Listen

Question:

In one small area of my studio near my mains circuit breaker/electrical panel, I have some type of electrical interference and all my microphones except the Neumann mics have hum when turned in a certain direction. If I turn the mics 90 degrees, the hum disappears.  

The Neumann mics (U87s and U64s) have no hum in any direction and work fine. So, before I write to the other mic manufacturers, I want to ask you if you use a special kind of shielding in your mics that prevent this type of hum from electrical interference. 

Answer:

The likely source of the problem is a hum field in your studio caused by the AC power lines. 

Any type of dynamic mic, like the Shure SM57 or SM58®, contains a voice coil that is very susceptible to picking up the hum field. Nothing can be added to these mics to control this. 

Some dynamic mics like the Shure SM7B contain an internal hum bucking coil that reduces hum pick-up. Reduces, not eliminates. Other dynamic mics, like the Shure Beta 58A have more effective shielding around the voice coil that also reduces hum. 

ALL dynamic mics will pick up this hum field to some extent. 

Condenser mics, like the Neumann's you have or the Shure KSM mics, do not have voice coils. These mics will perform much better in a hum field. But many of these mics have output transformers that, like a voice coil, will also pick up the hum. 

If the hum field cannot be eliminated, stick with high quality condenser mics with transformerless outputs. Shure KSM mics are condenser mics with transformerless outputs.

Listen

Question:

How do the theatre guys do it? I frequently have actors with lavs in their hairline or over their ears talk while hugging. I get a comb filter-like sound when they do that. Often I do not have time to reduce the volume to one of the mics to reduce the comb filter distortion. 

I know some people route the lavs to different speakers, but when you have many scenes like that with many lavs, the routing can be quite a headache. 

Answer:

There are basically 3 methods. 

1) Reduce the volume of one of the microphones. 

2) Route the microphones to different loudspeakers. 

3) Use an automatic mixer inserted into your main console so that the automixer can turn the microphones on and off as they are being used. Essentially, this is the same as #1, but it is much easier.

Listen

Question:

Please provide frequency information about cell phones and other wireless communication devices. I have encountered situations where such devices created interference in the sound system components: microphones (wired and wireless), cable snakes, mixers, and other pro audio equipment. Nothing seems immune!

Answer:

The actual frequencies used by cell phones varies by country and by carrier. There are four major digital transmission schemes in use: GSM, TDMA, CDMA, and iDEN — plus the analog AMPS system. To make things more confusing, the digital schemes can be configured to work in different frequency ranges depending on which carrier is selling the service and what frequencies they are licensed to use. 

The Blackberry devices are available from several different service providers. In North America, most use GSM and operate in 850 MHz band and 1.9 GHz band; some use CDMA on 800 MHz and 1.9 GHz; the ones sold by Nextel use the 800 MHz iDEN frequencies. Some models can also automatically switch to the Europe/Asia frequencies so they can be used in other parts of the world. 

A "carrier" or "service provider" is generally assumed to be a company (Verizon, Sprint, Cingular, etc.) that has licenses to use blocks of frequencies in various parts of the country, and that owns and maintains a network of antennas, towers, base stations, switching centers, interconnecting landlines, etc. That definition is changing. There are some new carriers in business who do not have any spectrum licenses or network infrastructure at all. They essentially buy huge blocks of minutes from the big carriers and resell them under their own brand name. These are called Mobile Virtual Network Operators (MVNO's); Virgin Mobile is an example -- they sell service using Sprint's CDMA network. Sprint has actively pursued this type of "OEM" business, but Verizon has not. 

Here is a good map with frequencies listed by country: 
 

Listen

Question:

I know that wireless microphones use the same frequencies at television stations, but where can I find out what stations are in my city?

Answer:

Wireless microphones do use the same frequencies as broadcast television stations.  In Chicago, we have television channels 7, 9, and 11 on air. We’d use wireless microphones on television channels 8, 10, and 12 to avoid the active TV channels. 

The hard part is knowing what television channels are active in your area. This becomes even harder due to the new DTV stations coming online in the U.S. There are a few DTV stations scheduled to come online within the next few years that have not yet begun broadcasting. Obviously, when choosing frequencies we want to avoid existing TV channels and future scheduled TV channels. The link below allows you to search for any city in the U.S. and recommends frequencies for that city. 

Search for your city name or zip code here. 

Every city will have a different set of occupied TV stations.