A comparison of reverberation times

Room 1 – Staircase

Part A

With a lot of hard surfaces, I expect the reverberation time not to be very short as the sound is rather boomy in this room. About 0.8s.

Part B

Reverberation time 0.44s.

There are also small echoes visible which might explain the boomy sound in the room.

Part C

Calculated Reverberation time: 0.73-0.81s

Cross Check Reverberation time online tool: 1.71 – 2.04s

Part D

	500Hz	1000Hz
Predicted (Part A)	0.8	0.8
Measured (Part B)	0.44	0.44
Sabine equation (Part D)	0.73	0.81
Cross-checked (Part D)	1.71	2.04

The predicted value was a lot higher than the measured part. After the experience of RT60 of 1.2s in the SoundLab in Bern last week, I could hear that it was shorter than that but felt like it was longer than the 0.6s that we also experienced in that room. However the measured value of 0,44s is even less than the 0.6s in the SoundLab, even tough it doesn’t sound as boxed as there. 

A reason, why the measured value was so low could be for example, that the microphone was put in front of tiny plants and I was standing 1m away from it. The plants and myself are very absorbing and big compared to the microphone receiver distance and we basically surrounded it which might explain more absorption than expected. The Sabines Equation hand calculation would support this point. It is very close to the predicted value, even tough a lot of parameters did not enter the equation. There are other absorbing objects in the room and the room does not end on that floor but continues four floors down which might also absorb some of the sound.

It is interesting to see that the Sabine equation predicts more reverberation time on the higher frequencies, which can also be heard. High frequency, shrilly sounds are quite desturbing in that room.

The huge difference between the online tool and the calculated values clearly is reasoned by the fact that the online calculation tool does not take into account furniture. With the absence of cussions, bookshelves an matrasses, the reverberation time is expected to be very high also in reality. However, probably still not as high as the online tool is perdicting. Because the online tool also does not take into account that the staircase is open towards the bottom and that some sound is also absorbed down below as mentioned above.

Room 2 – Bathroom

Part A

The bathroom has a clearly audible reverb and the sound is a bit opera-like. Estimation: RT60 = 1.0s

Part B

Reverberation time 0.52s.

There are also small echoes visible.

Part C

Cross Check Reverberation time online tool: 1.46-1.78s

Part D

	500Hz	1000Hz
Predicted (Part A)	1.00	1.00
Measured (Part B)	0.52	0.52
Sabine equation (Part D)	0.57	1.07
Cross-checked (Part D)	1.46	1.78

Sabine Equation is quite similar to the predicted value again. The reverb is clearly audible when speaking and can be „counted“ to one second. It does also make sense, that the sabine equation mainly is similar to the predicted values in the longer reverb of high frequencies as I do not have sensitive enough ears to distinguish between shorter and longer reverb that run parallel. 

I do not understand so well however, why according to the Sabine Equation low frequencies are absorbed better than high frequencies, as we have learnt that quite thick, porous materials are necessary for the absorption of low frequencies.That the absorption coefficient does not take into account the thickness of a material could be a reason.  

The measured value is with 0.52 similar to the low frequency reverb of the Sabine equation and much shorter than the high frequency reverb of the equation or the predicted value. A main reason for this could be, that the microphone was put on the chair visible in the photos above. The chair almost surrounds the microphone and consists of a material which might be strong in high frequency absorption, so mainly the low frequency reverb might remain visible in the recording.

Again, as in the example of the staircase, the cross-checking value of the internet calculation is much, much higher than all the other values. This might be, again, as it does not take into account  furniture, towels and people present in the room. This makes the prediction extremely abstract to a point where it does almost lose it’s interest.

The reverberation time the room currently has is quite suitable for its use, as people are mainly alone in the bathroom, it does not disturb speech intelligibility. As I like singing in the shower, the elevated reverberation time is even quite nice.