Desktop software · Windows · Free for study

ARAC: room acoustics calculator for auditoriums

ARAC (Architectural Room Acoustic Calculator) was programmed by the Department of Environmental Architecture, Hanoi University of Civil Engineering, together with the Institute of Information Technology in Civil Engineering – RDSIC. It helps you calculate the acoustic design of auditoriums with sound-absorbing and sound-reflecting materials.

6 Calculation tables

125–4000 Hz, 6 octave bands

6 Room functions

1.5.0 Version

Download ARAC 1.5.0 free Read the user guide

What ARAC does

From plan and section drawings to the room's reverberation time

You enter distances measured on the drawings, the room volume, the seating capacity and the area of each finish. ARAC checks the delay and level of reflected sound at the listening points, then calculates reverberation time in six octave bands and compares it with the optimum for that type of room.

Check the delay of sound reflected from the ceiling and both side walls
Calculate reflected and total sound level at each listening point
Calculate required and actual sound absorption
Calculate reverberation time with the Eyring formula and plot it against the ±10% band
Library of absorption coefficients for seats, curtains, carpets, ceilings, walls and absorbers
Save every table as a project file to reopen and edit later

The ARAC window with the list of calculation tables on the left — The ARAC window: calculation tables listed on the left, input and results side by side.

Calculation sequence

Six tables, worked through in order

Each table takes its inputs from the one before, so every value is entered once.

1
Reflection delay

From the direct path r₀ and reflected path r, delay Δt = (r − r₀) / 0.34 ms at each point.
2
Reflected sound level (2a, 2b, 2c)

Reflected level from the ceiling, side wall 1 and side wall 2, with the difference ΔL from direct sound.
3
Total sound level

Energy sum of direct and reflected sound: ΣL = 10·lg Σ10^0.1·Lᵢ.
4
Required absorption

From room function, volume V and optimum reverberation time: A_req = 0.164·V / T_opt in each band.
5
Actual reverberation time

From the area and absorption of each surface, the audience and the air, T by the Eyring formula.
6
Reverberation vs occupancy

Reverberation time at 500 Hz with the room empty, 50%, 75% and fully occupied.

User guide

Using ARAC, step by step

1. Getting to know the window

Click Bảng tính toán (Calculation tables) to pick a table. The list collapses each time you choose one; click the double arrow at its top to open it again.

The toolbar has four buttons:

Thêm dòng (Add row): adds a row when you need more than three data points.
Xóa dòng (Delete row): removes the last row, down to three rows.
Tính toán (Calculate): writes the results. After that the input area is locked.
Sửa số liệu (Edit data): unlocks the input so you can change it, then calculate again.

ARAC toolbar with the Add row, Delete row, Calculate and Edit data buttons — The toolbar is shared by every table.

Click Bảng tra vật liệu (Material table) to look up absorption coefficients at 125, 250, 500, 1000, 2000 and 4000 Hz. Materials are grouped into people and seats, curtains, floor coverings, openings and slots, reflectors and absorbers.

ARAC material table of absorption coefficients in six octave bands — Absorption coefficients for seats, curtains, carpets and more, in six bands.

2. Table 1: reflection delay

Enter the path lengths measured on the plan and section of the auditorium. Press Tab to jump to the next cell.
The r₀ you enter for ceiling reflections is copied to the side-wall tables automatically.
If the side walls are symmetrical, tick Tường đối xứng (Symmetrical walls): side wall 2 takes the results of side wall 1.
After typing in the last cell, click another cell so the value is accepted, then click Tính toán.

ARAC table 1: sound path inputs and reflection delay results — Example: r₀ = 4 m and r₁ = 5 m give a delay of 2.9 ms, shown next to the inputs.

Click Tiếp (Next) at the bottom to move on to table 2a.

3. Tables 2a, 2b, 2c: reflected level from ceiling and side walls

Table 2a covers the ceiling, 2b side wall 1 and 2c side wall 2. You do not enter r₀, r₁, r₂ or r₃ again; they come from table 1.

Click an absorption cell α to open the coefficient table and pick a value, or type the number in.
Enter the direct level of the source L₀ (dB), then click Tính toán.

ARAC table 2a: absorption coefficient alpha and source level inputs — The two inputs: coefficients α and source level L₀.

