Portable Power: The Secret Life Of Generators

Power is something often overlooked as long as its working we don’t really put much thought into it. Once the pigtails are handed to the electrician and all the voltages are checked the rest is forgotten till the end of the show. This turns into a very different situation when we are dealing with generators.  In some scenarios there is an electrician or a generator technician available but in many others the responsibility of hauling, operating and monitoring the generator can be appointed to us.

For starters lets break down the actual generator. A generator is a machine that converts rotating mechanical energy into electrical energy. It consists of four main parts the engine, generating unit, governor controls and distribution. Electricity is created when the engine turns a rotor inside a series of windings in the generator. The governor is responsible for controlling the speed of the engine so the generator is not overloaded. The distribution center is where we find all of our connectors and lugs along with extra protection by way of circuit breakers.

The generators we use or “Production” generators differ from standard construction generators by having great voltage regulation. For comparison our generators have around 1.5 percent tolerances while construction generators can have upwards of 25 percent tolerances. While this is fine for construction equipment it can be very harmful for our expensive electronics. They are also quieter than regular generators and have larger fuel tanks for longer operation. 

To ensure safe operation everyone should know the laws and codes concerning power systems. The National Electric code, sponsored by the National Fire Protection agency, is published every three years. The current edition is from 2011. The codes presented are merely advisory codes, it is up to local municipalities to adopt the current version of the National Electric Code to adopt it as laws. Some may adopt it exactly while others enforce stricter guidelines. The sections of interest to production companies are articles 520,525,590, and 640. 520 covers structures used for presentations and performances like theaters, assembly halls, and stages. 525 covers installation of portable wiring and equipment for carnivals, cruises, fairs and related functions. 590 covers installation of temporary power and lighting while 640 covers portable and/or temporary audio system installlations.

Several guidelines for generators include, parking the unit on a flat level site with easy access for refueling while also keeping it away from the general public. Secure the wheels to prevent rolling. Installation of a ground rod or tie the unit into an existing ground and most importantly meter the outputs before connecting your equipment.

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Anatomy of a Service Call

Even the most basic sound reinforcement systems can have problems.  The system at a chapel in a local retirement home is one of them. Distortion, poor gain before feedback and intermittent operation can be some of the troubles. 2 lavs that distort easily are the first offenders.  Although easily replaced by 2 wired mics there are no mic jacks and the owners do not want any visible cables. In determining possible solutions it is best to start at the input and work your way to the output of a system. First were the wireless mic connections. A balanced TRS output was present but a test with the TS-1 handset proved there was signal only on the tip , making it an unbalanced connection.  The mics sounded ok so they were ruled out as the source of distortion. The amplifier present was not appropriate for the application. It was low on power and had only 1 input channel that the installer summed both mics to. It also lacked the proper outputs to feed the auxiliary systems in the facility. The unit used to replace it had two balanced mic level inputs, an aux bus, and a line out.  Problems interfacing the receiver with the amp/mixer arose when the inputs were found to be labeled “low Z”. A quick check revealed the impedance was appropriate at 8kΩ but the level was unknown and presence of phantom power was suspected which was not good. This was the perfect situation for a step down transformer. One that would step down the output voltage of the receiver and would also block the DC voltage from the phantom power. The next factor down the line was the ceiling loudspeakers. Tests revealed high resistance coming from the loudspeakers, in a typical situation a high voltage amp output would be needed but this being a place where a lower level than usual is required the amp would be fine. It worked, the loudspeaker system provided adequate level while limiting when a speaker spoke too loud.  Driving the balanced input of the hearing assist systems also proved the system was functioning properly. A quick look at the wiring diagram showed it to be wired for a low voltage amp but being driven by a high voltage amp. The distortion created by that setup rendered the system useless. Lastly was the wireless mic reception.  1 of them worked fine across the whole area but the other had a few dropouts. The receivers lived in an adjacent room which blocked signal and with no other way to wire them in the main room the only solution was to leave the door open during service. These all proved simple and easy solutions to entry level systems. The perfect solution to many of these problems was to begin with adequate equipment. Consumers often go with low budget systems which later haunt them with problems.

http://www.prosoundweb.com/article/anatomy_of_a_service_call_even_simple_sound_systems_arent_plug_38_play/

Accuracy vs. Realism: Simulating The “Human” Side Of Audio Measurement

Although sometimes overseen, out auditory system has two inputs… Our left and right ears. This system that is responsible for our ability to locate sounds is called binaural.  According to some, playback systems may include upwards of 5 speakers while 2 will do fine recreating what we hear naturally. For years stereo mics have been used to capture sounds that simulate space and realism, which add depth to recordings. Acoustic measurement systems benefit from these because they keep the human characteristic.

While performing acoustic measurements there are two things to be considered, accuracy and realism.  This means we will have to choose between what is going on scientifically and what is being perceived by the human brain.

There are 2 factors in acoustic measurement the humans response ,which is constant, and the loudspeakers response , which can be position dependent but does not have to be.

Choosing whether accuracy or realism is more important comes down to purpose. If you are trying to calibrate an EQ or crossover then accuracy is more important. If you were trying to know what a room sounds like then realism is more important.

Mic placement when trying to achieve realism is much easier. The user simply places the mics in a spot where they would typically be listening. One of the most common mic techniques used is X/Y and more often than not this produces a convincing stereo image.  Another technique used is two spaced omnidirectional mics.  It is important to remember that all of this is relative and it all comes down to what you like, there are not rights or wrongs.   

Those looking for increased realism can opt for using a “head”.  It is not uncommon to use  Head Related Transfer Function (HRTF) by having an absorptive mass between the two mics.

Premade “heads” can  be purchased through some mic manufacturers but their price can prove impractical to some that will not be able to justify the high cost. Other alternatives exist like using your own head.  The In the Ear technique (ITE) was pioneered by Don and Carolyn Davis in the 1980’s.  The technique captures outer ear response by placing mics at the surface of ones ear.  A variation of this called the At the Ear increased practicality by placing mics at the entrance of the ear.

While a lot of measurements can be gathered it is essentially up to you to decide the perspective needed. The use of many techniques can provide insight as to what is going on both accurately and realistically.

http://www.prosoundweb.com/article/accuracy_versus_realism_simulating_the_human_side_of_audio_measurement1/av/P3/