(A) Synchronization must be accomplished at the start of each measurement day and checked at the end of each measurement day to minimize the effects of generator time drift. Any such drift must be documented and accounted for in processing.
(B) The use of Global Positioning System (GPS)-based measurement systems for acquisition of TSPI data is becoming commonplace. GPS receivers are capable of providing the user with precise time-base information (broadcast from the GPS satellite system), in some cases, eliminating the need for a separate time keeping device in the TSPI system. For noise data recording or for non-GPS-based-TSPI systems, dedicated IRIG B time-code generators are available that uses the GPS signal to constantly update and maintain time synchronization. Use of such a universal broadcast time-base can greatly simplify the logistics of time synchronization between measurement systems. It should be noted that there are two available time-bases for GPS-based systems, GPS Time and Coordinated Universal Time (UTC), whose values differ by more than 10 seconds at any given instant. Although the GPS signal includes both time bases, not all GPS receivers give the user access to both, therefore, the user should exercise caution in receiver selection.
(C) Many acoustical data recorders provide separate annotation channels in addition to the normal data channels. These channels are often not suitable for recording a modulated time-code signal because of limitations on dynamic range or bandwidth. In such cases, a normal data channel of the recorder must be dedicated to recording the time-code signal.
(D) When continuous time-code recording is used, analysis of the recorded acoustic data can be initiated by routing the time-code channel output into a time-code reader and triggering the analyzer based on readout time.
(ii) Recording of Single Time Marker: This method involves transmittal and recording of a radio "hack", or tone, usually used to indicate the "recorders on" or "overhead" time instant. This method typically requires a dedicated channel on both the noise and the TSPI recording systems. When such a system is used, analysis can be triggered manually by an operator listening for the hack, or by a detector circuit responding to the tone. When the operator wishes to start analysis at a time other than that of the time marker, a stopwatch or delay circuit can be used to delay triggering of the analyzer. When manual triggering is employed, the operator must use extreme care to perform the triggering as accurately as possible. Accuracy to within one-tenth of a second can be expected from a conscientious human operator.
(iii) Measurement of Interval between Recorder Start and Overhead: This method of synchronization involves use of a stopwatch or elapsed-time indicator to measure the interval between start-up of the noise data recorder and the instant that the aircraft position is overhead of the centerline noise measurement point. This method can be employed successfully as long as (1) the operator exercises care in timing, (2) the determination of the overhead instant is performed accurately, and 3) the start-up characteristics of the recorder (in both record and playback modes) are known and repeatable. Some recorders have variable startup times that cannot be predicted. Such recorders are not suitable for this method of synchronization.
(iv) Setting of Internal Time-Stamp Clock: Many digital recorders maintain a continuous internal time-of-day function by encoding time data in the recorded data stream. This method uses a digital recorder's sub-code time, synchronized to the time-base used for the TSPI data. Unfortunately, the time-setting function on many recorders does not provide for the necessary precision. The "second" digits cannot be made to "tick" in synchrony with an external clock. Such recorders are unsuitable for this method of synchronization. As with the continuous time-code recording method, synchronization by this method must be checked at the beginning and end of each measurement day, and any drift accounted for in processing.
(4) Additional Time-Synchronization Considerations: Regardless of the synchronization method used, all elements affecting time synchronization (such as analyzer start-up delay, head displacement between normal and annotation data channels on analog recorders, delays in automated triggering circuits, etc.) must be identified, quantified, and accounted for in analysis and processing. Whenever human response to a timing event is required, errors cannot be accurately predicted, and conscientious operation is required to minimize such
errors. The use of automated methods is preferred. Other methods, or variants of the listed methods may be appropriate, but the use of all methods and instrumentation is subject to prior approval by FAA.
