Hotto Engineering

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Active Bass Trap

 

The JAVA application (Download version 0.5 / 18.10.2013 / 9 KB / ActiveBassTrap.jar.zip) for Mac OSX and Windows enables a Subwoofer to act as an Active Bass Trap in a single Subwoofer configuration.

 

 

Functional Description:

Within a listening room even very good amplifier and loudspeaker create a frequency response, which varies up to 30 dB caused by room modes (resonances) and comb filter effects.
The most critical area is the low frequency range (20 Hz - 150 Hz). Unfortunately, low frequencies are difficult to tame because passive bass traps have to be very large, which renders them unpractical within a normal living room.
The application of equalizers is limited because they just work on peaks but cannot re-gain the energy lost by destructive interference of standing waves.

The on hand software uses 2 effects to reduce standing waves (modes):
1.) Applying a comb filter suppressing the axial modes of the room length
2.) Absorb the reflected sound by emitting a delayed counter signal to erase the sound wave before it can create a standing wave

Because the system targets the axial modes of one room dimension it is necessary to place the subwoofer at on of the walls limiting that dimension.

As soon as the subwoofer emits the bass signal it comes back at the subwoofer's position after passing two times the room length.
To avoid standing waves at frequencies of n x 343 m/s / (2 x Room Length [m]) it is necessary to generate a destructive (180° phase shifted) signal delayed by 2 x Room Length [m] / 343 m/s.

 

Software Setup:

The Active Bass Trap has been developed by using JAVA. Therefore it is necessary to install the free Java Runtime Environment (JRE) if not already available: http://java.com/en/download/index.jsp

To Apply the Digital Signal Processing (64 Bit Audio Engine) the software takes the audio signal from an audio source and routes it - after processing - to an audio sink.
If you want to use your audio software on the same computer then it is necessary to route the output signal into the input of the Active Bass Trap application.


Audio Routing Software:

1.) Mac OSX -> Soundflower (free software): http://cycling74.com/products/soundflower/
2.) Windows -> VB-Audio Software: http://vb-audio.pagesperso-orange.fr/Cable/


Configuration:

1.) Set the room length of the axial mode to your room length perpendicular to the wall where the subwoofer is located. The subwoofer's position should be directly at the wall, which is counter intuitive for a normal subwoofer setup.
2.) Select the audio input and output. Use audio routing software if necessary.
3.) Start the Digital Audio Processing by clicking the Activate-Button.
4.) The attenuation defines the level of the delayed and 180° phased shifted signal to suppress the standing waves. The default value of 3 dB seems to be a good choice.
5.) Use the Bypass-Switch to do an A/B-Comparison.

 

Measurements of a real Setup:

The following sequence of graphs presents the measurement results at the listening position within our test room without Active Bass Trap and with activated bass mode suppression:

 

The red curve represents the clean signal, whereas the green curve displays the measurement with a running Active Bass Trap.
All axial modes at aproximately 25Hz, 50Hz and 75Hz are reduced. Furthermore, as presented in the waterfall diagram (see below), the decay times are tamed too.
The dip around 50Hz created by the comb filter effect is not really audible.

 

The waterfall diagram and the spectrogram of the clean signal show long decay times for the room resonances (modes) around 25HZ, 50Hz and 75Hz as well as an uneven distribution.

 

The waterfall diagram and the spectrogram of the processed signal by applying the Active Bass Trap show good decay times for the room resonances (modes) around 25HZ, 50Hz and 75Hz as well as a more even distribution.

 

The solution cannot create a perfectly even frequency response and decay times like in a studio recording room but it improves the worst room resonances (modes) caused by standing waves enough to get a much better reproduction of bass notes to avoid the single note effect, where a wide range of bass notes just play on the room modes.

Furthermore, the need for huge passive bass traps is at least suppressed ;-)