Problem: Subwoofers with top sound quality Gerd Lommersum interviewed by Peter Strassacker (10/2004) Peter: Gerd, what's the secret of building a good sounding subwoofer?
Gerd: That depends on whether you want to build a hi-fi or home cinema subwoofer. A home cinema subwoofer should reproduce as low frequencies as possible including structure-borne noise. A large subwoofer with a high excursion (long throw) helps, since high volume, low frequencies require a high effective mechanical mass including air load, i.e. diaphragm size x excursion.
Peter: What about a hi-fi subwoofer? Many customers complain about the boom of long throw drivers with soft suspension.
Gerd: Well, a hi-fi subwoofer should only go down to 40 at the most 30 Hz. Otherwise the sub sounds loose and spongy. Wide surrounds and high excursion are also not helpful. Listening rooms of even 50 m2 having hard walls are not suited for 20 Hz. Large, special rooms with an acoustic open end are required.
Peter: Why are the requirements here so different compared to home cinema?
Gerd: In normal living rooms 20 Hz frequencies lead to an acoustic phenomenon that is perceived as boominess. The fact alone that a loudspeaker is designed to go down to 20 Hz, therefore showing a resonance at around this frequency leads to a dramatically deteriorated sound when 20 Hz frequencies are triggered by other frequencies.
Peter: What about the radiating area of a subwoofer driver?
Gerd: The larger the diaphragm the less the required diaphragm excursion at a given input. The smaller the excursion the easier it gets for the driver and the smaller the distortion. When 2 drivers are used the efficiency increases; for the same volume only half of the power output is needed.
The additional radiating area is mainly a standby, otherwise the driver could also be smaller.
Peter: Is a 20 cm driver sufficient for low volume in a 20 m2 listening room?
Gerd: Yes, that's absolutely sufficient.
Peter: Thank you very much for the discussion.
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