(5) 10 dB-down Period: This period is the portion of the airplane flyover in which the measured noise level is within 10 dB of PNLTM (i.e., the period to be used for the calculation of EPNL). Care should be taken during use of the flight path intercept method so that noise levels are outside the 10 dB-down period before flight path go-around procedures are initiated.
c. Procedures
None
123. Section A36.2.3.3
Position and performance data required to make the adjustments referred to in section A36.9 of this appendix must be automatically recorded at an approved sampling rate. Measuring equipment must be approved by the FAA.
a. Explanation
This section specifies requirements for measuring sufficient airplane position and performance data to permit adjustments from test to reference conditions. See section 6.6 of the appended ICAO TM. These data must be measured and recorded in permanent form, and the measurement systems used to record data must be FAA approved.
b. Supplemental Information
(1) Airplane and Engine Performance Parameters: Examples of parameters needed for measurement of airplane and engine performance include: airplane altitude, climb angle, airspeed and gross weight, flap position, landing gear position, engine thrust (power) setting parameters (e.g., compressor rotor speed, engine pressure ratio, exhaust gas temperature), and airplane accessory condition (e.g., A/C and APU "on" or "off"). Any other parameters that may affect measurement or adjustment of noise data and/or airplane or engine performance must also be recorded throughout the 10 dB-down period (e.g., SBV position, CG position).
c. Procedures
(1) Applicant's Responsibility: Aircraft performance parameters to be recorded and their range and tolerance should be proposed by an applicant in a noise compliance demonstration plan.
(2) Airplane Performance Measurements: Adequate airplane and engine parameters are to be recorded during all certification testing to ensure that airplane performance can be accurately determined. For example, for transport airplanes this may necessitate measurement and recording of airplane flap position, landing gear position, speed brake position, APU operation, and normal engine thrust (power) setting and associated airplane flight parameters. Determination and recording of adequate information enables validation of the test airplane configuration and correction of airplane performance and engine performance from test conditions to part 36 acoustic reference day conditions.
(3) Recorder Sampling Rate: The measurements of airplane position, airplane airspeed, airplane performance and engine performance parameters are to be recorded at an approved sampling rate sufficient to permit adjustments from test to reference conditions throughout the 10 dB-down period. An acceptable recording sampling rate for transport category airplanes is two to five samples per second.
124. Section A36.3 Measurement of Airplane Noise Received on the Ground
125. Section A36.3.1 Definitions
For the purposes of section A36.3 the following definitions apply:
126. Section A36.3.1.1
Measurement system means the combination of instruments used for the measurement of sound pressure levels, including a sound calibrator, windscreen, microphone system, signal recording and conditioning devices, and one-third octave band analysis system.
Note: Practical installations may include a number of microphone systems, the outputs from which are recorded simultaneously by a multi-channel recording/analysis device via signal conditioners, as appropriate. For the purpose of this section, each complete measurement channel is considered to be a measurement system to which the requirements apply accordingly.
127. Section A36.3.1.2
Microphone system means the components of the measurement system which produce an electrical output signal in response to a sound pressure input signal, and which generally include a microphone, a preamplifier, extension cables, and other devices as necessary.
128. Section A36.3.1.3
Sound incidence angle means in degrees, an angle between the principal axis of the microphone, as defined in IEC 61094-3 and IEC 61094-4, as amended and a line from the sound source to the center of the diaphragm of the microphone.
Note: When the sound incidence angle is 0°, the sound is said to be received at the microphone at "normal (perpendicular) incidence;" when the sound incidence angle is 90°, the sound is said to be received at "grazing incidence."
129. Section A36.3.1.4
Reference direction means, in degrees, the direction of sound incidence specified by the manufacturer of the microphone, relative to a sound incidence angle of 0°, for which the free-field sensitivity level of the microphone system is within specified tolerance limits.
130. Section A36.3.1.5
Free-field sensitivity of a microphone system means, in volts per Pascal, for a sinusoidal plane progressive sound wave of specified frequency, at a specified sound incidence angle, the quotient of the root mean square voltage at the output of a microphone system and the root mean square sound pressure that would exist at the position of the microphone in its absence.
