Art-Acoustic-Creation

AAC

Concepts of acoustics

Noise level dB (A)	Subjective impression	Situation	Conversation	Psychological aggression
0	Absolute silence	Difficult to realize		Faintness
20	Very calm with abnormal silence	Soundproof room, studio of recording, concert hall	With whispered voice
40	Calm, weak noise	Quiet office, residential district without traffic	With low voice
60	Current noises	Conversation aloud, office, mall	Aloud	Physiological incidence and possible physics Tire
80	Strong, bearable noises	Radio, noisy television, strong urban traffic	With strong voice	Physiological incidence and possible physics Tire
100	Noises very strong, painful, limits the bearable one	Horn, train, circular saw, power loom	While shouting	Field of the dangerous noises and traumatisants
120	Bearable noises a very short moment	Pneumatic drill, boiler making	Impossible	Threshold of the pain
140	Destruction of the ear	Engine, testing ground, explosion, rifle shot	Impossible	Threshold of the pain

THE FORMULA OF SABINE
Times of reverberation are calculated by frequency by means of the formula of Sabine:

OF WHICH:
T: time of reverberation in seconds
V: volume of the room (m3)
A: total acoustic absorption (m2 of open window). (An open window brings 100% of acoustic absorption. A material having a value of 1,0 also brings 100% of acoustic absorption.)
A=l' addition of the absorptivity of all surfaces and objects present. The way simplest to determine the quantity of acoustic absorption present is to measure the time of reverberation of the room, without acoustic installations, and to calculate absorption present by means of the formula of Sabine. The value has ceiling is calculated by multiplying the surface of the ceiling in m2 with its acoustic absorption coefficient and is expressed in m2 of open window (f.o.)).

The use of the formula in practice is subjected to some conditions, e.a.:
• the sound field in the room must be diffuse (diffusivity);
• the room must be more or less in the shape of cube;
• the absorbing material must be distributed uniformly in all the room.

In practice these conditions are generally not met. The height is often much smaller than the length and the width. The absorbing material is generally posed against only one surface, the ceiling, and uniformly is thus not distributed, so that the sound field is not diffuse.

The extent to which diffusivity is required determines the noise up to what point is presented to absorbing material and consequently up to what point qualities of acoustic absorption of the ceiling are exploited. In the case of a low diffusivity, p.ex in empty rectangular buildings with a rather high ceiling, the effect of the absorbing ceiling is tiny room of 50%.
In normal situations of offices, this percentage ranges between 70 and 80%.
The percentage per which added acoustic absorption (thus: the value has ceiling) must be corrected is called the factor of diffusivity.
Symbol: δ

To calculate the time of reverberation awaited after application of a ceiling having a very good acoustic absorption, one uses the formula of Sabine as follows:

OF WHICH:

T2	: time of reverberation after treatment (dry)
V	: volume of the room (m2)
A2	: total acoustic absorption after treatment in m2 of open window A2 =A1 + Aajouté
A1	: acoustic absorption before treatment in m2 of open window
Aajouté	: acoustic absorption added in m2 of
Aajouté	=O X αwx δ in m2 fenestrates open
O	=surface treated (m2)
αw	=coefficient of acoustic absorption by
δ	=factor of diffusivity

The time of reverberation is calculated by frequency (bands of octaves 125 Hz with 4.000 Hz).

What means that to obtain short times of reverberation, necessary to the good comprehension of the word, of the very high values of coefficients absorption acoustic are necessary.

For classrooms, halls of sport etc, one multiplies the acoustic absorption coefficient α with 1.000 Hz=0,85 (Rockfon Fibral) by the factor of diffusivity of 0,5, which gives an actual value of 0,43. For offices, this value α of 0,85 is corrected with 0,60-0,68.

Figure 6: first m2 of open window have much more effect over time of reverberation than last m2 of open window.

Figure 6 gives the chart of the relation between the time of reverberation and the total quantity of acoustic absorption. The improvement of the time of reverberation is equal (ΔT1=ΔT2). Acoustic absorption necessary A2 is definitely larger than A1.

