MIXING & MASTERING ARTICLE


ADDING BASS CLARITY
By Bob Dennis
FOUNDER, RECORDING INSTITUTE OF DETROIT

 • Related Article: Bass Guitar EQ

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The bass range of frequencies and the "bass instruments" of kick drum and bass guitar give the mixing engineer the most mixing challenges. This range is very important to many productions but hard to obtain clarity in mixdown. It is equally hard to judge how much of this range that you have in the mastering process. How do we get a clear yet powerful low-end in the mix and on our final master? In this article we will find that bass clarity in mixing depends on several factors, including how the instruments were recorded.

THE DIFFICULTY WITH BASS
The lowest octave that can be heard (20Hz to 40Hz) virtually has no energy in most music. The lowest note on a bass has a fundamental frequency of about 41 Hz. So this first octave contains such things a "room rumble" and the lowest notes of a pipe organ. The second octave (40 Hz to 80Hz) contains the fundamental frequencies of the low bass notes and the Kick drum. So this is the first octave we are really concerned with, unless we are reproducing earthquakes and the like.  This second octave is very difficult to judge, even for the most-experienced engineers. This is largely due to the speakers we use in listening.

THE DIFFICULTY WITH SPEAKERS
The average speaker that people listen to, does not properly reproduce the frequency range from 40 Hz to 80Hz. This is true even for many expensive "powered" speakers. You really can't hear this range of frequencies unless you have awfully big speakers or a sub-woofer on your system, and a subwoofer that doesn't "accent" this range. But the difficulties don't end there. Because some speakers don't reproduce this range, speaker manufacturers often "enhance" the low-end by boosting the sound output for the next octave (80 Hz to 160 Hz). This makes the speaker sound like it has more bass response but they are actually substituting extra energy in this octave for a lack in the lower octave (40 Hz to 80 Hz). This makes it hard to judge the sound, add equalization, etc. When subwoofers are added to speakers, there is still the boost in the higher bass octave and additionally there can be a "hump" around the point where the subwoofer and the speakers are both reproducing sound (around 100 Hz). The "truest" speakers are those huge studio monitors that no one can afford except large studios.

DIFFICULTY WITH ROOMS
In addition to all of these problems, you have problems with standing waves that make the bass uneven in the room. As you walk around most rooms, you will hear more bass in some spots and less bass in other areas. Usually the bass is louder near the walls and in the corners of a room. Also, speakers against the wall or in corners increases bass output. SO WHAT TO DO FIRST? Small speakers don't reproduce the very low frequencies very well but can become muddy sounding when they try. Therefore we often filter out the very lowest frequencies as a first step in mastering. We often will use a 32 Hz high-pass filter, which has the frequency response shown in figure 1.

FIGURE 2 - 32 KHZ FILTER RESPONSE  font>

Even though there are no fundamental frequencies below 40 Hz, there can be energy in this range caused by "rumble." In addition, however, the filter slightly reduces energy up to 64 Hz as shown in the graph. This reduction of the low bass energy has practically no effect on the bass sound except to make it a bit clearer and make the overall signal a little louder (after adjusting the level back to normal after filtering).

SO THEN WHAT DO I DO?
It is very necessary that you judge the amount of energy in both octaves that we have discussed. It is all to easy to "accidentally" have an abundance of energy in the low bass octave (40 Hz to 80 Hz) that goes "unnoticed" in your mixing but which makes the whole mix sound "muddy" on a big system and "low in level" on a smaller system. /span>

My advice for judging energy in this range would be to:

1. Always choose a "reference" cut from some national artist that "sounds good" and has good "bass clarity" on all systems. When trying to judge your bass clarity, compare the sound on your recording with the "reference."

2.  Listen to the mix on as many different systems as possible, including a "big" system or a system with a sub-woofer.

3.  Consider strongly adding a subwoofer system to your monitors, unless the specifications on your speaker show that you have adequate 50 Hz energy.

4.  Set up your "near-field" monitors in the room so that:
a. Your speakers are away from the wall by at least18 inches or so.
b. Realize there should be sound-absorbing material on the wall behind the speaker to prevent reflections from interfering with the "direct sound." This is especially important if the speaks cannot be 18 inches away from the wall.
c. Set up your "listening chair" so it is not against a wall, or in a corner.
d. I have developed a set-up that can be used in many small rooms that keeps the speakers working well for you (see right diagram above)

FFIGURE 2 - ROOM SETUP

 ADVANCED TIP
Another way to judge the amount of energy in the bass ranges of your mix involves using a Spectrum Analyzer. The spectrum analyzer shows the energy distribution in each octave. You play both your mix and the reference mix through the device and compare energy between the two for the ranges we have discussed. There are many models and many computer programs that will do this for you. The unit we use is in a program called Ozone by Izotope.

THE CLARITY FREQUENCIES
These bass-frequency instruments have harmonic frequencies that are in the midrange. This doesn't mean that you can generate these harmonics with your equalizer. It does mean that when you record you have to capture the harmonics of the instrument. When you do this, the amount of equalization you have to use will be less and clarity on your bass will automatically be better. When playing one pitch, instruments put out energy at the "tuned" frequency, called the fundamental frequency and also multiples of that frequency called harmonics. In general the harmonics are responsible for the "distinction" and "tone" of the instrument where the fundamental provides "pitch recognition" and "power." As a result, the harmonic frequencies are the frequencies that are the "clarity" frequencies. Equalization charts often identify frequencies that you may want to boost on the bass guitar as being 400 Hz, 800 Hz or 1.6 kHz. boosting one of these frequencies will often make the bass line more distinct and clear. You choose which frequency to use based on which one works most effectively in the particular mix you are doing.

