BASICS ACOUSTICS CONCERNING TO ISINAC

INTRODUCION

The human being has the capacity of discerning the sound at a certain frequency band, in case of a Young person with a normal hearing condition, it is included between the 20Hz and the 20KHz.

How we can see the human ear is little sensitive to low sound frequencies, it means, so that a bass sound has the same sound that a mid sound, the bass should have much bigger sound level. A sound pressure level from70dB to 20Hz, it produces the same level sound auditory sensation than a sound pressure level from 5dB to 1KHz.

The human being interprets the early reflections (until 50ms) as an extension of the real sound, so that the brain doesn´t discern these reflections with the direct sound. This effect is the already mentioned reverberation, or reverberant tail.

In fact, when it is trying to speak, these reflections help to improve the intelligibility of the word and increasing the sound of the message.

In case of the listener hears one reflection in a high volume with more than 50ms delay, the human ear will discern it as a different sound, as a consequence the intelligibility of the message will be affected in a negative way.

 

HOW TO ADD THE SOUND LEVELS?

For example, 30dB + 30dB it is not equal to 60 dB, otherwise it is equal to 33 dB as we will see coming up. If we want to add two decibels we can use the next equation:

Plus dB1 + dB2 = 10 log (10^(dB1/10) + 10^(dB2/10))

30 dB + 30 dB = 10 log(10^(30/10) + 10^(30/10) =
10 log(10^3 + 10^3) = 10 log ( 1000 + 1000) = 33 dB

The addition of two dB could never be more than 3 dB over the biggest of both. If the difference between both values to add is bigger than 10dB, the addition doesn´t have practical value, so it is taken the biggest value between both. For example, if we add 20 dB + 10 dB the result will be equal to 20 dB (approximate). Only the values that have a difference fewer than 10 dB are significant for the addition.

WHAT IS THE PROPAGATION SPEED OF THE SOUND?

The propagation speed of the sound through the air at 0º is the 331,6m/s and at 20º is the 343,5m/s

In the water the propagation speed is the 1500m/s.

 

 

WHAT IS THE REVERBERANT TIME?

When a sound source emits a wave into a room, the listener receives this wave in a two-different way: One of them is the direct sound which comes from the listener just like it would be in the propagation in an opening place; The other way is the reflected sound, which consists of the addition of the different reflections of the wave that they rebound in the surfaces of the room and they arrive to the receiver.

The Reverberant Time RT, it is the time that a signal gets, from it stops sounding, until it dismissed into a level of 60dB.

It is determinate for the room´s volume, and for the absorption coefficients of their surfaces, or it is rather for the surfaces which have a determinate absorption coefficient. As the fact that the absorption coefficient of the different materials are not the same for each sound frequency, the reflections generated inside the room will be different for each sound frequency, so that the room´s RT is different depending on the frequencies.

For make the calculation without making measurements it could be used the Sabine´s equation.

 

 

 

 

 

 

 

 

 

 

 

 

WHAT IS THE ABSORPTION COEFFICIENT OF ONE MATERIAL?

The absorption coefficient of a material is the relation between the absorbed energy for the material and the reflected energy from it. Due to this formulation the value will be always included between 0 and 1. The maximum absorption coefficient is determined for a value 1 where all the energy which has an impact on the material is absorbed by it, and minimum is 0 where all the energy is reflected.

 

GEOMETRIC ACOUSTIC

Normally the early reflections have a higher pressure level than the late contributions,

This is due to the fact that they have a lower order, and they are less affected by the absorption of the surfaces, just as for the wave diminishing for the distance travelled.

It must achieve some conditions of the room so that, in the practise, it would be a spectacular reflection.

  • Little reflective and smooth surface.
  • Big measurements (in comparison with the waves length to analyse)

If the measurements are equal or less than the wave length that we want to analyse it is produced the diffraction, due to the fact that the wave goes around of the surface and it continues spreading.

In case that the Surface has roughness with a similar size than the wave length, it is produced diffusion, due to have a reflection in some multiple directions.

