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Where best to place a subwoofer?

Peter Strassacker Dr. Peter Strassacker
involved in loudspeaker design since 1977
Books on material and field research
speakers like Lagrange 98, Laplace XT, Pascal, Sub 10-60

Peter Strassacker
interviewed by Torsten Bätz (12/2004).
 

Torsten:
Peter, where best to place a subwoofer?

Peter:
The closer the subwoofer is to the wall or corner the louder it will play. A subwoofer placed right in a corner is up to 9dB louder the a sub that's standing in the middle of the room (this sub then needs roughly nine times the power coming from the amp). Therefore, it makes sense to place a subwoofer in a corner.

Torsten:
...and what about sound quality?

Peter:
If the sub's cut-off frequency is approx. 80 Hz or lower, then the bass becomes omnidirectional. The sub may be placed anywhere you want.

If singing (bass driver with phase plug or bass reflex tube is too short) or non-linear distortion (harmonics of the base frequency are produced) become audible, then the bass can be located. This may be irritating the listener.

Depending on the quality, the subwoofer should be placed in-between the two front speakers or anywhere else in the room (under the condition that the sub doesn't produce any harmonics). In a high-end system with two subwoofers it's best to place one each close to the two front speakers. To avoid cancelling out of certain frequencies the two subs should have the same distance (+/-1 metre) to the listener.

Here is an example: if the distance to the first sub is 4 metres and to the second 2 meters, then the radiating sound reaches the listener with a skew of 2 metres. Frequencies around 85 Hz, having a wavelength of 340 m/s / 85 Hz = 4 meters, would then have exactly half a phase difference and would, therefore, cancel one another out partly.

This example is certainly not complete, since reflections coming from the walls and standing waves should be considered as well.

Torsten:
Is there a rule where best to place multiple subwoofers?

Peter:
They should best be placed symmetrical to longitudinal axis of the room. Here are some examples taking reflections and room design into consideration:Aufstellung der Subwoofer
Todd Welti from Harman International Industries Inc. showed a presentation with 80 slides, covering his comprehensive, theoretical and practical investigation. He came to the conclusion that configurations B and C, respect. D, E and F are especially advantageous. He believes that more than 4 subwoofers are of no use.

Torsten:
That means that all subwoofers are then close to the wall. What is the role of a rear bass reflex port, when the sub is placed close to the wall?

Peter:
As long as there is a 10 cm distance between the sub and the wall, it doesn't matter whether the bass reflex port is at the rear or the front. The further away the bass reflex port is from the bass driver, the better the added up sound coming from the port and the driver. The phase angle rotation is more favourable due to additional skew. The differences are, however, just marginal.

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