Matthew T. Neal

The room impulse response in time, frequency, and space: Mapping spatial energy using spherical array beamforming techniques

The auditory perception of rooms is a multi-dimensional problem. Our hearing system interprets time, frequency, and spatial information from arriving room reflections, but traditionally, only the time and frequency domains are considered in room acoustic metric and objective sound field analyses. This work aims to develop spatial visualizations of the energy in a room impulse response (RIR). With a spherical microphone array, a room’s energy can be mapped in full three-dimensions. First, beamforming techniques are used to generate a set of directional RIRs from the spherical microphone array measurement. This set of directional RIRs is analogous to using a microphone with a directional beam pattern response, oriented individually at all points around a sphere. Then, these directional or beam RIRs are time windowed and band-pass filtered to create spatial energy maps of the room. Comparisons between a plane-wave beam pattern and a Dolph-Chebyshev beam pattern will be demonstrated in the context of RIR beamforming. As well, different strategies for normalizing peak energy amplitudes to either the direct sound or a spherical spreading condition will be compared. With these considerations, final results of these spatial energy visualizations and directional RIR animations will be demonstrated. [Work supported by NSF Award 1302741.]The auditory perception of rooms is a multi-dimensional problem. Our hearing system interprets time, frequency, and spatial information from arriving room reflections, but traditionally, only the time and frequency domains are considered in room acoustic metric and objective sound field analyses. This work aims to develop spatial visualizations of the energy in a room impulse response (RIR). With a spherical microphone array, a room’s energy can be mapped in full three-dimensions. First, beamforming techniques are used to generate a set of directional RIRs from the spherical microphone array measurement. This set of directional RIRs is analogous to using a microphone with a directional beam pattern response, oriented individually at all points around a sphere. Then, these directional or beam RIRs are time windowed and band-pass filtered to create spatial energy maps of the room. Comparisons between a plane-wave beam pattern and a Dolph-Chebyshev beam pattern will be demonstrated in the context of RIR beam...

Architectural acoustics: From concert halls to classrooms

Architectural Acoustics (AA) finds its application in and around the built environment and, in doing so, touches and interacts with many different disciplines. Within ASA, the TCAA consistently hosts joint special sessions with the noise, musical acoustics, psychological and physiological acoustics, and signal processing technical committees. Apart from its involvement with special sessions, many unique subcommittees bring added diversity within architectural acoustics. These include subcommittees on classroom acoustics, speech privacy, green building acoustics, healthcare acoustics, building performance standards, and the concert hall research group (CHRG). The makeup of members in the AA group is quite varied. It includes those involved in education and research to those in consulting as well as those in industries that supply products to this segment. Along with published research which is typical of most technical committees, a large segment of the TCAA’s accomplishments are based on the projects prod...

Room measurement setup for accurate objective analysis and realistic subjective comparison of concert halls

When measuring a concert hall, a primary goal is to characterize the perceptual aspects of the room’s quality. Since subjective testing requires extensive time and resources, halls are often compared using objective metrics designed to correlate with perception. The goal of the present work is to create a concert hall measurement database that allows for both accurate objective and realistic subjective assessment of concert halls. A 32-element spherical microphone array is used to capture a spatial room impulse response (SRIR) within a hall. A three-way omnidirectional sound source is used for objective measurements, exhibiting omnidirectional behavior up to 5kHz. The omnidirectional SRIR can be used to calculate typical room acoustic metrics and analyzed using beamforming techniques. For a realistic auralization that resembles an orchestral performance, an alternate sound source was used: a compact 20-element spherical loudspeaker. This loudspeaker can be moved around a stage and the measurement signal c...

Effects of variable acoustic elements on the spatial sound field in multipurpose venues

The design of multipurpose venues typically includes variable acoustic elements which adjust the acoustics of the space to support the intended use. The most common approach to vary the acoustics of a venue is to use variable absorption, often in the form of heavy drapery and/or acoustic felt banners. These elements are often characterized by changes in reverberation time, but their position also impacts the spatial distribution of room energy. In order to study the effects of variable acoustic elements on the three-dimensional sound field at specific audience locations, spatial impulse response measurements were taken in a number of venues using a 32-element spherical microphone array. The venues ranged from a small recital hall of 400 seats to a typical multipurpose hall with 1300 seats to a 2500-seat concert hall with variable acoustics. Beamforming techniques were used to analyze the effects of the variable acoustic elements on the spatial and temporal distribution of sound energy at several receiver ...