131. Section A36.3.1.6
Free-field sensitivity level of a microphone system means, in decibels, twenty times the logarithm to the base ten of the ratio of the free-field sensitivity of a microphone system and the reference sensitivity of one volt per Pascal.
Note: The free-field sensitivity level of a microphone system may be determined by subtracting the sound pressure level (in decibels re 20 µPa) of the sound incident on the microphone from the voltage level (in decibels re 1 V) at the output of the microphone system, and adding 93.98 dB to the result.
132. Section A36.3.1.7
Time-average band sound pressure level means in decibels, ten times the logarithm to the base ten, of the ratio of the time mean square of the instantaneous sound pressure during a stated time interval and in a specified one-third octave band, to the square of the reference sound pressure of 20 mPa.
133. Section A36.3.1.8
Level range means, in decibels, an operating range determined by the setting of the controls that are provided in a measurement system for the recording and one-third octave band analysis of a sound pressure signal. The upper boundary associated with any particular level range must be rounded to the nearest decibel.
134. Section A36.3.1.9
Calibration sound pressure level means, in decibels, the sound pressure level produced, under reference environmental conditions, in the cavity of the coupler of the sound calibrator that is used to determine the overall acoustical sensitivity of a measurement system.
135. Section A36.3.1.10
Reference level range means, in decibels, the level range for determining the acoustical sensitivity of the measurement system and containing the calibration sound pressure level.
136. Section A36.3.1.11
Calibration check frequency means, in hertz, the nominal frequency of the sinusoidal sound pressure signal produced by the sound calibrator.
137. Section A36.3.1.12
Level difference means, in decibels, for any nominal one-third octave midband frequency, the output signal level measured on any level range minus the level of the corresponding electrical input signal.
138. Section A36.3.1.13
Reference level difference means, in decibels, for a stated frequency, the level difference measured on a level range for an electrical input signal corresponding to the calibration sound pressure level, adjusted as appropriate, for the level range.
139. Section A36.3.1.14
Level non-linearity means, in decibels, the level difference measured on any level range, at a stated one-third octave nominal midband frequency, minus the corresponding reference level difference, all input and output signals being relative to the same reference quantity.
140. Section A36.3.1.15
Linear operating range means, in decibels, for a stated level range and frequency, the range of levels of steady sinusoidal electrical signals applied to the input of the entire measurement system, exclusive of the microphone but including the microphone preamplifier and any other signalconditioning elements that are considered to be part of the microphone system, extending from a lower to an upper boundary, over which the level non-linearity is within specified tolerance limits.
Note: Microphone extension cables as configured in the field need not be included for the linear operating range determination.
141. Section A36.3.1.16
Windscreen insertion loss means, in decibels, at a stated nominal one-third octave midband frequency, and for a stated sound incidence angle on the inserted microphone, the indicated sound pressure level without the windscreen installed around the microphone minus the sound pressure level with the windscreen installed.
a. Explanation
Sections A36.3.1.1 through A36.3.1.16 provide technical definitions of a total noise measurement system, its microphone system, and terms that relate to the acoustical performance characteristics of the measurement system and/or its components.
b. Supplemental Information
(1) Applicability: These definitions may be applicable to measurement systems used in noise certification of other types of aircraft (e.g., small airplanes and rotorcraft).
c. Procedures
(1) Applicant’s Responsibility: An applicant’s noise compliance demonstration plan must take these definitions into account in proposing measurement systems for use in noise certification.
142. Section A36.3.2 Reference environmental conditions
143. Section A36.3.2.1
The reference environmental conditions for specifying the performance of a measurement system are:
a) air temperature 73.4°F (23°C);
b) static air pressure 101.325 kPa; and
c) relative humidity 50 %.
a. Explanation
This section specifies the required environmental conditions that serve as reference for defining the performance of the measurement systems used for noise certification.
b. Supplemental Information
(1) These environmental specifications correspond to the recommended requirements of the International Electrotechnical Commission (IEC).
c. Procedures
None
144. Section A36.3.3 General
Note: Measurements of aircraft noise that are made using instruments that conform to the specifications of this section will yield one-third octave band sound pressure levels as a function of
time. These one-third octave band levels are to be used for the calculation of effective perceived noise level as described in section A36.4.
