Hack57.Build a Sweet Sound


Hack 57. Build a Sweet Sound

Use frequency filters to adjust your recording to get the tone you want.

You can look at sound in two ways: either in the amplitude domain, or in the frequency domain. Frequencies are measured in hertz (Hz). The lower the frequency, the lower the pitch, and the higher the frequency, the higher the pitch. All sounds have a frequency, or a set of frequencies, with an amplitude at that frequency. Amplitude filters can alter the level of your sound, and frequency filters alter the character of your sound.

Human voices are strongest between 200 Hz and 3 kHz. Men's voices tend to have more amplitude in the low end and center a little lower in the spectrum than women's voices. Boosting the low frequencies will add more depth and warmth to a voice, and boosting the high frequencies will add detail and make the sound more crisp.

Voices also have under- and overtones that extend into the 15 kHz range and down to 50 Hz. Cutting the frequencies to between 500 Hz and 5 kHz results in boxy, phone-style sound that feels processed and unnatural. You can attenuate signals outside of this range to reduce rumble in the low end and hiss in the high end, but you shouldn't remove them altogether unless you are going for that boxy sound.

Table 8-1 shows a short list of the different frequencies in the human voice and the effect you get when you up the graphic equalizer (EQ) in that range.

Table Vocal characteristics at certain frequencies

Frequency

Characteristic

200 Hz

Enhances the bass in the voice

3 kHz

Adds extra clarity in the voice

5 kHz

Enhances vocal presence

7 kHz

Brightens vocals for lower voices

10 kHz

Brightens vocals for higher voices

15 kHz

Increases the breath sounds


Keep these ranges in mind as you are using equalizers to adjust your sound.

8.9.1. Graphic Equalizers

The most familiar frequency mechanism is the EQ. This filter breaks up the frequency spectrum into bands. You can add or remove amplitude in each band by changing the position of the slider. Most equalizer plug-ins [Hack #51] come with settings for common scenarios, adding or removing bass, adding detail, or cutting off the high end.

Figure 8-20 shows the 10-band equalizer from Audio Hijack Pro.

Figure 8-20. The graphic equalizer in Audio Hijack Pro


You can change the settings manually or pick from a handy drop-down list of predefined settings.

8.9.2. Parametric Equalizers

Parametric equalizers have a similar function to graphic equalizers, but they are more flexible. They usually have between two and ten control points that you can set to any frequency. You shape the equalizer by moving the control point up and down to add or remove amplitude at that frequency.

Figure 8-21 shows the Freq4 parametric equalizer from BIAS.

Figure 8-21. The BIAS Freq4 parametric equalizer


A Q value allows you to control the spread of the effect. A large Q value means that the effect will be centered strictly on the control point, and a smaller Q value means that the effect will be spread over a wider portion of the frequency spectrum.

In Figure 8-21, the first control point is centered around 100 Hz and has a low Q value, which is why the effect is spread out. The second control point is centered around 1.3 kHz and has a tighter Q value.

8.9.3. Low-Pass Filter

A simpler form of EQ is the low-pass filter that trims off the high frequencies and lets low frequencies "pass." You control the cutoff frequency, above which all frequencies will be attenuated. Some filters have a roll-off control that allows you to control the ramp of the cutoff, so frequencies that are closer to the cutoff aren't dropped as sharply as frequencies that are farther out.

This filter is ideal for cutting high-frequency noise such as the whine of a computer or the hiss from an air conditioner.

8.9.4. High-Pass Filter

The inverse of the low-pass filter is the high-pass filter. This filter lets high frequencies pass. Like the low-pass filter, you control a cutoff frequency. Below the cutoff, all frequencies will be reduced in strength.

A high-pass filter is good for reducing low rumbling noises such as the wind noise in a field recording, or the 50 Hz or 60 Hz cycle noise of a recording with ground loop noise.

8.9.5. Band-Pass Filter

A band-pass filter has two cutoffs, one for low frequencies and one for high frequencies. Frequencies in the band between the low and high cutoffs are allowed to pass. That's how this filter gets the name band pass.

This is more a description of a filter than an actual piece of software. You can use an EQ to implement a band-pass filter by notching down the low and high ends above and below your cutoff frequencies.

8.9.6. Tunable Noise Filtering

Frequency filters are often used to remove noise, such as hums or hisses, which occur at particular frequencies using notch filters (EQ filters set to very tight bands). Simple noise, such as the 50 Hz or 60 Hz hum from a power line, can be attenuated with a notched EQ filter. But complex noise, such as the sound from an air conditioner, is actually a multilayered noise source. You hear the whoosh of the wind moving, as well as clanking and rattling that come from the mechanism itself. As humans, we simplify this noise and reduce it, but microphones pick it up well.

A new advancement in digital noise reduction is the tunable noise filter that can tune itself to periodic noise, such as that of an air conditioner, and set up frequency filters to remove it. The trick is to reduce the noise without adding distortion to the original signal.

To use a tunable noise filter, you need a sample of just the noise from the environment. Usually this is the few seconds after you hit Record but before you start talking. Opinions vary on just how much of this sample noise is required, but to play it safe, you should record at least five seconds of it. However, you might want to record up to a minute of the environmental noise to use when editing [Hack #64].

BIAS SoundSoap 2 (http://www.bias-inc.com/), $99, is an excellent tunable noise reduction filter. You can run the filter in standalone mode or as a VST plug-in. When in standalone mode, select the input file with the noise sample at the front of the file. Then click the Learn Noise button and press Play. Figure 8-22 shows SoundSoap filtering the noise from a signal.

Figure 8-22. The SoundSoap sound reduction application


You can tune the filter using the controls to adjust the amount of reduction from the signal. Sometimes you don't want to block it all out because there is an ambience that you want to keep. When you have everything the way you want it, click the Apply button to apply the filter to the signal. Then use Save As to save the cleaned-up file with a new name.

The quality of tunable noise filters varies greatly. I wasn't able to get Audacity's [Hack #50] noise filter to remove noise without distorting the signal. Adobe's Audition sound editor for Windows has a high-quality tunable noise filter built in.

Tunable noise filters are similar in function to the noise reduction headphones popular with airline travelers. These microphones sample the outside noise and then create an antisignal to block out the noise selectively.

8.9.7. See Also

  • "Choose the Right Audio Tools" [Hack #50]

  • "Juice Your Sound" [Hack #51]



    Podcasting Hacks
    Podcasting Hacks: Tips and Tools for Blogging Out Loud
    ISBN: 0596100663
    EAN: 2147483647
    Year: 2003
    Pages: 144

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