diff --git a/test/torchaudio_unittest/prototype/functional/dsp_utils.py b/test/torchaudio_unittest/prototype/functional/dsp_utils.py
index fb0300a9d6..44c0cac3c3 100644
--- a/test/torchaudio_unittest/prototype/functional/dsp_utils.py
+++ b/test/torchaudio_unittest/prototype/functional/dsp_utils.py
@@ -1,4 +1,5 @@
 import numpy as np
+import numpy.typing as npt
 
 
 def oscillator_bank(
@@ -43,8 +44,8 @@ def freq_ir(magnitudes):
 
 
 def exp_sigmoid(
-    input: np.ndarray, exponent: float = 10.0, max_value: float = 2.0, threshold: float = 1e-7
-) -> np.ndarray:
+    input: npt.NDArray, exponent: float = 10.0, max_value: float = 2.0, threshold: float = 1e-7
+) -> npt.NDArray:
     """Exponential Sigmoid pointwise nonlinearity (Numpy version).
     Implements the equation:
     ``max_value`` * sigmoid(``input``) ** (log(``exponent``)) + ``threshold``