There is an acoustic phenomenon that has been present at every live concert since the beginning of live music: acoustic cancellations. So many people leaving a concert make wildly different comments about the sound quality of the show they have just seen; some blame the sound system, others the acoustics of the venue, and others even the sound engineer or the musicians.
In a stereo system, the sound reproduced by the left and right sides will arrive at the same time only for a small portion of the audience, which should include the sound engineer, provided the mix position forms an equilateral triangle with the PA system. So the only position with the best listening will be position A and the whole line running forward and backward from A.
Remember that there are several theories about where to place the mix position in order to avoid these acoustic cancellations; one of them is as follows:
If the distance between the left and right loudspeakers is D, the mix position should be placed between 1xD and 2xD on the horizontal plane. For example, if the distance between the speakers is 10m, then the mix position should be located between 10 and 20 m from the stage, with 20m as the maximum limit, as long as we don't exceed the critical distance, which is where reflected sound is greater than direct sound.
As we discussed in previous articles, sound travels at a certain speed, but not all frequencies are reproduced at the same time, so these time differences are more noticeable at high frequencies.
We can conclude, then, that it is impossible for everyone to hear the sound with the same frequency response. That's true! What's more, the sound system can be tuned perfectly for just one point and acceptably for several points, but not for the whole audience.
If we now look at the frequency response given by an RTA, we see, in this case, some cancellations and summations, specifically around 125Hz and 1KHz, respectively. The RTA cannot tell us whether these irregularities in the response are caused by differences in arrival time at the measurement microphone. For that, we would need to perform a transfer function; it is the only way to find out.
The famous comb filter effect is calculated as follows: if a signal arrives with a difference “x” expressed in milliseconds, this time is known as the offset; to find the cancellation frequency we must calculate the inverse of the offset, that is, 1/offset, and since a cancellation occurs every half cycle, the first one will occur at half the cancellation frequency.
Let's explain this with a practical example
If one signal reaches us at 41ms and another at 40ms, the offset will be the difference, which is 1ms. Converting 1 ms to seconds gives 0.001s; the cancellation frequency will then be 1/0.001, that is, 1000Hz.
This means we will have a cancellation every 1000Hz, with the first cancellation at half of those 1000Hz, that is, 500Hz. And the following cancellations at 1500Hz, 2500Hz, 3500Hz, 4500Hz,… as we can see in the figure above.
If we calculate the impulse response, we see that there are two impulses with a time difference, which in the previous example is 1ms; the first impulse corresponds to the signal that reaches us first, and is therefore higher, and the second impulse to the signal that reaches us later, and therefore has a lower level.
As the time difference decreases, the comb filtering tends to diminish. So if, following the previous example, we have a difference of 0.5ms, the cancellation frequency will be 2KHz and its first cancellation will be at 1000Hz. The cancellations will shift to the right.
In the next example we have increased the time difference to 2ms, which increases the cancellations, and they shift to the left. However, if the time difference is zero, in the impulse response we will see only a single impulse, and the frequency response becomes more linear, with the comb filter disappearing.
Remember that in previous articles we clearly explained that acoustic cancellations exist at low frequencies and are felt much more than at high frequencies. That is why, whenever we place sound systems in the classic way, left and right, especially the subwoofers, we will get a lot of SPL at the mix position and the classic "alleys," which are our cancellations, a consequence of distance and therefore of time.
From all this we conclude that it is not possible to have sound without cancellations for the entire audience, and that we will have to minimize them somehow by tuning the sound system for a specific position, which will be the sound engineer's. At that position the phase must be perfectly adjusted to allow for less phase misalignment, and therefore less time misalignment, across the audience.
Another option would be to sell the audience tickets for zones where certain frequencies are canceled; that way, someone who doesn't like a rock band's kick drum would buy a ticket for the spot where the frequencies around 60Hz are canceled, and so on. Just kidding!
