I mentioned in my previous article (What, Me Worry?) that I’ve been troubled by the lack of fundamental audio and acoustics knowledge often displayed in our industry.
And because of this, there are misconceptions that are difficult to stamp out.
So in an effort to help, I’ve whittled down a concentrated list of the top technical concepts I think everyone in our business should have under their collective thumbs.
Of course there are many more important ideas and concepts that we all need to know, but to me, these are the ones that translate very directly into a better product.
10. The Speed of Sound and Delay
First, we all benefit from understanding that sound waves move through a fluid medium - i.e., air - and thus the speed of sound is related to temperature and the density of the air, depending on altitude.
At sea level and “room temperature” (generally speaking 68 degrees Fahrenheit or 20 degrees Celsius), sound travels at about 1,130 feet or 343 meters per second.
That’s actually pretty slow when you think about it. In fact, it’s fairly easy to observe a lag between what you see and what you hear when located more than 40 or 50 feet from a sound source.
This is the reason why we need “delay towers” and not just “loudspeakers” when covering a very large area. Don’t forget that at higher altitudes, sound travels slower, and at higher temperatures, it travels faster.
As a result, it’s wise to carefully measure delay times for towers before setting them - the standard formula may not work for your particular setting, and also keep in mind that settings might need to change as the day wears on.
9. Loudspeaker Crossover Frequencies
Different loudspeaker drivers have different frequency ranges and need to be “crossed over” or properly filtered for the cabinets to work well.
But we should also be aware that different engineers have different approaches, and that different crossover schemes can have a radical effect on the ultimate sound of the system.
The exact corner frequencies chosen, the slope/s of the filters, and whether the crossovers are active or passive all make a difference.
For instance, a 2-way box that has an internal, passive crossover will sound different if bi-amplified with an external active crossover.
Some of this has to do with phase, and some of this has to do with filter slopes. I suggest reading up on the subject, starting with loudspeaker design guides. There’s a whole world of information available on this subject.
8. dB Relationships
First, we need to understand that the dB (deciBel, or 1/10th of a Bel) is a relative measurement.
In other words, it doesn’t mean anything without a reference of some sort. If someone says, “did you hear that show last night? It was so loud, it was probably 110 dB.”
Of course, the person is probably referring to 110 dB SPL (sound pressure level), but without the SPL, there is no reference.
There are also electrical references like dBv, dBu, etc., and it’s important to understand what they mean.
The other important concept about dB is that it is on a logarithmic scale (powers of 10). This means that if something is 10 dB more than something else, it is actually 10x the power! Interestingly, for us to perceive something as “twice as loud” as something else (a perception of SPL) we actually need 10x the power to achieve that effect.
A 3dB increase represents twice the power but only sounds “slightly louder”. So it truly boggles the mind to think about huge changes in dB, such as “the reverb tail was 60 dB down after 2.4 seconds”.
This means that 2.4 seconds after the last note ended, the sound level was reduced in acoustic power by 1 million times.
7. Grounding
You knew I’d bring this up! One of the main problems we all face is dealing with ground loops and the resulting audio problems, namely hum and/or buzz.
The basic concept is this: if there is more than one “path to ground” with different resistances, then there is a ground loop and the likely result is buzz.
I strongly recommend reading up on this subject in the Yamaha Sound Reinforcement Handbook and also possibly attending a SynAudCon workshop on hum, buzz and grounding.
6. RF Frequency Coordination
Many wireless microphone products have made this task easier the past few years. Some systems auto-coordinate, and some have “wizards” that step an operator through the process.
But I still believe it is important to understand what is happening “under the hood”.
And, frankly, most larger-scale systems at pro events are carefully hand-coordinated using specialized software.
Why?
Skilled operators can eek out a few more solid channels by using their superior knowledge and experience versus simply relying on a machine to do it.
Several manufacturers offer courses or workshops on the subject, and most are also more than willing to answer questions on the phone. Take advantage of these resources and you’ll be better off going forward.
5. Inverse Square Law
Like dB relationships, this is an important fundamental concept of acoustics that has powerful ramifications. In basic terms, the law is states: for every doubling of distance from the source (in free space, i.e., no reflections) the acoustic power is reduced by 6 dB (4x).
Mainly, this is important to understand within the context of coverage, the amount and type of loudspeakers needed, and the resulting amount of amplifier power required.
4. Power & Distribution
Do you know the difference between 110-volt and 220-volt AC power in the U.S., beyond the fact that 220 is basically twice as much voltage? How about 3-phase power?
While we’re at it, what’s “AC” actually mean? If you’re even slightly fuzzy on these concepts, time to get busy and learn.
I’ve found that understanding 110 versus 220 power has even made me more handy (and safe) around the house, since there are usually some 220 appliances around.
It’s also a good idea to be familiar with the peak and average current draw from all components in a sound system, and how to best manage the AC power. Remember the story about how Aerosmith lost power at Sturgis in 2009?
Someone plugged something (non-audio in this case) into a power distro and drew too much current, tripping a breaker.
The show was temporarily paralyzed, and Steven Tyler ended up falling off the stage and requiring hospitalization. All because someone ignored a power issue. Don’t be that guy.
3. Microphone Proximity Effect
Know the difference between omnidirectional and directional microphones with respect to proximity effect?
How about the difference of proximity effect between cardioid and hypercardoid mics? What do those little lines and dashes mean on a microphone frequency response plot below 100 Hz?
In a nutshell, proximity effect is something that directional mics exhibit, and true omni mics do not. The closer the microphone gets to the sound source, the more the low frequencies are boosted.
But don’t forget that it also works the other way - the further a mic is located from the sound source, the more the low frequencies are attenuated. The more directional the mic, the more proximity effect it will exhibit.
2. Polarity vs. Phase
These terms are often erroneously interchanged by those who either do not recognize the difference or don’t care enough to make the distinction. Some manufacturers even use “audio phase” where they mean “polarity”. So let’s straighten it out.
Polarity refers to swapping the positive for the negative, either in terms of speaker terminals, microphone signals, or the electrical signals within a console.
Phase refers to the relationship between signals in terms of “degrees of phase”. One place where these ideas appear to merge is when it is said that “flipping polarity is the same thing as the audio being 180 degrees out of phase”.
Yes, I suppose this is true, but it confuses the issue. Normally, a number of degrees of being “out of phase” refers to a specific frequency. In other words, you might be 90 degrees out of phase at 1 kHz, but it would be a different number at 2 kHz. Get the picture?
1. Gain Structure
I saved it for last because it’s the one I see done wrong more than any other. If you “use your gut” to set levels, and if you like to mix by the light of the clip indicators because your idol says, “I like to hit the buses hard” - you’re in for some trouble.
Unless you’re looking for a specific effect (i.e., some oversaturation in the sound), it’s best to gain stage the signal from start to finish so that you’re maximizing the dynamic range of the system, minimizing the noise floor, and avoiding distortion. There are many sources of good information and instruction about gain structure.
Learn it, know it, live it.
Karl Winkler is Director of Business Development at Lectrosonics and has worked in professional audio for more than 15 years. A good place to start doing homework on many of the subjects Karl mentions is the reference material in the Study Hall right here on ProSoundWeb
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