# Late Reverberation Synthesis Using Filtered Velvet Noise +
## 7 Subjective Evaluation  
The proposed advanced FVN method was evaluated using a subjective test. Three different concert halls impulse responses were approximated from pre-recorded RIR [26]. One of the RIRs was the Pori concert hall response used in the examples above, which has a reverberation time of 2.3 s at middle frequencies. The second hall was the Cologne Philharmonic, which has a shorter mid-frequency reverb time (1.9 s). Its RIR is quite dry, containing mainly the direct sound, a few reflections, and a relatively short reverberation tail. The third hall was the Vienna Musikverein, which has the longest reverberation time (3.2 s) of the selected halls. Its RIR sounds very reverberant, having a lot of early reflections soon after the direct sound. The beginning of each RIR approximation was taken from the measured RIR. Thus, the early-reflection part of the impulse responses remained the same as the original, and only the tail of the RIR was modified by the basic and advanced FVN approximations. The duration of each early-reflection segment was adjusted manually based on preliminary testing as follows: 110 ms for the Pori Concert Hall, 119 ms for the Cologne Philharmonic, and 52 ms for the Vienna Musikverein. Three different sound files were processed with the three reference (original) RIRs and their approximations produced using the advanced FVN method, which yielded altogether 18 (3 \(\times\) 6) sound files. The three test sounds contained drumming, slowly changing chords played with a synthesizer, and a cappella singing (the first 10 s of \"Tom's Diner\" by [PERSON]). The test type was ABX [27], which refers to a pair-wise test in which the subject always compares three sound files, A, B, and X, and is asked to identify whether sound X is the same as A or B. Additionally, in our test, the subjects had to evaluate the perceived difference between A and B on a five-point scale, a variant of the mean-opinion score. Figure 16 shows the user interface used in the listening test. The 18 test sounds were played in pseudo-random order, and they all appeared twice during the test, leading to 36 cases to be evaluated. Additionally, four extra cases were played in the beginning of the test, the answers of which were deleted from the data, since learning was assumed to occur during the first few cases, and only after this are the persons able to carefully and objectively evaluate the sounds. Thus, the total number of cases presented to the subjects in the listening test was 40. Twelve subjects with no reported hearing problems participated in the listening test. Their age varied between 23 and 41 years. All subjects had previously participated in listening tests. It took typically 30 to 40 min for the subjects to finalize the test. The test can be assumed not to have been too difficult or tiresome. Table 4 summarizes how the test subjects identified the synthetic reverberation from the original for different sound types. Since the subjects were allowed to listen to all sounds several times, detecting even the smallest differences turned out to be easy. Thus, in 86% of all cases, the persons identified the approximated RIR from the original. Detecting the difference in drumming, which contains transients, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Algorithm** & **FLOPs** & **Delay-Line Memory** & **Storage Memory** \\ \hline Direct convolution & 176,401 & **88,200** & 88,200 \\ Partitioned fast convolution & **399** & 176,400 & 88,200 \\ Basic FVN & 627 & 90,442 & 420 \\ Advanced FVN & 453 & **88,200** & **294** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Operation count, fast memory and storage memory consumption of various reverberation algorithms for modeling a 2-s RIR at a 44.1-kHz sample rate. The smallest numbers are in bold. FLOPS: floating-point operations. was the easiest, and the identification score was 99%. Chords were the most difficult case, as the sounds are mostly stationary and the synthetic sounds had a slow attack.