Sound octave

The sound absorption of materials is frequency dependent most materials absorb more or less sound at some frequencies than at others. Sound absorption is usually measured in laboratories in 18 one-third octave frequency bands with center frequencies ranging from 100 to 5000 H2, but it is common practice to pubflsh only the data for the six octave band center frequencies from 125 to 4000 H2. SuppHers of acoustical products frequently report the noise reduction coefficient (NRC) for their materials. The NRC is the arithmetic mean of the absorption coefficients in the 250, 500, 1000, and 2000 H2 bands, rounded to the nearest multiple of 0.05. 

Rotating vanes are provided in the rooms to ensure diffuse sound fields. Sound is introduced into the source room, the average sound pressure level is measured in one-third octave bands in both rooms, and the sound-transmission loss is calculated as follows, where and die average sound... 

A sound is generally not a pure tone, as the latter is only emitted from particular sources. It can be demonstrated that a sound can be divided into different pure tones (superposition method). The waves at different frequencies give the spectrum of the sound, which also describes its energy distribution. In frequency analysis, the spectrum is divided into octave bands. An octave band is defined as the frequency range with its upper boundary twice the frequency of its lower boundary. For every octave band, a central band frequency ( f. ) is defined as follows ... 

Octave bands are divided, on a logarithmic frequency scale, into three equally wide one-third octave bands. This is done often when more exact data of sound spectra are needed. Table 9.14 shows the standardized one-third octave band series. 

As regards the noise spectrum, the different situations can be analyzed ap proximately with NC (noise criterion) and NR (noise rating) curves (Fig. 9.6.3). NC and NR curves define the octave band limits of an acceptable back ground noise each of them is characterized by a number representing the sound pressure level at 1000 Hz. 

Octave band frequency The band in frequency scale that is split into bands, each assigned a sound power level, that is twice the power level of the lower limit. 

The simplest sound level meter consists of a microphone, an amplifier and a meter of some type. Sound level meters are graded according to British and international standards, and the most common type used for accurate measurement purposes it known as the Precision Grade or Type 1 meter. In practice, a basic sound level meter will incorporate weighting networks with either in-built octave filters or provision for connecting an external filter set (Figure 42.6). 

I really don t think it s wise to take it any further, she says, her voice dropping so suddenly, by an octave and several decibels, that I can tell she s forcing herself to sound reasonable. Fergus isn t in a position to make the right judgment, after all. He hardly knows the place. It s not as if he s ever lived there. ... 

Using harmonic overtones, it is possible to sound a tone which will cancel one or more of its octaves reflected in the harmonic scales above and below it. This is easily demonstrated on a cello Suppose a tone, say the open string A, is sounded. The sound is a wave-vibration of air molecules caused by the string, which then acts as a resonator. 

Dennis s rap drew on the theory of sound. If you pluck a string, it will sound in the octave in which it is struck, but it will also sound in octaves above and below its key. It has what are called harmonic overtones. If you strike the chord and then squelch it, you can still hear the harmonic overtones, a phenomenon that had fascinated Pythagoras. Dennis pointed out to us that one can use two sounds to cancel each other if the two sounds are exactly the same in relation to each other. 

If specified, the vendor shall provide both maximum sound pressure and sound power level data per octave band for the equipment. Control of the sound pressure level (SPL) of all equipment furnished shall be a joint effort of the purchaser and the vendor having unit responsibility. The equipment furnished by the vendor shall conform to the maximum allowable sound pressure level specified. ISO 3740, ISO 3744 and ISO 3746 may be consulted for guidance. 

Figure 3.4 Energy decay relief for occupied Boston Symphony Hall. The impulse response was measured at 25 kHz sampling rate using a balloon burst source on stage and a dummy-head microphone in the 14th row. The Schroeder integrals are shown in third octave bands with 40 msec time resolution. At higher frequencies there is a substantial early sound component, and the reverberation decays faster. The frequency response envelope at time 0 contains the non-uniform frequency response of the balloon burst and the dummy-head microphone. The late spectral shape is a consequence of integrating measurement noise. The SNR of this measurement is rather poor, particularly at low frequencies, but the reverberation time can be calculated accurately by linear regression over a portion of the decay which is exponential (linear in dB).

The element coming after any consecutive eight elements is somehow repetition of the first element like sound of an octave in music. ... 

The lowest frequency is called the fundamental, all of the other frequencies are multiples of the fundamental and are called harmonics. Doubling the frequency corresponds to raising a note by one octave. When a piano and a flute play middle-A, they both produce a distribution of sound waves with a fundamental frequency of 440 Hertz, but they sound different because the amplitudes of the different harmonics depend on the instrument. 

Mid-frequency of third-octave band (kHz) Sound pressure levels under which nearly all workers may be repeadly exposed without adverse effect. [One third-octave band level (dB re 20pPa)]... 

Another study was designed to determine how neural circuits are organized to provide a perception of sound. Specifically, Williamson wanted to find whether individuals have a tone map across the auditory cortex—that is, whether tones of different frequencies evoke neural activity at different locations. While Williamson s subjects listened to notes of a scale, the sensors hunted for magnetic field variations, and the researchers were able to monitor the movement of nerve impulses from cell group to cell group inside the brain. It appeared that the brain assigned equal numbers of neurons to each octave of the musical scale, much like the arrangement of keys on a piano. This... 

There are other instruments used for measuring noise including weighted-sound-levels and octave-band analyzers. These instruments measure the noises of different frequencies. 

Figure 8.11 is a comparison of the same sound spectrum as analyzed using three different band intervals—octave band, one-third octave, and narrow band. This comparison shows that the additional resolution provided by narrower band methods can be of vital importance. In this example the level at 1 kHz, as recorded by the octave band instrument is 90 dB, on the one-third octave instrument it is 85 dB, and on the narrow band instrument it is 70 dB. The lower resolution measurements produce higher measurements due to the spillover influence of the nearby peak at... 

Note The advantages of making broad analyses of sound using octave or one-third octave band filter sets are that less time is needed to obtain data and the instrumentation required to measure the date is less expensive. The main disadvantage is the loss of detailed information about the sound which is available from narrow band (FFT) analyzers. 

The overall sound level is calculated by adding the individual levels in the various octave bands. In col-unms 1 and 2 of Table 8.4, the burner sound curve has been split up into its component levels in each octave band. In column 3, the A-weighted correction has similarly been split up and listed. Colunm 4 gives the A-corrected values for the sound curve by simply adding colunm 3 to column 2. Now, the values in colunm 4 must be added to obtain the A-weighted overall sound level. 

Noise emissions are becoming increasingly important. With some refineries located near populated areas, it is important to keep noise to a minimum. Burner testing is usually conducted on a single burner and noise emissions are usually measured at a distance of approximately three feet from the burner air inlet. Data collected during the test includes an overall A-weighted sound pressure level and the sound level at each octave... 

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