TEH Subi

TEH Subi was Kinopoli's subwoofer, built by about 20 Audiopoli members in a single weekend in April 2008. The goal was to build Otaniemi's biggest subwoofer: the enclosure volume was over 1,500 litres, where ordinary hi-fi speakers typically stay under 50 litres. The subwoofer worked as the platform for the back-row seats in Kinopoli, and you could walk on top of the drivers, because they were covered with armoured glass and lit with controllable RGB LEDs. The Subi served the theatre until 2023, when it was replaced by Subi 4.0.

The project poster, 2008 (in Finnish)

Specifications

• Volume: over 1,500 litres, ported enclosure (tuned to about 10 Hz)
• Drivers: 4 × 18 inch
• Power amplifier: 2,400 W
• Lowest reproduced frequency: 7 Hz (−6 dB)
• Sound pressure level: about 114 dB at 10 Hz (the measurement was uncalibrated, so the real value is probably slightly higher)
• Build time: over 500 person-hours in a single weekend

Building

The idea had been brewing for a long time, and in January 2008 Audiopoli applied for a grant for the project from the Teekkaritoiminnan edistämisrahasto (TTER, a fund for student activities). During the design phase driver alternatives were compared and response curves modelled, until the choice fell on the 18-inch driver with the best value for money. Detailed CAD drawings were made of the structure, with every board modelled in advance. The original plan called for six drivers, but the grant turned out smaller than applied for, so four drivers were bought.

The actual building took place on 18–21 April 2008 in the TKY and HOAS workshops in Otaniemi. About 20 builders took part, and over 500 person-hours were put in over the weekend. The first sounds were heard shortly after three o'clock in the night between Sunday and Monday. The subwoofer was officially opened on 25 April 2008.

Kinopoli finally had sound reproduction covering the whole listening area, one that is distinctively student-engineer, showy and memorable. The Hifimaailma magazine wrote a feature on the project, and the blog kept during the build attracted about 16,000 downloads.

Measurements

The subwoofer's response was measured on 22 September 2008 with the REW software at three seating positions. The calibration of the measurement equipment was incomplete, so the decibel values are indicative. The results:

• The response varies a lot between seats. With such a small sample, the response was at its best close to the right-hand wall.
• The peak around 8 Hz is a good thing for movies, but the broad peak around 30 Hz is audible with some material.
• The peak was reduced with an equaliser: the filters at 32 Hz and 40 Hz gave at most 8 dB of attenuation. Based on the response, it would be good to set the crossover frequency to 60 Hz, to avoid the irregularities at higher frequencies.
• The port tuning frequency was measured from the minimum of the driver's motion: the sound pressure minimum was at 11 Hz, so the real tuning is about 10 Hz.

The subwoofer's response at three seats. In the curve names the first number is the seat row and the second the seat in the row.

The response at one seat before and after equalisation

The end of the Subi

TEH Subi was dismantled in Kinopoli's summer 2023 renovation after fifteen years of service. It was replaced by Subi 4.0, four sealed subwoofers in the corners of the auditorium.