Maximum Length Sequence Signal Research Articles

Basin scale propagation modeling of MLS signals from the Kauai Beacon source

Long distance underwater acoustic propagation is of interest for a variety of applications including underwater navigation, yet modeling such propagation is challenging due to the large degree of environmental uncertainty. In this presentation, the propagation of 75Hz center frequency maximum length sequence (MLS) signals emitted from a submerged source near Kauai and received at various sites throughout the Pacific Ocean Basin is modeled with the Bellhop ray tracing [1] and Monterey-Newport Parabolic Equation [2] models. Sites such as the Monterey Accelerated Research System (MARS) observatory and the Comprehensive Nuclear Test Ban Organization (CTBTO) hydroacoustic monitoring station near Wake Island are included in the discussion. The range-dependent environment is based on historical profiles from the World Ocean Atlas database and reanalysis data from the hybrid coordinate ocean model (HYCOM) [3]. Bathymetry profiles along the axis of propagation are interpolated from the General Bathymetric Chart of the Oceans (GEBCO) 2021 database [4]. Simulated broadband channel impulse responses transmission loss levels in the modeled environment are compared to selected receptions of the signal at the hydrophone sites. Additionally, fluctuations of the modeled transmission loss in the presence of an internal wave model [5] are discussed.

Basin-scale propagation modeling of MLS signals between Kauai and Monterey

Long distance underwater acoustic propagation is of interest for a variety of applications including underwater navigation, yet modeling such propagation is challenging due to the large degree of environmental uncertainty. In this presentation, the propagation of 75 Hz center frequency maximum length sequence (MLS) signals emitted from a submerged source near Kauai and received at the Monterey Accelerated Research System (MARS) observatory are modeled with the Bellhop ray tracing and the Monterey-Newport Parabolic Equation (MNPE) models. The range-dependent sound speed environment is based on historical profiles from the World Ocean Atlas database and reanalysis data from the hybrid coordinate ocean model (HYCOM). Bathymetry profiles along the axis of propagation are interpolated from the General Bathymetric Chart of the Oceans (GEBCO) 2021 database and the Naval Oceanographic Office Digital Bathymetric Data Base Variable Resolution (DBDB-V) data product. Simulated channel impulse responses are compared to observations for transmission loss and signal coherence.

Measuring the dynamic stiffness of resilient materials using ESS and MLS signals

Resilient materials are used to reduce vibration transmission in buildings, e.g. in floating floors designed to decrease the impact sound pressure level in the room below the floor. The performance of a floating floor can be estimated in advance knowing the dynamic stiffness per unit area of the resilient material, which is usually measured on a small sample according to ISO 9052-1. Thus, the reliability of results from ISO 9052-1 measurements is crucial. However, the published standard gives few indications on the measuring equipment and suggests measuring the resonance frequency of the fundamental vertical vibration of the test specimen and of the load plate by using either sinusoidal, white noise or pulse signals, considered as equivalent. In this work, a measuring apparatus following the ISO 9052-1 specifications with some hardware and software improvements is described. Then the scanty specifications about the excitation signal are complemented introducing two robust alternatives, compatible with the ISO 9052-1 prescriptions: the ESS (Exponential Swept Sine) and MLS (Maximum Length Sequence) signals. The two newly proposed signals are validated measuring the dynamic stiffness per unit area of different resilient materials, whose resonance frequency is estimated using the same measuring equipment and four different excitation methods: hammer generated impulses, white noise, ESS and MLS. The comparison shows that ESS and MLS signals, already in use since many years for other kinds of acoustic measurements, are viable alternatives to traditional techniques and may have advantages in term of repeatability, dynamic range, cleanness of the frequency response and background noise immunity.

Impulse Responses Measured with MLS or Swept-Sine Signals Applied to Architectural Acoustics: An In-depth Analysis of the Two Methods and Some Case Studies of Measurements Inside Theaters

