Hack3.5.Reduce Noise

Hack 3.5. Reduce Noise

Hum and hiss are annoying and distracting. Find out where the noise comes from and how you can reduce or remove it completely.

Hum and noise can ruin your recordings and turn off your listeners. What's even worse is that you are paying for noise [Hack #39], since a nice, clean signal will compress better. Better compression means a smaller MP3 file, which in turn means reduced bandwidth and disk space costs. Because software noise filtering distorts your sound, you will want to get rid of all the physical noise in your setup before turning to software noise reduction.

3.5.1. Find the Noise Floor

In a recording environment, the noise level is calculated in decibels. The studio's inherent noise is called the noise floor. The noise floor is the decibels of the noise in the room itself, which is called environmental noise. This also includes the noise of the sound recording system itself, which is called signal path noise. Since fewer noise decibels are better, you will want to lower your noise floor.

You should start by gauging your noise floor's current level. Then reduce your environmental noise, and then your signal path noise. Gauging your noise floor is fairly simple. You just record through your microphone [Hack #50] a sample of blank sound. Figure 3-8 shows a recording of the noise.

This is one of the few cases where you actually want a flat line.

The next step is to select the whole signal and use the View menu to plot the spectrum (see Figure 3-9).

The noise floor is the highest peak in the graph in Figure 3-9. In this case, it's near40 dB, which is pretty bad. For a clean sound, you will want at least60dB. Remember that less is better. A noise floor of120dB means the studio and signal path are very clean.

Figure 3-8. Audacity with a recorded noise sample

Figure 3-9. Spectral view of the noise floor

3.5.2. Eliminate Environmental Noise

Environmental noise is a little easier to track down than signal path noise. First, turn off anything you can hear. Ceiling fans, air conditioners, computers, refrigerators, air purifiers, and furnaces all create noise that humans tune out but microphones will detect. If you want to experiment with this, use some noise canceling headphones with no input to see just how much periodic noise you have in your own home. You will be amazed.

Office buildings are particularly bad for background environment noise. They have large A/C machines that generate an enormous amount of airflow. Additionally, some office buildings have started adding white-noise generators in areas with lots of cubicles to cut down on sound transmission.

All of these factors can lead to a low background noise in your recording. You should do as much as you can to eliminate these sounds during your recording session.

Another type of noise comes from the reflective surfaces in your room. Hardwood floors and desks bounce sound. The right angles in your room also reflect sound. Some simple ways to defeat these reflections are to close drapes, put carpeting on the floor, and hang or drape blankets on walls and horizontal surfaces.

Room size is also important. A standard-size bathroom is too small, and you will get quick sound reflections that will make the recording sound cramped. On the other hand, a cathedral will reflect sound and will give you an interesting reverb pattern that might not be what you expected. Try for something about the size of a small bedroom or den.

Basements are also ideal for recording, as they are below street level and thus have a protective barrier against outside noise. Recording late in the evening also cuts down on external noise.

Musical instruments can often sing just from the power of your voice. Cymbals and drums can bounce and distort sound even when you aren't playing them. So, if your podcast studio doubles as your home music studio, you should use blankets to cover up the instruments when you are recording podcasts.

Computers are horrible sources of noise. The internal fans are very noisy and their cases can leak high-frequency noise that can bleed into the audible spectrum. Laptops should be behind the microphone. Desktops should ideally be in another room, with cable extenders to get the keyboard, mouse, and video into the studio.

When you are recording out in the field be sure to use a windscreen for your microphone. A windscreen is the puffy black or red foam ball that goes on the live end of the microphone. This will cut down on wind noise.

3.5.2.1 Handling noise.

Handling noise, a deep thumping sound that comes from moving your fingers around on the microphone, is the easiest form of noise to fix. If you are in a studio or at a fixed interview setting, use a microphone stand. Even better, use an isolation mount so that if the table is bumped you won't hear it in the recording.

When you have to hold the microphone in your hand, you can use the handlebar grip from a mountain bike to wrap around the microphone. This will reduce handling noise because your hand will not be in direct contact with the microphone.

3.5.2.2 Places to record at home.

A home office is a good starting point when looking for a quiet place to record. You can use blankets to cover anything that could reflect sound. Another place to look is a walk-in closet filled with clothes. The clothes will dampen the sound. A garage could be OK, though you will need to look out for street noise, which can easily flow through walls that aren't insulated.

