Room Modes Research Articles

Wind turbine annoyance—A clue from acoustic room modes

When one admits that they do not know all the answers and sets out to listen to the stories of people annoyed by wind turbines, the clues can seem confusing. Why would some people report that they could get a better night’s sleep in an outdoor tent, rather than their bedroom? Others reported that they could sleep better in the basement recreation room of their home, than in bedrooms. That made little sense either. A third mysterious clue came from acoustic measurements at homes nearby wind turbines. Analysis of the sound signature revealed low frequency spikes, but at amplitudes well below those expected to cause annoyance. The clues merged while studying the acoustic room modes in a home, to reveal a remarkable hypothesis as to the cause of annoyance from wind turbines. In rooms where annoyance was felt, the frequencies flagged by room mode calculations and the low frequency spikes observed from wind turbine measurements coincided. This paper will discuss the research and the results, which revealed a finding that provides a clue to the annoyance, and potentially even a manner of providing limited relief.

Toward broadband electroacoustic resonators through optimized feedback control strategies

This paper presents a methodology for the design of broadband electroacoustic resonators for low-frequency room equalization. An electroacoustic resonator denotes a loudspeaker used as a membrane resonator, the acoustic impedance of which can be modified through proportional feedback control, to match a target impedance. However, such impedance matching only occurs over a limited bandwidth around resonance, which can limit its use for the low-frequency equalization of rooms, requiring an effective control at least up to the Schroeder frequency. Previous experiments have shown that impedance matching can be achieved over a range of a few octaves using a simple proportional control law. But there is still a limit to the feedback gain, beyond which the feedback-controlled loudspeaker becomes non-dissipative. This paper evaluates the benefits of using PID control and phase compensation techniques to improve the overall performance of the electroacoustic resonator. More specifically, it is shown that some adverse effects due to high-order dynamics in the moving-coil transducer can be mitigated. The corresponding control settings are also identified with equivalent electroacoustic resonator parameters, allowing a straightforward design of the controller. Experimental results using PID control and phase compensation are finally compared in terms of sound absorption performances. As a conclusion the overall performances of electroacoustic resonators for damping the modal resonances inside a duct are presented, along with general discussions on practical implementation and the extension to actual room modes damping.

Acoustics 101 for architects

A presentation of acoustical terminology and concepts that relate directly to the design and construction of an architectural space, non-technical explanations, descriptions, and examples. Architectural acoustics can be defined as the study and application of acoustic principles as they are applied to the inside of a building or structure. Usually, these are buildings that will be used for a live performance or the presentation of other useful information. This paper is written with the architect in mind; to help define and explain some of the most basic concepts relating to architectural acoustics. In today's pro/commercial audio marketplace, architectural acoustics must play a significant role in the design process for any programmed meeting or entertainment space. This work puts essential terms and concepts into the hands of the architect, owner, or anyone else who would like to have a better understanding of this topic. Contents: What is Sound? Sound Propagation Three Acoustical Tools Where Does All the Unused Sound Go After it’s Been Heard? Audio Volume Changes—What Do the Numbers Mean? The Inverse Square Law Room Geometry—The Good, Bad, and Ugly Reverberation and Echo Speech Intelligibility Noise Internal vs External Noise Room Modes Variable Acoustics Psychoacoustics Conclusion.

Active noise control in rooms

Maa published his ideas and achievements in the early days of the new branch of noise control based on using secondary sources to reduce the sound level of the primary sources by negative interference. He concentrated on active control of the reverberant sound in a room by a loudspeaker placed in one of the room corners and a microphone in its close vicinity. He examined the role of the complete solution of the wave equation consisting of a direct wave radiated from the sources and the reverberant field created by room modes. His work resulted in deriving a general formula for possible noise reduction that is independent of the shape, size, and the content of the room. He has shown noise reduction of 8 dB for a noise band centered at 100 Hz.

Optimization of electroacoustic resonators for semi-active room equalization in the low-frequency range

At low frequencies in listening rooms, standing waves cause large frequency-response variations within the whole space. These unwanted phenomena have a significant impact on the sound quality of an audio system rendering. Unfortunately, state-of-the-art soundproofing solutions cannot efficiently handle such low-frequency sound energy. To alleviate this problem, electroacoustic resonators can be used to damp room modes. This concept is based on the connection of direct-radiator loudspeakers to synthetic electrical loads allowing the passive dissipation of a certain part of the incoming acoustic energy of the sound field. Through judicious control of acoustic impedance, and depending of the placement of the electroacoustic resonators in the room, a significant damping of the dominant natural resonances can be achieved in order to meet the specifications of sound reproduction. This paper presents the design of prototypes of electroacoustic resonators and investigates their optimization and spatial arrangement in the perspective of semi-active room equalization.