 

DIRECT AND REVERBERANT FIELD

To the extent that the listener gets away from the source the direct component will decrease, however how it was said before, the reverberant field continues constant. This Hypothesis that the sound reflected is constant at any point is based in the statistical theory, which handles all the reflections at the same way, giving rise to results little exacts but taking simple calculations.

It could be delimited two areas in the room, one where exists a predominance of the reflected sound and other one where the predominance is the direct sound, the first one will be placed in the area most away from the source, and the second one in the nearest

The level of the direct sound will increase in 6dB every time it will be shorted in a half the distance from the source, the point where takes precedence the direct sound will be like an opening space situation

On the other hand, the points where the reverberant field takes precedence they will be the most away from the source and it will be delimited an area where the sound pressure level is constant.

 

INTELLIGIBILITY

Speaking, the vowels have bigger length and sound than the consonant, at the same time these are more bass in comparison with the consonants which are higher-pitched, as a consequence in a room with a lot of reverberation it would be made more powerful the length (therefore the sound level) of the vowels, since that how it was said before, the reverberation time in high sound frequencies is less than in low sound frequencies.

This situation generate a conflict due to that the vowel, when lengthing its length, is overlap with the consonant that follows it, consequently the simultaneity with the difference between the levels and the spectral difference on frequency, cause a disguising of the message and a difficulty in its understanding.

To sum up, the grade of intelligibility inside a room depends first and foremost on the right perception of the consonants, so that the disguising causing for a high reverberation suppose a decrease in the quality of the word of the room.

 

GEDDES METHOD

Among others there is the Geddes Method, of the wall double crosswise which shows vertical and horizontal angles that they are emphasize with the intention of destroy the strong axial and tangential ways, to become them in crosswise ways, less regular and more easily controllable. At the same time exist the scattering technique, which distributes the modal energy through wider super positions. These rooms had been frequently used together with monitors which directorate was fitted for each room, in the way that the sequence of the absorbent surfaces or reflective sources positioning, they would allow to achieve the ideal characteristic of RT/frequency inside them. These rooms usually include “bass traps”, absorbent traps of the bass frequencies, with a considerable volume, which were in charge of the less directorate frequencies below the 300 cycles.

 

ABSORPTION OF BASS

The Mathematical expression of the absorbent coefficient of the vibrant surfaces for a determinate frequency is the next one:

 

 

This equation is actually an expression which has only value when the denominator has a high value in comparison with the numerator. If we pay attention to the expression, we can only obtain a relatively significant expression in the low frequencies because when the frequency increases, the denominator will increase too and as a consequence the absorption value will be less.

The poli cylindrical diffusers are a group of flat surfaces, usually they are made of Wood, which have a convex figure that they are positioned sequentially, and they have a coverage radius minor than 5 metres.

If this radius would be over than 5 metres, the product has not behaved like a diffuser it´s just a reflector, on the contrary, if it is less than 5 metres the reflected sound wouldn´t be focus any more due to the fact that the coverage zone increases.

The Schroeder´s diffusers are selective diffusers which act only in a specific frequency range. They are based on the Manfred R. Schroeder theory and some different mathematical sequences, they are called RPG

The different types of RPG diffusers most important are

  • The MLS diffusers
  • The QRD diffusers
  • The PRD [1,2,3] diffusers

 

For make the calculate of the resonance frequency from a membrane resonator we will use the next formulae: The resonance frequency will be:

F=60/(raíz(m x d))

Being

  • m = mess per unit area of the panel (kg/m2)
  • d = separation from the rigid surface of work or forged (m)

We can calculate the resonance frequency of the Helmholtz multiple resonators with cavity in the next way:

F=5480 x raíz(z/(l’ x d))

Being:

  • E = percentage of perforation of the panel (en tanto por uno)
  • l’ = depth of the holes (cm)
  • d= thickness of the air layer (cm)

 

Its absorbent coefficient depends on:

  • The nature and thickness of the coating.
  • The shape and arrangement of the perforations.
  • The type of assembly.
  • The absorbent material placed on top.
  • Your index (%) of perforations.

 

 

SOURCES

  • Universidad politécnica de valencia. Diseño y acondicionamiento acústico de la sala de grabación musical de Basic Productions en Valencia . Dídac Corbí Albella
  • Acústica básica. Borja Azpiroz