diff --git a/tests/helpers.py b/tests/helpers.py
index b53ee53a..27d757da 100644
--- a/tests/helpers.py
+++ b/tests/helpers.py
@@ -43,12 +43,14 @@ def run_command(cmd: str) -> tuple[int, str]:
     # Subprocesses are expensive, but convenient, and so may be over-used in
     # the test suite.  Use these lines to get a list of the tests using them:
     if 0:  # pragma: debugging
-        pth = "/tmp/processes.txt"
-        with open(pth, "a", encoding="utf-8") as proctxt:  # type: ignore[unreachable]
+        pth = "/tmp/processes.txt"                      # type: ignore[unreachable]
+        with open(pth, "a", encoding="utf-8") as proctxt:
             print(os.getenv("PYTEST_CURRENT_TEST", "unknown"), file=proctxt, flush=True)

     encoding = os.device_encoding(1) or (
-        locale.getdefaultencoding() if sys.version_info < (3, 10) else locale.getencoding()
+        locale.getpreferredencoding()
+        if sys.version_info < (3, 11)
+        else locale.getencoding()   # type: ignore
     )

     # In some strange cases (PyPy3 in a virtualenv!?) the stdout encoding of
diff --git a/tox.ini b/tox.ini
index e0c5dbee..dbc0f1f9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,7 +28,14 @@ setenv =
     pypy3{,9,10,11}: COVERAGE_TEST_CORES=pytrace
     # If we ever need a stronger way to suppress warnings:
     #PYTHONWARNINGS=ignore:removed in Python 3.14; use ast.Constant:DeprecationWarning
-    PYTHONWARNDEFAULTENCODING=1
+    # We want to know about missing encoding arguments, but we need to silence
+    # some warnings that aren't ours.  We can't silence them in 3.9 because
+    # EncodingWarning doesn't exist yet, and it's hard to suppress them in some
+    # environments and not others.  So by default, don't warn, and will enable
+    # the warning in a handful of environments to catch the problems.
+    PYTHONWARNDEFAULTENCODING=
+    py3{10,11,12,13,14}: PYTHONWARNDEFAULTENCODING=1
+    py3{10,11,12,13,14}: PYTHONWARNINGS=ignore::EncodingWarning:pip._internal.utils.subprocess
     # Disable CPython's color output
     PYTHON_COLORS=0