Revenue and geographic information - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 294,160	$ 260,119	$ 210,173	$ 496,371	$ 272,940	$ 208,398	$ 186,720	$ 468,950	$ 1,260,823	$ 1,137,008	$ 992,526
Wireless speakers
Disaggregation of Revenue [Line Items]
Revenue									518,821	546,649	480,977
Home theater speakers
Disaggregation of Revenue [Line Items]
Revenue									489,602	418,416	348,899
Components
Disaggregation of Revenue [Line Items]
Revenue									188,861	150,436	151,965
Other
Disaggregation of Revenue [Line Items]
Revenue									63,539	21,507	10,685
Americas
Disaggregation of Revenue [Line Items]
Revenue									678,224	603,450	496,668
Europe, Middle East and Africa (EMEA)
Disaggregation of Revenue [Line Items]
Revenue									484,785	478,518	442,081
Asia Pacific (APAC)
Disaggregation of Revenue [Line Items]
Revenue									97,814	55,040	53,777
United States
Disaggregation of Revenue [Line Items]
Revenue									630,012	554,896	449,261
Germany
Disaggregation of Revenue [Line Items]
Revenue									124,385	121,546	111,065
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue									112,708	114,790	110,695
Other countries
Disaggregation of Revenue [Line Items]
Revenue									$ 393,718	$ 345,776	$ 321,505