Use these settings to efficiently investigate room modes with the impulse response module in Smaart©.
- FFT: 128k
- Avg: None
- Delay: 0 (doesn’t matter)
- Options > IR > Overlap: 0%
- Signal Generator: Pink noise, Pseudorandom, Drop IR Data Window
- Level: +10dB above noise floor
Measuring room modes sucks. It takes too long, pisses people off, and the ROI is debatable.
The last one is user dependent. Let’s work on the first two.
Why does it take too long?
This was user error on my part. I thought that multiple averages were required for sufficient dynamic range and actionable data. This meant that all of my measurements took twice as long because I had the averages set to 2.
While increasing averages does improve dynamic range, it is less important in researching standing waves because you’re going to average together at least 6 of them in the end.
Set averages to 0. Fixed.
Why does it piss people off?
- Wrong stimulus
- Unnecessarily high level
Use period-matched pseudo-random pink noise
The Pink Sweep stimulus pisses people off. People can get used to music and pink noise, but the sweep is surprising and painful. It’s the most common stimulus to enrage civilians.
While the Pink Sweep does improve dynamic range, the results I observed don’t justify the risk of someone telling you to stop. If you’ve got the room to yourself, though, do it.
From what I can see, using pseudo-random period-matched pink noise will get the job done at the same signal generator level and generate less complaints.
Set level +10dB above noise floor
I was using +20dB because according to the Smaart user manual you need to be at least 10dB above the noise floor, but 20dB is OK, and 30dB is preferred.
I ran through six tests at six different measurement positions:
- A: Pink Sweep at +20dB relative to noise floor
- B: Pink Sweep at +10dB
- C: Pink Noise at +20dB
- D: Pink Noise at +10dB
- E: Pink Sweep at 0dB
- F: Pink Noise at 0dB
Then I imported the batches of six measurements into Room EQ Wizard to be level matched at 100Hz, time aligned, and vector averaged. Below are the spectrogram graphs for each of the averages.
Even though dynamic range drops as the stimulus in the room is lowered, I don’t see any difficulty in identifying the first four peaks in the first four measurements. Only when the stimulus gets to 0dB compared to the noise floor do I start to have doubts.
Obviously the results will be different in larger rooms compared to my living room, but going through all of these tests gives me the confidence to try a -10dB level moving forward.
Verify signal generator level by comparing it to the noise floor. Switch over to an RTA graph. Measure the ambient noise floor. Offset it +20dB. Turn on the signal generator and match it at every frequency.
While you’re there, make sure that the input levels (Mic/Ref) are matched and within the yellow target zone.
What about wind?
In any scenario where there might be a possibility of any significant time variance during the measurement period, you would probably be better off increasing the measurement time window and/or using a period-matched stimulus signal rather than upping the number of averages.Rational Acoustics Smaart v8 User Guide
Pink Noise – Pseudorando
Use a 2-channel measurement with period-matched signal generator.
At first you might wonder, why would I ever use a single channel measurement? IR measurements go back a long way and can be completed with a balloon and stopwatch. This direct method may deliver mixed results, though, since it might not have an instantaneous envelope or uniform spectral content and it doesn’t tell you anything about the sound system.
The newer indirect method uses a 2-channel transfer function to mathematically estimate the response and combined with a test signal of matched length can simplify our goal of actionable data.
feed the DFT what it really wants to eat: a test signal that either fits completely within the measurement time window or cycles with periodicity equal to the length of the DFT time constant. Signals that meet these criteria can produce deterministic, highly repeatable measurements in a fraction of the time it takes to get comparable results using random signals.Rational Acoustics Smaart v8 User Guide
Unlike the Pink Sweep, pseudorandom cannot be triggered by the play button so you’ll need to start the signal generator before you press play or Smaart may crash, which is what happened to me the first time I tried it.
I was curious to see how the results of a pink sweep might differ so I tried that as well.
Ld = level of direct sound
Ln = level of noise
Ld – Ln = dynamic range of the measurement
The dynamic range of the measurement using pseudorandom noise is -44.73dB. The dynamic range of the measurement using pink sweep is -65dB. That’s an improvement of 20.27dB.
Unless you are in a very large or noisy room, 128k (2730ms = 2.7s) or 240k (5000ms = 5s) should be plenty. I normally use 128k.
If we want better dynamic range of our measurements we can either turn up the signal generator in the room, increase the averages, or increase the measurement time. Since my goal is speed, I’m looking for the lowest number of averages and measurement time.
When measuring with period-matched noise or sweeps, averaging is normally set to “None” or 2, although it is still possible that a higher setting could prove helpful if measuring in an extremely noisy environment.Rational Acoustics Smaart v8 User Guide
Normally this is set to 0% for maximum noise reduction, but can be set higher to save time. Let’s see if there’s any benefit to using more overlap in terms of saving time or improving SNR.
This image compares an IR measurement with 0 averages and 0% overlap in yellow with 2 averages and 99% overlap in blue, which took about the same amount of time to measure.
Doesn’t look like much of an improvement.
You only need to set the delay locator if you are using random stimulus.
The first is to delay the reference signal to match the timing of the measurement signal, so that the data windows line up. You should always do this when measuring with random signals.Rational Acoustics Smaart v8 User Guide
We are going to use a period-matched stimulus so we can ignore this step. Yay!
Big thanks to Rational Acoustics support. My laptop crashed a lot during my first tests while “Processing frame”, which really sucked when the signal generator was stuck on. I emailed support. John emailed me back and I found out that an update with a fix was coming out that day. It worked!