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A Method of Measuring Low-Noise Acoustical Impulse Responses at High Sampling Rates 137th AES Convention October 11th, 2014 ! Joseph G. Tylka Rahulram Sridhar Braxton B. Boren Edgar Y. Choueiri ! 3D Audio and Applied Acoustics (3D3A)

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SLIDE 1

A Method of Measuring Low-Noise Acoustical Impulse Responses at High Sampling Rates

137th AES Convention October 11th, 2014

!

Joseph G. Tylka Rahulram Sridhar Braxton B. Boren Edgar Y. Choueiri

!

3D Audio and Applied Acoustics (3D3A) Laboratory Princeton University www.princeton.edu/3D3A

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SLIDE 2

Applications

2

HRTF Measurements 3D3A Lab, Princeton University

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SLIDE 3

Objectives

  • Measurements at high sampling rates (>48 kHz)
  • Efficient, low-noise, and artifact-free measurements

3

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SLIDE 4

Approach

Initial Measurement Refined Measurement

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Processing

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SLIDE 5

Outline

  • Review: impulse response (IR) measurements
  • Measurements at high sampling rates
  • Proposed measurement procedure
  • Experimental results

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SLIDE 6

IR Measurements

  • Exponential sine sweep (ESS) [1, 2]
  • Deconvolution

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x(t) h(t) w(t) n(t) y(t)

+

[1] A. Farina (2007) Advancements in Impulse Response Measurements by Sine Sweeps [2] S. Müller and P. Massarani (2001) Transfer-Function Measurements with Sweeps

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SLIDE 7

Exact Deconvolution

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Frequency (Hz) Magnitude (dB) Frequency (Hz) Magnitude (dB) Frequency (Hz) Magnitude (dB)

+

=

Input spectrum Exact inverse

  • 3 dB/oct

+3 dB/oct

Signal Noise Signal Noise

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SLIDE 8

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Frequency (Hz) Magnitude (dB) Frequency (Hz) Magnitude (dB) Frequency (Hz) Magnitude (dB)

=

Input spectrum Time-reversed inverse [1]

  • 3 dB/oct

+3 dB/oct

Signal Noise Signal Noise

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Time-Reversed Deconvolution

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SLIDE 9

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Noisy Pre-response (16%)

SNR = 25 dB SNR = 32 dB

Settings: 96 kHz sampling rate, 5 second sweep from 20 Hz to 24 kHz

Exact deconvolution Time-reversed deconvolution

An improvement of 7 dB due to BPF

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SLIDE 10

Why high sampling rates?

  • Ultrasonic transducers
  • “Time-smear” [3]
  • Minimum interaural time difference ~ 10 μs [4]
  • Facilitate subjective tests

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[3] P. G. Craven (2004) Antialias Filters and System Transient Response at High Sample Rates [4] A. W. Mills (1958) On the Minimum Audible Angle

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SLIDE 11

Challenges

  • Signal-to-noise ratio (SNR)
  • Deconvolution issues
  • Transducer heating/damage

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SLIDE 12

Measurement Procedure

Initial Measurement Determine Pass-Band Refined Measurement

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Band-Pass Filter

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SLIDE 13

Defining the Pass-Band

  • Improved signal-to-noise ratio
  • Minimal filtering artifacts (PDA)
  • User preferences
  • Cost function?

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SLIDE 14

Optimal SNR

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Frequency (Hz) Magnitude (dB) Signal Noise Signal and Noise Pass-Band

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SLIDE 15

Optimal SNR

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Frequency (Hz) Magnitude (dB) Signal Signal and Noise Pass-Band Noise + 2.1 dB

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SLIDE 16

Phase II

Design and execute phase- controlled ESS

Phase I

START [5]

Example Implementation

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II Phase II

no yes Input preference: keep/reject PDA Is preference to reject PDA? Input max. PDA Determine constrained-PDA pass-band Determine optimal-SNR pass-band and estimate corresponding PDA STOP

Phase III

Design and execute refined ESS with fade-out Band-pass filter mic. signal Deconvolve mic. signal by input sweep to get IR

[5] K. Vetter and S. di Rosario (2011) ExpoChirpToolbox: a Pure Data implementation of ESS impulse response measurement

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SLIDE 17

Optimal SNR

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SLIDE 18

Results

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Sweep Length (s) Frequency Range Raw SNR (dB) BPF SNR (dB) Pre-response Peak (%) Initial Measurement ~1 23 Hz — 48 kHz 21 — — Optimal SNR 5 26 Hz — 40.6 kHz 24 37 <0.2 Conventional ESS 5 20 Hz — 24 kHz 25 32 16

Note: all measurements were performed with an output level of 75 dB SPL (1 kHz, 1 m)

Exact deconv. Time-reversed deconv.

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SLIDE 19

Summary

  • IR measurements at high sampling rates (>48 kHz)
  • Customizable measurement procedure
  • SNR improvement with minimal filtering artifacts

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SLIDE 20

Acknowledgements

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This work was conducted under a contract from the Sony Corporation of America.

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SLIDE 21

Thank You

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josephgt@princeton.edu

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SLIDE 22

References

  • 1. A. Farina, “Advancements in Impulse Response Measurements by

Sine Sweeps,” presented at the AES 122nd Convention, May 2007.

  • 2. S. Müller and P. Massarani, “Transfer-Function Measurements with

Sweeps,” J. Audio Eng. Soc., 49(6):443-471, 2001.

  • 3. P. G. Craven, “Antialias Filters and System Transient Response at

High Sample Rates,” J. Audio Eng. Soc., 52(3):216-242, 2004.

  • 4. A. W. Mills, “On the Minimum Audible Angle,” J. Acoust. Soc. Am.,

30(4):237-246, 1958.

  • 5. K. Vetter and S. di Rosario, “ExpoChirpToolbox: a Pure Data

implementation of ESS impulse response measurement,” presented at the 4th Pure Data Convention, 2011.

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