Guided　wave　structural　health　monitoring　offers　the　prospect　of　continuous　interrogation　of　large　plate-like　structures　with　a　sparse　network　of　permanently　attached　sensors.　Currently,　the　most　common　approach　is　to　monitor　changes　in　the　received　signals　by　subtraction　from　a　reference　signal　obtained　when　the　structure　was　known　to　be　defect-free.　In　this　paper　a　comparison　is　made　between　this　defect-free　subtraction　approach　and　a　technique　in　which　low-frequency　vibration　modulation　of　guided　wave　signals　is　used　to　detect　nonlinear　scatterers.　The　modulation　technique　potentially　overcomes　the　need　for　the　defect-free　reference　measurement　as　the　subtraction　is　now　made　between　different　parts　of　an　externally　applied　low-frequency　vibration.　Linear　defects　were　simulated　by　masses　bonded　onto　a　plate　and　nonlinear　scatterers　were　simulated　by　loading　a　similar　mass　against　the　plate.　The　experimental　results　show　that　the　defect-free　subtraction　technique　performs　well　in　detecting　the　bonded　mass　whereas　the　modulation　technique　is　able　to　discriminate　between　the　bonded　and　loaded　masses.　Furthermore,　because　the　modulation　technique　does　not　require　a　defect-free　reference,　it　is　shown　to　be　relatively　independent　of　temperature　effects,　a　significant　problem　for　reference　based　subtraction　techniques.