Frequency Spectral Characteristics Research Articles

Large eddy simulation for jet noise characteristics of liquid rocket engine with guide groove

The study of jet noise characteristics of liquid rocket engines is a prerequisite for evaluating the noise level of rockets and effectively controlling the noise. To obtain the effect of engine operating and structural parameters on jet noise, the Large Eddy Simulation combined with the FW-H integration method is applied for the scaled engine with guide groove under different chamber pressures and nozzle expansion ratios. The simulation results show that the noise field distribution of the jet with a guide groove is highly symmetrical and has obvious directivity. The noise caused by jet impingement on the guide groove is strongly correlated to the distribution of high vorticity regions. Remarkably, higher chamber pressure leads to longer potential jet length, higher noise sound pressure levels, and greater enhancement effect of impact on noise. However, the frequency spectral characteristics are basically invariant. When the chamber pressure is constant, the increment of the nozzle expansion ratio in a certain range results in lower noise peak frequency and weaker impact effect.

Approaches for Respiratory Sound Analysis in Identification of Respiratory Diseases

Purpose: Medical professionals throughout the world prefer to use conventional stethoscopes to listen to respiratory sounds. Listening to respiratory sounds through stethoscopes is a subjective matter, and proper diagnosis of the disease depends on the skills and ability of the doctor. Computerized analysis of respiratory sounds can help doctors and researchers to characterize different abnormal respiratory patterns and make informed decisions.
 Materials and Methods: This study includes previously reported work in different normal and abnormal respiratory sounds. The IEEE, PubMed, Google Scholar and Elsevier databases were searched and studies with the keywords of lung sound analysis, respiratory sound analysis, and respiratory sound classification were included. Detailed characteristics of normal and abnormal respiratory sounds are mentioned. In addition, Time-amplitude characteristics of different respiratory sound plots are obtained using MATLAB and ICBHI database. This study systematically discusses different approaches for respiratory sound analysis like visual analysis of the time-amplitude signals, frequency analysis, and spectral analysis using fast Fourier transform, statistical analysis, and machine learning approach. A list of relevant datasets is mentioned that can help researchers to do further analysis in this domain.
 Results: The careful observations and analysis show the possibility of predicting respiratory diseases by extracting suitable parameters such as the frequency response and spectral characteristics of the signal. Power spectral density can help us to calculate the maximum, median frequency over an extended period. Using machine learning we can estimate the energy, entropy, spectral features, and wavelets of the signals.
 Conclusion: Computer-based respiratory sound analysis can help medical professionals in making informed decisions. This will help in early diagnosis and devise effective treatment plans for the patients.

The Frequency Spectrum Analysis of Wideband Oscillations of Grid-Connected Voltage Source Converter

With the widespread application of voltage source converters (VSCs) in power systems with high renewable resource penetration, the problem of wideband oscillations caused by interactions between power converters and the power grid has drawn great attention. This paper presents a new methodology for simplifying the mathematical derivations of the impedance model of a VSC. Using the new simplified model, the stability and spectrum characteristics of the performance of the VSC can be clearly analyzed. The proposed second-order d-q impedance model of a grid-connected VSC in the presence of a PLL not only mathematically demonstrates that wideband oscillatory modes always occur in conjugate pairs, but also delivers a clear vision of the physical essence of these wideband oscillations. A classic modal analysis is executed in this paper to demonstrate the stability and frequency spectral characteristics of wideband oscillations in a VSC integrated system. The impact of the control parameters of the VSC on wideband oscillatory stability is thoroughly analyzed by modal analysis using ‘DIgSILENT PowerFactory’ as well as hardware-in-the-loop experiments. This methodology can be considered a powerful tool for the control design of VSCs in the modern power system.

