diff --git a/swash/config.ini b/swash/config.ini
index 08df32a..f815c62 100644
--- a/swash/config.ini
+++ b/swash/config.ini
@@ -18,7 +18,9 @@ mpi=4
 
 [post]
 inp=inp_post
+case=sws_spec_buoy.npz
 out=out_post
+#nperseg=1024
 dt=0.25
 x0=-1000
 t0=180
diff --git a/swash/processing/post.py b/swash/processing/post.py
index ec4977c..8a1ea30 100644
--- a/swash/processing/post.py
+++ b/swash/processing/post.py
@@ -26,7 +26,7 @@ inp = pathlib.Path(config.get("post", "inp"))
 root = pathlib.Path(config.get("swash", "out"))
 
 log.info(f"Reading bathymetry from '{inp}'")
-data = np.load(inp.joinpath("sws.npz"))
+data = np.load(inp.joinpath(config.get("post", "case")))
 x, t = data["x"], data["t"]
 
 # Cospectral calculations
@@ -36,14 +36,15 @@ t0 = config.getfloat("post", "t0")
 arg_t0 = np.abs(t - t0).argmin()
 dt = config.getfloat("post", "dt")
 f = 1 / dt
+nperseg = config.getint("post", "nperseg", fallback=None)
 log.info(f"Computing reflection coefficient at x={x0}")
 
 eta = data["watl"][t > t0, arg_x0]
 u = data["vel"][t > t0, 0, arg_x0]
 
-phi_eta = np.abs(sgl.csd(eta, eta, f))
-phi_u = np.abs(sgl.csd(u, u, f))
-phi_eta_u = np.abs(sgl.csd(eta, u, f))
+phi_eta = np.abs(sgl.welch(eta, f, nperseg=nperseg))
+phi_u = np.abs(sgl.welch(u, f, nperseg=nperseg))
+phi_eta_u = np.abs(sgl.csd(eta, u, f, nperseg=nperseg))
 
 R = np.sqrt(
     (phi_eta[1] + phi_u[1] - 2 * phi_eta_u[1])
@@ -72,16 +73,19 @@ fig_r, ax_r = plt.subplots()
 ax_fft = ax_r.twinx()
 
 ax_fft.plot(
-    fft.rfftfreq(eta.size, dt),
-    np.abs(fft.rfft(eta)),
+    *sgl.welch(eta, 1/dt, nperseg=nperseg),
     lw=1,
     c="k",
     alpha=0.2,
+    label="PSD",
 )
-ax_r.plot(phi_eta[0], R, marker="+")
+ax_r.plot(phi_eta[0], R, marker="+", label="R")
 ax_r.set(xlim=(0, 0.3), ylim=(0, 1), xlabel="f (Hz)", ylabel="R")
-ax_fft.set(ylim=0, ylabel="FFT")
+ax_fft.set(ylim=0, ylabel="PSD (m²/Hz)")
 ax_r.grid()
+ax_r.legend(loc="upper left")
+ax_fft.legend(loc="upper right")
+fig_r.tight_layout()
 
 fig_x, ax_x = plt.subplots()
 ax_x.plot(data["x"], -data["botl"], color="k")