An Online Database of Loudspeaker Polar Radiation Measurements

Joseph G. Tylka, Rahulram Sridhar, and Edgar Y. Choueiri

3D Audio and Applied Acoustics Laboratory, Princeton University

AES 139 Convention e-Brief: 230

Website: www.princeton.edu/3D3AThis work is sponsored by the Sony Corporation of America

4 .1 Directivity Indices

4 .2 Constant Directivity Metrics3 Signal Processing1 Introduction

2 Measurement Procedure

5 Data VisualizationThe directivity of a loudspeaker can have a significant influence on the interaction of the emitted sound with the environment and, consequently, the perception of that sound. It is an important characteristic to consider, for instance, when predicting the behavior of a loudspeaker in a room, a task which often requires detailed information about the loudspeaker's radiation. As part of an ongoing experimental survey of loudspeaker directivity, we have measured and compiled anechoic directivity data for a variety of loudspeakers into a freely available online database: http://www.princeton.edu/3D3A/Directivity.html

1. Place the loudspeaker on a computer-controlled turntable (Outline ET250-3D)

2. Align the high-frequency transducer with the point of rotation (see diagram below)

3. Align the microphone (B&K Type 4189) with the high-frequency transducer

4. Generate and record an exponential sine sweep [1] 5. Rotate the loudspeaker by 5° 6. Repeat steps 4 & 5 until the orbit is complete.

L = 1.6 m MicrophoneLoudspeaker

Turntable

Rec. sweep Input sweep

÷Deconvolve

Threshold

Truncate

Window

Smooth

Raw IRs

SPL data

1/24th-octave [2]

4 ms Tukey

16,384 samples

The processed SPL data are presented with four types of plots: 1. Frequency response: SPL vs. frequency 2. Polar: SPL vs. angle; normalized by on-axis

response at 1 kHz 3. Contour (shown below): SPL vs. frequency and

angle; contours every 3 dB; normalized by on-axis response

4. Waterfall: SPL vs. frequency and angle; 3-D surface; normalized by on-axis response

Loudspeakers with constant directivity should, generally, satisfy the following criteria: • Minimal coloration at off-axis listening positions

(ignoring broadband level differences) • Parallel and horizontal contour lines • Similar polar responses at all frequencies —

captured by normalized cross-correlation, Eq. (2)

We compute full and partial directivity indices (see Eq. (1)), each relative to a different section of the radiation pattern: • Full sphere • Front hemisphere • Horizontal (vertical) plane • Front horizontal (vertical) half-plane

These partial DI spectra allow certain sections of the radiation pattern (e.g., forward horizontal radiation) to be isolated when evaluating directivity.

DI(f) = 10 log10|H0 (f)|2P

n wn |Hn (f)|2 /P

n wn(1)

�(fi, fj) =

Pn Pn(fi)Pn(fj)⇣P

n |Pn(fi)|2 ·P

n |Pn(fj)|2⌘0.5 (2)

[1] Farina, A. “Simultaneous Measurement of Impulse Response and Distortion with a Swept-Sine Technique,” Presented at the AES 108th Convention, Feb. 2000.

[2] Hatziantoniou, P. D. and Mourjopoulos, J. N. “Generalized Fractional-Octave Smoothing of Audio and Acoustic Responses,” J. Audio Eng. Soc., 48(4):259–280, 2000.

References

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Contour PlotNorm. X-Corr. Plot

Average: 0.96

Average: 0.79 Avg. Front H DI: 7.23 dB

Avg. Front H DI: 2.32 dB