High sensitivity tracking of free-moving targets in time-varying complex backgrounds

Tracking of low-visibility targets in complex backgrounds is an important research field, where existing methods struggle to image low-visibility targets with irregular changes in moving direction and speed. Move contrast imaging can greatly improve the sensitivity of target tracking, which has achieved important applications in the field of X-ray imaging, including high-resolution imaging to microvessels in living rats with the help of contrast agents, <i>in-situ</i> dynamic observation of ion migration and redox reactions during electrochemical reactions, and water refilling along vessels in willow branch without resorting to agents. However, all these applications are limited to imaging with fixed trajectories or monotonous backgrounds. In principle, move contrast imaging is based on the frequency spectral characteristics of the time-domain grayscale signal and is highly sensitive to moving components, which is wavelength-independent. This paper extends the move contrast imaging to the visible light waveband for tracking free-moving targets in time-varying complex backgrounds. To meet the need for tracking imaging of free-moving targets in complex backgrounds, we develop a move contrast imaging (MCI) method based on continuous wavelet transform (CWT) and Hilbert-Huang transform (HHT) with high discriminatory capability for non-stationary signals. Selecting birds in the sky and forest for the tracking imaging, the irregular grayscale changes caused by natural light intensity in the wild field and random swaying of tree leaves result in complex imaging backgrounds. The tracing results of low-visibility free-moving targets show that FT-MCI method, CWT-MCI method and HHT-MCI method can improve the target tracing imaging sensitivity by 179.9 times, 175.8 times and 214.6 times compared with temporal subtraction imaging, respectively. The results of tracking imaging of free-moving targets in complex backgrounds show that compared with the FT-MCI method and CWT-MCI method, the HHT-MCI method can further effectively suppress the influence of background noise on tracking imaging of targets of interest, thus achieving high sensitivity imaging of free-moving targets in time-varying complex backgrounds. Combining the phase diagram of FT-MCI and the imaging parameters, we can further show the motion direction, the motion speed or the distance from the observation point. Therefore, the HHT-MCI imaging method developed in this paper is expected to provide a novel method for tracking free-moving targets in time-varying complex backgrounds.

Identification of fracturing behavior in thermally cracked granite using the frequency spectral characteristics of acoustic emission

The frequency characteristics of acoustic emission (AE) during triaxial compression of thermally cracked and unheated (“fresh”) granite samples were investigated with the aim of understanding the influence of pre-existing cracks on precursor information regarding macroscopic failure. The peak frequency during the damage process was the same for thermally cracked and fresh granites. Analysis of AE signals showed that signals with low peak frequency appeared before failure of the sample, implying the initiation of macrocracks with progressive growth of cracks. The peak amplitude of the frequency spectrum recorded in the thermally cracked samples was much lower than that in the fresh samples. This result suggests two reasons for the difference in peak amplitude: reduction in shear modulus and the attenuation filtering phenomenon caused by thermal cracks (i.e., the differences in overall bulk properties). In particular, the maximum value of peak amplitude in the low-frequency band for the thermally cracked samples was smaller than that for fresh samples. This characteristic can be related to the properties of source of AE signals, such as the stress drop and crack size. Assuming that pre-existing thermal cracks grow during the pre-failure stage, the events with low peak frequency and low peak amplitude in the heat-treated samples are interpreted as exhibiting a low stress drop for individual events. Therefore, although AE signals with low frequency can be considered as precursors to rock failure, cracking behavior suggested by events with low frequency depends on the initial damage condition of the rock sample.

Analysis and Optimization of the Operating Range of a Monopole Antenna Involving ‘Meander’ Type Slot In- homogeneities

Subject and Purpose. The paper presents results of numerical modeling and experimental studies of a disk-shaped microstrip antenna involving ‘meander’ type slotted inhomogeneities. The work has been aimed at optimizing the operating range of the antenna and matching it to external circuits through the use of additional structural elements and appropriate feeding techniques. Methods and Methodology. The design features a circular disk-shaped microstrip resonator containing within its plane groups of slotted inhomogeneities which form a segmented meander line, with the segments oriented relative one another at an angle of 120°. The antenna could be fed through a segment of a screened coplanar line. The location of the screening plane of the coplanar line, as well as its dimensions, were variable. Numerical simulation was carried out within the ‘semi-open resonator’ technique using the finite element method. The degree of optimization of the operating range was estimated, based on analyzing spectral characteristics of the antenna, for a variety of its geometric parameters, and the magnitude of the return loss over a given frequency range. Measurements of the VSWR were carried out with reflectometers. Results. Frequency and power characteristics of a monopole, disk-shaped microstrip antenna have been analyzed and optimized over a wide frequency range. Mechanical dimensions of the additional shielding plane and location thereof have been identified as factors having significant influence upon the frequency-dependent, polarizational and power characteristics of the antenna. Conclusions. The operating frequency range, spectral and power characteristics of a monopole, disk-shaped microstrip antenna have been studied both theoretically and experimentally. Numerical simulations were carried out with the use of the fi nite element method. Experimental studies of the frequency characteristics were performed using reflectometry techniques. The antenna considered can find practical application over a wide frequency range, either as a single radiating element in a device or system, or a constituent part of an antenna array.