ARAC table 2a results: reflected sound level and level difference — Results: reflected level L_px, direct level L_r0 and the difference ΔL.

4. Table 3: total sound level at the listening points

Click Tính toán to load the data from tables 1 and 2. ARAC adds direct sound to the ceiling and wall reflections to give the total level ΣL at each point. To give the seats enough sound energy, the total level must exceed background noise by at least 10 dB.

ARAC table 3: total sound level at three listening points — Total level at three listening points: 62.34 dB, 60.71 dB and 59.09 dB.

5. Table 4: inputs for reverberation time

Pick the room function: speech, church music, general music, multi-purpose hall, chamber music or cinema.
Enter capacity (people), volume V (m³), total surface S (m²) and the optimum reverberation time at 500 Hz.
Enter the factor K_f at 125 Hz and 250 Hz, then click Tính toán.

ARAC table 4: list of room functions — Six room functions to choose from.

ARAC table 4 results: required absorption in each octave band — A 456-seat chamber music room, V = 3,089 m³, T_opt = 1.25 s: required absorption 405.28 m² at 500 Hz.

6. Table 5: room surfaces and materials

Click Thêm dòng, then Chọn bề mặt (Choose surface) to pick a material from the library.
For a material that is not listed, choose Khác (Other) and enter its coefficients yourself.
Enter the area S_i of each material, the occupied seats and the empty seats.
Choose the air absorption factor at 2000 Hz and 4000 Hz, then click Tính toán.

ARAC table 5: material library tree opened from Choose surface — The library is split into ceilings, walls, floors, people and seats, doors, openings and slots.

ARAC table 5: entering coefficients for an Other material — Choose Other to type coefficients for all six bands.

ARAC table 5: area of each surface and air absorption — Area of each surface, plus air absorption at 2000 Hz and 4000 Hz.

Once calculated, open the chart to compare actual and optimum reverberation time. If it misses, click Quay lại (Back) to table 5 and change areas, add or remove materials until the calculated T is within 10% of the optimum. Then click Tiếp to go to table 6.

ARAC chart of actual reverberation time against the plus or minus 10% band — The yellow line is actual T; the band around it is the optimum ±10%.

7. Table 6: reverberation vs occupancy

Click Tính toán to get the 500 Hz reverberation time with the room 0%, 50%, 75% and 100% occupied, then open the chart.

ARAC table 6: 500 Hz reverberation time by audience occupancy — Table 6 works out audience absorption for each occupancy level.

ARAC chart of reverberation time by audience occupancy — 2.0 s with the room empty, 1.19 s when full.

Every chart you create is kept under the Đồ thị (Charts) tab.

8. Saving and reopening a project

Click the save icon at the top left, choose Tạo dự án (New project) and give a name and folder to store every table as one project. To continue later, click the same icon, choose Mở tệp (Open) and point to the saved folder. The menu also offers Xuất Excel (Export to Excel).

ARAC New project dialog for saving the calculations — The New project dialog: folder name and location.

Reference

Symbols used in the tables

Symbol	Unit	Meaning
r₀	m	Path length of direct sound
r	m	Path length of reflected sound
L₀	dB	Direct level of the source
L_r0	dB	Level at the listening point from direct sound
L_r	dB	Level at the listening point from reflected sound
ΔL	dB	Difference between reflected and direct level
ΣL	dB	Total level at the listening point
α	–	Absorption coefficient of a material
α_tb	–	Average absorption coefficient
n	–	Air absorption factor
S	m²	Area
A	m²	Sound absorption
A_f,cđ	m²	Fixed absorption at frequency f
A_f,tt	m²	Actual absorption at frequency f
A_f,yc	m²	Required absorption at frequency f
T	s	Reverberation time
T_f,tn	s	Optimum reverberation time at frequency f
T_f,tt	s	Actual reverberation time at frequency f

Download

Download and install ARAC

Version: ARAC 1.5.0
File: ARAC 1.5.0.rar, 14.7 MB, containing an .msi installer
System: Windows; the installer refuses Windows XP and older
Licence: Free for study

Extract the .rar with WinRAR or 7-Zip, run ARAC 1.5.0.msi and follow the installer.

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