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1© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Electroacoustic Measurements of
Headphones
Christopher J. StruckCJS Labs
San Francisco, CA – USA
2© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Overview• The Insertion Gain Concept• Orthotelephonic & Diffuse Field Responses• Acoustic Impedance• Couplers, Ear Simulators & Manikins• Electroacoustic Measurements
– Electrical Impedance– Frequency Response
• Manikin Response• Corrected Response• L-R Tracking• Crosstalk
– Non-Linear Distortion– Noise Isolation
• Noise-canceling Headphones
3© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
The Insertion Gain Concept
-10
0
10
20
30
100 1000 10000Frequency [Hz]
[dB
]
A: Loudspeaker Response Free FieldB: Loudspeaker Response At Ear DrumC: Earphone Response at Ear DrumD: Insertion Gain = B - C [in dB]
C
BD
A
The target is a “flat” Insertion Gain!4© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
-40
-30
-20
-10
0
100 1000 10000Frequency [Hz]
dB
Orthotelephonic Reference
MRP
ERP
pp
Acoustic transfer function intended to represent a typical conversation
Including distance attenuation and obstacle effect of the head
5© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Free Field to Drum Reference Point (DRP)On-Axis: 0° Azimuth, 0° Elevation
-5
0
5
10
15
20
25
100 1000 10000Frequency [Hz]
[dB
]
6© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
-10
0
10
20
30
100 1000 10000Frequency [Hz]
dB
To Ear Entrance
Ear Entrance To Ear Drum
To Ear Drum (DRP)
Components of theOrthotelephonic Response
“Obstacle Effect” of Head (& Torso)
Helmholtz Resonance of Ear Canal
7© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
-10
0
10
20
30
100 1000 10000Frequency [Hz]
dB
0°30°60°90°120°150°180°210°240°270°300°330°
Head Related Transfer Functionsvs. Azimuth Angle, 0° Elevation
8© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Diffuse Field to DRP Response
-5
0
5
10
15
20
25
100 1000 10000Frequency [Hz]
[dB
]
This is the typical target for headphones measured at DRP
Random Incidence ORPower Sum of ALL HRTFs in 3D
9© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Acoustic Impedance
UpZ A =
[ N·s/m5 (mks acoustic ohms)]
Ohm’s LawThévenin Equivalent Norton Equivalent
ZZS
+
-
U
p
|Z|·fdB
40
30
20
10
0
100 10kFrequency [Hz]
1k
IMPEDANCE x f OF OCCLUDED EAR
5k200 500 2k 20k
Z+-
pZS
U
log V
[cm3]
100
10
1
0.1
0.001
100 10kFrequency [Hz]
1k
EQUIVALENT VOLUME OF EAR
5k200 500 2k 20k
∴ ZA · f ∝ 1 / VEquivalent
Low ZS Pressure Source High ZS Volume Velocity Source
Sealed
w/ Leak or Vent
Measured
Transfer
10© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Acoustical Impedance of Headphones
High Acoustic Impedance
(Sealed)
Low AcousticImpedance
(Open)
11© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
The Effect of Leakage on Response
…assuming a high acoustic impedance source
Sealed
With Leak
Loss of low frequencies!
12© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Outer Ear & Pinna
Fossa
Helix
Crest of Helix
Crus
Canal
Tragus
Lobe
Darwin’s Tubercle
Anti-Helix
Anti-Tragus
Cymba
CavumConcha {
13© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Earphone Types
Circumaural Supra-aural Supra-concha Intra-concha Insert
14© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Acoustic Couplers• Couples measurement microphone to earphone receiver• Approximates (simplified) acoustic load of ear
Braun Couplerca. 1953
(DIN standard ca.1968)Telephone Handset
Testing (Legacy)
2cc Coupler1945 (standardized 1961)
Hearing Aid & Insert Earphone QC/QA
NOTE: No “reference point”, per se
NBS 9A (6cc)Couplerca. 1950
(standardized ca. 1961)Audiometer Calibration
A Brief History of some Legacy Couplers:
15© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Artificial EarIEC 60318 (ITU-T Type 1)ca. 1962-68 (standardized 1973)
* Ear Reference Point or Concha bottom
Acoustic |Z| x fat ERP*
• Audiometer Calibration• Telephone Handset Testing (Legacy)
16© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Ear SimulatorsIEC 60711 Ear Simulator
(1973, standardized 1979)ANSI S3.25-1989 Ear Simulator(a.k.a. Zwislocki Coupler, 1971)
Measurement Microphone Diaphragm is at DRP!
17© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
IEC 60711 Ear SimulatorAcoustic Transfer Impedance
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0
10
20
30
40
20 100 1k 10k 20k
Frequency [Hz]
5k2k50 200 500
18© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Pinna SimulatorsITU-T Rec. P. 57 Type 3 (all fitted to IEC 711 Ear)
Handset TelephonesBinaural Recording
Type 3.4Simplified Soft Anatomical
All Types. Most Realistic.Type 3.3Soft Anatomical (HATS)
Hard Earcap TelephonesType 3.2High Leak / Low Leak
Insert EarphonesType 2 (IEC 60711)Occluded Ear
Sealed High Z TelephonesAudiometer Calibration
Type 1 (IEC 60318-1/2)(Legacy)
Type 3.3
ca. 1996
Type 3.4
Type 3.2
19© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Ear & Cheek Simulator
• Uses IEC 711 Ear• No Crosstalk or Insertion
Response Testing• Cannot utilize actual
headband force• Retaining force arm may
occlude some open-back designs
• Serial Testing of L & R– Requires pinna substitution
20© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Test Fixtures
• Can be configured with IEC 711 Ear(s)
• Crosstalk & Insertion Response Test results may differ from manikin tests
• Positioning may be more consistent but less realistic than manikin, i.e., good for QC/QA
21© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Head And Torso Simulator
• IEC 60711 Ear Simulators• Anatomical Pinnæ (soft or
hard)• Simplified Geometry• Fulfills IEC 60959 and ITU-T
Rec. P.58 (and ANSI S3.36)22© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
-10
0
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20
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100 1000 10000Frequency [Hz]
dB
HATS Free Field Response
23© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
KEMAR
• Zwislocki OR IEC 60711 Ear Simulators
• Anatomical Pinnæ (soft or hard)
• Anthropomorphic Geometry
• Fulfills ANSI S3.36 (and IEC 60959 and ITU-T Rec. P.58
24© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
-10
0
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100 1000 10000Frequency [Hz]
dB
KEMAR Free Field Responsewith Zwislocki Coupler
25© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Test System
(adapted from IEC 60268-7)
Sine OR
Free or Diffuse Field Correction, if necessaryManikin
26© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Electrical Impedance
+
-VG
GND
IVS
+
-
VG – VS
+
-
For R << 0.1 |ZMIN(f)| (typically 0.1 Ω ):
R
)()()()(
fVfVRfVfZSG
S
−=
27© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Electrical Impedance
20 200 2k 20k
75
50
25
0
Ω
Frequency [Hz]
Frequency Response: MAGNITUDE
• Low Voltage (LINEAR!)• R40 40th decade (1/12 octave) stepped sine• Can also measure phase, if desired• Measured with headphones on manikin!
ZMIN ≥ 0.8 Z0 !
Z0 ≥ 1.25 ZMIN
28© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Test Conditions - 1• Characteristic Voltage: The sinusoidal (or IEC 60268-1
simulated program signal*) voltage at 500 Hz, applied through the rated source impedance (120 ohms), to obtain a sound pressure level of 94 dB SPL in the ear simulator (with or without A-Weighting).
• Rated Source Voltage: Maximum specified RMS voltage which should be applied to the headphone through the rated source impedance, during the reproduction of normal program signals. NOTE: For headphones complying with IEC 61938, Rated Source Voltage = 5 V. Preferably, should not exceed the Characteristic Voltage by more than 15 dB.
*only applicable to systems with integral signal processing
29© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Test Conditions - 2• Working Sound Pressure Level: SPL resulting from a
sinusoidal voltage at 500 Hz (or simulated program signal) through the rated source impedance (120 ohms), across the input connector of the headphone, at a level such that would cause 1 mW to be dissipated in a pure resistance equal to the rated impedance of the headphone. This is the SPL Sensitivity (or A-Weighted SPL) for 1 mW input.
• Maximum Sound Pressure Level: SPL produced in the ear simulator when the headphone is supplied with a sinusoidal voltage of the Rated Maximum Voltage at 500 Hz.