145. Section A36.3.3.1
The measurement system must consist of equipment approved by the FAA and equivalent to the following:
a) a windscreen (see A36.3.4);
b) a microphone system (see A36.3.5);
c) a recording and reproducing system to store the measured aircraft noise signals for subsequent analysis (see A36.3.6);
d) a one-third octave band analysis system (see A36.3.7); and
(e) calibration systems to maintain the acoustical sensitivity of the above systems within specified tolerance limits (see A36.3.8).
a. Explanation
This section specifies the types of components required for a noise certification measurement system.
b. Supplemental Information
(1) Measurement System Criteria: The specifications for a measurement system allow flexibility in applicant procurement of measurement system components. While on-site EPNL analysis may be useful for estimation of recording levels or for other diagnostic purposes, a true acoustical analysis requires that data be recorded in the field. This will allow for later reanalysis or auditing of acoustic data. A recording also facilitates later off-line processing of acoustic data, including application of adjustments for items such as system frequency response, microphone pressure response, and analyzer bandwidth error. Recording simplifies synchronization with other pertinent data, such as tracking and meteorological measurements. Such synchronization is necessary for proper application of many of the required adjustments to acoustic data, for elements such as microphone free-field response, windscreen incidence-dependent insertion loss, the influence of ambient noise, high altitude jet noise effects, non-reference flight performance, and non-reference meteorological conditions. Proper implementation of such adjustments in the field would be extremely difficult.
(2) Approval of Measurement System: FAA approval must be obtained for systems used for measurement, recording, and analysis of aircraft noise. Most of the currently available system components that are appropriate for aircraft noise certification use have already been approved, but implementations of new technology and variants or upgrades of existing components may require evaluation by VNTSC before FAA approval. Of special concern is the potential for a digital component’s functionality to change as a result of firmware or operating system upgrades or modifications. Applicants should be aware that approval of a particular component might be version-dependent.
c. Procedures
(1) Validation of Measurement System Configuration: FAA Order 8110.4B (reference 7) provides an FAA policy and procedure to promote uniform implementation of the noise certification requirements of part 36.
Each applicant must submit information to VNTSC about the measurement, recording, and analysis instrumentation and software used. VNTSC will determine whether any listed components require evaluation for approval by FAA.
(2) Changes in Measurement System Configuration: If an applicant makes changes to the approved instrumentation, VNTSC must be notified before aircraft noise certification testing, to determine whether additional evaluation and FAA approval are required.
146. Section A36.3.3.2
For any component of the measurement system that converts an analog signal to digital form, such conversion must be performed so that the levels of any possible aliases or artifacts of the digitization process will be less than the upper boundary of the linear operating range by at least 50 dB at any frequency less than 12.5 kHz. The sampling rate must be at least 28 kHz. An anti-aliasing filter must be included before the digitization process.
147. Section A36.3.4 Windscreen
148. Section A36.3.4.1
In the absence of wind and for sinusoidal sounds at grazing incidence, the insertion loss caused by the windscreen of a stated type installed around the microphone must not exceed –1.5 dB at nominal one-third octave midband frequencies from 50 Hz to 10 kHz inclusive.
a. Explanation
This section specifies the methodology for determining windscreen insertion loss and insertion loss tolerances for purposes of noise certification.
b. Supplemental Information
(1) Determination of Data Adjustments for Windscreen Insertion Loss: The physical condition of a windscreen can significantly affect its performance, and manufacturer-provided data for windscreen insertion loss are valid only for new or clean, dry windscreens. Insertion loss data adjustments for windscreens may be obtained by free-field calibration in an anechoic chamber.
c. Procedures
(1) Applicant's Responsibility: The applicant must specify proposed windscreen types and their insertion loss characteristics in a noise compliance demonstration plan.
