A subwoofer (English pronunciation [ ˈsʌbˌwʊfə (ɹ) ] for bass box , in German mostly [ ˈsʌbˌwuːfɐ ]) is a monophonic loudspeaker box whose construction principle is optimized for the sole reproduction of low-frequency sound waves and usually in combination with specifically adapted speakers - Satellites - which emit frequencies in the medium and higher frequency range, are used.
The separation of bass and treble into separate loudspeaker systems, which is practiced in subwoofer satellite systems , along with the possibility of relatively independent installation of the bass system, is based on the fact that the source of sound at low frequencies below 100 Hz can hardly be localized by the human ear. With regard to the technical implementation, this means that the mono signal of the subwoofer is limited to the higher frequencies by means of active or passive crossovers - upper limit frequency of the subwoofer - and as a result is increasingly covered by the more dominant stereophonic signals from the satellite boxes. Furthermore, there are also various design options with regard to the shape of the housing to generate an upper limit frequency towards the satellites.
Active and passive switching
Basically, there are two operating modes for loudspeakers in general and for subwoofers in particular. In a passive circuit , the frequency band is divided after the power amplifier by means of a crossover equipped with RC and RL filters, consisting of coils , capacitors and resistors . In relation to the application in the field of subwoofer technology, the increased performance requirements that are placed on the components used, along with the resulting larger dimensioning of them, are to be regarded as problematic in this operating mode. For stereo operation with only one subwoofer, a special chassis with a double voice coil may also be necessary. On the other hand , better results can be achieved by means of active circuits which consist of electronic components and are connected upstream of the power amplifier, since it is now possible in a simple manner to divide up the frequency band and, if necessary, to correct it.
Upper and lower limit frequency
Some active subwoofers have the option of setting the upper limit frequency, from which the subwoofer should no longer reproduce frequencies. In practice, this means that, for example, an upper limit frequency, which can be switched in three stages - 60 Hz, 120 Hz and 180 Hz - results in the audio signals above these set frequencies to an increasing extent, depending on the slope of the filter used , are attenuated, i.e. less and less audible.
Such a setting option for the upper limit frequency of the subwoofer does not generally affect the frequency reproduction of the satellite systems connected to the subwoofer, for example, with their mid-range and tweeter speakers, which ultimately means that the satellite systems radiate into the lower frequency range of the subwoofer.
If, on the other hand, an active low-pass and high-pass filter for the subwoofer and the satellites is installed in the loudspeaker boxes - an active external crossover can be used in PA systems - the satellite loudspeakers can, for example, have a cutoff frequency of 80 Hz, which means that they no longer reproduce frequencies, which are below 80 Hz, while the subwoofer no longer reproduces frequencies above 80 Hz. Both components - subwoofer and satellite - each experience a separate active frequency separation by means of an upper and lower limit frequency.
In contrast, these setting options do not apply to passive subwoofers due to the fixed and unchangeable dimensions of the components used. This means that the upper limit frequency of the subwoofer, usually matched to the frequency response of the satellites, cannot, or can only be adjusted with difficulty, to the local conditions and the personal preferences of the audience. The same applies to loudspeaker systems that have passive frequency separation by means of an upper and lower cut-off frequency - separate for subwoofers and satellites.
Active subwoofer amplifier module
One of the most widespread systems for operating subwoofers in the hi-fi and home entertainment sectors is that of an active amplifier module for direct installation in the subwoofer itself. In terms of functionality, such a module works classically with an active component that influences the frequency is connected upstream of the power amplifier. Depending on the equipment of the module, this results in a multitude of options for individual sound control of the subwoofer, the most important of which are described below.
- The volume controller takes on the task of adapting the input signal to the individual needs of the audience with regard to the output power of the module and the resulting volume. The signal coming from a preamplifier can be transmitted asymmetrically using cinch connections and symmetrically using XLR connections . Furthermore, with many active modules there is the option of signal transmission using the already amplified output signals of a power amplifier, which are converted within the module according to its electronic specifications.
- The frequency controller takes on the task of determining the upper limit frequency of the subwoofer, from which the amplifier module lowers the frequency with a certain slope. Depending on the design of the module, this can be fixed or freely selectable in steps or infinitely. Most active amplifier modules have a slope of 12 dB (decibel) and 18 dB, special modules 24 dB per octave.
- The phase controller takes on the task of ensuring that the sound waves emitted at the listening position - if the subwoofer and satellites are at different distances from it - do not result in any annoying cancellation or addition of the sound waves generated . The phase regulator is also used to compensate for "holes and bumps" in the frequency response that arise when subwoofers and satellites emit sound in the same overlapping frequency range.
Principles and variations
Depending on the application scenario, a wide variety of subwoofers are used, the physical principles of which and the resulting acoustic properties are based on the elementary construction principles of loudspeaker technology. The most important construction classes and their modes of operation, such as those of closed, bass reflex and bandpass systems, are described in more detail below. In addition, there are horn systems that are used in PA technology.
|Bass reflex system|
|simply ventilated bandpass system|
|double ventilated band pass system|
further information, see loudspeaker housing
- Frontfire and downfire subwoofers
- Frontfire and downfire subwoofers represent a variation in the design of the closed system construction principle , whereby the principle of the bass reflex system is used as a supplementary element to expand the deep bass . Basically, only the main direction of sound radiation changes in the individual systems. In the first case, the sound is radiated forward in the traditional direction, in the second case the sound is radiated towards the ground, accompanied by a phase shift to the main signal of the satellites aligned in the listening direction.
With regard to the place of installation and the resulting sound properties of a subwoofer, statements can only be made within a narrowly limited framework due to the diverse influences, such as the size and division of the room and its furnishings - subject area room modes . The possible installation locations described below can therefore only be viewed as a rough guide for the positioning of subwoofers. In practice, an experimental procedure for determining the optimal location or measuring using the measuring devices provided for this purpose is the most practical and preferable.
- Location with a subwoofer
- Locations with two subwoofers
- Locations with four subwoofers
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- Götz Schwamkrug, Reiner E. Römer: Loudspeaker Poetry and Truth. 3rd edition, Elektor-Verlag, Aachen 1989, ISBN 3-921608-83-X
- Berndt Stark: Loudspeaker manual. 7th edition, Richard Pflaum Verlag GmbH & Co. KG, Munich 1999, ISBN 3-7905-0807-1
- Michael Dickreiter: Handbook of the recording studio technology. 8th edition, Walter de Gruyter GmbH, Berlin 2014, ISBN 978-3-11028-978-7
- Thomas Görne: Sound engineering. 4th edition, Carl Hanser Verlag GmbH & Co. KG, Munich 2014, ISBN 978-3-44643-964-1
- G. Schwankrug, R. Römer: Speakers Poetry and Truth, Elektor Verlag, Achen, ISBN 3-921608-45-7 (PDF download of the book)