Recroding Room Impulse Responses

https://www.prosoundweb.com/what-is-an-impulse-response/

What are Room Acoustics?

A room acoustic is the sound that is reflected from a source. Known as a Room Impulse Response (RIR) the walls, floor, ceiling and other surfaces in the room reflect sound back to the listener, which is interpreted by the listener as “acoustic space” or “reverb”.

If you’ve entered a large space you will have heard early reflections as 'echoes' and the subsequent wash of reverb. This is diffuse reflections where the signal gets split into its constituent parts due to rougher surfaces such as unpolished stone or concrete.

Recording an RIR

An impulse is a short, but loud signal that can be used to excite a room. This way we can listen to the room’s response and make observations. Recording this ‘Impulse Response’ can allow us to impart the space's sound on other signals. This is done through the process of ‘convolution’.

There are two methods most commonly used to record the acoustics of a room:

Impulse Method: Using natural physical acoustics we can excite the environment with either a clap of the hands, a gun-shot, or a balloon being popped, (or sending an impulse out of a speaker)
Exponential Sine Sweep (ESS): Using a loudspeaker and a sine wave sweep can reduce distortions and noise, as instead of all frequency bands being excited at the same time it is split over a longer length of time.

Experimenting with the Impulse Method

The Impulse method relates to a specific signal, however an ‘Impulse’ or a loud short sound can mimic the workflow.

Take yourself into an interesting space such as a stairwell or large indoor area and produce a loud short sound. A clap or a balloon popping is usually used to demonstrate this. Try to count how many sections it takes for the room’s response to decay till you can’t hear it any more.

Mental Checkpoint 1: Think about what sort of purpose a long decay time could have, also think about what purpose a short decay could serve.

Reaper ReaVerb - ESS

The DAW we will be using is Reaper as it is fairly cheap and very accessible to learn. It also contains multiple stock plugins, one of which is ReaVerb, the ‘in house’ convolution reverb unit.

Outcomes:

At the end of this session you should be able to:

Record a room impulse response using ESS method
Describe two different types of impulse response techniques
Understand the application of acoustic measurements
Understand the creative application of Impulse Responses

Requirements:

A base level knowledge of Reaper and it's functions:

Setting up audio IO
Adding Tracks
Adding Inserts/FX units
Recording Audio

A base level knowledge of Audio Recording Concepts:

Using audio interfaces
Microphone Placement
Speaker Outputs

STEP 1: Setting Up Tracks

Open reaper and add 3 tracks from the menubar ‘track -> insert new track’
Name the tracks 'ReaVerb' 'SineSweep' and 'Record'.
Save your file on the desktop.

Make sure to save your file at this point, if you don't, finding files later will become very difficult.

Add the 'ReaVerb' plugin to the 'ReaVerb' channel.

This is what your mixer window should look like after naming the tracks.

Step 2: Setting up ReaVerb

Click on 'Add -> File', then click “cancel” when asked to select a file.
Click on 'Generate test tone', navigate to the desktop and save a file named 'Test Tone'. This should produce a .wav of an exponential sine sweep (ESS).
Bring the test tone (.wav file) into the 'Sine Sweep' Channel and place onto the timeline, make sure to leave a bar or silence before.

This is what your right-hand side of the ReaVerb window should look like when 'file' is selected.

Step 3: Recording the ESS

Now record arm the 'Record' track by clicking the round red button on the track or in the mixer. Record the sine sweep (test tone you generated) out of a speaker in the room.

Make sure the track does not have record monitoring turned on as this will produce a feedback loop.

Remember to be very quiet as any noise in the recording will cause errors in the final result.

Mental Checkpoint 2: Write down why you think sweeping through the frequencies works better than displaying all frequencies at the same time.

STEP 4: Processing and Deconvolve

This is what the recorded signal looked like for a living room. Yours may look different due to recording a different room.

Now trim the clip to the size of the 'Test Tone' signal, by clicking and dragging from the sides of the clip.
Right-click the clip and click 'Glue items': this will create a new file of the exact length of the sinesweep signal.
Open up ReaVerb and click 'Deconvolve' on the right side of the window. It will first ask for the glued file (named ‘xxx-glued’) in the previous step which should be found in the project directory. Then it will ask for the 'Test Tone' which should be found on the desktop.

Reaper will then deconvolve the glued signal and save a new file to the desktop

Add some loops from the sample pack in the first activity to listen to different musical material in the space.
To listen to more of your recorded space, bring up the 'wet' fader in ReaVerb.

Your IR will be imported into the ReaVerb.

Mental Checkpoint 3: Write down how you think the length of a sine sweep would affect the quality of your measurement.

Mental Checkpoint 4: Write down how you would go about controlling external noise in a busy city centre.

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