Mechanism and fundamental models of gas–liquid surging for valve columns

AbstractThe gas–liquid streams in valve columns are likely to surge at low gas velocities, which degrades tray performances and threatens production safety. Therefore, the gas–liquid surging mechanism and vibration properties were investigated in this article. First, the theory of gas–liquid unstable flow at the seal point was proposed based on the principle of lowest energy. Then, the evolution mechanism, gas–liquid flow characteristics, and essential condition of surging are proposed. Meanwhile, experiments were carried out in a Φ580 mm valve column; the frequency spectral and hydraulic characteristics of surging were in accordance with the expected surging mechanism. Moreover, the surging vanish point criterion and safe operating superficial gas velocity for valve columns were advanced. Finally, fundamental models for surging frequency and amplitude were established. The models are in close accordance with the experimental data, which suggests that they could accurately describe the surging behavior.

SHAKING TABLE TESTS ON THE SEISMIC RESPONSE CHARACTERISTICS OF DOUBLE-COLUMN PIERS WITH SHEAR BEAMS

Shaking table tests were carried out to study the influences of shear beams on the seismic response of double-column piers that were designed to support the girder of a full cable-stayed bridge model. The double-column pier with shear beams included two columns and five shear beams equally installed between the two columns. A full cable-stayed bridge model of 1/70-scale that included the pier (a double-column pier and a double-column pier with shear beams), tower, girder, pile group, and artificial soil was designed and tested on a shaking table array. The seismic response characteristics were compared between the double-column piers with and without shear beams when they were subjected to various earthquake waves of various frequency contents, such as the artificial, El Centro, and Mexico City waves. The experimental results show that the maximum strains at the bottom of the double-column pier with shear beams were less than those of the double-column pier under the artificial, El Centro wave, and Mexico City waves of various shaking amplitudes. The additional shear beams significantly mitigated the strain responses at the bottom of the double-column pier, and decentralized the inner force of the columns. The maximum accelerations at the bottom of the double-column pier with shear beams are 1.4-1.8 times those of the shaking table when the bridge model was subjected to the artificial, El Centro, and Mexico City waves with 0.1 g, 0.15 g, and 0.2 g. The pile-soil-structure interaction significantly affected the seismic response of the double-column pier with shear beams and increased its longitudinal acceleration responses. The seismic responses, such as the acceleration, displacement, and strain responses of the double-column pier with and without shear beams were significantly affected by the frequency spectral characteristics of the various earthquake waves. The extent of the influence was dependent on the relationship between the frequency contents of the earthquake waves and the frequency spectral characteristics of the piers.

Frequency noise reduction of delay-coupled quantum cascade lasers.

This work theoretically investigates the frequency noise and spectral linewidth characteristics of mutually delay-coupled quantum cascade lasers, which are operated in the stable locking regime. We demonstrate that the mutual injection significantly reduces the frequency noise at proper coupling phases. However, the relative intensity noise is insensitive to the mutual injection. Influences of the pump current, the linewidth broadening factor, the coupling phase, and the delay time on the frequency noise are discussed as well. In addition, it is found that the appearance of multiple compound laser modes can deteriorate the frequency noise performance of the lasers.

A study of spectral behaviors of turbulent wall-pressure fluctuations

Turbulent wall-pressure fluctuations are major sources of noise and their numerical calculations have been important topics in engineering fields. In this paper, large eddy simulation is conducted to study spectral behaviors of turbulent wall-pressure fluctuations. The roughness was simulated by using an array of 10 off-set square columns extending in the vertical directions on the bottom surface, and local downstream aerodynamic fields were numerically investigated. The wind speed fluctuations in the flow field and the pressure fluctuations at the bottom of the flow field are monitored at a certain cross-section when varying square column heights and wind turbulence conditions at the wind speed inlet. Frequency spectral characteristics are analysed and the influence factors of wall-pressure fluctuations are explored.