The measurement of impulse responses inside of a room or a theatre is a critical task in architectural acoustics. The impulse responses are usually measured using either Exponential Sine Sweep (ESS) or Maximum Length Sequence (MLS) signals. The theory behind MLS generation and use is well developed and does not involve computational problems. When a non-linear system is measured, the distortion appears in the deconvolved impulse response as a series of peaks distributed over time, in deterministic positions. In case of presence of noise during the measurement, the phase of any spurious noise, even an impulsive one, is randomly distributed over the entire length of the recovered impulse response. The ESS signal shows some advantages over MLS, such as a better signal to noise ratio (SNR) and a robust non-linearity rejection. Typically, in the same conditions, the ESS has a dynamic range of about 15dB higher than MLS. Nonetheless, the generation of an ESS signal and the subsequent analysis of impulse responses involve some problems whose solutions are not yet common practice and short impulsive noises can contaminate the sampled data, causing bad effects on the deconvolved impulse response, in form of a frequency decreasing sweep. For the above mentioned reasons ESS is generally more suitable than MLS for use in architectural acoustics, but in some cases the use of MLS signal is still to be preferred. Differences and advantages between impulse response measurements obtained by means of MLS or ESS, are analyzed and discussed in this paper and an in-depth analysis of both measurement methods is presented. Some case studies of impulse response measurements performed inside historical Italian opera houses using both MLS and ESS, in non-ideal conditions (for example in presence of spurious noise), are presented and examined, highlighting differences, advantages and pitfalls of both measurement methods.

Sound reflection measurements on noise barriers in critical conditions

It is often necessary to check the intrinsic acoustic characteristics of installed noise barriers, like sound reflection and airborne sound insulation, to verify their compliance to design specifications or their quality after some years of life. These characteristics may be measured in-situ following CEN/TS 1793-5. These guidelines have been substantially improved in the frame of the European project QUIESST (2009–2012), which is now under consideration by the relevant CEN working groups to produce new European standards. The new method for measuring sound reflection specifies the usage of an electroacoustic sound source and a microphone grid, in order to obtain a set of impulse responses; these are processed by means of improved algorithms to compute the required results. The impulse responses are acquired using MLS (Maximum Length Sequence) or ESS (Exponential Swept-Sine) as test signals. While the acoustical characteristics of a noise barrier obtained using the two signals are generally equivalent, in critical conditions – e.g. excessive background noise or local meteorological variability – some discrepancies may occur. Moreover, different type of background noise (broadband or impulsive) give different effects on the final result, using MLS or ESS test signals. This paper presents a series of experiments designed to put in evidence the differences between Reflection Index measurements performed in the mentioned critical conditions, according to the QUIESST guidelines, done using MLS or ESS signals. The relative advantages and drawbacks are analysed and discussed in detail. Conclusions are drawn on the selection of the best test signal for each situation.

M系列信号を用いた新型超音波バイオテレメトリーシステムの開発

M系列信号を用いた新型超音波バイオテレメトリーシステムの開発

머리전달함수 측정법의 실험적 비교

Three methods(the stepped sine method, the statistical method(random excitation method) and the maximum-length sequence(MLS) method) for head-related transfer functions(HRTFs) are experimentally compared in view point of accuracy and efficiency. First, the stepped sine method has high signal-to-noise ratio, but low efficiency. Second, the statistical method is fast measurement speed, but weak to noise than the other methods. Finally, the MLS method shows both good efficiency and high signal-to-noise ratio, but it needs additional software or equipment such as MLS signal generator. For comparison of measurement accuracy, HRTFs of KEMAR dummy are measured for various azimuths and elevations. Error norms for magnitude and phase of HRTFs are defined and calculated for the measured HRTFs. The calculated error norms show that the methods give similar results in magnitude and phase except a little phase difference in the MLS method.

An Application of Time‐Reversal Acoustics to Sound Insulation Measurements in Buildings

This paper considers the possible application of time reversal acoustics (TRA) to airborne sound insulation measurements in buildings. In TRA, an array of transducers is set up to form a time‐reversal 'mirror' or 'cavity'. Using this array, the sound radiated from an initial source is collected and refocused spatially and temporally, thereby being reproduced at the initial source position with high signal‐to‐noise ratio (S/N). Most previous studies of TRA have been conducted underwater with ultrasonic sound sources, with only a few in the audible range in real buildings. The technique is best suited to non‐dissipative systems, raising the question of whether any advantage could exist for transmission between rooms. This study applies TRA experimentally in the audible range using maximum length sequence signals for transmission between two rooms. Comparison is made between conventional measurements (with and without impulse response deconvolution) and TRA in terms of effective S/N and apparent level differ...

The fast measurement of head related impulse responses for all azimuthal directions using the continuous measurement method with a servo-swiveled chair

This paper proposes a method for the fast measurement of head related impulse responses (HRIRs) for all azimuthal directions using the continuous measurement method with a subject sitting on a servo-swiveled chair (SSC). The subject faces all azimuthal directions with a constant angular speed as the SSC rotates and hears corresponding spatial sounds. In our continuous measurement method, we use a maximum length sequence (MLS) as the excitation signal, record binaural signals using tiny microphones placed in the subject’s ear canal around 5 mm from the entrance, and feed them, along with the MLS signal, into a PC so that HRIRs can be extracted for all azimuthal directions and discrete elevations. In this paper, the underlying principles behind the method are described and conditions for successful implementation are examined through numerical simulations. Further, the validity of the proposed method is verified using a physical realization and empirical measurements.

