Revenue and geographic information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Mar. 30, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 210,173	$ 186,720	$ 706,544	$ 655,670
Wireless speakers
Disaggregation of Revenue [Line Items]
Revenue	71,499	85,933	297,368	359,318
Home theater speakers
Disaggregation of Revenue [Line Items]
Revenue	89,006	68,790	299,243	217,220
Components
Disaggregation of Revenue [Line Items]
Revenue	38,499	28,977	89,405	71,247
Other
Disaggregation of Revenue [Line Items]
Revenue	11,169	3,020	20,528	7,885
Americas
Disaggregation of Revenue [Line Items]
Revenue	116,344	92,985	375,071	324,723
Europe, Middle East and Africa (“EMEA”)
Disaggregation of Revenue [Line Items]
Revenue	79,307	83,941	295,387	299,205
Asia Pacific (“APAC”)
Disaggregation of Revenue [Line Items]
Revenue	14,522	9,794	36,086	31,742
United States
Disaggregation of Revenue [Line Items]
Revenue	109,852	86,120	346,964	296,143
Germany
Disaggregation of Revenue [Line Items]
Revenue	20,085	19,517	76,140	77,208
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	15,729	16,227	70,226	70,490
Other countries
Disaggregation of Revenue [Line Items]
Revenue	$ 64,507	$ 64,856	$ 213,214	$ 211,829