룸 튜닝에 의한 실내 공진의 최소화

스피커를 실내에서 재생하면 공진에 의해서 저역에 피크와 딥이 생겨서 좋은 저음을 재생하기 어렵다. 이러한 공진을 줄이기 위해서 어느 특정 지점에서 전기적으로 이퀄라이징을 하지만, 이는 그 지점에서만 유효하다. 그리고 이퀄라이징으로 주파수 특성을 평탄하게 조정할 수 있지만, 특정 주파수에서 울림(ringing)은 제거되지 않는다. 다른 방법은 실내 공진이 여기되지 않은 지점에 스피커를 설치하는 것이다. 그러나 현실적으로 실내에서는 여러 가지 제약 때문에 원하는 지점에 스피커를 실제로 설치하기가 어렵다. 따라서 실내 공진에 의한 저주파수의 문제를 해결하기 위한 이상적인 방법은 베이스 트랩을 사용하는 것이다. 본 논문에서는 이퀄라이징에 의한 룸 튜닝과 스피커를 설치하는 위치의 최적화를 논의하고, 공진 모드에 의한 저주파수의 문제는 베이스 트랩을 실내에 설치하여 해결하는 것이 최상의 방법이라는 것을 제시하였다. The room modes were showed as the peaks and dips on the low frequency response of the loudspeaker, and were one of the biggest obstacles to reproduce the sound. In order to reduce the occurrence of resonance, equalizing is performed at one position, and the frequency response to be equalized is valid only at that position. Performing the equalization would improve the flatness of the frequency response a little, but it cannot eliminate the ringing. Another method is that it is located the speaker at the specific position where the room resonances were not frequently happened. However, there may be the practical limitation that you may not be able to install the speaker at the position to be wanted. One of the practical solutions to decrease the production of resonance in room is the use of bass trap. In this paper, the practical limit for the room tuning using an equalizer and the room optimization will be discussed. The use of bass traps to solve the resonance problem that is always happened in the room is also proposed.

Methods of discovering problem room modes

Determining the best course of action to eliminate problem room modes in the Kansas Public Radio Live Performance studio required a way of measuring room modes and determining the problem dimensions. This case study focuses on how the room modes where measured and the determination of the problem modes in the KPR studio.

Evaluation of room-acoustic modal characteristics from single-point measurements using Bayesian analysis

Room mode analysis is an important element of architectural acoustic design. In small rooms especially, well-separated low frequency room modes can cause unpleasant aural effects, such as undesirable resonances and flutter echoes. Traditional room mode analysis is generally done using the discrete Fourier transform. This work proposes a time-domain modal analysis method by which the amplitudes, frequencies, and damping constants of the room under test can be directly determined for low modal frequencies by using a Bayesian inference algorithm on a single room impulse response (RIR). The method’s time-domain, model-based approach allows the number of modes present in the RIR, as well as the amplitudes, frequencies, and damping constants for each mode, to be determined using Bayesian model selection and parameter estimation, respectively. The method uses Skilling’s nested sampling algorithm to infer parameter values. Results indicate that the method is especially useful in rooms with closely-spaced modes at low frequencies.

Adaptive Control of Low-Frequency Acoustic Modes in Small Rooms

The work presents the implementation in software of an adaptive method for reducing the non-uniformity of the frequency-response-curve, by covering certain walls of small rectangular rooms, with a particular type of a passive acoustical treatment, consisting of porous absorbents covered with perforated sheets, in which each perforation behaves independently as Helmholtz resonators. The method employs new developments of another publication for finding the modal parameters, namely natural frequencies and associated damping constants, of rectangular rooms with arbitrary (uniform) wall-impedances. The adaptive software is capable of continuously improving the implemented type of acoustical treatment and would allow the implementation of new types of treatments as desired. Some testing results are presented, demonstrating the ability of the software for finding best parameters of the acoustical treatment, giving as input data the room dimensions and source/receiver locations only.