3.5.2.3 Places to record on the road.

When you scout out a location, first make sure it's legal for you to be there. For example, ask permission from café owners, and make sure that you have the correct pass if you are in an area with restricted access. Then look for a room that's carpeted, has blinds to draw over the windows, and has linen-covered furnishings, such as beds or couches.

If you are recording yourself on the road in a hotel room, you can use the bed sheets and blankets to make a mini sound booth where you can record and isolate the outside noise.

If all else fails, try an outdoor location such as a park or a football field. It might actually be less noisy than your hotel room.

3.5.3. Eliminate Signal Path Noise

Everything between the microphone and your ears is called the signal path. Hum, hiss, crackle, and some types of pops are often created by problems with your signal path. One source of noise is particularly common: the 50 Hz or 60 Hz cycle noise that comes from power sources. This comes from the waveform used to transmit power via alternating current. In Europe, 50 Hz is used. In the U.S., 60 Hz is used. Either way, it comes out as a very low hum that's easily distinguishable.

These types of noises often have simple causes and easy solutions:

Don't mix audio cables with power cables

Laying audio cables on top of A/C cables is a sure way to get a low, 50 Hz or 60 Hz hum. Keep your cables separated. If you have to cross two cables, you should cross them at 90-degree angles.

Use the shortest-length cable you can

Don't run cables any longer than you need to. Cables are like antennas, and they will pick up noise. The longer the run the more noise they will pick up.

Buy good-quality cables

Use shielded XLR cables for your microphones and all of your audio interconnects. Make sure the metal connectors on the ends of the cables are tight and that none of the connector's points makes a connection with the shielding. XLR cables are balanced, which means the signal is kept on separate lines from the shielding. This drastically reduces the amount of interference injected by the cabling.

Connect all A/C units to the same ground

This avoids ground loops, which are another common source of 50Hz or 60Hz noise. If you can't connect everything to a single ground, you can add a transformer that should isolate the noise.

Use XLR microphones

An XLR cable is less prone to interference than the two conductor cables of cheaper microphones. Quality microphones use XLR cables. Don't settle for anything less.

Another source of noise is the equipment itself. Microphones, preamps, and mixers all have inherent noise. With microphones, this is called self-noise and you can test this by leaving the mic on with no input and then bringing up the gain to its maximum point. In general, the better the quality of the microphone, the lower the self-noise. Of course, in this scenario it could also be your preamp. The only way to test that is to try your microphone on a different preamp to gauge the difference in noise levels.

In a complicated, multidevice signal path, additional noise can come from multiple gains. Each gain in the path will multiply the noise from the previous gain, and so on. A good rule of thumb is simply to remove any devices you aren't using from the signal path during a recording session.

If all of these quick tips failed and you still have some noise, try powering off each unit in the signal chain. Take it all the way back to the microphone and the preamp. Check for the noise there. If it's there, it's either a bad preamp or a noisy microphone. If it's not there, add your other signal processors and filters, one by one, until you find the one that's adding the noise.

Compressors, which are used to avoid the high peaks of vocal work, cause their own noise. For every dB of compression you use you get a corresponding dB of noise added to the noise floor. This is one reason to limit or remove compression from your signal path and to concentrate on controlling your vocal work.

3.5.4. Software Filtering Options

If all else fails, you can remove noise in post-processing by using software filters. High-quality software filters are available for removing hum, hiss, and A/C cycle noise, with little or no distortion to the signal. To help these filters you will need to record several seconds of empty sound at the beginning of your session. This section of empty sound will train the filter as to the ambient noise in the room.

Also, many software tools [Hack #57] will help you eliminate noise from your recordings.

3.5.5. Good Noise

Not all noise is bad noise. Some sounds, such as the clanking of coffee mugs in a café or the background music in a bar, can add ambience to your pod-cast [Hack #64]. This gives the listener a sense of place. Two tips will help you effectively use background ambient noise:

• Record some of it. That will help you both eliminate the noisy part of the signal later in post with a noise training filter [Hack #51], and give you some content to talk over as you fade into the segment.

• Random sounds are great, but sequential sounds aren't. For example, if you are at a ballgame, try not to record the score or other elements that will clue listeners in to when you made the recording. These audio cues are distracting and make it difficult to edit and rearrange the segment later.

3.5.6. See Also

• "Train Your Voice" [Hack #19]

• "Record and Add Background Ambience" [Hack #64]

• "Assemble a Small Recording Rig" [Hack #69]

• "Podcast at an Event" [Hack #71]