149. Section A36.3.5 Microphone system
150. Section A36.3.5.1
The microphone system must meet the specifications in sections A36.3.5.2 to A36.3.5.4.
Various microphone systems may be approved by the FAA on the basis of demonstrated equivalent overall electro-acoustical performance. Where two or more microphone systems of the same type are used, demonstration that at least one system conforms to the specifications in full is sufficient to demonstrate conformance.
Note: An applicant must still calibrate and check each system as required in section A36.3.9.
151. Section A36.3.5.2
The microphone must be mounted with the sensing element 4 ft (1.2 m) above the local ground surface and must be oriented for grazing incidence, i.e., with the sensing element substantially in the plane defined by the predicted reference flight path of the aircraft and the measuring station. The microphone mounting arrangement must minimize the interference of the supports with the sound to be measured. Figure A36-1 illustrates sound incidence angles on a microphone.
Grazing incidence 90°
60° 120°
O° 180°
Sensing element
Figure A36-1: Illustration of Sound Incidence Angles on a Microphone
152. Section A36.3.5.3
The free-field sensitivity level of the microphone and preamplifier in the reference direction, at frequencies over at least the range of one-third-octave nominal midband frequencies from 50 Hz to 5 kHz inclusive, must be within ±1.0 dB of that at the calibration check frequency, and within ±2.0 dB for nominal midband frequencies of 6.3 kHz, 8 kHz and 10 kHz.
153. Section A36.3.5.4
For sinusoidal sound waves at each one-third octave nominal midband frequency over the range from 50 Hz to 10 kHz inclusive, the free-field sensitivity levels of the microphone system at sound incidence angles of 30°, 60°, 90°, 120° and 150°, must not differ from the free-field sensitivity level at a sound incidence angle of 0° (“normal incidence”) by more than the values shown in Table A36-1. The free-field sensitivity level differences at sound incidence angles between any two adjacent sound incidence angles in Table A36-1 must not exceed the tolerance limit for the greater angle.
Table A36-1 Microphone Directional Response Requirements
a. Explanation
This section specifies the required performance characteristics of microphone systems that may be used during noise certification testing.
b. Supplemental Information
(1) Grazing Incidence: The purpose of the grazing incidence specification is to minimize the effects of variations in the microphone response during measurement of airplane noise. By orienting the microphone so that the airplane is substantially within the plane of the microphone diaphragm (grazing incidence) during the 10 dB-down period, all sound from the airplane impinges on the microphone at approximately the same angle relative to its axis.
Normal Incidence (0 º) Grazing Incidence (90 º )
Nominal midband frequency kHz
Maximum difference between the free-field sensitivity level of a microphone system at normal incidence and the free-field sensitivity level at specified sound incidence angles dB _______________________________________________________________________
Sound Incidence angle degrees
0.05 to 1.6
2.0
2.5
3.15
4.0
5.0
6.3
8.0
10.0
0.5
0.5
0.5
0.5
0.5
0.5
1.0
1.5
2.0
0.5
0.5
0.5
1.0
1.0
1.5
2.0
2.5
3.5
1.0
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
6.5
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
7.5
(2) Microphone Characteristics: These specifications are based on the performance characteristics of typical one-half-inch condenser microphones designed for nearly uniform frequency response at grazing incidence. Other microphones may be used, provided they meet the specified performance requirements. For example, pre-polarized (electret condenser) free-field microphones exist that greatly minimize the possibility of arcing in humid environments and do not require an external polarization voltage. Although many of these microphones are intended primarily for use in normal-incidence free-field applications, they can be used in airplane noise certification testing if their performance at grazing incidence meets the requirements of section A36.3.5.
