Revenue and geographic information - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 208,398	$ 223,078	$ 864,069	$ 778,431
Wireless speakers
Disaggregation of Revenue [Line Items]
Revenue	93,867	92,797	453,185	389,300
Home theater speakers
Disaggregation of Revenue [Line Items]
Revenue	66,732	83,928	283,952	266,972
Components
Disaggregation of Revenue [Line Items]
Revenue	42,283	44,160	113,530	114,007
Other
Disaggregation of Revenue [Line Items]
Revenue	5,516	2,193	13,402	8,152
Americas
Disaggregation of Revenue [Line Items]
Revenue	122,982	114,707	447,705	388,620
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenue	75,585	95,802	374,790	349,327
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	9,831	12,569	41,574	40,484
United States
Disaggregation of Revenue [Line Items]
Revenue	113,558	106,364	409,701	352,194
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	17,930	20,500	88,421	88,505
Germany
Disaggregation of Revenue [Line Items]
Revenue	18,512	25,973	95,720	86,308
Other countries
Disaggregation of Revenue [Line Items]
Revenue	$ 58,398	$ 70,241	$ 270,227	$ 251,424