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

2. 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

3. 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)

4. 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)

5. 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

6. 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

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

8. 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).

9. 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.

10. 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)

11. Test Signal – x(t)

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

13. 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)]

14. Result of the deconvolution1° 2° 3° 5°
The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders

15. Maximum Lenght Sequence vs. Sweep

16. 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

17. 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)

18. 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)

19. 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

20. 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

21. 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.

22. 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

23. 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

24. 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

25. 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