(3) Microphone Specifications: Table A36.1 specifies the maximum permitted differences between the free-field sensitivity of a microphone at normal incidence and the free-field sensitivity at specified sound incidence angles for sinusoidal sound waves at each one-third-octave-band nominal midband frequency over the range of 50 Hz to 10 kHz. These differences are larger at higher frequencies, allowing for the effect of the microphone body in a free-field environment.
c. Procedures
(1) Microphone Orientation: For microphones located directly under the flight path, an orientation angle of 90 degrees from vertical is appropriate regardless of target altitude. For lateral noise measurements, applicants may wish to reorient the microphones for grazing incidence for each target altitude in order to maintain substantially grazing incidence throughout the 10 dB-down periods. In many cases, this reorientation can eliminate the need to apply data adjustments for varying-incidence, since the incidence angles will be more likely to be contained within ± 30 degrees of grazing incidence.
(2) Applicant's Responsibility: The applicant must specify proposed microphone types and orientation for noise certification testing in a noise certification compliance demonstration plan.
154. Section A36.3.6 Recording and reproducing systems
155. Section A36.3.6.1
A recording and reproducing system, such as a digital or analog magnetic tape recorder, a computer-based system or other permanent data storage device, must be used to store sound pressure signals for subsequent analysis. The sound produced by the aircraft must be recorded in such a way that a record of the complete acoustical signal is retained. The recording and reproducing systems must meet the specifications in sections A36.3.6.2 to A36.3.6.9 at the recording speeds and/or data sampling rates used for the noise certification tests. Conformance must be demonstrated for the frequency bandwidths and recording channels selected for the tests.
156. Section A36.3.6.2
The recording and reproducing systems must be calibrated as described in section A36.3.9.
a) For aircraft noise signals for which the high frequency spectral levels decrease rapidly with increasing frequency, appropriate pre-emphasis and complementary de-emphasis networks may be included in the measurement system. If pre-emphasis is included, over the range of nominal one-third octave midband frequencies from 800 Hz to 10 kHz inclusive, the electrical gain provided by the preemphasis network must not exceed 20 dB relative to the gain at 800 Hz.
157. Section A36.3.6.3
For steady sinusoidal electrical signals applied to the input of the entire measurement system including all parts of the microphone system except the microphone at a selected signal level within 5 dB of that corresponding to the calibration sound pressure level on the reference level range, the
time-average signal level indicated by the readout device at any one-third octave nominal midband frequency from 50 Hz to10 kHz inclusive must be within ±1.5 dB of that at the calibration check frequency. The frequency response of a measurement system, which includes components that convert analog signals to digital form, must be within ±0.3 dB of the response at 10 kHz over the frequency range from 10 kHz to 11.2 kHz.
Note: Microphone extension cables as configured in the field need not be included for the frequency response determination. This allowance does not eliminate the requirement of including microphone extension cables when performing the pink noise recording in section A36.3.9.5.
158. Section A36.3.6.4
For analog tape recordings, the amplitude fluctuations of a 1 kHz sinusoidal signal recorded within 5 dB of the level corresponding to the calibration sound pressure level must not vary by more than ±0.5 dB throughout any reel of the type of magnetic tape used. Conformance to this requirement must be demonstrated using a device that has time-averaging properties equivalent to those of the spectrum analyzer.
159. Section A36.3.6.5
For all appropriate level ranges and for steady sinusoidal electrical signals applied to the input of the measurement system, including all parts of the microphone system except the microphone, at one-third-octave nominal midband frequencies of 50 Hz, 1 kHz and 10 kHz, and the calibration check frequency, if it is not one of these frequencies, the level non-linearity must not exceed ±0.5 dB for a linear operating range of at least 50 dB below the upper boundary of the level range.
Note 1. - Level linearity of measurement system components may be tested according to the methods described in IEC 61265 as amended.
Note 2. - Microphone extension cables configured in the field need not be included for the level linearity determination.
160. Section A36.3.6.6
On the reference level range, the level corresponding to the calibration sound pressure level must be at least 5 dB, but no more than 30 dB less than the upper boundary of the level range.
161. Section A36.3.6.7
The linear operating ranges on adjacent level ranges must overlap by at least 50 dB minus the change in attenuation introduced by a change in the level range controls.
