pianoexpert wrote:
Beats are caused by frequency differences. If we have an A tuned to 440 and its nearby string at 441, we would say that one beat per second is produced.
The concept of beating is well explained mathematically by performing the sum of two periodic vibrations (such as sound) with very close frequencies f1 and f2. Anyone who has studied a bit of mathematics can easily derive the expression for the sum of two elementary vibrations, which immediately shows the nature of the resulting vibration; unlike the first two, it does not have a constant gain (to mean the volume of a radio), but also an oscillating gain: what we hear as the result is a "cyclic sound" and not a nice, steady amplitude (volume).
The frequency of the resulting wave can be quantified just as its gain can be quantified by observing the expression of the sum function. Without going into details, it is obtained that given two vibrations of close frequency (440 and 441 Hz for example), the composite vibration is characterized by a frequency f = (f1+f2)/2 [example (440+441)/2=440.5 Hz] while its gain oscillates at a frequency F = (f2-f1)/2 [example (441-440)/2=0.5 Hz].
A vibration of 1 Hz performs one cycle (after which it repeats identically) in 1 second (the oscillation period is given by the inverse of the frequency) and in each cycle, the oscillation contains "two peaks" over time: a maximum and a minimum.
And this same phenomenon, invisible to the human eye, is found in our home electrical grid; the grid frequency is 50 Hz, which means that 50 are the complete oscillations per second of the current, meaning the light bulb turns on and off 100 times in a second (each cycle consists of one turning on and one turning off).
Having clarified this, it is clear that a beating frequency of 0.5 Hz makes our ear perceive "half a cycle per second" in terms of "volume" (the complete oscillation would occur in 2 seconds) and therefore an amplitude variation of the sound in one second, i.e., 1 beat per second. The variation in the amplitude of the resulting vibration is called the envelope by mathematicians; the number of peaks in one second is equivalent to the number of beats.
There are curious cases, and I don't know what effects they have on an unison for practical purposes; I have never had the chance to hear if something pleasant can be derived from certain combinations, for example:
suppose we have two strings of the same pitch tuned at 439 Hz and the other at 441 Hz. The resulting frequency in terms of "sound pitch" would be (439 + 441)/2=440 Hz, which is equivalent to the oscillation frequency of the central A. However, we wouldn't have a nice, steady A with constant amplitude, but an oscillating sound with a frequency equal to (441-439)/2 = 1 Hz, meaning with 2 beats... by bringing the frequencies of the two strings closer and closer, one would have increasingly slower oscillations; by moving them further apart, one has faster oscillations instead...
For example, listening to Horowitz's piano, I have always wondered how the hell... the beats were placed in that case....the single note was never a blasted straight sound, it oscillated slowly....just listen to the notes of the song here!! Franz Mohr, explain to us!!! I cannot ask my tuners these things since it is already too much if when they leave the house the octaves sound good...
And don't tell me that it depends on the quality of the instrument and the recording; pink noise does not introduce oscillations... don't even bring up string length because in the example reported on the notes of the song, they are not extremely long strings... I have shown you with numbers that an A can oscillate in infinite different ways, I showed you in a video that unisons already oscillate at the attack of the sound, sometimes they seem like rubber bands... the instrument is excellent....like this one, for that matter:
but you don't hear the same effect; the touch is obviously different but it doesn't affect the specific phenomenon, the instrument is different but that seems like a negligible factor to me....the tuner is different.... opinions?
Finally, the performance which I consider the best; listen to how even here the notes fluctuate in a particular way, more delicate... needless to say, great pianist and great tuner: