reverberation time with low degrees of absorption

[Atriplex82]

Hello,
while trying to simulate a room, I noticed the measured reverberation time differs from the one calculated with Sabine. When setting the coeffs of the walls to 0.5 the reverberation time seems plausible. Why is that?

Here's the code:

import numpy as np
from scipy.io import wavfile
import IPython
import pyroomacoustics as pra

# Define the materials array
ceiling_mat = {
    "coeffs": [0.45, 0.50, 0.5, 0.4, 0.35, 0.35],
    "center_freqs": [125, 250, 500, 1000, 2000, 4000],
}
floor_mat = {
    "coeffs": [0.16, 0.24, 0.56, 0.69, 0.81 , 0.78 ],
    "center_freqs": [125, 250, 500, 1000, 2000, 4000],
}
wall_mat = {
    "coeffs": [0.02, 0.02, 0.03, 0.03, 0.04, 0.06],
    #"coeffs": [0.5, 0.5, 0.5, 0.5, 0.5, 0.5],
    "center_freqs": [125, 250, 500, 1000, 2000, 4000],
}


height = 3.75

corners = np.array([[0,0], [0,8], [7,8], [7,0],]).T  # [x,y]

mat = pra.make_materials(*[(wall_mat,) for i in range(corners.shape[1])])

room = pra.Room.from_corners(
    corners,
    materials=mat,
    max_order=-1,
    ray_tracing=True,
    air_absorption=True,)

me = pra.make_materials(floor=floor_mat, ceiling=ceiling_mat)
room.extrude(height, materials=me)

# Set the ray tracing parameters
room.set_ray_tracing(time_thres=10,)

# specify signal source
fs, signal = wavfile.read("german_speech_8000.wav")

# add source and set the signal to WAV file content
room.add_source([1., 1., 0.5], signal=signal)

# add microphone 
room.add_microphone([6.0, 6.0, 0.5])

room.simulate()

rt60_tgt = room.rt60_theory()
room_dim = [8, 7, 3.75]
e_absorption, max_order = pra.inverse_sabine(rt60_tgt, room_dim)
print("absorption_mean:",np.mean([ceiling_mat["coeffs"],floor_mat["coeffs"],wall_mat["coeffs"]]))
print("absorption_theory:",e_absorption)
print("max_order_ism:",max_order)
print("RT60_theory:",rt60_tgt)
print("RT60_measure:",room.measure_rt60()[0, 0] )

,thanks

[fakufaku]

Hi @Atriplex82 , sorry for the late reply! Yes, I have also noticed discrepancy between Sabine's formula and measured RT60. I think this may come from deviation between the hypothesis used to derive Sabine's formula and actual simulation.

[Atriplex82]

Could it be that, it's beacause Sabine's formula needs a diffuse sound field? By setting medium sized scattering coefficients (I hope I did that correctly) the reverberation time decreases significantly.