Measurement of Reverberation Time and Calculation of Absorption Coefficients - Lab Report Example

Measurement of Reverberation Time and Calculation of Absorption Coefficients Introduction Good acoustics are a prerequisite for a high quality publicaddress system, broadcasting or recording studios. Reverberation Time and Absorption Coefficients are the key parameters towards providing acoustic treatment in studios, auditoriums, meeting rooms and other such places requiring use of audio equipment. The reverberation time depend upon the kind of usage for the room. For a music concert or a drama performance we need to have a higher reverberation time while for a meeting room, talks studio it should be minimum. In fact in modern day houses as well, professionals take pride in explaining how diligently they have worked towards providing adequate acoustic parameters. In any enclosed space the sound remains in air even after the source of sound is taken off. This 'hanging on' of the sound in a room is known as Reverberation and the time required for the sound to decay to one millionth of its initial value (or 60 dB) after the source has stopped, is termed as 'Reverberation Time'. The R/T of any room depends upon the shape and size of the room, sound frequency, and the amount of absorption offered by the boundary surfaces. This experiment aims to find out how the boundary wall formation and other parameters affect the R/T and absorption coefficients of a room and its significance in acoustic treatments. Procedure For this lab experiment we have a microphone and the B&K type 2133 digital frequency analyser. The steps are as follows: Step-1: Measure the surface area S and volume V of the room. Step-2: get the details of the surfaces (absorbents) to be placed in the room. Step-3: Set up the frequency analyser. Step-4: Set the analyser to excite the room with a stationary broadband noise, which is suddenly stopped. This will provide relevant information to the analyser from which the reverberation time, in one-third octave bands can be calculated. The Octave bands could be centred at 125, 250, 500, 1000, 2000, and 4000 Hz. Step-5: Introduce the sound absorption material into the room and repeat the measurements as above. Do this for four acoustic batts in a 2x2 arrangement on the floor. Step-6: Having obtained the room absorption coefficients with and without the sound absorption material, calculate the absorption coefficient of the material. Step-7: Draw a graph of the absorption coefficient Vs one-third octave band frequency for the material. Results: Room Details: Length = 7.9 meters Width = 6.4 meters Height = 4.1 meters Volume of the room = 7.9 x 6.4 x 4.1 = 207.3 m3 Measurement of Reverb Frequency (Hz) Reverberation (Bare Room) Reverberation (With absorbents) 40 50 63 80 100 11.5 7.25 125 8.92 5.10 160 7.07 3.65 200 5.41 3.04 250 5.41 2.92 315 4.98 2.03 400 5.79 1.79 500 5.26 1.90 630 5.80 1.90 800 5.22 2.06 1k 5.26 2.06 1.25k 5.01 2.02 1.6k 4.76 2.12 2k 4.30 1.96 2.5k 3.91 1.92 3.15k 3.27 1.82 4k 2.74 1.57 5k 2.27 1.42 Discussion From the above mentioned experiment it is quite evident that reverberation and absorption are inextricably linked with each other. Reverberation figures in a room with bare walls are more than a room fitted with absorbent surfaces. During the experiment it was also observed that the volume of sound increases due to the reverberation, which at times complements the audibility of the sound, but at the same time it has been observed that quality of sound suffers adversely when there is too much of reverberation. Prolongation of sound was observed with the reverberation. In fact if we can make use of reverberation in the most efficient manner, particularly during musical or theatre performances, it results in a soothing blending of one sound with the next, thus producing a very pleasant continuity in the flow of music. It is worthwhile here to mention that if the extent of prolongation increases it results in quality deterioration. It is also worth mentioning here that during entire lab experiment all positions of the microphones, the equipment, furniture and other fittings were marked in order to have uniformity in results. The furniture, fittings or the human body for that matter also function as absorbents to some extent, which has the potential of affecting the end result. Negligible reverberation was observed at the lowermost frequencies while this figure was the highest at around 100 Hz, nearer to the initial frequency range audible to the human ear. The reverberation time was measured lower at the higher frequencies. This goes on to prove that while the absorbents have not proved to be much effective at the lower frequency ranges, the higher frequencies are better absorbed by the surfaces, walls and other absorbent material in the room. It is also observed through the experiment that starting from 100Hz, the reverberation figure goes down till about 1Kz, where it sees slight increase once again for some higher frequencies, before finally going down once again. This property of R/T is made use of while designing studios and auditoriums. For example, rooms meant for musical performances are designed in such a manner that reverberation figures are greater at the lower and higher frequencies than the mid-frequencies. This helps in ensuring that the aural rate of decay of pure tunes will be approximately the same for all frequencies. The absorption coefficient of an absorbent or a surface is the measure of the extent to which the material is able to absorb the sound waves. The coefficient in general is a figure between 0 and 1. For example in open air, there is no possibility of sound reflecting back to the source, thus implying an absorption coefficient of 1. But in hilly terrain this doesn't hold good, reason being we do experience some amount of reflection from the hills. Similarly in a closed room, the reflection of sound waves depends upon the walls of the room. If the walls are bare we have more reverberation and less of absorption i.e. absorption coefficient approaching zero, but if we have walls with absorbent surfaces, reverberation is less while absorption coefficient tries to approach unity. Conclusion: Sound waves coming out of a source in general are propagated in all directions. In their journey, these waves are subject to reflection, refraction and absorption, depending upon the kind of obstacles encountered by the waves. In order to make maximum use of the energy being propagated through these waves, acoustic treatment is provided to classrooms, meeting rooms, lecture halls, studios, auditoriums/ concert halls, cinema halls, theatres etc. The extent of echo, reverberation and absorption takes place depending upon the purpose of the room and the structure and shape of the obstacles and the frequency of the sound waves. In this manner we are able to modify the physical characteristics of the sound waves before these waves actually reach the human ear, thus creating the desires effect. This is one of the finest examples of the manner in which we are able to have the desired impact on our audience and can in effect modify the reach and properties of the sound waves. Since the reverberation time of the room is dependent upon the frequency. Therefore, the phenomenon of reverberation tends to modify the frequency characteristics of the total sound field inside the room. High reverberation time at middle and higher frequencies is said to lead to increased 'liveliness', while a similar arrangement at lower frequencies increases the 'warmth'. Read More