Validation of the pyramid tracing algorithm for sound propagation outdoors: comparison with experimental measurements and with the ISO–DIS 9613 standards

The aim of this work is to test the accuracy of numerical previsions of the sound propagation outdoors made with the newly developed pyramid tracing algorithm (Farina A, RAMSETE—a new pyramid tracer for medium and large scale acoustic problems, Proceedings of the Euro-Noise 95, Lyon, France 21–23 March 1995, 1995;1:55.; Farina A, Pyramid tracing vs. ray tracing for the simulation of sound propagation in large rooms, in COMACO'95, Proceeding of the International Conference on Computational Acoustics and its Environmental Applications, Southampton, England, 1995, Computational Mechanics Publications:Southampton 1995: 109.) Pyramid tracing has already been proposed and validated for indoor calculation, both for the study of the sound quality in concert halls or other Sabinian spaces, and for noise reduction in factories or other non Sabinian spaces (Farina A, Verification of the accuracy of the pyramid tracing algorithm by comparison with experimental measurements of objective acoustic parameters, Proceedings of the ICA'95, Trondheim, Norway, 26–30 June 1995, 2;1995:445.; Farina A, Auralization software for the evaluation of the results obtained by a pyramid tracing code: results of subjective listening tests, Proceedings of the ICA'95, Trondheim, Norway, 26–30 June 1995, 1995;2:441.) As the algorithm allows for the evaluation of sound passing through sound insulating panels, taking also into account the edge diffraction over free boundaries of screens, it is natural that it can be used also for the simulation of sound propagation outdoors. Anyway it must be noted that the actual implementation of pyramid tracing, available in the Ramsete package, does not take into account the interference effects caused by propagation at grazing incidence over the soil, neither the ray curvature caused by temperature or air velocity vertical gradients. In this paper a comparative experiment is described, in which the pyramid tracing code was tested against experimental measurements (made with an innovative Maximum Length Sequence signal, previously used only for room acoustics). The sound source was a directive loudspeaker, which Sound Power Levels and Directivity Balloons in Octave Bands were previously measured in free field conditions. The measurement in each point was obtained through asynchronous cross-correlation of the signal coming from a standard Sound Level Meter (recorded for convenience on a DAT tape recorder) and the original MLS sequence, through a fast-Hadamard algorithm, yielding the Impulse Response between the Source and the Receiver positions. With proper synchronous averaging of the incoming signal, a great improvement in the signal-to-noise ratio was achieved, making it possible to make measurements almost not affected by background noise even in highly shielded positions. The comparison is made also with an Image Source code, built up around the computing formulas contained in the new ISO–DIS standard 9613, kindly made available by Luigi Maffei (Farina A, Maffei L, Sound propagation outdoor: comparison between numerical previsions and experimental results, in COMACO'95, Proceedings of the International Conference on Computational Acoustics and its Environmental Applications, Southampton, England, 1995, Computational Mechanics Publications: Southampton 1995:55). The test case was chosen in an area containing all the most interesting acoustic phenomena: large distance propagation over absorbing and reflecting soil, shielding by embankments and buildings, multiple reflections on buildings facades. Only adverse atmospheric conditions were not taken into account (strong wind, inverted temperature gradient). Both the experimental and numerical data were used to build graphical plots, enabling a direct comparison of the results: they show that the capability of accurately modeling the source directivity produce generally a better estimate using the pyramid tracing algorithm, but the shielding effects and excess attenuation are more accurately modeled by the ISO9613 code.

Variations in the surface textures of choral reflectors

The commission to design a portable choral reflector seemed a simple process of finding a balance between acoustical and structural requirements. Initial inquiries, however, found a lack of information on the acoustical effect of different surface textures normally used. An experiment was developed to assess both the subjective and objective performance of varying degrees of surface roughness. Eight possible surfaces were chosen and applied to standardized panels. The impulse response of each panel was measured in an anechoic room using a maximum length sequence signal. Early results indicate significant differences in the reflected energy and frequency response of the respective panels. An attempt to obtain a subjective evaluation of the reflections by convolving choral and solo voice recordings with the impulse response of the panels will be reported. A prototype choral reflector based on this work will be described. [The support of the Acoustics Research Centre for this project is acknowledged.]