0
00
02
0
22
0316.0
001.0
,'
ZV
ZPZV
PZV
ZV
RVP
VIPIRVLawsOhm
=
⋅==
=
==
==
30© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
IEC 60268-1 Simulated Program Spectrum
-40
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-10
0
10
10 100 1000 10000 100000Frequency [Hz]
[dB
]
Filtered Pink Noise
31© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Maximum Voltage• Maximum [RMS ?] voltage of the IEC 60268-1 program
noise signal, clipped to a crest factor between 1.8 and 2.2, through the rated source impedance, which the headphone can tolerate without permanent damage*
• Rated Long-Term Maximum Voltage:– Signal applied for 10 periods of 60s ON, 120s OFF– *No change in specs after 4 hours of storage
• Rated Maximum Permanent Noise Source Voltage:– Signal applied continuously for 100 hours!– *No change in specs after 24 hours of storage
NOTE: THIS IS A DESTRUCTIVE TEST!32© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
60
20 200 2k 20kFrequency [Hz]
70
80
90
100
110
dB SPL
Frequency Response1 mW Input
LeftRight
• Ear Simulator(s) must be calibrated!• R40 (1/12 octave) stepped sine, 1 mW (or 94 dBSPL)• Test/Re-Test: dB Average (or Median) of 5 re-positions
– Reject re-positions > ± 2.5 dB (500Hz – 8 kHz)– Consider using a jig…– Check low frequency response for leakage– Measurement typically not robust above 10 kHz!– Curve smoothing optional
33© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Diffuse and Free Field Corrected Response
85
90
95
100
105
110
100 1000 10000Frequency [Hz]
dB S
PL fo
r 1 m
W
Diffuse Corrected Response
Free Field Corrected Response
HFFC (f) = H (f) – HFF 0°,0° (f) [in dB]
HDFC (f) = H (f) – HDiffuse (f) [in dB]
Right Ear
Only 100-10kHz!OK to apply smoothingSimplifies visualization and tolerance application
This is the Insertion Gain!
34© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Left-Right Tracking
-10
-5
0
5
10
100 1000 10000Frequency [Hz]
dB
HL-R (f) = HLeft (f) - HRight (f) [in dB]
Responses 1/3 octave power averaged
L(f) = 10log10 Σ 10Li /10
L(f) = 10log10 Σ 10Li /10
N
i=1
1
N
In each 1/3 octave band:
35© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Crosstalk
70
80
90
100
110
120
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
• Requires 2 Ear Simulators• Measurement S/N is poor:
– Use 94 dB SPL sinusoidal test signal– Convert to 1/3 octave as power summation*– Check background noise level ( ≤ -10 dB in each band!)
• Can measure sequentially – Best measured simultaneously
Left (other ear)
Right (w/signal)
L(f) = 10log10Σ 10Li /10
L(f) = 10log10Σ 10Li /10
N
i=1
In each 1/3 octave band:
36© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Crosstalk
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-20
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0
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100 1000 10000Frequency [Hz]
dB
CL-R (f) = GL/R (f) – GR/R (f) [in dB] for signal applied to RIGHT Ear
37© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
0
20 200 2k 20kFrequency [Hz]
20
40
60
80
100
dB SPL
Harmonic Distortionat Rated Voltage
Fundamental2nd Harm.3rd Harm.
38© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
THD vs. Frequency
20 200 2k 20kFrequency [Hz]
-80
-60
-40
-20
0
dB
0.01 %
0.1 %
1 %
10 %
∑
∑
=
== N
nn
N
nn
A
ATHD
1
2
2
2
39© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Intermodulation Distortionf2
-2nd
: f1
–f 2
• f2 low frequency fixed tone at -1.9 dB re: Rated Voltage at 70 Hz• f1 tone at -14 dB (4:1) at 600 Hz OR stepped or swept• f2 is the frequency interval between distortion products• IM products follow f2 as a group
f1
2nd: f
1+
f 23rd
: f1
+ 2f
2
-3rd
: f1
–2f
2
10 dB/div
Relative Frequency
40© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Difference Frequency Distortionf2
-2nd
: f1
–f 2
• f1 & f2 each at -6 dB re: Rated Voltage, stepped or swept.• Δf = f1 – f2 = 80 Hz• Δf is the frequency interval between distortion products• Odd-order products follow as a group with f1 & f2 . Even order products fixed.
f1
3rd: 2
f 1–
f 2
-3rd
: 2f 2
–f 1
Δf
dB
Relative FrequencyΔf
41© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Frequency
dB
Distortion Measurementsin Band Limited Systems
Harmonic Distortion– Most Harmonics above
passbandFrequency
dB
Frequency
dB
f1
f1f2
f1f2
Intermodulation Dist.– f2 below passband OR IM
products outside passband
Difference Frequency Dist.– Odd-order products within
passband– Even-order products may be
inside or outside passband42© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
40
60
100
120
dB SPL80
100 200 500 1k 2k 5k 10k
40
60
100
120
dB SPL80
100 200 500 1k 2k 5k 10k
DF vs. Harmonic Distortion
Effective cutoff: fMAX / Nwhere N=Harmonic order
Underestimates distortion at high frequencies
Harmonic Distortion
Difference Frequency DistortionΔ f = 80 Hz
No inherent high frequency cutoff!
