the following steps:
1) Measure an impulse response, typically 5–6ms of which is reflection-free following the initial response of the loudspeaker, and obtain the frequency response..
2) Apply a minimum phase filter to the impulse data such that the frequency response becomes flat to DC and, optionally, a high-pass filter with a corner frequency significantly above that of the loudspeaker.
3) Truncate the impulse response such that all room reflections are removed. The resulting frequency response will have high-pass characteristic at a higher corner frequency.
4) Apply an inverse filter to that of step 2.
Now the impulse response has the low frequency persistent decaying oscillation portion extending cleanly beyond the first reflection arrival time.
There are several impulse response windowing methods and filter types that can be used. John explained that a rectangular window introduces ripples into the frequency response, while other types cause data to be lost towards the end of the truncated impulse response.
Methods of shortening the impulse response Filter types discussed were the Backmann method and the Fincham method. The Backmann method of flattening the frequency response to DC causes the impulse response to have a very long but zero-valued tail, making it suitable for truncation. The Fincham method, which raises the apparent corner frequency of the loudspeaker’s LF roll-off, shortens the impulse response, again allowing truncation to be applied without significant loss of data in the tail. As originally described, the Fincham method seemed to applyies the step 2 filter to the test signal, which results (when the inverse filter is applied) in increased contamination of the acoustic measurement by low frequency noise. This can be avoided by applying the step 2 filter to the measured impulse response instead, and apparently this was the method actually employed.
Results obtained from a mid-size test speaker measured in a reverberant space were presented to show that reflections contaminate the measured frequency response if not windowed out. If they are windowed out conventionally, however, the frequency response at low frequencies is inaccurate because the impulse response is truncated prematurely. Whereas if the impulse response is processed using a 5ms rectangular window and Fincham filtering the result is a much more accurate frequency response below 200Hz.
Design of the Fincham filter requires knowledge of the loudspeaker’s bass alignment, which can be obtained either from analysis of its impedance versus frequency behaviour or from a near-field acoustic measurement. Accuracy of the frequency response obtained from the processed impulse response is not too dependent on the alignment parameters used.being known within tight tolerances.
John explained that the resulting low frequency response has a strong imprint of the model applied but argued that the result is still useful because we have good knowledge of the behaviour of loudspeakers at low frequencies. He also demonstrated that cabinet diffraction does not compromise the method, whereas it does provide difficulties for Prony Method modelling of the impulse response because diffraction cannot be modelled as an exponentially decaying oscillation.
John concluded the lecture by showing that conventionally gated impulse responses have validity at mid and high frequencies, so that obtaining the low frequency response using the method described gives a final measurement result which is in large part free of imperfections caused by room reflections across the entire audible frequency range. John ended the lecture by encouraging all present to try this methodology for themselves.
Report by Matthew Neighbour and Keith Howard