MORE BASIC
More basic to the use of equalization is to make sure that you have recorded sufficient harmonic frequencies. There has to be harmonic frequencies sufficiently present in the recording for the equalization to boost. An equalizer does not generate harmonic frequencies, but simply makes them louder or softer. It is interesting that when the recording is rich in harmonics, the need for equalization diminishes.

RECORDING THE BASS GUITAR
The bass guitar we record is an electric instrument. It makes sound by sending it though a bass-instrument amplifier. You can record the bass by putting a microphone on the bass amplifier's speaker, and you can record it with a "direct" box, not using any microphone. Alternately, you can record it both ways, blending the two together. In my experience, the best way to record the bass is is the last choice. This is especially true for tunes that have a lot of guitar work (like rock) and when there are horns or low synthesizer parts recorded on the production. In my experience the amplifier sound, blended with the direct sound, gives you a bass that can't be killed with lower speaker volumes (see "Fletcher Munson Effect"). In addition the "recommended" equalization seems to have much more effect. This is due to the harmonic frequencies that are generated in the bass guitar's instrument amplifier.

RECORDING THE KICK (BASS) DRUM
The Kick drum is a low-frequency drum. It consists of a skin which generates the "note" and a shell which amplifies the skin sound by actually "generating" additional sound at some frequencies. We almost always record the back of the drum (away from the drummer) though a hole cut in the back skin. The drum, by the way, sounds better when there is an 8" hole in the skin rather than having the whole back skin removed. Drummers often put a pillow in the Kick drum to "damp" the skin so that it doesn't "ring out" as much.
 

1. If possible, use a "dead ringer" type damper rather than a pillow.  "Dead Ringer" is probably a trademark but we are talking about a strip  of foam that has glue on one side. This strip is attached to the shell,  right where the shell and the skin meet. The foam presses lightly  against the shin all the way around the shell and damps skin vibration  without changing the tone of the instrument much.span lang="en-us">

2. Put the microphone at the end of the shell (back end), almost touching the shell and pointing at the skin. See the diagram on the right. Make sure that there isn't any damping material between the skin of the drum and the microphone. - as shown in figure 3. 

images/ftmic.gif (1809 bytes)

FIGURE 3

This way of micing the Kick drum gives you the "shell sound" as well as "skin sound." The result is that the drum sounds more like a drum, is easier to hear at low listening volumes and responds to equalization better.

THE REAL WORLD
Recording the bass or the Kick drum correctly makes mixing much easier. In the real world, however, you will often have to mix things that weren't recorded correctly. So what do you do then?

RE-RECORDING
Re-recording is a way many professional recording engineers achieve the desired sound. As an example, lets take the bass guitar. Let's say that you need to mix a tune where the bass guitar was recorded with a direct box and you want to "blend" a mic sound that was never recorded. One thing you could do is to send the "direct" sound from the recorder to a bass amp in the studio. Now as the multitrack tape plays, the bass amp will get a signal like it would get from the instrument. You then put a mic on the bass amp and bring the signal back into the console to blend it with the direct sound.

TRIGGERED SAMPLES
Another way to achieve the "best" drum sound would be to use good-sounding samples of drums. You trigger the sampled drum sound off of the actual sound you have [You send you recorded Kick drum to the "trigger input" of the sample playback unit, drum machine or keyboard - so that the sample will play only when the Kick drum you have recorded plays]. The sound you wind up with is a blend of the "real" Kick sound and the sample.

"BUILDING" A SECOND MICROPHONE
One common electric guitar mic placement technique is to place a microphone 8 inches behind a microphone right against the grill cloth of the guitar amplifier. One mic is used as the "left" mic and the other mic is used as a "right" mic. So let's say you have a guitar that was only recorded with one microphone and you want the sound that would be obtained with 2 mics. You can attain something close to this by adding delay, EQ and reverb.

My specific recommendations for building that second microphone would be:

1. Put your guitar into two channels of the console, using a "Y Cord" or some such device. Put a delay on the second channel of about 0.75 ms. (one Kick is about 1 ms. and 8 inches would be about three-quarters of a ms.).).

2. Reduce the high frequencies of the delayed microphone about 3 dB with shelf response equalizer, using a corner frequency of about 3 kHz. This simulates the sound traveling though some air before reaching the microphone.

3. Add a slight amount of reverb to the delayed microphone to simulate the increased "room sound" of the second microphone. If you add reverb to both mic channels (which you normally would), always add a little more reverb to the delayed channel.

WHAT'S THE POINT?
Well the point is that the mixing engineer has a lot of techniques that can be used to get the sound that involve more than just bringing up faders. The professional engineer working on big budget productions uses techniques like this to get the sound. The techniques are not that difficult for a home recordist to use, but you need to know that they exist and that this is how those national records get that "fantastic" sound.

 
 

Copyright © 2003, 2010 BY ROBERT DENNIS - ALL RIGHTS RESERVED
PUBLISHED IN RECORDING ENGINEER'S QUARTERLY™ / ALEXANDER MAGAZINE™ WITH PERMISSION
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