T E S T
H O M E

 

 

 

 

 

 

Istituto Elettrotecnico Nazionale

Galileo Ferraris

 

Sede di Corso Massimo d'Azeglio, 42 - 10125 TORINO - Telefax (+39) 011.650.76.11

Sede di Strada delle Cacce, 91 - 10135 TORINO - Telefax (+39) 011.34.63.84

Telefono (+39) 011.3919.1 (selezione passante) sito Internet: http://www.ien.it

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RELAZIONE

 

 

 

 

N. 34380 constituted of n. 4 pages released in date 20/09/2001

 

to Mr. Michele Greci. via XX Settembre 98/E - 00187 Roma [RM] - Italy

 

conforming to the application prot. IEN 6747/01 in date 18/09/2001

 

 

Object: comparative measurements of sound pressure levels on two guitars.

 

1)Preliminary statements

 

The customer has asked for a comparative measurement of sound pressure levels on two guitars which he furnished himself:

 

 

Purpose of the measurements is to establish the superiority of the sound level produced by the Greci guitar.

 

 

2) EXECUTION OF MEASUREMENTS

 

With the purpose to eliminate from the results the human performance procedures on the instrument, a mechanical device was conceived and created at the university La Sapienza in Rome, Faculty of Engineering, under the direction of professor Enzo Marchetti of the Department of Mechanics and Aeronautics. The guitar is placed on a horizontal surface; a cylinder, which carries a projecting part similar to a plectrum, is set in slow rotation by an electric scaling engine, which at every turn pinches the string of the guitar with a procedure analogous to that of a human performance but highly repeatable. It is also possible to shift the guitar horizontally so that it offers its six strings consecutively to the point of stimulation of the plectrum.

 

The device, set on a bearing surface, has been situated in a semi-anecoic chamber. Above it, at a distance of 1 metre from the soundboard of each guitar and on the perpendicular of the point of stimulation, has been situated a omnidirectional mesure microphone for free field, turned toward the instrument. By means of the microphonic preamplifier, the microphone has been connected to a real time two channel digital frequency analyser of 1/3, 1/12 and 1/24 of octave.

 

Measuring instrumentation is therefore the following:

 

 

With the described sequence, measurements of the level of sound pressure have been effected through acquisition of the spectrum of 1/24 of octave and at the same time measurements have been effected of global, linear and ponderable A level; measurements have been repeated three times on each open string of each guitar, so as to verify repeatability. The analyser was started using the trigger on the global signal: when a particular sound level was reached, varying according to the string and the instrument, correspondent to the first beginning of the sound, the acquisition began, with linear integration time equal to 4 seconds, such as to include the part of the sound decadence distinguished by greater energy. Transferring on a calculator the levels of each band of 1/24 of octave and elaborating them on a spreadsheet, the energetic average value and the standard deviation of sound pressure level of the three measures have been then calculated, for each band of 1/24 of octave. The datum of standard deviation regarding average value calculated on global levels (linear and weighted A.) has been, in the worst case, lower than 1,5 dB, with values on the average lower than 1,0 dB, thus confirming repeatability. Also the level of residual noise has been measured (due to rotation of the electric engine of the device for mechanical stimulation), so as to check that it didn't interfere with the sound produced by the instrument. The result was a difference between the level of the background noise and that of each string of each instrument constantly higher than 20 dB for every band of the spectrum, for each string and each instrument, so as to confirm that in no case the measure effected on the guitars has been influenced by background noise (error lower than 0,1 dB).

 

 

 

3) PRESENTATION OF MEASUREMENTS RESULTS

 

As previously expressed, the data of the spectrums of 1/24 of octave (three for each string and for each guitar) have been transferred on a spreadsheet so as to calculate the energetic average value and the standard deviation. We have so obtained 12 average spectrums (six for each guitar and for each instrument, of which the data are expressed in dB (A) and dB corresponding to the global levels, weighted . and linear, in tab.1).

 

 

Table 1): gives the comparison of the global and weighed sonorous levels A of the two guitars, in dB.

( NOTA BENE: A 3dB HEIGHTENING MEANS DOUBLE SOUND POWER )

 

 

Greci Guitar

Classic Guitar

NOTA (Note, string)

A

Linear

A

Linear

MI (E sixth)

69,7

73,7

60,7

65,7

LA ( A fifth)

66,2

72,1

61,7

67,8

RE (D fourth)

68,9

73,6

60,4

64,5

SOL (G third)

63,0

68,3

58,9

63,1

SI (B second)

65,8

69,6

60,7

63,5

MI Cant. (E first)

65,3

66,5

58,4

59,7

 

The data of the table are also expressed in the diagrams of the two accompanying tables (fig. 1 and fig. 2), which set a comparison, respectively, between the global weighted A and linear levels of all the strings of the two instruments on histograms and diagrams.

 

The data regarding the spectrums of 1/24 of octave have been here expressed exclusively in graphic form due to the great number of data (194 values for each string and each instrument). In figures 3- 8 in the two accompanying tables the spectrums of each instrument are compared, each string separately, on a diagram with lines of different colours, according to the instrument. The numerical data are, in any case, available on the floppy disk enclosed, which is integral part of this paper, as a file (chitarre.xls) in Microsoft Excel 2000, which contains the whole spreadsheet used for the elaboration.

 

 

 

 

 

4) INTERPRETATION OF THE RESULTS

 

 

The results regarding global levels both linear and weighted A show that, depending on which note, the Greci guitar has in the same conditions and played on the same string, a superior sound level for a value between 4.1 dB (Sol - G, weighted A.) and 9.1 dB (RE - D, linear) compared with the classic guitar. Examining the frequency one can see that all harmonic components of the Greci guitar produce a superior sound level, except sporadic exceptions; the increase of level becomes particularly elevated in the most higher harmonics, producing a richer and brighter sound timbre.

 

 

ENCLOSURES

 

N. 8 TABLES