Sound Quality Assessment IR measurements and AQT University of Parma Sound Quality Assessment IR measurements and AQT

Problems with traditional measurements methods Traditional measurement are based on steady-state signals - they provide a frequency response curve - this does not provide significant match with listening experience in cars It was attempted to employ transient analysis (impulse response) based on the same parameters employed for concert halls (reverberation time, clarity, definition) - also these do not correspond with the short time scale of time transients inside cars In-car listening tests are useful, but difficult to standardize General lack of understanding of the relationship between subjective perception and physical facts

Solutions Reliable in-car measurement technique (MLS, sweep) Transient analysis and equalization (AQT) Virtual listening for comparative tests Subjective (IPA) and objective (IPM) evaluation indexes for audio quality assessment New metric based on all above for rating with a sigle number the sound quality (IQSB)

Methods: Reliable in-car measurement technique Impulse Response (IR) as standard measurement for characterization of the car sound Hardware: B&K type 4100 binaural head and torso Hardware: Notebook PC + Edirol FA-101 Firewire interface Software: Adobe Audition and Aurora for IR measure AQT as advanced analysis (instead of FFT)

Standardized Position of the Dummy Head Reliable in-car measurement technique Standardized Position of the Dummy Head ca. 25o 70 cm 30 cm verticale volante verticale a piombo asse del dorso (sul piano dorsale) tettuccio

Hardware: PC and audio interface Reliable in-car measurement technique Hardware: PC and audio interface Edirol FA-101 Firewire sound card: 10 in / 10 out 24 bit, 192 kHz ASIO and WMA

Software Reliable in-car measurement technique Aurora Plugins Generate MLS Deconvolve MLS Generate Sweep Deconvolve Sweep Convolution Kirkeby Inverse Filter Speech Transm. Index

Measurement process The desidered result is the linear impulse response of the acoustic propagation h(t). It can be recovered by knowing the test signal x(t) and the measured system output y(t). It is necessary to exclude the effect of the not-linear part K and of the background noise n(t).

Test signal: Log Sine Sweep x(t) is a sine signal, which frequency is varied exponentially with time, starting at f1 and ending at f2.

Deconvolution of Log Sine Sweep The “time reversal mirror” technique is employed: the system’s impulse response is obtained by convolving the measured signal y(t) with the time-reversal of the test signal x(-t). As the log sine sweep does not have a “white” spectrum, proper equalization is required Test Signal x(t) Inverse Filter z(t)

Test Signal – x(t)

Measured signal - y(t) The not-linear behaviour of the loudspeaker causes many harmonics to appear

Inverse Filter – z(t) The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]

Result of the deconvolution 1° 2° 3° 5° The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders

Maximum Lenght Sequence vs. Sweep

Post processing of impulse response Specific XFM modules have been developed for the computation of STI and for performing standard analysis according to ISO-3382

AQT Analysis Goal: Substitute classical analysis based on FFT with a new kind of dynamic transient analysis Reason: FFT characterizes a only in steady state condition BUT Hearing is more sensible to transients Musical signal is not a steady state signal Solution: AQT analysis (Acoustic Quality Test)

AQT Analysis Features: Use transient stimulus (wavelet) instead of steady-state (periodic sine) Find a frequency response curve more related to perceived curve than FFT response Find resonances and tails that affect hearing Useful during car development and for design of optimal equalization Output: “Dynamic” frequency response curve of the sound Articulation curve (transient capability at each frequency)

AQT Analysis Methods: The test signal is a sequence of sine bursts of increasing frequency Each burst is 200 ms long. The gap between a burst and the next is 33 ms The instantaneous RMS level is plotted versus time (or frequency) The same result can be also obtained convolving the test signal with the IR

AQT Analysis Details: The envelope of the maxima is the dynamic frequency response of the The depth of the valley between two bursts shows the dynamic transient capability of the Universiy of Parma

AQT Analysis Temporal masking according to Zwicker The analysis of a single burst shows the level variation due to a single burst, and the overshoot phenomenon at the beginning and end of the burst.

AQT Analysis AQT Tool: An automatic tool was developed to perform a quick processing This tool can also generate the coefficients for digital FIR filters The filters are designed for producing a prescribed optimal frequency response

AQT Equalization results Fiat STILO Above: measured AQT curve not equalized vs. target curve (red) Below: measured AQT curve after equalization vs. target curve (red) Results (IPA listening test): Not equalized Traditional FIR Inverse AQT 7.18 7.20 7.50

The new AQT plugin for Audition The new module is still under development and will allow for very fast computation of the AQT curve from within Adobe Audition

Conclusions The sine sweep method revealed to be systematically superior to the MLS method for measuring electroacoustical impulse responses In fact, it is now employed from top-grade measurement systemss, includiong the Audio Precision (TM) Traditional FFT-based analysis of the measured impulse response did noy show good correlation with subjective perception It was tehrefore necessary to derive new physical descriptors, better correlating with the human listening experience inside a car. Three physical parametrs revealed to be more correlated with subjective responses: IACC, STI and AQT