Note that, as seen in Figure 1, the trumpet remains to be 12 to fifteen dB above the background at this frequency; so the energy seen at fifty five kHz, although non-harmonic, remains to be trumpet sound. The decrease trace exhibits the background with the trumpet silent; that is dominated by the microphone's 'self-noise,' as proven in section VIII, beneath. Of course this background is current when the trumpet performs; and that's the reason the upper hint is recognized as 'Trumpet + Background.' The mere existence of this vitality is the purpose of this paper, and many of the discussion simply explains why I think that the spectra are correct, throughout the limits described beneath.
Recording engineers usually use this sound to check their tools, and one can see that it is indeed demanding, with the vitality at an elevated level from 7 kHz to above 40 kHz. The fourth excerpt exhibits that by 55 kHz, the harmonics are vanishing.
At the tip, nevertheless, I cite others' work on notion of air- and bone-performed ultrasound, and supply a couple of remarks on the attainable relevance of our spectra to human perception and music recording. Harmonics of French horn can lengthen to above ninety kHz; trumpet, to above 80; violin and oboe, to above forty; and a cymbal crash shows no signal of working out of power at 100 kHz.
If I assumed that the background had been dominated by room sound, I would 'right' each trace to allow for the unflatness of its explicit microphone frequency response. I first considered the way to right the background spectra (the lower curve in Figure 1, for instance). I reflected that if the microphone had no 'self-noise,' then the background I measured would consist solely of ambient sound in the room. In different words, if the random-incidence response curve equipped by the manufacturer is down by 5 decibels at a sure frequency, I ought to elevate the background spectrum by 5 dB at that frequency to get an accurate studying.
At very high frequencies, due to the shortness of the wavelengths and the acoustic 'liveness' of the room, the instrument
may be picked up extra in a random-incidence trend. If this be the case, then as a result of the random-incidence response is down 9½ dB at 70 kHz, I should raise the measured spectrum by 9½ dB at that frequency, and by acceptable quantities at other frequencies.
Every recording engineer knows that speech sibilants , jangling key rings , and muted trumpets can expose problems in recording gear. If the issues come from power under 20 kHz, then the recording engineer simply needs higher equipment. It is even attainable, contrary to my assumption of two paragraphs in the past, that the on-axis curves will not be the appropriate ones to make use of in any respect.
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