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Measurements

Category: Measurements

Analysis

Page p1 ESSENTIALS with graphs S1 to S6

In the “Test” page, you can see all form values used for calculation.

Real graphs

Ideal graphs (simulated)

Page 1, informations of measured system and SPL levels in dB weighted B and C
S1 Frequency response is smoothed to 1/20th octave under 200Hz and 1/6th above and represents the global balance of the loudspeakers measured with MMM method. Black curve corresponds to L+R in low frequencies. Green curve is the personnalised target caculated with measurement results and volume, distance and directivity.
S2 This response is more detailed because smoothed at 1/20th octave on the whole spectrum with a scale in conformity to CTA-2034 recommandations (25dB for a frequency decade). Indicated is Smoothess of InRoom response (SM_IR) for both L and R channels betwen 0 and 100%, 100% being the ideal value. And also indicated is Wide Band Deviation (WBD_IR) of InRoom response, 100% being ideal. At the moment, you cannot directly compare to those found in the AudioScienceReview website, here are real measured values while in ASR, the responses are estimated and not exactly calcutated the same way.
S3 Blue curve (left) and red (right) represent low frequeny response under 200Hz. Here we can see room modes near 35,60 and 100Hz. Those modes may be corrected by EQ, parametric or FIR, but dips at 55 et 70Hz are difficult to improve.
S4 Comparison of in-phase L+R and opposite-phase L-R : normally L+R should be much higher than L-R . But here at 90 or 120Hz, L-R is higher than L+R. This can lead to a sense of missing low frequencies because those frequencies are generally recorded in mono (L+R).
S5 RT60 is representing reverberation time in seconds. This measurement is not done in conformity to RT acoustics standard but gives a good indication of the sound field decrease. It is better that the curve shows no increase to the right (higher frequencies).
S6 ETC Energy Time Curve, shows the first 20 milliseconds, to display early reflections. It is recommended that both L and R curves stay under the recommanded AES limits in green. Here we see reflections at 8 and 9ms.
Page 2 for temporal aspects and phase, graphs S7 to S12
S7 and S8 show impulse responses. In an impulse response, it is mostly the high frequencies that are visible.  In this graph, we see a reflection at 4.8ms which correspond to a diffrence of distance of about 1.6m (4.8×0.34m).
S9 et S10 the step response is totally equivalent to impulse response, but with energy better dispatched on the frequency spectrum, it better shows the whole spectrum and it is easier to see some informations : here we see high frequencies starting before mids and lows (typical of a standard crossover).
S11 Phase : measurement being done at listening position, phase is retrieved from a frequency dependant window. The ideal response should be flat but we know that phase response is less important than amplitude.
S12 Group delay corresponds to phase variations : there is no clear limit of audibility but a response between the two green lines should be ok, it corresponds to +-0.5 periods. In above example, we se a peak at 1.2kHz due to the crossover.
Page 3 for other temporals, localisation and distortion, graphs S13 to S18
S13 et S14 Temporal evolution of frequency response : the time window starts at 2ms up to 100ms so we can see the evolution of some reflections.
S15 Pre-echo is a signal starting before the real signal (0ms) that may come from FIR equalisation : here some pre-echo can be seen near -7ms at -60dB.
S17 With wavelet display, we can see the spectrum of the pre-echo signal.
S16 Localisation : a well centered soundfield should stay near the green line for all frequencies but it depends on LR balance and loudspeakers distances.  Here we see a progressive shift to Left in low frequencies. ITD Interaural Time Difference and ILD Interaural level Difference are indicated on the graph.
S18 Total Harmonic Distortion (THD) : due to the short length of test signal sweeps, and depending on noise in the recording, distortion graphs may not allways be representative of the true distortion of measured loudspeakers. In this case, a measurement with stepped sine wave would be more effective (use REW or similar softwares).
Page 4, other temporals, graphs S19 to S24
S19 and S20 Spectrogram : this view is similar to S7 but detailled in frequencies : here are some reflections at 3, 8 and 9ms.
S21 et S22 Waterfall may give indication of room modes in low frequencies. In this case, a mode is seen at 40Hz.
S23 et S24 Wavelet visualisation
Comparable to spectrogram S13 but this kind of analysis gives better resolution in low frequencies.   Note that the horizontal scale is in periods. The 40Hz mode is clear. It is interesting to know that resonnant modes stay horizontal but reflexions are seen as oblique lines going up to the right.
Graphs p5 for temporal alignement
The perfect temporal alignement is when all crossings to level zero are at time 0 for all frequencies.

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Category: Measurements

If field “Measure and correct” is validated, subdirectory “Correction” is created to contain.wav files for FIR correction, respectively Left and Right in linear phase and minimal phase : xxx-hyblinL.wav, xxx-hyblinR.wav, xxx-hybminL.wav, xxx-hybminR.wav. Those files can be directly used for corrections.

Pages p7 to p9 are also created.

p7 Correction
Separated C1 measurements L and R and also L+R (C2 black)
FIR corrections C3 calculated from L, R and L+R and hybrid corrections C4 (same in lower frequencies and separated above)
C5 Phase correction
Simulated responses of L, R and L+R after FIR correction : C7 for separated corrections and C8 for hybrid correction
p8 Simulated ETC energy-time curve after correction for pre-echo visualisation
p9 Simulated wavelets for pre-echo visualisation

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Category: Measurements

For many people, most curves and graphs are not so easy to understand so some of you have asked about a simple performance rating. We have tested the ratings proposed by Sean Olive in AES papers 6113 and 6190 but for some reasons, it was not totally satisfying. Those ratings are based on anechoic room measurements extended to Predicted In Room results. With our method, we only measure InRoom values and we have to quantify performance based only on those real measurements.

We get the score from three main factors :

  • SM_IRR SMoothness of InRoom Response between 125 and 11500Hz : the proposal of Olive is not very intuitive (Pearson coefficient) and this value is not used by us
  • NBD Narrow Band Deviation of InRoom Response between 125 and 11500Hz (6.5 octaves) : measured surface difference between 1/20th octave curve and 1/2 octave curve, so it is not related to target and general slope
  • WBD Wide Bandwidth Deviation of frequency response from target curve : it is a value based on area difference (so related to variance) between the measured response and the target response between 125 and 11500Hz
  • LFD Low Frequencies Deviation is based on area difference between the measured response and the target response between 25 and 125Hz (2 octaves) but calculated on a linear frequency scale
  • please notice that the displayed mean value is the lowest of L and R values

It is important to understand that the rating is only based on measured amplitude response and is missing other factors that may influence audible quality : max levels, directivity, distortions, phase and time response, etc… So be carefull when you compare ratings of different systems, ithe highest may not be the best ! But compare numbers before/after equalisation/correction is certainly valid.

 

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