Jan 22, 2013 2:30 PM, By Bob McCarthy
Sound systems and the rooms they live in
This requires a moment to break down this time-sensitive mechanism that affects our sound system so dramatically. This is the summation effect of multiple arrivals of the same signal, which may or may not be in time (and therefore may or may not be in phase). These effects occur whenever we hear a reflection, or whenever two speakers play the same signal, or even between your own voice and a copy of it coming through the sound system. The early reflections and reverb tail are members of this family but vary by relative level and timing. The interaction of multiple speakers will create the same family of frequency response effects if the relative timing and levels of the reflections are the same.
This effect goes by many names with “comb filtering” being the most common and easy to digest. Here is what you need to know right now to get the picture: 1. The peaks and dips created by comb filtering are strongest when the mixed signals are close in level. 2. The width (in octaves) between the peaks and dips is the reciprocal of the number of wavelengths (cycles) the two sources are apart in time. So if we are one wavelength apart then we have octave spacing. If we are 1,000 wavelengths late the width is 1/1,000 octave. Wide peaks and dips (such as octave, half octave, and one third octave) are heard as a tonal shift, which we might be inclined to modify with equalization. When we hear peaks and dips that are 1/100 of an octave wide, we don’t perceive this as a tonal modification but rather a separated event such as an echo or part of the reverb tail.
Let’s do a quick and easy example to put this into perspective. A strong wall reflection arrives 10 milliseconds late. In the acoustician’s terms, this is an early reflection. To the sound system, it is early, medium, and late, depending on frequency. For 1kHz, the effect is a peak with dips on either side than span 1/10 octave wide (medium). For 10kHz, the peak is surrounded dips that are spaced 1/100 of an octave wide (late) and finally 100Hz, which is an octave wide (early). The 100Hz peak would clearly be perceived as tonal distortion, while 10kHz would be far too narrow for even someone who believes audiophile marketing materials to imagine they can hear as tonal.
Once we realize that a 100-millisecond reflection creates 1/100 octave-wide peaks and dips at 1kHz, it becomes clear that the late room reflections are far out of the tonal range for all but the lowest frequencies. Even something as short as a 1-millisecond reflection will make 1/10-octave-wide havoc to our high driver, so just forget about the room helping us up there.
We can see that it is best for the speakers to go it alone in all but the low end. So why do we care so much about tone and frequency response? Frequency response is the second most common metric of sound performance (first place goes to dB SPL bragging rights). The reason we care so much is that our speakers, the combined mix of the music in the show, and anything that leaves a strong tonal stamp on our response does so to all of the instruments and singers passing through. A strong early reflection added to the speaker path colors the sound for the entire orchestra at your location, and worse, it paints a different set of discolorations to the other seats. You might say, “Why the big peak at 400Hz?” while your friend in the next seat wonders why 500Hz stands out of the crowd.
How is this pathological version of the early reflection different from the beneficial reflections that acousticians love? Consider that the band is spread out over the stage and their individual direct sounds all hit the walls at different times. Each instrument gets some color, but none of them get the same. For the sound system, a single wall can create a 6dB peak in the response for a huge percentage of the people in the coverage area. To duplicate this undesirable feat with natural room acoustics for everything coming from the stage would be the ultimate challenge for an acoustician.
The next level may help to see why we keep our focus on the speakers and view the walls as someone in the room you prefer to avoid talking to: speaker arrays. Clusters of speakers are the ultimate ERSDs (Early Reflection Simulation Devices), an idiotic acronym, which I just made up at this moment. The multiple speakers of an array couple together in close proximity and arrive almost at the same time as the leading speaker. Almost is the key word here because whatever amount of time difference we have is a factor of 100X greater comb filter challenge for 10kHz as it is for 100Hz. As stated before, our margin of error is terribly small. A four-speaker array with a spread of different arrival times can have peaks of up to 12dB. Few acousticians would ever face this kind of frequency response deviation by virtue of room acoustic design, but this is an everyday event for the sound system engineer. Because of the great potential for frequency response uprisings among our own people (the speakers), we are in no hurry to expand the population we are trying to control by adding strong reflections from the walls.
So let’s get back to the original premise of how much differently we would design a sound system for a live or dead room and consider a series of questions. If you designed a sound system with perfect coverage in a room you were told had a 1.5- second reverb time and then found out it was actually 1.0 or 2.0, what would you change?
If there was a big flat reflective balcony front in a reverberant room, would you try to avoid it? How about the same balcony in a dry room? When you are told that a room has “perfect acoustics,” do you think “Easy Street” or is it more like “Danger Zone”?
So where does this leave us? Pretty much right where we started, but hopefully a little clearer on why we do things the way we do. The biggest difference between our approach to live and dead rooms is the amount of fear we have about spilling any sound out of the seats. The live room requires a more surgical approach, but the shape of the direct sound target is exactly the same whether the building is just a wire frame or the walls are filled in.
Sound engineers are generally aware that they are not acousticians and yet a sound engineer has to live with the fact that everyone has two jobs: theirs and sound. We respect the training and knowledge required for an acoustics degree. But knowledge of acoustics does not automatically mean knowledge of sound system engineering. Many sound engineers lack formal training and yet we have a huge and valuable volume of experience about how speakers perform in rooms. We can spot a newly built room that was designed with an 18th century approach to speaker systems. And likewise, we can spot (and are infinitely grateful for) the rooms that are designed with the needs of modern speaker systems in mind. Speakers are here to stay—mostly because the only shows that make any money are the ones with speakers, so let’s get realistic about designing rooms that are going to work with and not against them.
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