Psycho-acousticsThe human ear, and brain, is capable of great
sensitivity and ability to filter nuances from what we hear. This is a
wonderful ability that enhances listening to music but it can be very
deceptive when attempting to use hearing to measure acoustics. Our ears
don't lie, but we often hear only what we want to hear or even hear what
we thing is there but is not. MeasurementsPrecision,
measurement instruments are necessary tool which, when used correctly
and knowledgeably, can give us a more UN-biased perspective. RPlusD / ETF-5This is a very useful application for measuring
room acoustics from Acoustisoft; written by Doug Plumb. However, room
acoustics measurement is not a completely intuitive process and the
application can feel complex because of it's many features. So, here is a
simplified view along with a set of directions which may help to more
quickly get some useful information about a room. The ConceptFundamentally,
ETF sends a complex signal of pink noise and a swept-frequency sine
wave to one of your monitor speakers and collects the room response
signal via a flat-response measurement microphone. Note a few details
about this: - Note: Your monitor speaker is assumed to have an ideal,
flat-frequency response. It doesn't ... but it is likely much better
than the room's response -- so ETF ignores any speaker error. It is also
very difficult to measure a speaker because you need a near ideal acoustic space and a precision, calibrated measurement microphone. This is another reason why good near-field monitors are well worth the investment.
- All the various plots are
derived from the same measurement data. So, after everything is set-up,
take a measurement and save the data. That data can be recalled later
for further analysis.
- You don't need an expensive (>$1k) mic to get measurements of sufficient accuracy and resolution to find problems with room acoustics. The $50 price class Behringer ECM8000 does very well.
- Generally,
take 3-5 measurements which all look similar; avoid or reject the
really 'odd' plots as noises can disrupt the measurement. As long as you
can easily get a 3-5 series of plots that look really similar, the data
is probably a valid representation of room acoustics -- at that spot.
- Average 3-5 plots. Their differences are meaningless and just part of the noise.
- Measurement
position matters more before the room has been acoustically treated.
So, it is best to just place the measurement mic at the "listening
position" and that will give you easier comparisons.
- An initial
"loop-back" configuration with the sound card in your computer IS
NECESSARY. This is how a reference is established for all additional
measurements.
- All measurements are the ratio between the Left Channel Reference (hard-wired loop-back) and the Right Channel Measurement (using the microphone).
The SequenceThe
following sequence has been useful as a quick-reference guide for
making ETF measurements. When it can be weeks between measurements (It
takes time to construct new acoustic treatments like 8 bass traps!) it
is easy to forget a step. - PC / Audio System Setup
- Sound Card
- SigmaTel
- 48 kHz Sample Freq
- Loop-back Test Config
- PC LEFT OUT --> PC LEFT IN
- PC RIGHT OUT --> PC RIGHT IN
- Start : Programs : Accessories : Entertainment : Volume
- PC Volume Control:
- SPKR: 3.5 (1-7 scale) [REF]
- WAVE: 3.5 [REF]
- PC Recording Control
- LINE: 3.0 (1-7 scale) [REF]
- Mic: MUTE
- Mic
- Behringer ECM8000
- 48V phantom power
- PreAmp
- Studio Projects VTB1
- Input Gain (listening Position): 40, Speaker Test: 30
- Blend: Full Left (none)
- Output Level: 0 (mid)
- ETF App Sequence
- Initial Setup
- Options : Mixer : Off
- View : Frequency Response : Bode/FFT
- Set display colors
- White (Background)
- Blk, DrkBlu, DrkGrn, Drk Vio, DrkOrg, Red, DrkGry, LtGry
- Set display view range
- single click = max zoom out
- boxed area = zoom in
- boxed area > graph size = zoom out
- Config Loop-Back Test
- PC LEFT OUT --> PC LEFT IN
- PC RIGHT OUT --> PC RIGHT IN
- Run Loop-Back Test
- Verify the following levels (loop-back reference check)
- Reference Levels:
- -6 dB Mic Right (-6.1 meas)
- >25 dB SNR Right (31.2 SNR meas)
- -6 dB Ref Left (-6.4 meas)
- >25 dB SNR Left (30.7 SNR meas)
- 0 meters mic/spkr distance
- Config for Room Measurement
- PC LEFT OUT --> PC LEFT IN (Reference)
- PC RIGHT OUT --> Right Pwr Amp --> Right Spkr
- PC RIGHT IN <-- VTB-1 Mic preamp out
- Position Mic at listening position
- Config PC SoundCard Levels
- PC Volume Control:
- PC Recording Control
- Run Measurement
- Verify the following levels (loop-back reference check)
- Measured Levels:
- -5 dB Mic Right (-1.2 meas)
- >25 dB SNR Right (25.0 SNR meas)
- 5 dB Ref Left (4.5 meas)
- >25 dB SNR Left (30.9 SNR meas)
- 2 (check that it's reasonable) meters mic/spkr distance(1.922)
- Take 3 to 5 measurements
- File : Save : Measurement Data
- PC / Audio System Reset
- Start : Programs : Accessories : Entertainment : Volume
- PC Volume Control:
- PC Recording Control
- Frequency Response Plot:
- View : Freq Response : FractOct
- Set display range as needed
- Impulse Plot:
- View : Time Response : Impulse
- Set display range as needed
- Waterfall Plot
- View : Freq Response : Bode plot
- Select Averaging
- Set display range
- 10 Hz - 400 Hz
- +10 dB - -80 dB
- Gate time:
- 259 mS (long gate) (Set to 2x what you want to view)
- View : Waterfall : ok to re-calc
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