Effects of periodically arranged absorptive materials on acoustic potential energy in rectangular rooms

A modal analysis method is applied to compute the acoustic potential energy of a three-dimensional rectangular room, where sound absorptive materials are periodically arranged on the boundaries of the rectangular room. This paper studies the influence of two parameters, including the frequency of the sound source and the number of absorptive material strips, on the acoustic potential energy in detail. This study reveals that both of the two parameters have notable effects on the coupling behavior of room modes. A main result of this paper is that using a periodic arrangement of absorptive materials can be more efficient than totally covering of absorptive materials on corresponding surface in reducing acoustic potential energy under certain conditions. The coupling behavior of room modes can well explain why excess potential energy reduction can be achieved by using periodically arranged absorptive materials.

Questionnaire about Low Frequency Noise Measurements in Rooms

Room modes at low frequencies generate large sound pressure level differences within enclosures. The resulting sound field is complicated and the sound pressure level measurements become very uncertain. Consequently, the measurements of environmental noise in dwellings, of occupational noise exposure, and of sound insulation of façades, are not accurate at low frequencies. A special measurement methodology exists in a few countries. Measurement methods aim to be accurate and reliable, but they can not determine, at the same time, both the existing and the experienced sound field. A questionnaire was created to gather opinions on how measurements should be performed from several experts with experience in low frequency noise measurements in rooms. Twenty-three experts took part in this survey. The results from the questionnaire are presented in this paper. Most participants considered it necessary to standardize a reliable measurement method, but there was no unanimity about what the method should be. The most controversial points of discussion were the locations where the measurements should be taken, and how the measured data should be analyzed. A brief discussion of these points is presented.

Modeling treatments to reduce sound transmission through an open window into a room: Effect of window thickness

Sound transmission loss through an exterior wall is limited by its weakest structure, generally a window, especially if it is open. Considering the acoustic modes within a window and a room are important because much of the acoustic energy from noise sources such as traffic and large wind turbines is in the low to middle frequency range. Previous models of open windows have either neglected the thickness of the window or the influence of room modes on the sound transmission. The present investigation considered a baffled rectangular aperture of finite thickness backed by a rigid walled cavity. An impedance/mobility approach was used to study the effect of the thickness of an open window on the insertion loss and sound pressure levels inside the cavity. The insertion loss study was confirmed using FEM modeling, and the difference in sound pressure levels was compared to experimental results. Increasing window thickness decreases the amount of sound transmitted at frequencies below the second order modes of the cavity for both window sizes under investigation. Using the impedance/mobility approach was effective in this study and allows the model to be easily extended.

Generalized acoustic energy density

The properties of acoustic kinetic energy density and total energy density of sound fields in lightly damped enclosures have been explored thoroughly in the literature. Their increased spatial uniformity makes them more favorable measurement quantities for various applications than acoustic potential energy density (or squared pressure), which is most often used. In this paper, a generalized acoustic energy density (GED), will be introduced. It is defined by introducing weighting factors into the formulation of total acoustic energy density. With an additional degree of freedom, the GED can conform to the traditional acoustic energy density quantities, or it can be optimized for different applications. The properties of the GED will be explored in this paper for individual room modes, a diffuse sound field, and a sound field below the Schroeder frequency.

Teaching room modes and diffraction using COMSOL MULTIPHYSICS.

The concepts of room modes and diffraction are fairly easy to explain in a qualitative way but the math involved to develop the physics of both can be difficult. When explaining the concept of room modes in more detail, many educators start with the idea of a rectangular room with rigid boundaries to get analytic solutions. However, convincing students that room modes exist in rooms of other shapes is not always easy since analytic solutions cannot be found. Similarly, diffraction is easy to explain as a concept but the functions involved in mathematically explaining diffraction are advanced and difficult to understand. In order to get meaningful solutions to both room mode and diffraction, some researchers and educators turn to finite element software. In this paper, the use of Comsol Multiphysics, a general purpose finite element program, in teaching room modes and diffraction, is shown. COMSOL can be used to draw rooms of an arbitrary shape and generate resonance frequencies and room modes in a matter of minutes. COMSOL can also be used to generate animations of sound waves diffracting around objects. Examples of both uses will be shown.

Room Modes With Regard to Speech Intelligibility in Classrooms: Comments on Knecht et al. (2002)

To comment on the article "Background Noise Levels and Reverberation Times in Unoccupied Classrooms: Predictions and Measurements" by Knecht, Nelson, Whitelaw, and Feth (2002). Schroeder frequencies to determine the transition from individual resonances to many overlapping normal modes are calculated and discussed. Schroeder frequencies ranged from 67 Hz to 141 Hz. Knecht et al. could have validly measured reverberation time at 250 Hz in all classrooms and at 125 Hz in the majority of classrooms because, above the Schroeder frequency, measurement of reverberation time is not significantly affected by room modes. Furthermore, Knecht et al. selected 5 different positions in each classroom for measurement of reverberation time, but in small volumes more measurement positions must be selected due to the effects of room modes compared with large volumes.

