An exquisite violin is easily identified by the trained ear. In fact, even untrained ears can spot a good violin upon a mere glance at the price tag: the greatest instruments of seventeenth century Italian luthiers are priced in the millions, while a cheap but playable violin can be bought for less than a hundred dollars. The difference in quality is undeniable, for the perfection behind the making of a Stradivarius – deemed the greatest of all violins – is not easily attained.
In this experiment, the quantitative differences between various violins, ranging greatly in price and sound quality, was determined. For each violin, open strings were recorded and a Fourier transform was used to generate the spectrogram and determine the percent of sound in each harmonic. The three highest quality instruments tested were the Stradivarius, Schleske, and Marcello Villa violins. The cheapest and poorest quality violin tested was the eBay violin.
First, sound waves must be examined to achieve an understanding of violin tone. As sound travels, air molecules compress and expand. When sound waves reach the ear, eardrums vibrate and through the series of processes that follow, a nerve impulse is sent to the brain and sound is "heard." The height of a sound wave (called amplitude) is related to the volume and amount of compression and rarefaction, or decrease in density, of the air molecules, so a louder sound has larger amplitude. Another essential element of a sound wave is its frequency, the number of times the sound wave cycle repeats in one second, measured in Hertz. For example, A, the note to which an orchestra tunes, has a wave cycle that repeats 440 times in one second. The frequency of the wave is known musically as pitch. Higher pitches have higher frequencies.
The most basic sound wave resembles a sine graph and has one pure frequency. The complex sounds of instruments, however, contain an infinite mixture of sine and cosine waves. This means that not one, but infinite frequencies, are sounded when a note is played. The reason we hear one pitch and not chaos is because each frequency has a different amplitude, and the main pitch we hear (the fundamental frequency) has the greatest amplitude. The other frequencies involved, called harmonic overtones, peak in amplitude at integer multiples of the fundamental on the graph. When a note is played on the violin, the listener identifies the fundamental frequency as the pitch, but also hears the frequencies of the overtones. These overtones give sound complexity and allow for the differentiation of similar sounds. Different instruments, for example, have different strengths in each harmonic. This is how a trumpet can be distinguished from a flute by ear. This is also how two different violins, even when both are playing the same note, can be distinguished from each other.
Thus, in order to compare two violins, the strength of its various frequency components must be analyzed, which can be done through a Fourier transform. Essentially, the Fourier transform breaks a complex wave into its sine and cosine components to determine the specific amplitude of each frequency. A spectrogram represents this graphically with time on the horizontal axis, frequency on the vertical axis, and amplitude on 3rd axis (often represented by color).
A variety of violins sounding open A (440 Hz) were recorded and compared to each other and to a computer-generated A. On the spectrograms, the computer-generated A had only one line because it is one pure frequency. The A on the violins, however, had many frequencies - overtones. From the spectrograms, it was noted that as the harmonic overtones on the violin become higher, its strengths also become weaker. The strength of the harmonics varies depending on the violin. The higher quality violins had similar wave patterns and had more strength in the fundamental note. Using the Fourier transform, it was shown that these violins (namely, the Stradivarius, Schleske, and Marcello Villa) had approximately 92, 70, and 82 percent of the sound in the fundamental, respectively. Meanwhile, the Fourier transform revealed that the eBay violin had only 46 percent of the sound in the fundamental for the A string. The huge gap between the high quality and low quality instruments is most likely a reason why some instruments are deemed "high quality" and others are not.
It must be noted that greater strength in the fundamental harmonic is not always favorable. The computer-generated sine-wave A has 100 percent of sound in the fundamental; however, this pure frequency is incomparable to the violins. Nonetheless, the fact that the eBay violin had only 46% in the fundamental compared to the 92% in the Stradivarius does show that purity of a sound is one factor in determining the quality of violin. Evidently, more research is needed to determine the extent that purity of sound (percent of fundamental) plays in the perception of quality. However, the percentage of sound in the fundamental may have been altered the quality of recording as well. A higher quality recording system may have picked up sounds in higher harmonics with more precision.
Moreover, the highest quality violins tested had higher relative strengths of the fourth harmonic. The Stradivarius had 4.2% of the total sound in the fourth harmonic, while the Schleske and Marcello Villa violins had 15.1% and 9.0% of the sound in the fourth harmonic, respectively. Most of the other harmonics had relatively low strengths, with a thousandth or hundredth percent of the total sound in the high quality instruments. In the lower quality violins, no significant correlation could be made between the strengths of the various harmonics, with certain harmonics having a higher strength in some instruments and not in others. The "randomness" of the distribution of the strength of the harmonics in the lower quality violins probably contributes to their poor tone.
By scientifically analyzing the tones produced by the various violins, numerous differences between the high and low-quality instruments were found. The hope is through examining the basis of violin sound, instruments of Stradivarius quality may be one day be replicated with ease and be accessible to musicians around the world.
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