The previous chapters have plotted the course of a piano tone, from the moment of impact between hammer and string, to the last dying sound. This sound output is characterized by a sharp amplitude increase, a peak, and a long sustain, with changing timbre and harmonic proportions along the way.
The way in which this sound is radiated and projected into a room presents another scenario, which includes a multitude of variables which much be dealt with in order to capture the true sonic quality of the piano. Just as no two points are alike during the "formation" of piano tone, no two points within the "listening area" are similar either. Therefore, in order to record the full dynamics of the piano, it is essential to know exactly how the sound is radiated, in order to take some of the guesswork out of microphone selection and placement.
Acoustic instruments are inherently complex radiators, that is they project in all directions but with varying amplitude, and strong directional characteristics for many frequencies. These characteristics can be used as a guideline in predetermining the acoustical outcome of a recording. Pianos of course vary from one to another, as does listener preference and room acoustics, and a certain amount of experimentation is therefore unavoidable.
The strongest directional patterns in a grand piano are those produced by the open lid. In this case, the sound travels upwards from the soundboard, and is then reflected off of the bottom of the lid and out into the room.
The floor also plays a fundamental role in the direction of sound, as sound vibrations are radiated downward by the soundboard as well as upward. The relative size and proximity of the floor to the soundboard, makes it an important variable in determining the overall projection of the piano. Floor material should therefore be selected accordingly.
There is also some direct radiation into the room from the strings themselves, which in combination with the floor and lid, characterizes the directional properties of a given piano.
Pitch is also to be considered, because the pattern radiated by the soundboard is also influenced by the point or the keyboard in which the stimulation occurs. The different patterns for the low, mid, and high sections are shown in Fig. 1.
The low, middle, and high registers are typified as follows: lid open:
1. Sound radiation symmetrical with an approximate 4 DB drop of
fundamental in the horizontal to 20 degree range.
2. The level behind the grand is about 5 DB higher than in front of the open lid in the area of 250 HZ. This is because there is no interference between sounds coming from the top and the bottom of the board, which are out of phase with each other. The listener behind the piano localizes sounds below the instrument.
Here there is more directivity, predominately between the horizontal to 55 degree range. The directional response upward is lessened, producing a dull timbre there. There is a decrease in the fundamental tones between the angles of 90 and 180 degrees, but never goes below 10 DB of the maximum.
In the high ranges, the lid plays a much greater role in directing the sound. The directivity is especially strong between 15 degrees and 30 degrees, which corresponds to the angle of the open lid.
Measurements taken at 4000 HZ showed levels in the ranges 0 © 5 degrees, and 60 © 180 degrees were more than 10 DB below the maximum. The brilliant sound is therefore quite restricted to a narrow range of angles. Figure two compares levels in the front and back of the piano in the high registers. The levels in back are not nearly as strong. In addition to amplitude decrease, the wave structure is less ordered. This is primarily due to more predominant noise components (impact noise etc.) which are not weakened as much as the harmonic components.
The back of the piano has no essential directional change. In the front there is an obvious change between 0 and 90 degrees, with a decrease in the highs of about 10 DB. The radiation of the highs are mainly directed towards the player, and are virtually the same here as with the lid open. This is due to the opening between the lid and the music stand.
Lid Half Open
The short prop holds the lid at an angle of 10 degree. One might expect there to be a more restricted angle of radiation, due to the much lower stick angle, but the primary radiation is between 10 and 60 degrees, similar to that of a full stick. There is, however, a decrease in amplitude, especially at the higher frequencies. At 4000 hz, the level is about 6 db less than with a full stick. Therefore, the short stick lacks the brilliancy and clarity of the full stick.
The directional characteristics towards the back of the instrument remain the same, compared with an open or closed lid.
The directional characteristics of grand piano with no lid are fairly well balanced in the low and middle registers. The radiation of these components are basically omni directional on the horizontal plane.
There is a loss of low frequencies below 250 Hz near the horizontal plane. At 65 Hz there is an average decrease of 5 db. Higher pitches have a strong upwards tendency, and in the area of 4000 Hz there is a 3©5 db increase in this direction as compared with the lid half or fully open. Since the general radiation of a topless piano is stronger upward, there is consequently a reduction in the horizontal plane. As a result, the higher frequencies are more or less lost in the strong skyward radiation, and the resultant sound becomes relatively dull.
The last section illustrated the many different sound radiation patterns
of the grand piano, producing a wide variation in amplitude and timbre, depending
upon the location of the listener and/or microphone.
As one moves farther away from the piano, however, phase and time relationships among the various parts of the piano proved to be different, as compared to a closer proximity. The net effect results in a perceived change in dynamics, or transient character.
For more on recording the piano, and the full book version of Vital Sound, click here.
©2011 Don Kulak All rights reserved