Bricks of dimensions: 16 cm × 4 cm × 4 cm were made, according to the proportions of Table 1 and materials below: a metal brick mold (see Figure 1(a)); a manual concrete press (see Figure 1(b)) with its accessories for compaction; a KERN KB-type electronic scale (range 10,100 g, d = 0.1 g).

The particle size of dry and untreated kambala wood chips (KWC) used in the manufacture of composite bricks ranges from 0.5 to 2 mm [5] [8].

The soil dried at 105˚C and sieved with AFNOR 34 module sieve (diameter: 2 mm) [12] was also used for making the bricks.

Twenty four (24) bricks per percentage of WW were produced according to the following procedure: a sieved clay mass and cement corresponding to the

formulation to be manufactured, was taken, weighed and poured into a tray; then knead with the trowel. Then, a quantity of WW corresponding to the same formulation was taken, weighed, poured into the tank containing the preceding mixture (clay earth + cement). Then knead again until we obtain a homogeneous material. The mixture was gradually moistened with the mixing water, the percentage (22%) of which was obtained by the Proctor Test according to standard NF-P 94-093 [13] ; then kneads again. The molds (Figure 1(a)) were loaded; then worn on the press (Figure 1(b)). The compaction was carried out with a pressure force of 75 KN.

The bricks thus compacted are deposited on the bench of the laboratory (Figure 1(c)) where they are dried up to 28 days at room temperature (25˚C to 27˚C) [9].

Two experimental devices were used for sound measurement: the sound level meter (Figure 2(a)) and the Portable Ultrasonic Non-destructive Digital Indicating Tester (PUNDIT7) ultrasound device (Figure 2(b)). The measurements were taken at the physics laboratory of the “National Teacher Training College”, away from any other sound source that could skew the results.

The length of the bricks was measured using a vernier calliper at 1/50th (0.02 mm accuracy). A Tesla-meter was used to measure the magnetic field induced by composite bricks.

We measured the sound of a radio transmitter placed 160 mm from the sound meter in the open air first; then, placed at the same distance (Figure 3(a)), in a hermetically sealed box, built with stabilized clay bricks (SCB), mixed with KWC

at different levels, and with a clay mortar (Figure 3(b)) according to standard NF EN 61672-1 [14].

The speed being the quotient of the length of the specimen on the propagation time of the sound in the brick [8], we measured the time of travel of the wave in the brick placed between the two converters of the ultrasonic device PUNDIT7 (see Figure 3(c)) according to standard NF EN 12504-4 [15]. The measurements were carried out in the laboratory of the Office of Building Control and Public Works in the abbreviation “BCBTP” in Brazzaville.

Earth has a geomagnetic field due to the movement of molten rocks within the Earth’s core; a field whose intensity varies according to where we are on the surface of the earth. The magnetic induction of cement-stabilized clay bricks mixed with KWC at different grades was measured using a Tesla-meter. The measurement was done by placing the activated device on the brick according to standard NF EN 62233 [16].

Consider a composite material clay + cement + wood. Let Φ_(g+c) be the mass proportion of the clay + cement mixture which constitutes the stabilized mixture (stabilized clay with 6% cement) and, Φ_b the mass proportion of the wood chips (WC) in the clay + cement + wood mixture. By definition, the mass proportion of WCΦ_b in this composite is given by:

Φ b = m b m g + c + m b × 100 (1)

where m_b and m_(g+c) are respectively the masses of WC and the stabilized mixture clay + cement. In the same way, the mass proportion of the clay + cement mixture, noted Φ_(g+c) in the composite clay + cement + wood is given by:

Φ g + c = m g + c m g + c + m b × 100 (2)

We study the influence of the WC content on the acoustic properties of composite bricks (clays + cement + wood) according to the following composition (see Table 2).

To study the effect of adding WC in the clay + cement mixture, the mixture B1 containing 0% wood chips is considered as a reference composite.

In order to allow an easy discussion of the results, the measured acoustic quantities (noise level, sound velocity and magnetic induction) in the composite containing WC are standardized with respect to the reference acoustic quantities, measured in the medium containing 0% of WC.

Call V₀, B₀ and L₀, respectively the speed of sound, the magnetic induction and the noise level in the reference composite (clay + cement), and V, B and L respectively, the speed of sound, the magnetic induction and noise level in the composite with addition of WC. We define by V^*, B^* and L^* respectively the sound velocity, the magnetic induction and the noise level dimensionless (normed with respect to the acoustic quantities chosen as references) given by:

V * = V V 0 (3)

B * = B B 0 (4)

L * = L L 0 (5)

We discuss in the following, the effect of the addition of WC on these adimensional quantities.

Table 3 summarizes the attenuation values of acoustic properties of CB stabilized at 6% cement as a function of KWC content. We took 110 dB reference noise level and 2194 m/s reference sound speed for the brick without adding KWC.

The sound velocity measured in the reference brick B1 (without the addition of KWC) is 2194 m/s, which is about 6 times greater than the rate of propagation of sound in the air. The addition of 8% of KWC in the reference composite B1 results in a reduction of this ratio to 4.5 (i.e. 1560 m/s for brick B5).

Figure 4 shows the variation of the adimensional speed V^* of the sound as a function of the WC content Φ b . A linear decrease of the speed of sound is observed with the increase in KWC content.

This linear decrease in the average sound velocity as a function of the increase in wood fiber content by ultrasonic testing was also reported by Mekhermeche [17] in the conclusions of his study on composite bricks with date palm fibers.

This linear decay is described by the mathematical model (8):

V * = V 0 * − I a Φ b (6)

where V 0 * is the adimensional speed of sound measured in the reference brick (clay + cement) without adding KWC ( Φ b = 0 % ) and I a = 0.036 is the attenuation coefficient of sound propagation in the brick following the addition of KWC. Recall that Φ b is the KWC addition content defined by Equation (1). The value of the attenuation coefficient I a is the result of a linear adjustment of the experimental data made with the Gnuplot software via the Marquardt-Levenberg method.

We can define from the relation (7), an optimal content of KWC Φ b o p t , which would lead to a speed of propagation of zero sound in the brick.

Φ b o p t = V 0 * I a (7)

This optimum content of KWC that gives a perfect acoustic insulation is estimated at about 27.56% according to the relation (7) resulting from the predictive model (6). According to this theoretical model, a KWC content of about 23% would make a composite brick that would give a speed of sound propagation close to that of air.

Figure 5 shows the variation in the noise level generated by a transmitter source, as a function of the KWC proportion in the CB stabilized at 6% cement.

This curve shows that this level decreases rapidly, then more and more slowly as the KWC content increases. The addition of 2% KWC results in an attenuation of approximately 36% of the noise level whereas this attenuation does not exceed 10% when the KWC content varies from 2% to 8% (Table 3). The addition of KWC in the reference brick (clay + cement) leads to an acoustic insulation that can reduce the noise level up to 62% for the proportions studied (from 110 dB to 68 dB, respectively for Φ_b = 0% and Φ_b = 8%).

The decay of the noise level with the increase in KWC content observed in Figure 5 can be described by the model (8).

L * = 0.909 e − 0.05 Φ b (8)

The sound attenuation observed in Figure 4 following the addition of KWC in the composite brick (clay + cement), could be explained by an increase in the porosity of the medium, induced by the alveolar structure of the wood consisting of microscopic cavities organized in a network that would trap air [1].

This sound attenuation was also observed by Cérézo [18] who showed in the conclusions of his thesis work from the Kundt tube test that the absorption coefficient of the sound wave increases with the increase in hemp fiber content.

In the rest of the paper, we propose a correlation between the evolution of the acoustic properties and the density of CB stabilized at 6% cement.

Figure 6 shows the influence of KWC content on magnetic induction. Magnetic induction increases with the addition of KWC. However, a 6% addition of KWC results in only 1% increase in magnetic induction. For these contents, the incorporation of the KWC in the reference brick does not seem to significantly disturb the magnetic field of the medium. However, at 8% of KWC, the magnetic field disturbance reaches 5.56% compared to the magnetic field of the reference brick. The increased increase in magnetic induction could be explained by an increase in the porosity of the brick, induced by the alveolar structure of the introduced wood, consisting of microscopic cavities organized in a network that would trap air. The increase in pores, created by the addition of wood, leads to an increase in the magnetic permeability, and therefore in the magnetic induction.

Figure 7 shows the speed of sound propagation and noise level dimensionless as a function of the density of the bricks.

This figure shows that the speed of sound propagation and the noise level are inversely proportional to the density of the bricks. The increase in the density of the bricks with the KWC content leads to an attenuation of the propagation speed of the sound and the noise level. The speed of sound propagation in bricks is well correlated with the density of bricks by the power law (9).

V * = [ ρ ( Φ b ) ρ 0 ] − α (9)

With ρ₀ = 2.07 g/cm³ is the density of the composite brick B1 (without addition of KWC) and α = 4 ± 0.05 is the slope of the power law which reflects the attenuation rate sound propagation in bricks.

Figure 8 shows that there is a correlation between magnetic induction and the level of noise reception through SCB, incorporating KWC. In fact, the level of noise reception decreases with the increase in the magnetic field of the bricks for WC contents of less than 6%, whereas it seems to reach an asymptotic regime at high levels.