Early reflections calculator
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Is a MS Excel document. Calculates side, back and back->side reflection delays, amplifications and instrument tracker panning position. Refl. amplitude calculation includes a material absorption coefficient (frequency independent) and natural absorption by air. Ver. 1.3 has graphical presentation, better parameter description and sorting of results.

download er calculator document

Problems of ER simulation:

If we had to precisely follow the physical model, it would be necessary to calculate also an interaural phase shift, angle of incidence and head shading (lowpass filtering) for each reflection. As practical implementation seems to be impossible with the free DSP effects available today, I recommend to try the following simplified model.

Direct sound head shading amount should be lower than appropriate since multitap delay is using the filtered channel as a source for the reflections (they should stay unfiltered)

Since lateral reflections arrive from wide angles, they can be hard panned to left or right. This solves head shading problem for the cost of loosing interaural phase shift accuracy (trapezoidal ground plan simulations are more accurate in this aspect - their lateral reflections arrive from wider angles).

Rear wall and ceiling reflections head shading and interaural phase shift are omitted because most of them have a narrow angle of incidence.

Step by step example of application in Modplug tracker using freeware plugins:

Step 1 - Take JS PanDelay, place it into FX1 and set the apropriate IPD delay using the slider. If the slider control is too coarse, remember you can add one more digit to the 0.xx number and click the set button to get closer to the desired value. If the instrument is to the left of the centre, you will use the "delay right" slider, of course.

Step 2 (optional) - Place BlueDelay VST into FX2, set LP-Mode to 6dB, Lowpass to 1.5 kHz, Feedback to zero (both sliders), Delay to minimum (both sliders), Mix slider of the channel at the other side than where the instrument is located to zero. Now use the the second Mix slider - values higher than 20 are not recommended since this output will be used by a multitap delay, which should ideally work with unmodified signal.

Step3 - Place BlueMultiTap VST into FX3 and select the number of taps. 3 (left, right, ceiling) is often enough for instruments that do not lie near the rear wall. 4 (left, right, rear, ceiling) for instruments that lie near the rear wall and so should sound that way. 5 (left, right, rear->left, rear->right, ceiling) for instruments that do not lie near the rear wall and finally 6 (left, right, rear, rear->left, rear->right, ceiling) for those that do. Lowpass filter can be switched off but sometimes, it sounds better if LP-Mode is set to 6dB and Lowpass to 5kHz. The Mix sliders should be theoretically set to 50, but I can't help it - it sounds much better at 30 (In software simulation, the reflected and dry signals are practically identical so it's better that way). Once you've selected the number of taps, pan them - left, right, rear->left and rear->right reflections are always panned completely to the sides, rear and ceiling reflertions are panned according to Rr pan and Cr pan. This order allows you to compare results given by different number of taps easily: 1st tap = left reflection, 2nd = right reflection, 3rd = ceiling reflection, 4rd = rear->left reflection, 5th = rear->right reflection, 6th = rear reflection. Now the last part... If you look to the left of the parameter slider in Modplug, there's a roll out menu with unnamed parametres. 86 - 8D are delays, 8E - 95 is volume control. So if your first tap needs 12ms delay with amp 37, you select parameter 86, type 0.012 into the window to the right, click the "set" button, then choose parameter 8E, type 0.37 into the window and click the "set" button again. This way you have to adjust all the remaining taps.

Step4 - Set the FX1 output to FX2, the FX2 output to the FX3 and the FX3 output to the reverb effect (if you use it). That's all.

All used plugins can be found here

Hall parametres copyed from Nagata acoustics