Add --cut-slope option for filter steepness (default 12 dB/oct); cascade biquads for steeper slopes; update docs

README.md

@@ -7,7 +7,7 @@ A CLI tool for processing WAV files to generate impulse responses (IR) from swee
 - **Fast FFT-based deconvolution** for accurate IR extraction
 - **Automatic input conversion:** Accepts any WAV sample rate, bit depth, or channel count
 - **Optional output IR length:** Specify output IR length in milliseconds with --length-ms
-- **Optional low-cut and high-cut filtering:** Apply 2nd-order Butterworth filters to the recorded sweep before IR extraction (--lowcut, --highcut)
+- **Optional low-cut and high-cut filtering:** Apply Butterworth filters to the recorded sweep before IR extraction (--lowcut, --highcut, --cut-slope)
 - **Automatic fade-out:** Linear fade-out at the end of the IR to avoid clicks (default 5 ms, configurable with --fade-ms)
 - **96kHz 24-bit WAV file support** for high-quality audio processing
 - **Multiple output formats** with configurable sample rates and bit depths
@@ -79,6 +79,14 @@ You can apply a low-cut (high-pass) and/or high-cut (low-pass) filter to the rec
 
 This applies a 40 Hz low-cut (high-pass) and 18 kHz high-cut (low-pass) filter to the recorded sweep.
 
+You can control the filter steepness (slope) with `--cut-slope` (in dB/octave, default 12). For example:
+
+```sh
+./valhallir-deconvolver --sweep sweep.wav --recorded recorded.wav --output ir.wav --lowcut 40 --highcut 18000 --cut-slope 24
+```
+
+This applies a 40 Hz low-cut and 18 kHz high-cut, both with a 24 dB/octave slope (steeper than the default 12).
+
 ### Different Output Formats
 
 Generate IRs in different sample rates and bit depths:
@@ -141,6 +149,7 @@ Generate IRs in different sample rates and bit depths:
 | `--fade-ms` | Fade-out duration in milliseconds at end of IR (default 5) | 5 | No |
 | `--lowcut` | Low-cut filter (high-pass) cutoff frequency in Hz (recorded sweep) | - | No |
 | `--highcut` | High-cut filter (low-pass) cutoff frequency in Hz (recorded sweep) | - | No |
+| `--cut-slope` | Filter slope in dB/octave (12, 24, 36, ...; default 12) | 12 | No |
 
 ## File Requirements
 
@@ -173,8 +182,9 @@ Generate IRs in different sample rates and bit depths:
 - You can change the fade duration with `--fade-ms`
 
 ### Filtering
-- You can apply a 2nd-order Butterworth low-cut (high-pass) and/or high-cut (low-pass) filter to the recorded sweep before IR extraction
+- You can apply a Butterworth low-cut (high-pass) and/or high-cut (low-pass) filter to the recorded sweep before IR extraction
 - Use `--lowcut` and/or `--highcut` to specify cutoff frequencies in Hz
+- Use `--cut-slope` to control the filter steepness (12 dB/octave = gentle, 24+ = steeper)
 
 ### Deconvolution Process
 1. **FFT-based deconvolution** of recorded signal by sweep signal