A concert hall database of US and European halls: Preliminary measurements and results

Subjective perception in the realm of concert hall acoustics, specifically overall impression, is a difficult problem to approach. Ideally, this type of work should be done using realistic concert hall auralizations, allowing direct comparison of a wide variety of rooms from around the world. Currently, measurements are being taken in concert halls across the United States and in Europe. Halls to be measured were selected from an online survey of researchers and consultants around the United States and Europe. Final selections were made to ensure that a wide variety of hall shapes, sizes, and reverberation times were included in the database, given the available travel resources. Measurements have been made using a 32-element spherical microphone array, a three-way omnidirectional sound source, and a directional sound source. The directional sound source is a 20-element compact array that can reconstruct the frequency-dependent radiation patterns of different orchestral instruments. An overview of the hal...

Investigating energy integration time limits for listener envelopment perception using a real-time adaptable hybrid impulse response method

Listener envelopment (LEV), the sense of being immersed in a sound field, is a common perception in concert hall acoustics, but more work is needed to establish a metric. The objective of this research is to further investigate LEV utilizing custom software building on Dick & Vigeant’s previous research to understand what contributes most to LEV in a concert hall (JASA 140:3175 2016). For this study, spatial room impulse response (SRIR) measurements were obtained using a 32-channel spherical microphone array in different performance venues. These SRIRs were used in a subjective listening test and processed for 3rd order Ambisonic reproduction over a 30-loudspeaker array. Utilizing a custom testing interface developed in Max 7, pairs of equal-length SRIRs with contrasting high and low LEV were time-windowed and summed together to create a hybrid SRIR. A dial in the interface controlled the hybrid SRIR’s time transition point, and subjects were asked to adjust the timing dial to the point at which they perc...

Design of a compact spherical loudspeaker array for simulating accurate instrument directivities for concert hall measurement and auralization

An omnidirectional sound source is often used as a repeatable source to measure impulse responses in concert halls to obtain room acoustics metrics. The limitation of using an omnidirectional source is that real sound sources, such as musical instruments, do not exhibit omnidirectional radiation patterns. For an orchestra, many instruments with unique directional radiation patterns are distributed across the stage. To achieve more realistic auralizations, a 20-element compact spherical loudspeaker array was constructed to simulate the frequency-dependent radiation properties of instruments. This loudspeaker array is capable of achieving a truncated third-order spherical harmonics representation of an instrument’s radiation pattern. Once the source is moved to different instrument locations around stage, frequency-dependent weighting factors for each driver are controlled to match the source’s radiation pattern to that instrument. Full three-dimensional impulse response measurements of the source were made...

Connecting the sense of envelopment to specific components of the sound field using perceptually motivated auralizations

Envelopment is known to be a key attribute related to overall room impression. Despite this importance, limited research has been done to identify the specific components of the sound field that contribute to envelopment. The goal of this study was to determine the timing and spatial distribution of reflections contributing to envelopment. A subjective study was conducted using a range of simulated auralizations, which were played back over a three-dimensional loudspeaker array in an anechoic chamber. For each auralization, subjects rated their perceived envelopment. A real-time acoustic simulation program was developed in Max to generate the signals, which simulated early sound with the image-source method and late sound with statistical reverberation. When creating the stimuli, the program produced immediate auditory feedback in response to adjusting the input parameters. The signals were quantified through impulse response measurements to ensure a wide range of conditions. The subjects’ envelopment rat...

Evaluation of a three-way omnidirectional sound source for room impulse response measurements

Commercially available omnidirectional loudspeakers, which are used to measure room impulse responses, typically have a limited bandwidth and are only omnidirectional below approximately 1kHz. To extend the operating bandwidth, a three-way omnidirectional source was built consisting of a subwoofer, a mid-frequency dodecahedron, and a high-frequency dodecahedron. The subwoofer contains two 10 in. drivers in a sealed box. The mid-frequency dodecahedron uses 12 4 in. mid-bass drivers. The high frequency dodecahedron was made with 12 closely spaced 3/4 in. dome tweeters, providing omnidirectional behavior up to approximately 5 kHz. The directivities of the dodecahedrons were validated by taking measurements in an anechoic chamber. The performance of the sources in a realistic setting was assessed using measurements taken in Eisenhower Auditorium, located at Penn State. Measurements were made with the three sources placed individually on the stage in the same location and compared to measurements made with the...

The construction and implementation of a multichannel loudspeaker array for accurate spatial reproduction of sound fields

The spatial distribution of sound has a strong impact upon a listener’s overall impression of a room and must be reproduced accurately for auralization. In concert hall acoustics, directionally independent metrics such as reverberation time and clarity index simply do not predict this impression. Late lateral energy level, lateral energy fraction, and the interaural correlation coefficient are measures of spatial impression, but more work is needed before we fully understand how the directional distribution of sound should influence architectural design decisions. A three-dimensional array of 28 loudspeakers and two subwoofers has been constructed in a hemi-anechoic chamber at PSU, allowing for accurate reproduction of sound fields. For the array, closed-box loudspeakers were built and digitally equalized to ensure a flat frequency response. With this facility, subjective studies investigating spatial sound in concert halls can be conducted using measured sound fields and perceptually motivated auralizati...