head phone measurements

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


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


The Insertion Gain Concept

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[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]

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dB

Orthotelephonic Reference

MRP

ERP

pp

Acoustic transfer function intended to represent a typical conversation

Including distance attenuation and obstacle effect of the head


Free Field to Drum Reference Point (DRP)On-Axis: 0° Azimuth, 0° Elevation

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[dB

]


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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


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dB

0°30°60°90°120°150°180°210°240°270°300°330°

Head Related Transfer Functionsvs. Azimuth Angle, 0° Elevation


Diffuse Field to DRP Response

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[dB

]

This is the typical target for headphones measured at DRP

Random Incidence ORPower Sum of ALL HRTFs in 3D


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


Acoustical Impedance of Headphones

High Acoustic Impedance

(Sealed)

Low AcousticImpedance

(Open)


The Effect of Leakage on Response

…assuming a high acoustic impedance source

Sealed

With Leak

Loss of low frequencies!


Outer Ear & Pinna

Fossa

Helix

Crest of Helix

Crus

Canal

Tragus

Lobe

Darwin’s Tubercle

Anti-Helix

Anti-Tragus

Cymba

CavumConcha {


Earphone Types

Circumaural Supra-aural Supra-concha Intra-concha Insert


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:


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)


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!


IEC 60711 Ear SimulatorAcoustic Transfer Impedance

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20 100 1k 10k 20k

Frequency [Hz]

5k2k50 200 500


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


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


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


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]

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HATS Free Field Response


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


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KEMAR Free Field Responsewith Zwislocki Coupler


Test System

(adapted from IEC 60268-7)

Sine OR

Free or Diffuse Field Correction, if necessaryManikin


Electrical Impedance

+

-VG

GND

IVS

+

-

VG – VS

+

-

For R << 0.1 |ZMIN(f)| (typically 0.1 Ω ):

R

)()()()(

fVfVRfVfZSG

S

−=


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


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


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

=

⋅==

=

==

==


IEC 60268-1 Simulated Program Spectrum

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[dB

]

Filtered Pink Noise


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]

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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


Diffuse and Free Field Corrected Response

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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!


Left-Right Tracking

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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:


Crosstalk

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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:


Crosstalk

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CL-R (f) = GL/R (f) – GR/R (f) [in dB] for signal applied to RIGHT Ear


0


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dB SPL

Harmonic Distortionat Rated Voltage

Fundamental2nd Harm.3rd Harm.


THD vs. Frequency


-80

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-40

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0

dB

0.01 %

0.1 %

1 %

10 %

∑

∑

=

== N

nn

N

nn

A

ATHD

1

2

2

2


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


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


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

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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


Simulated Diffuse Noise Field• 8 Uncorrelated Sources:

– Independent noise generators OR– Noise signals recorded independently OR– Generator with multi-tap delay line

Noise Isolation


Noise Isolation

Open Ear Response

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Frequency [Hz]

dB S

PL

G 1 (f) Noise Cancelation OFF!

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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!

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Frequency [Hz]

dB S

PL


Noise Isolation - 1



Noise Isolation - 2

Open Ear Response

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Frequency [Hz]

dB S

PL



Noise Isolation - 3

G 1 (f) Noise Cancelation OFF!

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100 200 400 800 1600 3150 6300 12500

Frequency [Hz]

dB S

PL



Noise Isolation - 4


G 2 (f) Noise Cancelation ON!

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Frequency [Hz]

dB S

PL


Noise Isolation

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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]


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”

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