AES San Francisco 2010
Paper Session P3

P3 - Acoustical Measurements

Thursday, November 4, 2:30 pm — 6:30 pm (Room 236)

John Vanderkooy

P3-1 Methods for Extending Room Impulse Responses beyond Their Noise FloorNicholas J. Bryan, Jonathan S. Abel, Stanford University - Stanford, CA, USA
Two methods of extending measured room impulse responses below their noise floor and beyond their measured duration are presented. Both methods extract frequency-dependent reverberation energy decay rates, equalization levels, and noise floor levels, and subsequently extrapolate the reverberation decay toward silence. The first method crossfades impulse response frequency bands with a late-field response synthesized from Guassian noise. The second method imposes the desired decay rates on the original impulse response bands. Both methods maintain an identical impulse response prior to the noise floor arrival in each band and seamlessly transition to a natural sounding decay after the noise floor arrival.
Convention Paper 8167 (Purchase now)

P3-2 On the Use of Ultrasound Transducer Arrays to Account for Time-Variance on Room Acoustics MeasurementsJoel Preto Paulo, ISEL- Instituto Superior de Engenharia de Lisboa - Lisbon, Portugal, CAPS – Instituto Superior Técnico, TU Lisbon, Lisbon, Portugal; José Bento Coelho, CAPS – Instituto Superior Técnico, TU Lisbon - Lisbon, Portugal
In real room acoustical measurements, the assumption of time-invariant system is usually not verified. A measurement technique was set-up with the purposes of monitoring the acoustical media, searching for time variance phenomena, and for low SNR situations. A probe test signal in the ultrasonic band is sent to the room by using a parametric loudspeaker array, with high polar pattern directivity, simultaneous with the test signal frames. The relevant parameters to establish time-variance and associated thresholds are then estimated from the acquired ultrasonic sound. The valid test signal frames, which pass the thresholds test, are labeled with a weighting factor depending on its significance. Otherwise, the frames are rejected, not entering on the averaging process. Results are presented and discussed herein.
Convention Paper 8168 (Purchase now)

P3-3 Impulse Response Measurements in the Presence of Clock DriftNicholas J. Bryan, Miriam A. Kolar, Jonathan S. Abel, Stanford University - Stanford, CA, USA
There are many impulse response measurement scenarios in which the playback and recording devices maintain separate unsynchronized digital clocks resulting in clock drift. Clock drift is problematic for impulse response measurement techniques involving convolution, including sinusoidal sweeps and pseudo-random noise sequences. We present analysis of both a drifting record clock and playback clock, with a focus on swept sinusoids. When using a sinusoidal sweep without accounting for clock drift, the resulting impulse response is seen to be convolved with an allpass filter having the same frequency trajectory form as the input swept sinusoid with a duration proportional to the input sweep length. Two methods are proposed for estimating the clock drift and compensating for its effects in producing an impulse response measurement. Both methods are shown to effectively eliminate any clock effects in producing room impulse response measurements.
Convention Paper 8169 (Purchase now)

P3-4 Quasi-Anechoic Loudspeaker Measurement Using Notch Equalization for Impulse ShorteningRichard Stroud, Stroud Audio Inc. - Kokomo, IN, USA
The length of the impulse response of a typical piston driver is largely determined by the characteristic second-order high-pass response of the driver. This time response makes anechoic (i.e., gated) measurement difficult in non-anechoic environments, as reflections must be suppressed to returns of 30 ms. or more. This paper outlines a quasi-anechoic frequency and phase response modification technique using a tuned notch, or band-cut, equalization that shortens the impulse response and allows correct full-range loudspeaker measurement in moderately sized non-anechoic rooms.
Convention Paper 8170 (Purchase now)

P3-5 Estimating Room Impulse Responses from Recorded Balloon PopsJonathan S. Abel, Nicholas J. Bryan, Patty P. Huang, Miriam A. Kolar, Bissera V. Pentcheva, Stanford University - Stanford, CA, USA
Balloon pops are convenient for probing the acoustics of a space, as they generate relatively uniform radiation patterns and consistent “N-wave” waveforms. However, the N-wave spectrum contains nulls that impart an undesired comb-filter-like quality when the recorded balloon pop is convolved with audio. Here, a method for converting recorded balloon pops into full bandwidth impulse responses is presented. Rather than directly processing the balloon pop recording, an impulse response is synthesized according to the echo density and frequency band energies estimated in running windows over the balloon pop. Informal listening tests show good perceptual agreement between measured room impulse responses using a loudspeaker source and a swept sine technique and those derived from recorded balloon pops.
Convention Paper 8171 (Purchase now)

P3-6 Complex Modulation Transfer Function and its Applications in Transducer and Room Acoustics MeasurementsJuha Backman, Nokia Corporation - Espoo, Finland
Modulation transfer function in audio applications describes well the clarity of sound, but conventional definitions and measurement methods are not easily applicable to transducer measurements, low-frequency acoustics, or capturing effects of narrow-band phenomena. A revised definition of modulation transfer function, taking into account the magnitude and phase of modulation transfer for each carrier and modulator frequency combination is presented. This function is derived from the complex frequency response by analyzing the response at the carrier frequency and at the modulation sidebands. Also the distortion of modulation envelope arising from the asymmetry especially in the phase transfer properties is discussed. Examples of the use of the complex modulation transfer function are presented for simple filters, anechoic response measurements of loudspeakers, and for loudspeakers in rooms.
Convention Paper 8172 (Purchase now)

P3-7 Practical Implementation of Perceptual Rub & Buzz Distortion and Experimental ResultsSteve Temme, Pascal Brunet, Brian Fallon, Listen, Inc. - Boston, MA, USA
In a previous paper [1], we demonstrated how an auditory perceptual model based on an ITU standard can be used to detect audible Rub & Buzz defects in loudspeakers using a single tone stimulus. In this paper we demonstrate a practical implementation using a stepped sine sweep stimulus and present detailed experimental results on loudspeakers including comparison to human listeners and other perceptual methods.
Convention Paper 8173 (Purchase now)

P3-8 Measurement of Turbulent Air Noise Distortion in Loudspeaker SystemsWolfgang Klippel, Robert Werner, Klippel GmbH - Dresden, Germany
Air leaks in the dust cap and cabinets of loudspeakers generate turbulent noise that highly impairs the perceived sound quality as rub and buzz and other loudspeaker defects do. However, traditional measurement techniques often fail in the detection of air leaks because the noise has a large spectral bandwidth but a low power density and similar spectral properties as ambient noise generated in a production environment. The paper models the generation process of turbulent air noise and develops a novel measurement technique based on asynchronous demodulation and envelope averaging. The technique accumulates the total energy of the leak noise radiated during the measurement interval and increases the sensitivity by more than 20 dB for measurement times larger than 1s. The paper also presents the results of the practical evaluation and discusses the application to end-of-line testing.
Convention Paper 8174 (Purchase now)

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