Realistic estimate of high frequency distortion
Fundamental2nd Harmonic3rd Harmonic4th Harmonic
Fundamental
-3rd Order DF3rd Order DF
-2nd Order DF
43© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Simulated Diffuse Noise Field• 8 Uncorrelated Sources:
– Independent noise generators OR– Noise signals recorded independently OR– Generator with multi-tap delay line
Noise Isolation
44© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation
Open Ear Response
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
G 1 (f) Noise Cancelation OFF!
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
90 dBSPL Pink Noise w/ Real-time Filter Analysis OR FFT + 1/3 Octave Synthesis
G 2 (f) Noise Cancelation ON!
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
45© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation - 1
90 dBSPL Pink Noise w/ Real-time Filter Analysis OR FFT + 1/3 Octave Synthesis
46© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation - 2
Open Ear Response
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
90 dBSPL Pink Noise w/ Real-time Filter Analysis OR FFT + 1/3 Octave Synthesis
47© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation - 3
G 1 (f) Noise Cancelation OFF!
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
90 dBSPL Pink Noise w/ Real-time Filter Analysis OR FFT + 1/3 Octave Synthesis
48© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation - 4
90 dBSPL Pink Noise w/ Real-time Filter Analysis OR FFT + 1/3 Octave Synthesis
G 2 (f) Noise Cancelation ON!
60
70
80
90
100
110
100 200 400 800 1600 3150 6300 12500
Frequency [Hz]
dB S
PL
49© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Noise Isolation
-40
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-20
-10
0
10
100 1000 10000Frequency [Hz]
dB
Passive AttenuationActive + Passive AttenuationActive Attenuation Only
LPassive (f) = G1 (f) – GOpen Ear (f) [in dB]
LActive + Passive (f) = G2 (f) – GOpen Ear (f) [in dB]
LActive (f) = LActive + Passive (f) – LPassive (f) [in dB]
50© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
Conclusion• Insertion Gain – This is the TARGET!
• Orthotelephonic & Diffuse Field Responses
• Acoustic Impedance• Low Acoustic Z = Open• High Acoustic Z = Sealed
• Test System and Tests• Requires a manikin equipped with calibrated ear simulator(s)• Sine AND Noise stimuli may be required• FFT Data requires 1/3 octave synthesis (power averaging)• Most post processing is simple dB subtraction (Excel?)• All results shown as GAIN!• Present data using the ANSI/IEC preferred aspect ratio:
– 10, 25, or 50 dB = 1 decade• Curve smoothing is OK for corrected responses!
51© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected] 52© Copyright 2009 CJS Labs – San Francisco, CA USA – www.cjs-labs.com Email: [email protected]
References• IEC 60268-7 – Sound System Equipment. Part 7: Headphones,
International Electrotechnical Commission, Geneva, Switzerland.• J. Borwick, Loudspeaker and Headphone Handbook 3rd Ed. 2001• L. L. Beranek, “Acoustics”, McGraw-Hill, 1954 (Revised Edition –
Acoustical Society of America, 1993).• ANSI Standard S3.25-2009, “Occluded Ear Simulator”.• IEC 60711 Standard (to be superseded by IEC 60318-4) –
Occluded Ear Simulator, International Electrotechnical Commission, Geneva, Switzerland.
• Møller, H. et al, “Design Criteria for Headphones” J. Audio Eng. Soc., Vol. 43, No. 4 – April 1995.
• Burkhard, M.D., editor, “Manikin Measurements”, Industrial Research Products, Inc., Elk Grove Village, Illinois, U.S.A. (1978) –available as a PDF from G.R.A.S., Denmark
• IEEE Standard 1652-2008 “Standard for the Application of Free Field Acoustic Reference to Telephony Measurements”
• ISO 4869-3: 2007 “Acoustics – Hearing protectors -- Part 3: Measurement of insertion loss of ear-muff type protectors using an acoustic test fixture”