Estimation of sand water content using GPR combined time-frequency analysis in the Ordos Basin, China

Abstract Groundwater is the key factor of determining the growth of vegetation. Identifying the characteristics of groundwater is an important basis to formulate a management plan for water resources and develop the technology of controlling desertification in arid areas scientifically. It is also important to the environmental protection in China. Ground penetrating radar (GPR) signals produce a special response to the changes in water content during propagation, thus it is essential to study the propagation of electromagnetic pulse in aeolian sand with different water content. The GPR tests of aeolian sand samples with different water content were conducted by a GPR system, dielectric constant meter, and conductivity meter. The temporal and frequency spectral characteristics of electromagnetic signals measured from aeolian sand samples were analyzed. The results show that the dielectric constant and conductivity of aeolian sand increase nonlinearly as the water content increases, and the attenuation coefficient of electromagnetic pulses increases parabolically. Meanwhile, the temporal waveform fluctuates significantly at the boundary of different media, and its two-way travel time increases nonlinearly as the water content increases, the pulse velocity decreases nonlinearly. Furthermore, the peak frequency of the spectrum for a signal propagating through aeolian sand decreases as the water content increases. The peak frequency is concentrated in the 1000 to 1400 MHz range, but the shape and bandwidth of the spectrum are less affected by water content. The above-mentioned correlations can provide a water content estimation of aeolian sand with direct value to the local authorities which are involved in the decision-making process for effective water management in arid and semi-arid area.

Effect of Guide Vanes on Flow and Vibroacoustic in an Axial-Flow Pump

An effect of guide-vane numbers on pressure fluctuations and structural vibroacoustics induced by unsteady flow is performed by a hybrid numerical method. A 3D flow field is simulated in axial-flow pump with four impeller blades, in which three diffuser models with 5, 7, and 9 vanes are devised to match, respectively. A full scale structural vibroacoustics coupled model is solved using LMS acoustics software. The results show that the blade-passing frequency (BPF) is dominated frequency of the vibration acceleration of pump, which is consistent with frequency spectral characteristics of pressure pulsation. The unsteady pressure fluctuating becomes strong as the flow discharge decreases from 1.0Qv to 0.6Qv, the circumferential unsteady behavior of which is more severe due to flow nonuniformity induced by the suction elbow at partial operation. Generally, the pressure fluctuating increases slightly when the flow discharge increases from 1.0Qv to 1.3Qv. Moreover, pressure fluctuations amplitude on the pump with 9-vane diffuser is small relative to other two models and the vibrating accelerating and radiation sound field at BPF are also slight relatively, which indicates that appropriate guide-vane numbers contribute to suppress pressure fluctuations and vibroacoustics in axial-flow pump. The conclusions in the present paper can provide theoretical guidance for low vibration pump design.

Prediction and Analysis of Structural Noise from a U-beam Using the FE-SEA Hybrid Method

With urban rail transit noise becoming an increasingly serious issue, accurate and quick analysis of the low to medium frequency spectral characteristics of this noise has become important. Based on the FE-SEA (Finite Element - Statistical Energy Analysis) hybrid method, a vibration prediction model of a U-beam was established using a frequency-dividing strategy. The frequency domain and spatial characteristics of the vibration and structural noise of the U-beam within the 1.25-500 Hz frequency range, when subjected to vertical wheel-rail interaction forces, were analyzed. Compared with other methods described in the literature, the proposed FE-SEA hybrid method improves the calculation efficiency while ensuring better accuracy for a wide frequency range of structural noise and vibration. It was found that the excitation frequencies of the wheel-rail force dominate the spectra of the vibration and structural noise of the U-beam. Therefore, the frequency band containing the excitation frequencies should be the target for noise and vibration reduction when implementing strategies. The results show that the bottom plate contributes the most to the sound pressure level at all prediction points, and therefore should be the focus for noise and vibration reduction.

