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Fundamental Acoustics Foundations

Sep 23, 2013 4:31 PM, By Bob McCarthy

The facts and fictions of modern audio engineering.

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Figure 3: An intellectual discussion of room acoustics by Trap and Zoid (from questions 10 and 11). Click here to see a larger image.

8. Did you know it is possible for two speakers to be out of polarity and in phase? How? Reverse polarity of one (180 degrees) and delay the other one half a wavelength. This is often done in two-way crossovers. Polarity has no time or frequency component, only normal (0 degrees) or inverted (180 degrees) for the entire frequency range. Phase changes are related to time. A fixed offset of time for all frequencies creates a different phase shift per frequency. If the native two devices are 180 degrees apart at crossover then a polarity reversal will bring them together.

9. Have you heard people talk about the need to turn the system up loud enough to “excite the room?” It is indeed challenging to get a room excited. They are notoriously dull. But if these walls could talk, things would get interesting. Many people believe that there is a trigger threshold where there is enough direct sound to get the reflections going. Imagine a wall with a policy like an amusement park ride: “You must be at least this loud to be allowed to reflect off this wall.” This is another case of “it’s in your head.” The reflections are always there. If we raise the direct sound level, we also raise the reflection level. Where does such an urban legend come from? It relates to how we can more clearly perceive the reflections when they are louder than the noise floor. Reverberation time in rooms is quantified as an RT60 (RT=reverberation time) value: the time it takes to fall 60dB in level after the direct sound finishes. But you will not hear all 60dB of decay if the direct sound is not at least 60dB more than the noise floor. As an example, we will use one second. If the noise floor was 30dB SPL and our direct sound 90dB SPL, then we would hear one second of reverberation before it was lost in the noise (90-30=60dB of decay). Our perceived RT would be the same as the measured RT. If the direct sound were only 60dB SPL, then the room would sound drier than before since we will lose half of the reverb in the noise. If we turn it up to 120dB SPL, then the reverberation would still be 30dB more than the noise floor after one second of decay. The room still measures with an RT60 of one second but our perception is much longer.

10. There is another legend about rooms out there. In this one, our powerful sound systems can “overdrive” the room or drive the room into saturation. This viewpoint seems to think of rooms as having acoustical limits in the manner that our power amplifiers have electrical limits. So overdrive and saturation would be the result of hitting the mechanical limits of the walls. The more likely outcome of reaching the mechanical limits of the walls would be the roof collapsing on your head, but fortunately our sound systems do not have that much power. The perception of overdrive and saturation are real though. They are combinations of distortion and compression in the sound system and your ears, as well as the perceived extension of reverberation time that results from the high acoustic levels.

11. Do you think that we might achieve matched amplitude and phase through the acoustic crossover when we use unmatched high-pass and low-pass filters in an electronic crossover? This is much more likely than if the filters are matched. Are the speakers you are crossing together a matched pair? It’s pretty unlikely that your two-way speaker is comprised of a 12in. front-loaded low driver and a 12in. front-loaded high driver. When we are crossing between two very different speaker components (which is the main reason for having a crossover), it is extremely unlikely that the native roll-off response of the two drivers is matched. Also they might not be mechanically aligned on the same plane. Acoustic asymmetry needs to be met with electronic asymmetry to get the two devices to play nicely at the meeting point.

12. Did you know that there is no such thing as a “phase problem” between two speakers? If two speakers driven by the same source arrive at our ears at different times, there will be peaks and dips in the frequency response. This is often called a “phase problem,” so you wouldn’t think I could solve this with an amplitude solution, would you? Simple. Turn off one of the speakers. Phase problem is gone.

13. The real problems are “phase + amplitude” problems, and there are plenty of them. The severity of phase + amplitude damage is predictable based on two factors: how close the two phase responses are and how close the two amplitude responses are. If you are close in level, you better be close in phase (time). If you are far apart in phase (time), you better get far apart in level. Two speakers are like two children: when they play nicely together, they can stay close together. But if they want to fight, send each of them to their rooms.

14. Remember how Spinal Tap likes to turn its amps up to 11 just to get that little bit more over the top? We can do even better with digital audio, because it goes to “111111111111111111111111.” What level is that? That is called “full-scale digital” or 0dBFS. But if they want that little bit more, it is just too bad. There are no 2’s or 3’s, and we can’t just add another digit on because the next device in the chain won’t know to read it. If you try to go beyond full-scale digital, you have chewed off more than you can bit. What level exactly is “full scale?” This follows audio industry standard practice, which is to say we practice setting a lot of different standards. Full-scale digital (0dBFS) can be anything since it is just a mathematical construct inside a number-crunching machine until we finally reach the outside world where audio exists in an actual medium such as electricity: the A/D or D/A converter. It is at this stage that the dBFS value is given a voltage value such as 0dBFS = 10V (+20 dBV) or another voltage of the manufacturer’s or end-user’s choice. It is highly recommended that you read the spec sheet and find out the full-scale conversion number for both input and output of your digital audio devices.

15. Did you know that the equal-loudness (Fletcher-Munson) curves have no application to the equalization settings of a live sound system? Those are the curves that explain how our ears change their response to frequency when the source level changes. Remember the “loudness” button on an old hi-fi receiver? If we listen too quietly, it sounds too midrange-ish, so the loudness button bumps up the LF and HF regions. It takes a good 20dB change in level for the response differences to really matter. The reason it doesn’t matter to live sound is because it’s live. First, this means that our mixer has full control of the tone of the program material, and secondly, we never have quiet parts in live shows anymore because the moving lights are too noisy. The mix engineer sets the level and adjusts the tone to sound right for the given song. When a ballad is performed, the mix is modified to sound right at the new level. It is a closed-loop system that does not need a retuning of the PA every time the band creates some dynamics. Now what if the levels are different between the front and rear of the house? Simple answer: If you have 20dB differences between the front and rear, you have bigger problems than Fletcher-Munson can solve. So why is it valid for playback? Because the studio mix assumed a certain level for playback, which may or may not be the level you are listening to at home.

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