Acoustics Test with Audio Precision

Transcription

1 Acoustics Test with Audio Precision

2 Why hardware matters Critical aspects of measurement mics The front end of an audio analyzer

3 Limitations Of Sound Card Based Measurement Systems Sound Card Maximum input levels typically very low A few volts Not enough for speaker testing without external attenuators Sound Card Noise and distortion may not be adequate Especially for modern electronics No input ranges; poor dynamic range Sound Card minimal input and output protection Sound Card - No calibration Sound Card I/O may be affected by PC OS Your test equipment should be better than your device in every respect: noise, distortion, reliability, repeatability, and accuracy.

4 Audio Analyzer Front End High Voltage inputs to test at SPL incurred in field 1 Watt test conditions specify 2.83 VRMS into 8 ohm Peak levels may be much higher For high test levels, may need 10 to 20 Vpk across driver Low noise and distortion Analyzer noise can be limiting factor in quiet room measurements High-order harmonics are audible at very low levels Output and input ranging required for large dynamic range Robustness and reliability Inputs and outputs need to be protected - it s easy to connect the output of a power amp to a signal generator output by mistake Long-term reliability and stability

5 Why Use Measurement Microphones? Measurement microphones provide: confidence in results reassurance of repeatability long-term stability Margin for error is non-existent : the microphone has to work today, tomorrow and years from now.

6 1) Visual Inspection 2) Cleaning 3) Pin holes 4) Sensitivity 5) Frequency Microphone Calibration G.R.A.S. Calibration

7 Audio Precision Calibration Adjustment The process of restoring optimum instrument performance by resetting internal electrical components and stored software constants to correct for the inevitable effects of aging and drift. No Cal certificate, so the traceability chain is broken for customer s quality program Calibration Provides documented and traceable verification that instruments meet or exceed all of their published specifications. Traceability an unbroken chain of comparisons known measurement uncertainty documentation competence reference to SI units calibration intervals Accredited calibration Adds a further rigor by incorporating review and on-site assessment by recognized independent experts. AP is accredited by A2LA to the ISO:17025 standard

8 Speaker Measurements in Non-ideal conditions

9 Problem: Acoustic Reflections Measurements in ordinary rooms are contaminated by reflections OPTIONS Measure Outdoors Anechoic Chamber Time selective (quasi-anechoic) techniques Combination Direct sound Reflected Reflected

10 Problem: Acoustic Reflections Anechoic = an (no) + echoic (echos or reflections) No Reflections! Anechoic response is implied! Anechoic response is rarely measured!

11 Problem: Acoustic Reflections Low frequency measurements require a LARGE, EXPENSIVE chamber! 100 Hz chamber requires cones that are 0.86m 20 Hz chamber requires cones that are 4.3m λ = c/f c = 343 m/s f (Hz) λ (m) λ (ft) 1.0 k

12 Problem: Acoustic Reflections Anechoic chamber An anechoic chamber is best if you have it (for Mids and HF) Be aware that there will still be some reflections Off stands, lights, etc. Off the floor, unless it s suspended Mount the speaker face-up in the floor Not all chambers are equally quiet Anechoic and noise-free are not the same! HVAC, equipment noise, outside noise increase noise floor Always measure your noise floor Chamber has a minimum anechoic frequency Determined by wedge size Below cutoff frequency, chamber becomes echoic Go outside for low frequency measurements! In a small chamber, this might be below 500 Hz

13 Problem: Acoustic Reflections Surprise reflections Example of setup issue: Mic stand makes a difference (any object will) less reflections, diffraction, absorption more ideal sound field

14 Problem: Acoustic Reflections Use Far-Field Measurements for High Frequencies Most accurate at mids and high frequencies Far-Field measurement Microphone greater than 3 X M from Driver Problems with reflections reflections & noise Not accurate at low frequencies nearest reflecting surface poor resolution Smoothing ~ 1 T