Some examples of absorption values

Coefficients of Sabine

* MATERIALS *************************	Unit	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	5000Hz
Slate	m2	0,010	0,010	0,030	0,040	0,050	0,070
Rough concrete	m2	0,010	0,010	0,015	0,020	0,050	0,070
Wood	m2	0,090	0,110	0,100	0,110	0,080	0,080
Varnished wood	m2	0,050	0,040	0,030	0,030	0,030	0,030
Bricks	m2	0,020	0,022	0,030	0,040	0,050	0,070
Painted bricks	m2	0,010	0,010	0,020	0,020	0,020	0,020
Tiling	m2	0,010	0,015	0,020	0,025	0,030	0,040
Coarse rough coat	m2	0,010	0,030	0,040	0,050	0,080	0,170
Stuck plastic flagstones	m2	0,020	0,020	0,040	0,030	0,020	0,020
Smooth cement rendering	m2	0,010	0,010	0,020	0,020	0,020	0,030
Racks	m2	0,270	0,330	0,300	0,250	0,280	0,400
Fenestrate + jealousies	m2	0,050	0,100	0,150	0,250	0,400	0,600
Fenestrate + net curtain	m2	0,050	0,080	0,100	0,180	0,300	0,450
Fibragglo against the wall	m2	0,130	0,110	0,220	0,540	0,850	0,710
Projected fibres of rock	m2	0,120	0,430	0,760	0,880	0,850	0,710
Liege aglomere 10 mm for wall	m2	0,150	0,260	0,220	0,220	0,200	0,200
Natural cork 10 mm	m2	0,040	0,050	0,080	0,180	0,210	0,200
Liege in flagstone lasts for the ground	m2	0,040	0,030	0,050	0,110	0,070	0,020
Linoleum on felt	m2	0,080	0,080	0,090	0,100	0,120	0,120
Marble	m2	0,010	0,010	0,010	0,010	0,010	0,010
Thick fitted carpet	m2	0,120	0,200	0,250	0,450	0,400	0,350
Close-cropped fitted carpet	m2	0,100	0,120	0,150	0,300	0,350	0,320
Fitted carpet on carpet felt	m2	0,140	0,320	0,450	0,450	0,400	0,350
Foam of clay	m2	0,380	0,770	0,870	0,990	0,880	1,240
Paper + painting	m2	0,010	0,015	0,030	0,080	0,170	0,250
Wallpaper	m2	0,010	0,020	0,040	0,100	0,200	0,300
Stuck parquet floor	m2	0,030	0,040	0,080	0,120	0,120	0,170
Parquet floor on damping	m2	0,200	0,150	0,120	0,080	0,100	0,150
Plaster gross	m2	0,040	0,030	0,030	0,040	0,050	0,080
Plaster painted	m2	0,010	0,010	0,020	0,030	0,040	0,050
Carry out of wood traditional	m2	0,100	0,110	0,100	0,090	0,080	1,000
Door planes out of wood	m2	0,120	0,220	0,170	0,090	0,100	0,100
Light curtains (cotton)	m2	0,040	0,050	0,110	0,180	0,300	0,440
Heavy curtains (velvet)	m2	0,100	0,340	0,400	0,520	0,500	0,550
Staff	m2	0,100	0,020	0,030	0,050	0,040	0,050
Surfing. Upholstered	m2	0,150	20	0,400	0,450	0,400	0,450
Carpet long wool	m2	0,150	0,300	0,400	0,500	0,600	0,800
Thick pane	m2	0	0,060	0,040	0,030	0,020	0,020
Glaze normal	m2	0,350	0,250	0,180	0,120	0,070	0,040
* VARIOUS ***************************	Unit	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	5000Hz
Settee	Part	0,800	1,200	0,400	1,600	2,000	2,500
Upholstered chair	Part	0,150	0,200	0,320	0,350	0,350	0,300
Simple chair	Part	0,030	0,040	0,040	0,050	0,050	0,060
Leather easychair	Part	0,350	0,450	0,600	0,700	0,800	0,900
Nobody sat on an armchair	Part	0,200	0,300	0,360	0,440	0,500	0,550
Nobody sat on a chair	Part	0,100	0,150	0,032	0,420	0,550	0,550
Nobody upright	Part	0,150	0,250	0,350	0,045	0,550	0,550
Plant out of pot (large)	Part	0,025	0,050	0,100	0,200	0,300	0,400
Plant out of pot (small)	Part	0,005	0,010	0,020	0,040	0,070	0,100
Count of living room	Part	0,130	0,160	0,150	0,120	0,120	0,160
Count normal	Part	0,220	0,280	0,250	0,200	0,200	0,280
* ACOUSTICAL SLABS ************	Unit	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	5000Hz
Plywood 5 mm has 20 mm of the wall	m2	0,070	0,120	0,280	0,110	0,080	0,080
Plywood 5 mm has 50 mm of the wall	m2	0,470	0,340	0,300	0,110	0,080	0,080
Compressed wood fibres 230 Kg/m2 20 mm	m2	0,150	0,440	0,450	0,440	0,530	0,590
Painted compressed wood fibres	m2	0,150	0,430	0,440	0,400	0,420	0,400
Hard Isorel has 50 mm of the wall	m2	0,320	0,150	0,090	0,090	0,090	0,090
Soft Isorel 12 mm	m2	0,060	0,110	0,330	0,400	0,400	0,430

AAC Rue Walcourt,111 - 1070 Bruxelles +32 (2)521.71.59 - +32 (2)521.71.59 info@art-acoustic-creation.com