Note: It is possible for a measurement system to have level range controls that permit attenuation changes of either 10 dB or 1 dB, for example. With 10 dB steps, the minimum overlap required would be 40 dB, and with 1 dB steps the minimum overlap would be 49 dB.
162. Section A36.3.6.8
An overload indicator must be included in the recording and reproducing systems so that an overload indication will occur during an overload condition on any relevant level range.
163. Section A36.3.6.9
Attenuators included in the measurement system to permit range changes must operate in known intervals of decibel steps.
a. Explanation
Sections A36.3.6.2 through A36.3.9 specify the performance characteristics required for a recording and reproducing system used to store airplane noise signals recorded during noise certification testing for subsequent analysis.
b. Supplemental Information
(1) Recorder Types: An applicant has a choice of recorder types that will satisfy the FAA requirement for recording “the complete acoustic signal” during certification testing. In addition to a magnetic tape recorder, other means of obtaining a “true” acoustic recording include digital audiotape (DAT), recordable compact disc (CD-R), and direct-to-hard-disk recording. The applicant should be aware that systems that use data compression techniques that result in substantial data loss, such as Mini-Disc (MD) or digital compact cassette (DCC), are not acceptable.
c. Procedures
(1) Frequency Range for Recordings: The time-varying waveform produced by the microphone response to noise signals during certification tests must be recorded. If there are questions about the data observed during the tests, the recording can be replayed, multiple times if necessary, to verify the results.
Recorded data, whether digital or analog in nature, must allow reproduction and reprocessing of an analog signal over the frequency range of 40 Hz to 12.6 kHz. A dynamic range of at least 60 dB is recommended. Many typical instrumentation DAT recorders feature a nominal 10 kHz bandwidth operating mode in which the attenuating response of the antialiasing filter intrudes within the 10 kHz one-third-octave passband. In such cases, the recorder must be operated in a nominal 20 kHz-bandwidth mode, which may reduce the number of available channels or the duration of available time per tape.
Note: Although the one-third-octave bands of interest are those with nominal center frequencies of 50 Hz through 10 kHz, to ensure that the entire actual bandwidth of the uppermost and lowermost bands is included, the center frequencies of the one-third-octave bands immediately outside this range are specified.
(2) Digital Recording Levels: The overload characteristic of a digital system is determined primarily by the limits of the analog-to-digital-conversion. Since such an overload condition is characterized by an abrupt, catastrophic type of distortion, the level range should be set so that the anticipated maximum signal level is at least 10 dB, and preferably 20 dB, below the upper boundary of the linear operating range.
(3) Dynamic Range Limits-Digital Recorders: The lower limit of a digital recording system’s usable dynamic range is more often determined by amplitude non-linearity (due primarily to “quantization error”) than by the presence of a noise floor. Digital devices (such as recorders or analyzers) that are to be used for aircraft noise certification purposes must be tested to determine the extent of such non-linearity.
(4) Consider a 16-Bit Quantization System: The theoretical dynamic range of such a system is usually assumed to be near 96 dB (i.e., 20*log10 (2^16)). At the lower limit of this range, there is a potential for 6 dB error in the digitized signal versus the analog input signal that it represents. IEC 61265 imposes a ±2.0 dB limit on acceptable linearity error outside of an instrument’s linear operating range. As amplitude levels are increased above the lower quantization limit, the linearity error is reduced. If the guidance for setting the level range is followed, the usable dynamic range is further decreased. Significant improvement of amplitude linearity can be obtained by system designers via implementation of techniques such as over-sampling and dithering.
Therefore, testing must be performed to determine the actual limits for each digital recording system.
Assumptions based on experience with analog systems do not always apply.
(5) Pre-emphasis Systems: Use of pre-emphasis will only be allowed if the system also employs complementary de-emphasis. Attempts to compensate for the effects of a pre-emphasis filter by applying onethird-octave-band de-emphasis adjustments (either numerically to analyzed data via a pink noise correction or on