15 Problem: Acoustic Reflections Time-selective Techniques for accurate Far-Field Measurements Far-Field measurement T = 2d R d c Reflections nearest reflecting surface T Reflections poor resolution Smoothing ~ 1 T

16 Level (db) Frequency (Hz) Problem: Acoustic Reflections Use Near-Field Measurements for Low Frequencies Microphone as close as possible to LF driver Eliminates reflections & noise Valid at low frequencies where driver behaves like a rigid piston Theoretical Upper Frequency Limit for Near-Field Measurements 1,000 c/πm Frequency Major Source Dimension M (in)

17 Level (db) Problem: Acoustic Reflections Simulated Free-Field Measurements Near-Field Measurement overlap region Far-Field Measurement 20log d n d f 1/T c/πm Frequency

18 Simulated Free-Field Measurements Ports and/or multiple LF drivers Multiple measurements Add contribution from each (complex math) Room must be large in relation to speaker cabinet for overlap region Improve your chances of a good measurement Quiet room => turn off HVAC, if possible Averaging Keep device under test away from surfaces More info: AES paper no by Struck & Temme

19 DRIVERS: Impedance and TS Parameters

20 Driver Evaluation Issues Objective: Evaluate different drivers for your product Verify manufacturer specs Is the driver really this flat? Correct Thiele-Small parameters? Make sure max SPL in data sheet matches reality Unit-to-unit variance - get sample from supplier to see if they can clone drivers Export impedance curve or T&S parameters to cabinet modeling software APx supports CSV, XLS, and *.mat formats Measurements SPL, Frequency response, Distortion Impedance / Thiele-Small Polar plots & waterfall charts Rub and buzz Frequency response also to determine cone breakup

21 Driver Impedance Rated quantity for drivers and speaker systems Measured minimum value (DC - 20 khz) must be within 80 % of rated value Drive level Small enough to ensure operation in linear region Basis for determining Thiele-Small parameters Rated (nominal) 8 Ohms

22 Driver Impedance What are the Thiele-Small parameters? Electromechanical parameters that define low frequency driver performance Published specs used to help designers of cabinets and crossovers Small signal parameters usually measured and reported Mechanical components are non-linear; properties change With temperature Over time

23 Production Test Measurements

24 Measurements for Production Test SPK-PT Measurement speed is critical Impedance + Microphone QC measurements Frequency response relative to Golden Unit Impedance Subset of Thiele-Small parameters derived from one measurement Polarity Rub and Buzz to detect defects Voice coil rubbing on magnet Particles in the gap Poorly dressed tinsel leads ticking Box and grille rattling Air leak detection Critical for bass response in small box designs

25 Golden units and variations in mounting Measure known good drivers Measure N number of drivers/finished products from a pilot run and append on a single graph Calculate the theoretical ideal Use Derived Results function to calculate the Mean of all curves (excluding outliers)

26 Variations Golden unit target curve Export this Geometric Mean curve to a file for reference Compare production units to Golden unit target curve visually or create limits from golden curve

27 Compare function Compare Each driver s response compared to golden unit Compare subtraction of magnitude in db (division in linear units) Good driver appears as a flat line centered in limits

28 Testing Powered Speakers

29 Test Standards AES 2 Methods of measuring and specifying the performance of loudspeakers for professional applications CEA-426-B Recommends the maximum power rating for an amplifier to be connected to the loudspeaker BS EN Designed to protect users of portable audio devices from exposure to excessive sound levels IEC Sound system equipment Part 5: Loudspeakers IEC Sound system equipment Part 7: Headphones and earphones Power supply (if powered) Impedance Input voltage Input power SPL Frequency response Harmonic distortion Intermodulation Distortion (IMD) Difference-frequency distortion (DFD) Unwanted sound radiation (leakage) Sound attenuation Crosstalk

30 Powered speaker signal chain Sometimes a single IC

31 Class D (Switch Mode) Amplifiers Active components are ON or OFF 100% for brief pulses, dissipating very little power. Efficient, but outputs typically have high-amplitude, high-frequency noise. Noise signals have very high slew rates which can cause erroneous distortion and noise readings in analyzer measurements. In some cases audio analyzer circuits can be forced into unstable states or even suffer damage. Filter Required!

