Room modes can make your mix sound flabby and are most prevalent in small rooms. A few properly placed notch filters can help and in this article I am going to show you how to measure your room and place the filters using Smaart.

In general, the process is simple. The audibility of room modes is determined by duration so all you need to do is observe a Spectrograph or Waterfall of your room’s impulse response. The tricky part is getting the right impulse response.

The quality of the impulse response is important because notch filters are very narrow. You need accuracy. The more measurements you take, the higher the accuracy.

Once you take all of the measurements, you could simply look at them one at a time and attempt to find the trends among them, but a faster way is to create an average. Unfortunately, Smaart doesn’t have this functionality. Fortunately, I have a workaround for you.

First, we need data. Let’s measure the impulse response.

Measure the impulse responses in Smaart

Without too much explanation for why I have chosen these settings, here’s what I recommend for the impulse module in Smaart.

Settings

• FFT: 128k
• Averages: 2
• Signal: Pink sweep triggered by IR
• Level: 20dB above the noise floor from 20Hz to critical frequency* (Don’t stress about this. If you were already taking transfer function measurements and getting actionable data through the LF, then your sig gen level is probably fine.)

Steps

1. Place mic at head height anywhere in the audience. Room modes are not distance dependent.
2. Press play.
3. File > Save impulse response.
4. Repeat six times at six random locations. If the audience and room are symmetrical, you only need to measure half of it.

This should be one of the final steps of your system tuning work. You’ll want the entire system ready to go since you are measuring its interaction with the room.

Here’s the sound system I’ll use for this article.

Here are the mics. They look closely spaced because the audience was only that deep.

Here’s what the first measurement looked like.

Now let’s create the average.

Workaround #1 – Easy/Expensive

The easiest, yet most expensive ($1,400) workaround would be to buy FIR Capture. Pat Brown used it to teach me this process at his OptEQ seminar during InfoCOMM 2019. Let’s go through it.

1. File > Import first impulse response (IR). Repeat for all IRs. No time window necessary.
2. Normalize all at 100Hz.
3. Create power average. (Or optionally, right click on the IR plot and choose Sum Multiple IR to normalize arrival times and preserve the IR)
4. Observe waterfall plot. If necessary, adjust time window for better resolution.

Here’s all of the IRs imported.

Here’s the average.

And here’s the waterfall plot where I have identified two room modes.

Workaround #2 – Medium difficulty/cheaper

If you don’t want to make the financial investment and the time to learn a new piece of software right now, I have an alternative for you: Averager.

This is an app from Eclipse Audio who you might know if you use FIR Creator. For $50, it’s a nice utility to get this job done.

1. Preferences > Transform size > 131,072 (maximum)
2. Load > Select directory with your IRs.
3. Uncheck any files you don’t want to use.
4. Optional: Select the IR with the greatest distance from the source as the reference. Delay > Time Align all to Reference.
5. Gain > Normalize to a frequency range of 90-110Hz.

1. Average > Averaging mode > Power
2. Save > File > Format > WAV
3. Set IR end to maximum (1,365ms).
4. Save

Back in Smaart…

1. File > Load impulse response and choose the file you just saved.
2. Calculate Spectrograph at 16kHz with 99% overlap.
3. Adjust upper and lower thresholds to discover room modes.

This method is more challenging than the previous because the graph is harder to read.

Workaround #3 – Medium difficulty/cheapest

Room EQ Wizard is a free app with some great functionality. Although it can create a power average, it will not generate a waterfall or spectrogram of the average. Your two options are to generate a vector average instead or open the average you created in Averager. You’ll get slightly different results, but essentially the same frequency information. It looks like this.

I still call this one medium difficulty because of the hoops you have to jump through.

Treat the room modes with EQ filters

Now that you have identified the room modes, treat them with narrow band (Q > 10, BW < 1/6oct) filters and listen to the results.

Here’s what my filters looked like in Vu-Net.

As you can see, I decided to audition a bunch of different filters.

Here’s the average IR of my room post EQ in FIR Capture.

And here it is in Smaart.

And here it is in REW.

WARNING: YOUR RESULTS MAY VARY

While listening in the audience, I auditioned each filter and discovered how easy it was to overdo it. A little help from the notch filter tightened up the mix, but too much and it lost its life and excitement.

What’s a room mode?

Resonance: wavelengths that “agree” with a volume.

a pressure wave that decays more slowly than those of the surrounding frequencies

daytonaudio.com

Standing wave: non-propagating, it’s “standing” in space because it’s reflecting back and forth between two surfaces or nodes.

In physics, a standing wave, also known as a stationary wave, is a wave which oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with time, and the oscillations at different points throughout the wave are in phase.

Wikipedia

Room mode: now we take duration into account. If the standing wave is standing around for longer than its neighbors, it might be a room mode.

Room modes are the collection of resonances that exist in a room when the room is excited by an acoustic source such as a loudspeaker.

Wikipedia

*What’s critical frequency?

Critical frequency is a milestone in the transition of room acoustics from lower density modal behavior to higher density geometric behavior. It can be estimated with by dividing 3,390 by the room’s smallest dimension (3c / RSD).

If you are working in arenas all the time, you’ll never need to worry about it, but if you are working in small rooms it can give you some insight into the behavior of your room.

Also, the first mode can be found dividing the speed of sound by twice the room’s longest dimension (c/2L).

Questions I didn’t answer

Wouldn’t the ceiling need to be 25ft away or shorter to have a room mode at 132Hz?

Yes, because 3,390/25=137Hz. At the time, I neglected to consider this. I never measured the ceiling, but it was probably closer to 30 or 40ft, which would put the critical frequency at about 113 or 85Hz.

Why was I seeing resonances above the critical frequency?

My only idea is that critical frequency is a milestone for a transition not a true/false verification.

What do you think? (read Michael’s response below)

Also, have you tried measuring and treating room modes? What were your results?

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