ISO 13472-1-2022 pdf free download – Acoustics — Measurement of sound absorption properties of road surfaces in situ — Part 1: Extended surface method.
5Test system
5.1Components of the test system
The test equipment shall comprise an electronic signal generator, a power amplifier and a loudspeaker,a microphone with amplifier and a signal analyser capable of performing cross-correlation andtransformations between the time and the frequency domains.
A sketch of the essential components of the measuring system is shown in Figure 1.
The complete measuring system shall meet the requirements of at least a type 2 instrument inaccordance with IEC 61672-1.For the purposes of this document, the measurement frequency range isdisplayed in one-third-octave bands, from 250 Hz to 4 kHz.
5.2Sound source
The loudspeaker shall
一 have a single loudspeaker driver,
– be constructed without any port, e.g. to enhance low frequency response,
be constructed without any electrically active or passive components (such as crossovers) whichcan affect the frequency response of the whole system,and
have a smooth magnitude of the frequency response without sharp irregularities throughout themeasurement frequency range, resulting in an impulse response under free-field conditions with alength not greater than 3 ms.
NOTE As the sound power reflection factor is calculated from the ratio of energetic quantities extractedfrom impulse responses taken using the same loudspeaker and microphone within a short time period, thecharacteristics of the loudspeaker frequency response are not critical, provided a good quality loudspeakermeeting the above prescriptions is used.
5.3Test signal
The test signal shall consist of a repeatable short signal with a low peak-to-RMS ratio, typically below2, and an almost flat spectrum that covers the one-third-octave bands from 250 Hz up to 4 kHz withan acceptable S/N ratio. Several signals may be used, such as maximum-length sequences(MLS) orexponential sine sweep (ESS), see ISo 18233.
6Data processing
6.1Calibration
The measurement procedure described in this document is based on the power ratio of two transferfunctions extracted from the same electro-acoustical chain.An absolute calibration of the measurementchain with regard to the sound pressure level is，therefore,unnecessary.However, a referencemeasurement as described in Annex B is required.
6.2Sampling frequency
The subtraction principle implies knowledge of the exact wave form, especially for checking change oftime delays in the measurement chain. The sampling frequency,f, shall therefore have a value greaterthan 40 kHz.
NOTE1 A time delay is often introduced in the measurement chain when using an audio card connected to aportable computer.This time delay is compensated before performing the signal subtraction technique describedin 6.3.
NOTE2 Although the signal is already unambiguously defined when the Nyquist criterion for the samplingfrequency is met (see Reference [18)), higher sampling frequencies facilitate a clear reproduction of the signal.Errors can be detected and corrected more easily, such as corrections needed to account for time shifts due totemperature changes.
6.3Temporal separation of the signals
Before measurements, a preliminary check shall ensure that no parasitic signals appear in the temporalwindows (see 7.3).
The separation of the direct and the reflected signals is obtained by applying the signal subtractiontechnique (see 4.2).
The low-frequency limit of the analysis is proportional to the reciprocal of the length of the narrowesttemporal window used and dependent on the window shape. For measurements in the 250 Hz one-third octave band be valid, the low-frequency limit is 220 Hz, which implies a minimum time windowlength of approximately 5,9 ms.For more details see Annex F.
In every case the shape and the lengths of the selected temporal window shall be reported in the testreport.
For the purpose of this document, windowing operations in the time domain shall be performed usinga temporal window, called Adrienne temporal window, with the following specifications (see Figure 3);- a leading edge having a left-half Blackman-Harris shape and a fixed length of 0,5 ms (“pre-window”);- a flat portion (“main body”);
a trailing edge having a right-half Blackman-Harris shape;
– the lengths of the flat portion and the right-half Blackman-Harris portion shall have a ratio of 7/3.The Adrienne window having a total length Tw,ADR = 5,9 ms (standard length), is made of:
– a leading edge having a left-half Blackman-Harris shape and a fixed length of 0,5 ms (“pre-window”); a flat portion having a total length of 3,78 ms (“main body”);
a trailing edge having a right-half Blackman-Harris shape and a length of 1,62 ms.ISO 13472-1 pdf download.