32 Class D Amp: Typical Test Set Up

33 Testing headphones Key standards Good test setup Test challenges

34 Sound system equipment Part 7: Headphones and earphones Characteristics & measurements Headphone Performance Standards Power supply (if powered) Impedance Input voltage Input power SPL Frequency response Harmonic distortion Intermodulation Distortion (IMD) Difference-frequency distortion (DFD) Unwanted sound radiation (leakage) Sound attenuation Crosstalk IEC

35 Key points Headphone Safety Standards Information Technology Equipment - Safety Part 1: General Requirements Based on a sound limit of 85 db considered safe under normal conditions of use. The user can choose to override the limit so that the sound level can be increased up to maximum 100 db. If the user overrides the limit, warnings about the risks must be repeated every 20 hours of listening time. Transition period ends on 24 January After this date, industry is expected to apply the standards to their products. Standards are voluntary. However, conforming products benefit from a presumption of conformity to the safety requirements of the applicable EU legislation on the risk of hearing damage. EN

36 Test set up: Headphones Why KEMAR vs. a stand-alone microphone Challenge What: Loudspeakers on your head trying to duplicate loudspeakers in a room Need: Dedicated measurement equipment to account for changes in transducer placement and Sound perception. Why: Head and Torso Effect, Concha gain, Ear Canal resonances, form and fit.

37 Why we need Ear Simulators & Manikins Ear Simulators and Manikins = Replica Human Auditory System Acoustic Impedance Couplers, couplers, ear simulators 711 down to the micron Acoustic Diffraction 1972 was the year of the KEMAR People in the sound field - Binaural recordings Quality in, Quality out. Great complement to Golden Ear subjective tests How are your products used

38 Challenges for headphones Head-related transfer function (HRTF) for each person uniquely shaped vs. Average Joe KEMAR High frequency (10+kHz) variations - in people and equipment small modal artifacts (room modes) arise when wearing headphones move headphones around to achieve spatial averaging Headphone/earphone positioning Pro-tip: play a 80Hz square-wave as you mount the headphones to check for fit. Pro-tip: Play a pink noise signal and look for bass response. Pro-tip: Listen to the signal yourself.

39 Magnitude (db) Head Related Transfer Function (HRTF) Frequency Response of "Flat" Speaker in Free-Field Microphone KEMAR 0 microphone incidence ,000 10,000 Frequency (Hz)

40 Magnitude (db) Translation of measurement data from in-ear to Free-Field Correct measured response Subtract HRTF in db (division in linear units) DRP to 0 Correction Single curve used for both ears Differences due to pinna deformation alone exceed coupler variation Telephony applications IEEE P1652 Orthotelephonic reference: Face-to-face conversation in free field at 1 m distance ,000 10,000 Frequency (Hz) Hearing damage studies e.g., BS EN Hearing loss SPL data is based on Free-Field measurements.

41 Golden Curve and spatial averaging Start with ideal fit. Then take 8 additional non-ideal measurements. Finish with ideal fit, to have a total of 10 samples. Measuring your headphones Measure ideal response and your many bad responses. Show variations on a single graph

42 Headphones : Safety & Isolation

43 Challenge: Isolation Leakage Are you disturbing your environment? (library, office, public space or transport) How much noise do YOU make? 1) Measure ambient conditions using stand alone mic with all systems off. 2) Play pink noise through headphones at 90dBSPL 3) Measure ambient conditions with stand alone mic. High Leakage score means you contribute to the acoustic pollution(!) in the world.

44 Isolation EPA requirement for NRR! products that are designed and sold on the basis of their ability to reduce the level of sound that may enter the ears... determine the performance and properly label them with their effectiveness rating (Noise Reduction Rating, or NRR) for legal entry into U.S. commerce. Challenge: Isolation