Room Modes With Regard to Speech Intelligibility in Classrooms: Response to Shalkouhi (2010)

Purpose To respond to comments by Shalkouhi (2010) on the article “Background Noise Levels and Reverberation Times in Unoccupied Classrooms: Predictions and Measurements” by Knecht, Nelson, Whitelaw, and Feth (2002). Method The Schroeder frequencies calculated by Shalkouhi (2010) were compared with the octave-band weighting factors used by Steeneken and Houtgast (1980) in their calculation of the Speech Transmission Index. Results To assess the potential impact of invalid reverberation times measured at 125 Hz on speech understanding in the classrooms, we note that the weight given to the octave band at 125 Hz is 0.0. Conclusions Since the octave band of background noise centered on 125 Hz has no impact on speech understanding, we suggest that reverberation time measures at 125 Hz are unnecessary.

A Study on the Method of Measurement of Heavy-Weight Floor Impact Sound Considering the Room Mode in Low Frequency Ranges

Most of receiving rooms for the measurement of floor impact sound have rectangular shapes with couple of meters of dimension, with reflective finishing, no furniture and no curtains. Modal overlaps in those conditions are the major reason for the low reproducibility, and as a matter of course, low credibility in measurement. Especially the standard sound source that is currently being used for measurement of heavy-weight floor impact sounds, would cause many social problems due to the excessive low-frequency energy created, within a range from 63 to 125 Hz, and is difficult to evaluate and measure. To solve this problem, space-time average SPL’s through combinations of rotating microphones was suggested in the preceding research. In this study, measurement method using rotating machine and fixed microphones were compared and analysed by the field measurement. The measurement method using rotating machine has good consistencies with average SPL of entire receiving room. This method is expected to reduce the measurement time and measurement deviation as well as to support building regulation/standard development in Korea for Korean apartment buildings.

Measurement of Sound Field for Floor Impact Sounds Generated by Heavy/Soft Impact Sources

Field measurements of floor impact sounds at low frequencies give poor repeatability due to spatial sampling problems. In this study, the spatial distribution of heavy-weight impact sound pressure levels was investigated by field measurements using heavy/soft impact sources, which have been specified as a standard heavy-weight impact source in 140-11. The results of the present study showed that the vertical and horizontal distribution of measured sound pressure levels showed a significant room mode effect at low frequencies. Among the existing standards for measurement of a heavy-weight impact sound, KS F 2810-2 and JIS A 1440-2 provide an appropriate estimation of the room average sound pressure level for bare slab and floating floor, with respect to a low bias error and 95% confidence interval. The guideline for measurement of a heavy-weight impact source was recently proposed in order to complement the limitations of KS F 2810-2 and JIS A 1440-2, and it has been shown that the guideline appropriately performed in the test building and in living rooms of tested apartment buildings.

Room mode bass absorption through combined diaphragmatic and Helmholtz resonance techniques

An acoustical bass absorber which reduces peak and dip frequency response errors caused by interference from naturally occurring axial standing waves in rectangular rooms. The apparatus uses two forms of simple harmonic resonance: pistonic diaphragm resonance and Helmholtz cavity resonance. The pistonic diaphragm resonance is achieved by attaching a rigid planar membrane to metal springs. The Helmholtz cavity resonance is achieved by constructing an enclosed chamber attached to an open cylindrical tube. Coupling these two dissipation devices leads to several-fold improvement in absorption and total room mode attenuation.

Reverberation time measuring methods

In this paper different well-established methods of reverberation time measurement are compared. Furthermore, the results obtained using these methods are compared to the results provided by some additional methods which could serve as an in situ tool if, for any reason, the reverberation time measurements cannot be carried out using the standardized methods. The methods compared in this paper include the standardized methods (EN ISO 3382:2000), namely the impulse response measured with pink noise, exponential sweep, MLS, but also pistol shots of different calibers, balloon bursts, gated external pink noise, and the B&K filtered burst method. In order to make the comparison, the measurements were performed in four acoustically very different spaces - a rather small and well-damped listening room, a much bigger damped listening room, a rather reverberant atrium, and a large and very reverberant shoebox-shaped room. The results were evaluated according to signal-to-noise ratio criterion as well. Special attention has been given to the influence of room modes on measurement results.