Research of Anomaly in Fix-Point Deformation Prior to Qinghai Menyuan MS6.4 Earthquake in 2016

Based on the observation data of the fix-point deformation precursor in Gansu, Qinghai, Ningxia and Inner Mongolia, this study investigates the Qinghai Menyuan MS6.4 earthquake which occurred in January 21, 2016, and applied the S-transform temporal-frequency analysis method to calculate and extract the short-impending high-frequency anomalies in five stations prior to this earthquake: borehole strain of Menyuan station, borehole tilt of Huangyuan station, borehole title of Sitan station, borehole strain of Haiyuan station and borehole tilt of Liangshui station. On the basis of the interference elimination and the data quality assessment, the frequency spectral characteristics and temporal-spatial distribution characteristics of high-frequency anomalies prior to the earthquake are analyzed and studied. The study results indicate that prior to the Menyuan earthquake, on the basis of the background signals, massive signals with a period of about 10-60mins emerged in multiple stations. In addition, the spatial distributions of the signals conformed to the openness and nonlinearity characteristics of the deformation anomalies while the energy amplitude expanded as the earthquake approached, and then attenuated gradually subsequent to the earthquake. Concerning the temporal characteristics, as the epicentral distance became shorter, the occurrence and duration period of the anomalies gradually shortened, and the impending earthquake anomalies gradually increased. Besides, the anomalies migrated from the peripheral region to the source region. The study conclusions are of great significance to the temporal-spatial evolutionary characteristics of short-impending anomalies prior to strong earthquakes.

Numerical investigation on vibration and noise induced by unsteady flow in an axial-flow pump

Full-scale structural vibration and noise induced by flow in an axial-flow pump was simulated by a hybrid numerical method. An unsteady flow field was solved by a large eddy simulation-based computational fluid dynamics commercial code, Fluent. An experimental validation on pressure fluctuations was performed to impose an appropriate vibration exciting source. The consistency between the computed results and experimental tests were interesting. The modes of the axial-flow pump were computed by the finite element method. After that, the pump vibration and sound field were solved using a coupled vibro-acoustic model. The numerical results indicated that the the blade-passing frequency was the dominant frequency of the vibration acceleration of the pump. This result was consistent with frequency spectral characteristics of unsteady pressure fluctuation. Finally, comparisons of the vibration acceleration between the computed results and the experimental test were conducted. These comparisons validated the computed results. This study shows that using the hybrid numerical method to evaluate the flow-induced vibration and noise generated in an axial-flow pump is feasible.

Acoustic characteristics of clearly spoken English tense and lax vowels.

Clearly produced vowels exhibit longer duration and more extreme spectral properties than plain, conversational vowels. These features also characterize tense relative to lax vowels. This study explored the interaction of clear-speech and tensity effects by comparing clear and plain productions of three English tense-lax vowel pairs (/i-ɪ/, /ɑ-ʌ/, /u-ʊ/ in /kVd/ words). Both temporal and spectral acoustic features were examined, including vowel duration, vowel-to-word duration ratio, formant frequency, and dynamic spectral characteristics. Results revealed that the tense-lax vowel difference was generally enhanced in clear relative to plain speech, but clear-speech modifications for tense and lax vowels showed a trade-off in the use of temporal and spectral cues. While plain-to-clear vowel lengthening was greater for tense than lax vowels, clear-speech modifications in spectral change were larger for lax than tense vowels. Moreover, peripheral tense vowels showed more consistent clear-speech modifications in the temporal than spectral domain. Presumably, articulatory constraints limit the spectral variation of these extreme vowels, so clear-speech modifications resort to temporal features and reserve the primary spectral features for tensity contrasts. These findings suggest that clear-speech and tensity interactions involve compensatory modifications in different acoustic domains.

Numerical investigation on flow-induced structural vibration and noise in centrifugal pump

A full scale structural vibration and noise induced by flow was simulated by a hybrid numerical method. An interior flow field was solved by large eddy simulation firstly. The sliding mesh technique was applied to take into account the impeller-volute interaction. A sensitivity analysis on effects of near-wall grid size and sampling time on amplitude of pressure pulsations was performed to impose appropriate vibration exciting source. Computed modal of pump components was validated by experimental results, before the volute vibration and sound field were simulated using a coupled vibro-acoustic model. The numerical results indicated that the amplitude of pressure fluctuation, especially on those points located at near the volute tongue, strongly depended on near-wall grid size. The dominated frequency of the vibration velocity of volute was also blade-passing frequency (BPF), which was in according with frequency spectral characteristics of unsteady pressure fluctuation. Directivity distribution of radiation acoustic field excited by volute vibration was typical dipoles. This study shows that it is feasible to use the hybrid numerical method to evaluate the flow-induced vibration and noise generated in centrifugal pump.

