Octave bands

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. 

Face velocity, m /min Octave band center frequency, H2 ... 

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 ... 

For example, the lowest octave band corresponds to a frequency range between 22 and 45 Hz. Its central value is... 

In Table 9.13 the standardized octave band series are shown. 

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. 

TABLE 9.14 Standardized One-Third Octave Band Series (ISO 226 ... 

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. 

If it is necessary for engineering purposes to know the tonal make-up of a noise, several approaches are possible. A bandpass filter can process the noise. The most common filters are octave band filters, and the agreed center frequencies are as follows ... 

If there are tonal noise problems the local authority may use more complex measurements to specify the required reduction. Noise rating may be used or octave or one-third octave band levels specified. 

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).

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)]... 

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

To illustrate the suppression technique on a full-scale engine, the frequencies are divided by a factor of 12 (assuming the full-scale nozzle diameter = 0.6 m) and then the frequency spectrum is converted into a discrete one-third-octave spectrum. In order to do this, one must first determine the center frequency of each octave band and then its lower and upper limits. The corrected pressure spectrum is then integrated over these limits to determine the SPL value at the respective center frequency. A-weighting is then applied to the entire spectrum to properly reflect subjective judgments of the noise as commonly used in the literature. 

Since this program is in the early stages of development, this chapter will discuss recent progress associated with a planned flight test of a military aircraft to acquire narrowband acoustics. The majority of full-scale acoustic studies have been performed to establish the environmental impact on a community by estimating ground contours to set Duration Index Levels. As a result, these studies are generally performed in Vs-octave bands. 

A flight test of a military aircraft will be conducted at NAS Patuxent River. The purpose is to obtain narrowband acoustic data to identify jet-noise sources. This same data can be analyzed in Va-octave bands and used to evaluate predicted ground-based contours. 

The wide range of frequencies in our hearing range may be convenienfly handled by breaking it up into octave bands. Each octave band represents a doubling in frequency. Table 8.2 shows the 10 octave bands that cover the hearing range and the center frequencies that can be used to represent the octave band. Each octave band extends over seven fundamental musical notes. 

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