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Mixtape 5 is a concert venue in the centre of Sofia, Bulgaria. It started in 2011 as a nightclub but rapidly diverted to become a music venue. Now it is considered one of the best places for contemporary amplified music and stage for international events. Before its acoustic refurbishment in 2015, the venue had an extended reverberation tail in the lowest octaves and was subjectively perceived as boomy sounding hence it lacked resolution. The audience in front of the stage were suffering from an absence of low-frequency energy while overwhelmed in other areas. Acoustic Waves was addressed to solve the issue. 



Several reverberation measurements were performed at different points using the venue's PA. The acoustic environment was analysed, and the result showed substantially different reverberation times in the 50 - 200Hz one-third octave bands at various positions. The reverberation time was over two seconds at some positions, too long for a venue with a volume of 1700 cubic meters. The average reverberation time in the mid octave bands was below 0.5 seconds, while the high bands reached 0.9 seconds. The unbalance was due to excessive mid frequency absorption of medium density mineral wool placed on the ceiling and walls of the venue. There was no low-frequency treatment – the tightly packed audience provides some low-frequency absorption, but far off from being sufficient to balance the reverberation time. A large part of the mineral wool surrounded the stage dampening most early reflections making it uncomfortable for the artists to perform on it. Another problem of the venue due to the low ceiling height and presence of terrace levels was the difficulty to create proper PA coverage. This generated strong differences between direct and reflected sound with some spots with either too high either too little direct sound. Given these circumstances, it was agreed to target for a lower reverberation of 0.5 seconds. A more suitable reverberation time that would suit the needs of contemporary amplified music. 



After considerable statistical calculations and ray tracing simulations, it has been concluded that in order to balance the time domain and reach the target reverberation time the venue needed to cover 377 square meters of its surface with tuned low / high resonant absorbers. Most of the acoustic treatment was installed on the ceiling to reduce the modal ringing of resonances due to the height of the room. Even though the venue’s medium size, the ceiling supported strong axial resonance that increased the reverberation in the lowest octave bands. The rest of the treatment was installed in other strategic places and behind the main PA. The treatment behind the main PA also improved and diffuse early reflections to the musicians, creating a bit more liveness on stage.

photo credits: Creative Visual Solutions






The results in the graphs show the average values of 8 measurements made in the venue for T30, EDT and D50, expressed respectively in third-octave bands and octave bands. Red shows the venue’s parameter before the renovation, blue after the acoustic treatment completion. The dotted red indicates the maximum reverberation time measured before the treatment. The targets were achieved. The time domain and the rate of direct to reverberant sound in the venue are now spectrally balanced, yielding better resolution and control. 



Room Modes: Are series of modal resonances that are formed in an enclosed space when excited by a sound source. The resonances are related to the geometry of the room and affect certain frequencies whose wavelengths fit between room boundaries. They are responsible for an uneven distribution of sound energy in a room. 


Reverberation: It is the natural decay of the sound in an enclosed space. It is formed by repeated reflections at the room boundaries. The reverberation time depends on the size of the room and the acoustic properties of the walls. 


T30: It measures the rate at which the reverberation decays to inaudibility. It is defined as the time it takes for the sound to decrease with 60dB. What is usually being measured is the decay of the first 30 dB after the sound source has stopped. The result is then multiplied by 2.


EDT: It is just another way of measuring 60dB reverberation decrease in intensity. It takes into account the first 10dB after the sound source has stopped. The result is then multiplied by 6. EDT correlates better than T30 with the human's perception of reverberance and it describes the liveness of the room.


D50: It is a ratio that compares early to late reflections for a given position.  Reflections arriving at the first 50ms are considered early whereas the ones that arrive latter are defined as late.  The early reflections have an influence on humans impression of clarity for rhythmic sounds.


... i can't believe how nice it sounds in this room. I feel like I'm in a studio. This place is absolutely perfect!

Brandy Whynne, Bass at Ozric Tentacles 

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