The Filtered IR graph allow octave and one-third octave filters to be applied to the measurement. It is primarily aimed at examining decay behaviour in different frequency bands and analysing the results per ISO 3382. In addition to the filtered impulse response itself this graph includes traces of the impulse response envelope (ETC) and the Schroeder integral.
Octave and One-Third Octave Filters
Octave and 1/3 octave filters can be selected from the box in the lower left corner of the graph. The selected filter is applied to the Impulse Response upon selection. The filter remains active until "No Filter" is selected. The measurement name on all graphs is shown with an indication of the applied filter, for example "Auditorium [250Hz 1/3]".
The Schroeder Integral is a curve obtained by backwards integration of the squared impulse response, ideally starting from a point where the response falls into the noise and applying a correction (a starting value for the integral) which assumes the rate at which the Schroeder curve is falling continues for the whole response. REW uses an iterative procedure to estimate the best starting point for the integration, often called "Lundeby's Method" (from the paper by A. Lundeby, T. E. Vigran, H. Bietz, and M. Vorländer, “Uncertainties of Measurements in Room Acoustics,” Acustica, vol. 81, pp. 344–355 (1995)). The slope of this curve is used to measure how fast the impulse response is decaying, deriving a figure for "RT60" which is the time it would take sound to decay by 60dB. The curve shown on the Impulse graph is for the currently applied filter, if any. When calculating decay data for the octave and one-third octave RT60 results the impulse is first filtered to the corresponding bandwidth and centre frequency before the Schroeder Integral for that band is determined and the various RT60 measures calculated.
Filtered IR Controls
The control panel for the Filtered IR graph has these controls:
The impulse response may be plotted with or without normalisation to its peak value according to the setting of the Plot Responses Normalised control. When normalised plotting is selected the peak will be at 100% or 0dBFS.
The Time Reversed Filtering control applies the octave band filters backwards in time, this reduces the filter's own contribution to the measured decay. When using 1/3 octave filters at low frequencies the filter decay time can be significant, over 200ms for a 100Hz 1/3 filter, for example. Applying the filter in reverse reduces this decay to less than 50ms, but it does affect the response somewhat, such that Early Decay Time (EDT) figures using Time-Reversed filters may not be valid.
Zero Phase Filtering applies 4th order bandpass filters in two passes through the data, one forwards and one reversed, to give a response with overall zero phase shift. This may be useful when comparing the locations of reflection peaks in filtered impulse responses or the ETCs derived from them as the peaks will not be shifted in time by the effect of the filter's phase response.
The Show Data Panel control shows a panel on the graph containing the results for the decay values. The RT60 figures include the decay range over which they have been calculated and an "r" value, the regression coefficient, which measures how well the data corresponds to a straight line. A value of -1 would indicate a perfect fit, values lower in magnitude than -0.98 indicate the corresponding decay figure may not be reliable. Unreliable figures are italicised and shown orange. The parameters available are:
- Early Decay Time, based on the slope of the Schroeder curve between 0dB and -10dB.
- Decay time based on the slope of the Schroeder curve between -5dB and -25dB.
- Decay time based on the slope of the Schroeder curve between -5dB and -35dB.
- An "optimal" decay time based on the slope of the Schroeder curve over a variable range chosen to yield the best linear fit. If the early decay time is much shorter than T30 the Topt measure uses a start point based on the intersection of the EDT and T30 lines, otherwise it uses -5dB. REW tests every end point in 1dB steps to the end of the Schroeder curve and chooses the one which gives the best linear fit.
- (T30/T20 - 1) expressed as a percentage, providing an indication of how the slope of the decay curve is changing. Values from 0 to 5% are typical, higher than 10% is suspicious and may indicate that the room has a two-stage decay curve. If curvature is negative the results should be treated with caution as they may be in error.
The graph can also show the "Regression Line", which is a line obtained by carrying out least squares linear regression on the Schroeder curve over the range applicable to any particular decay parameter. The selector for which regression line is to be shown is next to the Show Regression Line check box.
ETC Smoothing is used to smooth the envelope (ETC) trace using a moving average filter of the duration specified in the spinner.