Construction of a HATS and its HRTF measurement for 3-D sound

Based on the anthropometric data on the head and torso dimensions in Korean male adults, a head and torso simulator (HATS) was constructed. Data, which have no counterparts in the Korean standards, were used with ANSI S3.36-1985. Measuring microphones can be positioned at the eardrum or at the ear canal entrance according to measuring purposes. Head related transfer functions (HRTFs) for the HATS, i.e., binaural impulse responses, were measured at 710 points on a spherical surface of radius 1.55 m using a burst maximum length sequence (MLS) signal of 65 535 samples in an anechoic chamber. The measurement system consists of one part to generate the MLS signal and to drive a Boss 101A loudspeaker and another part to record the output signal of a microphone in the HATS and consequently to measure the impulse response. Also measured were the impulse responses of the driving loudspeaker and some headphones for 3-D sound reproduction in order to get the exact HRTF of the HATS-alone. The impulse-version HRTFs at the sampling frequency of 44.1 kHz, which have filter lengths of 512 points with minimum phase characteristics and can be used for 3-D sound, were finally obtained through a postprocessing procedure.

Average acoustical characteristics of ten atrium spaces

Extensive acoustical measurements were made in ten different atrium spaces varying in volume from 3 300 to 45 000 m3. Impulse responses were measured at between 18 and 71 combinations of source and receiver position in each atrium depending on the size of the atrium. The computer-based measurement system (RAMSoft-3) used a maximum length sequence signal and a fast Hadamard transform procedure to obtain 5-s-long impulse responses. Octave band values of decay times, sound levels, and energy ratios were calculated for each impulse response. The average values of each quantity in each atrium were used to characterize typical acoustical characteristics of untreated atria. These allow rough estimates of the acoustical conditions in other atria as a function of the volume of the spaces.

Propagation over rough finite impedance boundaries

Previous studies of the boundary wave associated with near-grazing propagation over randomly rough surfaces have been concerned with high-frequency (2.5–30 kHz) pulses and acoustically rigid surfaces [see, for example, Medwin etal., J. Acoust. Soc. Am. 66, 1131–1134 (1979)]. Continuous wave measurements of the excess attenuation spectrum by means of maximum length sequence signal analysis have been carried out. These have enabled study of roughness effects in the frequency range 500 to 2500 Hz. Data have been obtained over rough wooden surfaces and over a thin polymer foam layer to which rectangular 3 cm by 0.5 cm strips have been attached. These data have been compared with predictions due to Howe [J. Sound Vib. 98, 83–94 (1985)] modified for effects of roughness shape according to Tolstoy [J. Acoust. Soc. Am. 66, 1135–1143 (1989)]. [Work supported by S.E.R.C. (UK).]

Measuring and predicting the effect of an audience

Previous research has shown that the sound absorption of arrays of occupied and unoccupied theater chairs is related to the ratio of perimeter-to-area of a sample of chairs [J. Acoust. Soc. Am. 91, 1514–1524 (1992)]. This work has been extended by using sound absorption measurements of various sized samples of occupied and unoccupied chairs to predict the absorptive effect of audiences in auditoria. In the auditoria, a full range of modern room acoustics quantities were measured both with and without the audience present. This was accomplished using a maximum length sequence signal and fast Hadamard transform technique to obtain impulse responses without significantly disturbing the audience. The influence of the audience was measured in terms of changes to: reverberation times, early decay times, sound levels, early-to-late sound ratios, and lateral energy fractions. The effect of the audience on reverberation times was compared to the changes in the other quantities. The changes in reverberation times and the added absorption due to the audience is predictable from the reverberation time measurements.

More experimental data collected by comparison between the Schroeder’s back integration method and a new method based on sound intensity for measuring reverberation time

The experimental comparison between the Schroeder’s and the intensity-based method for measuring reverberation time [D. Stanzial etal., J. Acoust. Soc. Am. 91, 2368(A) (1992)] has been improved both in the enumeration of cases and in its methodology. The sound field was generated inside various test rooms of different volume by means of a reference isotropic source driven by a maximum length sequence signal. The measured (pressure) impulse responses of the rooms as well as the active and reactive sound intensity for the same input–output configuration of the acoustic system was then used for the reverberation time evaluation. The collected data by comparison between the two methods are reported here.

M系列信号を利用した船舶操縦性指数の適応同定

M系列信号を利用した船舶操縦性指数の適応同定