Impulse Response vs Transfer Function

Impulse Response vs Transfer Function

Q: Wondering if I could get a hand with using Impulse Response to tune PA as opposed to using TF with Pink Noise.

I’m aware of the “Common way” blasting Pink noise, Finding the delay and then storing traces and averaging and making the adjustments. I’m just not familiar with using Impulse Responses. Is there an easy tutorial to follow or an article that I’ve missed that could help me understand it’s purpose? For instance using sine sweep etc….

A: In Smaart, IR mode is conceptually very similar to RT mode – in fact, you’ll notice it uses the same TF engine configuration (the TF engines you have configured in RT will follow you into IR mode). The acquisition is the same basic idea, just with an IFT at the end so we end up with time domain data rather than frequency domain data.

IR mode tends to use much larger FFTs (seconds vs milliseconds) and as a result, has much longer acquisition times. The main benefit is producing a measurement with much higher SNR than a RT measurement.

IRs are most useful if we want to investigate the acoustic properties of an environment (studying the decay of sound in the space). Doing this accurately requires us to expose 50 dB+ of the room’s reverberant decay, which is aided by longer FFTs.

Just as with RT mode transfer functions, IR mode can acquire a dual channel measurement using any broadband stimulus you like (pink noise, music, etc). It is not the case that pink noise must be used for RT measurement and sweeps must be used for IR. Smaart can acquire measurement data independent of test signal choice in both modes (they are both dual channel, and therefore signal-independent). It’s just that in IR mode, certain period-matched test signals offer some benefits.

Choosing period-matched pseudorandom noise allows us to drop the data window and further lower the noise floor of the measurement, and a period-matched log sweep (pink sweep) increases the dynamic range further mostly due to the signal’s low crest factor.

You can, of course, use these same stimuli in RT mode, however there is typically not much benefit from doing so – generating Magnitude, Phase and Live IR in realtime doesn’t require having such a high SNR. Likewise, if you acquire a large (high SNR) IR measurement and view the resulting magnitude response (Frequency plot) you will see that it gives you pretty much the same answer that the RT measurement gave you, it just took longer to there. For this reason, IR mode is not the best choice if your goal is to tune a PA system (IR mode also doesn’t produce a phase plot).

To tune a PA, a realtime measurement with Live IR, Magnitude, Phase and Coherence would be the preferred approach. For acoustic analysis, when long acquisition times are not a hinderance, IR measurements provide the mechanism by which we can penetrate deeper into the noise to better characterize the reverberant decay, and the high SNR data that results is important for accurate calculation of the Schroeder curve and the associated RT metrics.

Take a look at the user guide – there are two chapters dedicated to working with IR mode. You can also find some video content on our YouTube page if you feel like listening to me talk: https://youtu.be/brEgFZpsqCs

 

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