From 36bab27edf48f3abd20c341b349b7ff5f9605f16 Mon Sep 17 00:00:00 2001
From: Tom Donoghue <tdonoghue.research@gmail.com>
Date: Sun, 12 Apr 2026 13:22:09 +0100
Subject: [PATCH 1/3] add knee constant func

---
 specparam/modes/funcs.py | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/specparam/modes/funcs.py b/specparam/modes/funcs.py
index 625a3b4a..fa4b6a75 100644
--- a/specparam/modes/funcs.py
+++ b/specparam/modes/funcs.py
@@ -170,6 +170,31 @@ def double_expo_function(xs, *params):
     return ys
 
 
+def knee_constant_function(xs, *params):
+    """Knee function with a constant, for fitting aperiodic component.
+
+    Parameters
+    ----------
+    xs : 1d array
+        Input x-axis values.
+    *params : float
+        Parameters (offset, exp, knee, constant) that define the function.
+
+    Returns
+    -------
+    ys : 1d array
+        Output values for the fit function.
+    """
+
+    ys = np.zeros_like(xs)
+
+    offset, exp, knee, constant = params
+
+    ys = ys + np.log10(10**offset * (1 / (knee**(exp) + xs**(exp))) + constant)
+
+    return ys
+
+
 def linear_function(xs, *params):
     """Linear fitting function.
 

From d3574e2f693885cb09ad3e072e873b47807422bb Mon Sep 17 00:00:00 2001
From: Tom Donoghue <tdonoghue.research@gmail.com>
Date: Sun, 12 Apr 2026 13:23:07 +0100
Subject: [PATCH 2/3] rewrite offset, for consistency

---
 specparam/modes/funcs.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/specparam/modes/funcs.py b/specparam/modes/funcs.py
index fa4b6a75..1a6b234d 100644
--- a/specparam/modes/funcs.py
+++ b/specparam/modes/funcs.py
@@ -190,7 +190,7 @@ def knee_constant_function(xs, *params):
 
     offset, exp, knee, constant = params
 
-    ys = ys + np.log10(10**offset * (1 / (knee**(exp) + xs**(exp))) + constant)
+    ys = ys + offset - np.log10(1 / (knee**(exp) + xs**(exp)) + constant)
 
     return ys
 

From c88292b9699dd1c8430c84890c60d5d022d15ac2 Mon Sep 17 00:00:00 2001
From: Tom Donoghue <tdonoghue.research@gmail.com>
Date: Sun, 12 Apr 2026 13:42:39 +0100
Subject: [PATCH 3/3] add knee constant mode

---
 specparam/modes/definitions.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/specparam/modes/definitions.py b/specparam/modes/definitions.py
index 7d5e7036..4af316d2 100644
--- a/specparam/modes/definitions.py
+++ b/specparam/modes/definitions.py
@@ -75,6 +75,28 @@
     powers_space='log10',
 )
 
+## AP - Knee with Constant Mode
+
+params_knee_constant = ParamDefinition(OrderedDict({
+    'offset' : 'Offset of the aperiodic component.',
+    'exponent' : 'Exponent of the aperiodic component.',
+    'knee' : 'Knee of the aperiodic component.',
+    'constant' : 'Constant value which the aperiodic component decays to, after the exponent.',
+}))
+
+ap_knee_constant = Mode(
+    name='knee_constant',
+    component='aperiodic',
+    description='Fit a Lorentzian function that decays to a constant.',
+    formula=r'XX',
+    func=knee_constant_function,
+    jacobian=None,
+    params=params_knee_constant,
+    ndim=1,
+    freq_space='linear',
+    powers_space='log10',
+)
+
 
 # Collect available aperiodic modes
 AP_MODES = {