EMG frequency spectral characteristics of inspiratory accessory muscles during the shoulder lifting motion in throwing athletes

Background: Dysfunctions related to the shoulder joint are very common in throwing athletes, due to the overhead generated inrepetitive movements with your arms above your head. In addition, in handball, the movements are of high intensity, which generatesa large metabolic demand and high ventilatory demands. Studies show that highest work of the respiratory muscles, coupled withhigh mechanical requirement in the upper limbs, generates a dual requirement under the inspiratory muscles in these athletes,leading to limitations related to muscle fatigue. Objective: The objective of this study was to evaluate the EMG frequency spectralcharacteristics (fatigue index) to accessory muscles of inspiration, through the induction of shoulder elevation movement in handballathletes. Method/Design: The sample consisted of 25 male athletes with an average age of 20 years. Fatigue index was measured bysurface electromyography (mean and median frequency - Hz), in which the volunteers performed isometric movement of elevationof the shoulders, and after collection, the data were processed off-line on Matlab R2014a software. For statistical analysis used theShapiro-Wilk test to test the normality of the data, followed by the comparison of data through the Student t test for independent samples.Significant difference was considered for p value <0.05. Results: The results showed a significant difference between the mean andmedian frequency values for the sternocleidomastoid muscle right compared to the left sternocleidomastoid muscle. Conclusions: Weconclude that during isometric movement of shoulder elevation, there was asymmetry in the frequency spectral characteristics of onlyone accessory muscles of inspiration, presenting asymmetry, and greater fatigue in the sternocleidomastoid muscle dominant side inrelation to the non-dominant side.

Accuracy of Combined EEG Parameters in Prediction the Depth of Anesthesia

BackgroundThe importance of proper qualitative evaluation of EEG parameters during surgery has been recognized since many years. Although none of the characteristics based on the frequency, entropy, and Bi spectral characteristics have been regarded as a good predictor for detection of the depth of anesthesia alone. So it seems necessary to study multiple characteristics together.ObjectivesIn this study we tried to introduce the best combination of the mentioned characteristics.Materials and MethodsEEG data of 64 patients undergoing general anesthesia with the same anesthesia protocol (total intravenous anesthesia) were recorded in all anesthetic stages in Shohada Tajrish Hospital. Quantitative EEG characteristics are classified into 4 categories: time, frequency, bi spectral and entropy based characteristics. Their sensitivity, specificity and accuracy in determination of the depth of anesthesia are yielded by comparison with recorded reference signal in awake, light anesthesia, deep anesthesia and brain death patients. Then, with combining 2, 3, 4 and 5 of characteristics and using coded algorithm we determined the error degree and introduced the combination yielding the least error.ResultsFifteen vectors (of dimension two to five) which yielded the best results were introduced. Vectors combined of entropy based characteristics obtained 100% specificity and sensitivity during all 4 stages.ConclusionsThe combination entropy based characteristics had high accuracy in predicting the depth of anesthesia. Reevaluation of classic indices cortical status index and BIS seems necessary. The next step is to find a system to simplify the evaluation of this information for technicians.

Near equivalence of human click-evoked and stimulus-frequency otoacoustic emissions

Otoacoustic emissions (OAEs) evoked by broadband clicks and by single tones are widely regarded as originating via different mechanisms within the cochlea. Whereas the properties of stimulus-frequency OAEs (SFOAEs) evoked by tones are consistent with an origin via linear mechanisms involving coherent wave scattering by preexisting perturbations in the mechanics, OAEs evoked by broadband clicks (CEOAEs) have been suggested to originate via nonlinear interactions among the different frequency components of the stimulus (e.g., intermodulation distortion). The experiments reported here test for bandwidth-dependent differences in mechanisms of OAE generation. Click-evoked and stimulus-frequency OAE input/output transfer functions were obtained and compared as a function of stimulus frequency and intensity. At low and moderate intensities human CEOAE and SFOAE transfer functions are nearly identical. When stimulus intensity is measured in "bandwidth-compensated" sound-pressure level (cSPL), CEOAE and SFOAE transfer functions have equivalent growth functions at fixed frequency and equivalent spectral characteristics at fixed intensity. This equivalence suggests that CEOAEs and SFOAEs are generated by the same mechanism. Although CEOAEs and SFOAEs are known by different names because of the different stimuli used to evoke them, the two OAE "types" are evidently best understood as members of the same emission family.