diff --git a/synth/config_test.go b/synth/config_test.go
new file mode 100644
index 0000000..9c2cffe
--- /dev/null
+++ b/synth/config_test.go
@@ -0,0 +1,50 @@
+package synth_test
+
+import (
+	"testing"
+
+	"github.com/netsynth/netsynth/classify"
+	"github.com/netsynth/netsynth/synth"
+)
+
+func TestAllClassesHaveConfig(t *testing.T) {
+	for _, class := range classify.AllClasses() {
+		if _, ok := synth.ClassFreqConfigs[class]; !ok {
+			t.Errorf("class %q has no entry in ClassFreqConfigs", class)
+		}
+	}
+}
+
+func TestFrequenciesInRange(t *testing.T) {
+	for class, cfg := range synth.ClassFreqConfigs {
+		if cfg.BaseHz < 60 || cfg.BaseHz > 800 {
+			t.Errorf("class %q BaseHz=%.1f is out of range [60, 800]", class, cfg.BaseHz)
+		}
+	}
+}
+
+func TestFrequenciesUnique(t *testing.T) {
+	seen := make(map[float64]classify.TrafficClass)
+	for class, cfg := range synth.ClassFreqConfigs {
+		if prev, exists := seen[cfg.BaseHz]; exists {
+			t.Errorf("classes %q and %q share the same BaseHz=%.1f", prev, class, cfg.BaseHz)
+		}
+		seen[cfg.BaseHz] = class
+	}
+}
+
+func TestHarmonicsNonEmpty(t *testing.T) {
+	for class, cfg := range synth.ClassFreqConfigs {
+		if len(cfg.Harmonics) < 2 {
+			t.Errorf("class %q has fewer than 2 harmonics (got %d)", class, len(cfg.Harmonics))
+		}
+	}
+}
+
+func TestPanPositionsInRange(t *testing.T) {
+	for class, cfg := range synth.ClassFreqConfigs {
+		if cfg.Pan < -1.0 || cfg.Pan > 1.0 {
+			t.Errorf("class %q Pan=%.2f is out of range [-1.0, 1.0]", class, cfg.Pan)
+		}
+	}
+}
diff --git a/synth/layer_test.go b/synth/layer_test.go
new file mode 100644
index 0000000..21bffd9
--- /dev/null
+++ b/synth/layer_test.go
@@ -0,0 +1,88 @@
+package synth_test
+
+import (
+	"testing"
+
+	"github.com/netsynth/netsynth/synth"
+)
+
+func TestEMAAmplitudeRise(t *testing.T) {
+	cfg2 := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}},
+		Pan:       0.0,
+	}
+	layer := synth.NewLayer(cfg2, synth.SampleRate, 1.0)
+
+	// Force the layer to have been seen and set target to 1.0
+	layer.UpdateTarget(100, 100)
+
+	// After 1 second (SampleRate samples) with tau=1.0, currentAmp should be > 0.5
+	// EMA: after tau seconds, amplitude reaches ~63% of target
+	for i := 0; i < synth.SampleRate; i++ {
+		layer.AdvanceSample()
+	}
+	if layer.CurrentAmp() <= 0.5 {
+		t.Errorf("expected currentAmp > 0.5 after 1 second rise, got %.4f", layer.CurrentAmp())
+	}
+}
+
+func TestEMAAmplitudeDecay(t *testing.T) {
+	cfg := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}},
+		Pan:       0.0,
+	}
+	layer := synth.NewLayer(cfg, synth.SampleRate, 1.0)
+
+	// Set up: layer has been seen (currentAmp starts at 1.0) and target is whisper floor
+	// We'll manually prime by updating target with 100/100 first then re-route to whisper
+	layer.UpdateTarget(100, 100) // mark as seen, target=1.0
+	// Force currentAmp to 1.0 by running a few cycles at target 1.0
+	for i := 0; i < synth.SampleRate*3; i++ {
+		layer.AdvanceSample()
+	}
+	// Now decay: set count=0 (whisper floor kicks in)
+	layer.UpdateTarget(0, 100)
+	// After 1 second, currentAmp should be < 0.5
+	for i := 0; i < synth.SampleRate; i++ {
+		layer.AdvanceSample()
+	}
+	if layer.CurrentAmp() >= 0.5 {
+		t.Errorf("expected currentAmp < 0.5 after 1 second decay, got %.4f", layer.CurrentAmp())
+	}
+}
+
+func TestWhisperFloor(t *testing.T) {
+	cfg := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}},
+		Pan:       0.0,
+	}
+	layer := synth.NewLayer(cfg, synth.SampleRate, 1.0)
+
+	// Mark layer as seen by passing count=1, then set count=0
+	layer.UpdateTarget(1, 100)
+	layer.UpdateTarget(0, 100)
+
+	// Target should be whisper floor (not zero) because seen=true
+	if layer.TargetAmp() < synth.WhisperFloor {
+		t.Errorf("expected targetAmp >= WhisperFloor (%.2f) for seen layer at zero count, got %.4f",
+			synth.WhisperFloor, layer.TargetAmp())
+	}
+}
+
+func TestWhisperFloorNotSeenIsZero(t *testing.T) {
+	cfg := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}},
+		Pan:       0.0,
+	}
+	layer := synth.NewLayer(cfg, synth.SampleRate, 1.0)
+
+	// Never seen — UpdateTarget with zero count
+	layer.UpdateTarget(0, 100)
+	if layer.TargetAmp() != 0.0 {
+		t.Errorf("expected targetAmp == 0.0 for unseen layer, got %.4f", layer.TargetAmp())
+	}
+}
diff --git a/synth/oscillator_test.go b/synth/oscillator_test.go
new file mode 100644
index 0000000..7708013
--- /dev/null
+++ b/synth/oscillator_test.go
@@ -0,0 +1,59 @@
+package synth_test
+
+import (
+	"testing"
+
+	"github.com/netsynth/netsynth/synth"
+)
+
+func TestOscillatorAdvance(t *testing.T) {
+	harmonics := []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}, {Ratio: 2, Amplitude: 0.5}}
+	osc := synth.NewOscillator(440.0, synth.SampleRate)
+
+	hasPos := false
+	hasNeg := false
+	for i := 0; i < 100; i++ {
+		s := osc.Advance(harmonics)
+		if s > 0 {
+			hasPos = true
+		}
+		if s < 0 {
+			hasNeg = true
+		}
+	}
+	if !hasPos || !hasNeg {
+		t.Errorf("expected both positive and negative samples; hasPos=%v hasNeg=%v", hasPos, hasNeg)
+	}
+}
+
+func TestOscillatorPhaseWrap(t *testing.T) {
+	harmonics := []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}}
+	osc := synth.NewOscillator(440.0, synth.SampleRate)
+
+	for i := 0; i < synth.SampleRate; i++ {
+		osc.Advance(harmonics)
+	}
+	p := osc.Phase()
+	if p < 0 || p >= 1.0 {
+		t.Errorf("expected phase in [0, 1), got %.6f", p)
+	}
+}
+
+func TestOscillatorDistinctFreqs(t *testing.T) {
+	harmonics := []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}}
+	osc65 := synth.NewOscillator(65.0, synth.SampleRate)
+	osc440 := synth.NewOscillator(440.0, synth.SampleRate)
+
+	same := true
+	for i := 0; i < 100; i++ {
+		s65 := osc65.Advance(harmonics)
+		s440 := osc440.Advance(harmonics)
+		if s65 != s440 {
+			same = false
+			break
+		}
+	}
+	if same {
+		t.Error("expected oscillators at 65 Hz and 440 Hz to produce different sample sequences")
+	}
+}