Simulation

Blog Post #1 : Coupling vs Comb Filtering(Part 1)

Hey Everyone! Thank you so much for tuning in. This is the very first technical post on my website and i have decided that while we are discussing systems and how to design and optimise them, there are some basics we need to be very clear about and i’ll be touching a few of them at regular intervals. The topic that i have decided for this month is : Coupling vs Comb Filtering!

        Here’s a video simulation of the things I’m gonna talk about in this post.

Coupling and Comb Filtering are the two most important extremes we come across when it comes to sound.Hence, knowing these interactions becomes very crucial to us. This will help us strengthen our basics on this subject. These basics will lay a strong foundation and help positively influence our decision making!

So let get started!

What is the most common setup we come across :

  1. A Signal Generator : Could be a console,a sound card, etc.
  2. The PA consisting of : Left PA , Right PA, Subs.
  3. Listener.

We will be using Pink Noise as the test Signal.

Also check out the details of the latest test signal ‘Meyer Sound’s M – Noise‘ which isn’t yet available to us.

The Factors that affect the interaction between two identical signals are :

1)Relative Amplitude.

2)Relative Phase.

For Part 1 of this topic, we are gonna restrict ourselves to using 2 identical signals with same Amplitude.

Leaving us with only variable : Phase.

Effect of Variable phase on Signal Interaction.Comb Filtering.
Effect of Variable phase on Signal Interaction.

As sound travels, every frequency,depending on its time duration is in different position on the phase cycle.

When 2 Identical signals of same amplitude, arrive in Phase, it results in a 6 dB gain in amplitude. The gain in amplitude is experienced till the phase difference reaches 120º, which is considered the additive side of phase. This phenomenon is called as Coupling. And the region where it occurs is the Coupling Zone.

Preferred FOH Mix Position.
Listening Poistion A : The Coupling Zone!

However ,when the same 2 signals of same amplitude,arrive with a time delay between them, we do not get Coupling at all frequencies as the individual frequencies are at different positions in their phase cycle. As a result, we end up with a  resultant interaction varying from 6 dB gain to 3 dB gain to no gain (0 db gain) to loss of 3 dB to complete cancellation and again followed by rising gain and then the cycle keeps repeating itself.

Let’s see what happens when the time delay between arrival of 2 Identical signals of same amplitude is 1 ms.

At 1 ms the first frequency that will arrive in phase (360º apart), and will show a 6 dB gain (coupling), will be the one which has completed one complete cycle. This basically means the frequency whose time period is 1 ms.

F(Hz) = 1/t(in sec) = 1/1ms = 1/0.001sec = 1000Hz.

Similarly, at 1 ms the first frequency that will be 180º out of phase will be 500 Hz, showing complete cancellation.

So now ,we understand that all frequencies below 1kHz (1000Hz) would have completed less than one complete cycle,and all above would have completed more than 1 cycle.

All those frequencies, that arrive on completing whole cycles will show coupling, while those frequencies which arrive 180º apart will show complete cancellation. And as previously mentioned the cycle will keep repeating itself giving rise to a pattern ranging from 6 db gain to complete cancellation again to 6 db gain. When this repeating pattern in seen on a linear scale, it appears like a comb. Hence the name Comb Filtering.

Comb Filtering
Resemblance of Linear scale of 1ms interaction with the comb above.

Considering this, while on listening position A, if Left PA and Right PA are not arriving in time (due to delay on the processor or because the two sides are not rigged at the same distance,or we are standing off-center, etc) this position will end up being what is called as the Combing Zone.(Simplified for the purpose of this artcile.Detailed specific discussion on the same in upcoming blogs.)

1ms Comb Filtering.
Listening Position A : A Combing Zone due to difference in arrival time.

It’s clearly evident that the pursuit for coupling comes with the risks of combing!

The same process applies to different time delays,and depending on the time difference different frequencies will show coupling and cancellations at regular intervals, leading to comb filtering.

For eg.For time delay : 5ms.

First coupling  at F = 1/0.005 = 200Hz. So, first cancellation at  F = 100Hz.

Coupling frequencies : 200 Hz, 400 Hz, 600 Hz, …..

Cancelling frequencies : 100 Hz, 300 Hz, 500 Hz, …..

Watch my video, to see the comb filtering effects for different time delays.

As mentioned in the video, posting the screenshot of all the traces here :

Comb Filtering.
Screenshot from my video showing Comb Filtering at 10ms(pink), 1ms(blue) and 0.1 ms(light green) and Coupling(Bright green).

Here, I’ll conclude part 1 of this series. In this part we only used phase as a variable factor and we observed how a small time delay affects the interaction between PA. I hope this article helps you all in understanding what coupling and comb filtering is.

In the upcoming part of this series, we will observe the effects of varying amplitudes on PA interaction and what are its implications.

Till then,stay tuned.

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