David A. Dick

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 new metric to predict listener envelopment based on spherical microphone array measurements and ambisonic reproductions

The objective of this work is to propose a new metric for listener envelopment (LEV), the sense of being immersed in a sound field. Current LEV metrics, for example, Late Lateral Energy Level (LJ), are based only the late sound field. However, recent studies indicate that the late component of the impulse response (IR) alone is inadequate for predicting LEV. For this study, room IR measurements were obtained with a 32-element spherical microphone array in a total of nine halls ranging from 400-2500 seats. A subset of these IRs were selected for a subjective listening test and processed for 3rd order Ambisonic reproduction over a 30-loudspeaker array. Two sets of modified IRs were also generated for the listening test by combining parts of IRs from different halls and by bandpass filtering to select certain frequency ranges, respectively. The ambisonic IRs were convolved with anechoic orchestral recordings and subjects rated the perceived LEV for each stimuli. The IRs were also analyzed objectively via pla...

Relating listener envelopment to specific time segments in early and late portions of sound fields

Listener envelopment (LEV) in concert halls is a function of both the spatial and temporal properties of the room impulse response (IR). This study used measured spatial IRs that were modified to study which segments of the early and late parts of the IRs contribute the most to the perception of LEV. Measurements were obtained in the Peter Kiewit Concert Hall in Omaha, NE, using an Eigenmike 32-element spherical microphone array, and processed for third-order Ambisonic reproduction over a 30-loudspeaker array. A subset of the IRs were identified with either an exceptionally high or low amount of LEV. These IRs were modified such that some time segments of the IRs were reproduced in full 3D (e.g., the early energy), while other segments were played only through a single loudspeaker in front of the listener (e.g., the late energy). Additional stimuli were generated that contain time portions of both the highly enveloping IRs and the unenveloping IRs. A subjective listening test was conducted in which listen...

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...

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...

Investigation of listener envelopment using spherical microphone array measurements and ambisonics playback

An important aspect of overall room impression is listener envelopment (LEV), the sense of being immersed in a sound field. Current LEV objective metrics are primarily based on lateral reflections, typically measured with a figure-of-eight microphone. The purpose of this study was to investigate LEV using measured impulse responses (IRs) taken with an Eigenmike 32-element spherical microphone array as an initial step toward creating a new metric for LEV. The spherical array enables a spatial analysis with higher resolution than traditional methods. Impulse response measurements were made in the Peter Kiewit Concert Hall in Omaha, NE, in several seat positions and hall absorption settings. Auralizations were generated by convolving the measured IRs with anechoic music excerpts and then processing the signals for third-order ambisonics playback. The signals were played over an array consisting of 32 loudspeakers in an anechoic chamber. A subjective study was run in which musically trained listeners rated th...

Measurement repeatability of late lateral energy level and lateral energy fraction

Late lateral energy level (GLL) and lateral energy fraction (LF) are two room acoustics measures that have been shown to correlate with certain aspects of spatial impression in concert halls. The purpose of this study was to investigate the repeatability of GLL and LF measurements. A custom microphone stand was built that can be adjusted in each spatial dimension separately to allow for accurate and precise microphone placement. Room impulse responses (IRs) were measured at six receiver locations in a 2500-seat auditorium using two different methods to obtain the lateral energy IR: the beamformed dipole response from a spherical microphone array, and a studio-grade figure-of-eight microphone. Three sets of IR measurements were taken at each receiver location. In between sets, the microphone stand was removed and the various adjustment points were randomly repositioned. The stand was then replaced in the same position to re-measure the IRs. The variability between the measurements at each receiver location was found to be relatively low for GLL (the standard deviation ranged between 0.22 and 0.73 JNDs for the 125–1000 Hz sum), and higher for LF (the standard deviation ranged between 0.49 and 2.80 JNDs for the average over the 125–1000 Hz octave bands). [Work supported by NSF Grant 1302741.]

Dynamic behavior of microwave-cured guitar wood

An ongoing investigation has been conducted to measure and analyze vibration properties of guitar-body wood samples with different wood finish curing methods for Taylor Guitars. The current method uses ultraviolet radiation to cure a lacquer finish. Virginia Techs (VT) Material Science & Engineering Department has proposed an alternative method using microwave radiation to cure the finishes, now in its third stage of development. Microwaves (MW) have longer wavelengths than ultraviolet (UV) radiation, allowing greater penetration depth and increased diffusion between wood and lacquer molecules. To investigate the effect of the finish on dynamic behavior, modal analyses have been conducted at the University of Hartford Acoustics Engineering Laboratory using unfinished, UV-cured, and MW-cured guitar wood samples (spruce, maple, ash, and rosewood). Samples were characterized at bending and torsional modes up to 3.2 kHz. Overall results to date demonstrate that both UV-cured and MW-cured samples have higher ...

Examining the relationship between total acoustic absorption and late lateral energy (GLL).

Listener envelopment, the sense of being immersed in the sound field, has been shown to correlate with the objective parameter late lateral energy (GLL). Barron proposed that GLL is primarily a function of the ratio of reverberation time to auditorium volume or the total acoustic absorption [Barron, Appl. Acoust. 62, 185–202 (2001)]. An investigation has been conducted to examine this proposed theory with measurement data and predicted values from a room acoustics computer modeling program. Room impulse response measurements have been taken in the Belding Theatre in downtown Hartford, CT. Features of the hall include variable absorption in the form of curtains, both within the hall and also within small adjacent coupled volumes. Measurements were taken in several absorption configurations. A detailed room acoustics computer model of the hall has been created in ODEON v9.22 to examine the accuracy of the model’s prediction of GLL. The model was initially validated using the measured parameters of reverbera...