Short time delays are those that are are less than 20 ms.  
1.  Short time delays can be used for time compensation for distances between microphones and distances between speakers as discussed in part 4.
2.  Short time delays affect the direction that you hear a sound source - even more than volume effects hearing directionality.  As a result, short delay effects can be used in creating or modifing the stereo image in the mix. 
3.  When a short delay is mixed with the direct (undelayed) sound, a cancellation of a group of frequencies occurs.  This can be used to create a variety of flange effects.   
Hearing Directionality With Short Delays 
Haas Effect:
The Haas Effect is, simply stated, a factor in human hearing where delay has a much bigger effect on human perception of direction than level does. Helmut Haas in Germany showed that although identical audio sources sent to two speakers at equal level resulted in a center image, a 5 - 20 ms delay to one of the signals shifted the image to the non-delayed side, and the delayed side had to be made 10 dB louder to get the image to shift back to the center. This is also called Precedence Effect.

Figure 1 - Haas Effect

Using The Hass Effect
Sometimes in mixing you get into an "unbalanced" situation where one meter reads higher than the other.  Lets say, for example, you have a "hot" right side to your mix because you have one rhythm guitar and it is panned right.  You can correct the imbalance caused by sending a delay of the guitar to the other side of the mix using the panning shown in figure 2.  

Figure 2 - Correcting Balance

Lets say in the above example that before you added the delayed guitar, that the imbalance was 3 dB.  Slowly bring up the delayed guitar until until the imbalance is only 1 dB.  Keeping this delayed guitar lower in level will help the guitar tone if the mix should be played in mono.  (See the information on Comb Filters that follow).  
Adding a delayed guitar with a short delay panned opposite to the un-delayed guitar will give more guitar sound to the mix. In other words the guitar will sound fuller and add a "wideness" to the sound, without over-driving meters and without changing the pan location of the instrument to any great degree.   Go here to hear a demo on the Haas Effect.
The Comb Filter
When short delays are mixed with the direct sound, a frequency response called a "comb filter" is obtained, as shown in figure 3. This is the frequency response of phase-cancellation. This response curve gets its name from the fact that it looks similar to a pocket comb. The figure shows a 50/50 mix of the direct signal mixed with a 1/2ms. delay of the signal.

Figure 3 - Comb Filter (0.5 ms delay)

The frequencies that cancel have a harmonic relationship (odd harmonics) compared to the lowest-frequency null.  The frequencies that will null (cancel) is determined by the delay time, according to the following chart:

Figure 4 - Notch Frequency For Different Delay Times

Reducing Comb Filter Cancellations
The cancellation or "nulls" of the comb filter can be made less noticeable by adjustment of levels and adjustment time delay times (changing the frequency of the nulls).  Equalization can also help.
Finding The Best Null Frequencies
Taking our example above, the guitar was panned right and a 10 ms delayed guitar was panned partially left.  When a mix is played in mono, the left and right signals are combined.  Since the signals are combine, there can be a comb filter effect that reduces certain frequencies from the guitar sound - according to the chart 50 hz, 150 Hz and 250 Hz would be reduced.  If you varied the delay time slightly, the frequencies canceling will vary.  There will be an ideal delay time, close to 10 ms, that gives the the least interference with the specific guitar part.  It is a good idea to find the best delay time to use by performing  the following procedure: 
1.   Solo the guitar and the guitar delayed track.  Bring the levels up so that they are the same and panned center.  You will hear the comb-filtering affect the guitar sound. 
2.   Adjust the delay time up and down from 10 ms. until you hasve the maximum interference with the guitar sound (usually making it thinner sounding). 
3.   Reverse the phase on the delayed guitar channel - the frequencies that cancelled will now add - giving you the fullest guitar sound. 
Once you establish the best delay time (using the above procedure) - use this setting to correct the balance of the mix.
Mixing Levels To Reduce Cancellations
The deep null cancellations of the comb filter are caused by equal but out-of-time signals being mixed together.   If the levels are made dissimilar, there will be less cancellation as shown in figure 5. 

Figure 5 - Comb Filter With Signal Level Adjustment

If the level difference between the delayed and un-delayed signal is kept at or exceeding 6 dB, the amount of cancellation will be reduced to about 3 dB - making them much noticeable.  This is why it was suggested that the delayed signal be brought up lower than the undelayed signal when using short delays to correct balance problems. 
Equalization to Reduce Comb Filter Cancellation
Equalization can also be applied to reduce the delayed signal at certain frequencies (like 150 Hz in the guitar example) and thereby reducing the cancellation of the most noticeable null. 
Stereo Effects With Combs
An artificial stereo effect can be achieved with comb filters.  When a direct & delayed signal are mixed together, certain frequencies will cancel.  When the delay is phase-reversed, the frequencies that cancel will now add and the "in-between frequencies will cancel.  In figure 3, the frequencies that cancellec were 1000 Hz, 3000 Hz, 5,000 Hz, etc.  If the delay had been phase-reversed, the frequencies of cancellation would be 2000 Hz, 4000 Hz, etc.
If you made two comb filters with the same delay time - one for the right side and one for the left side, you would get a stereo effect by reversing the phase of the right side comb filter.  Whatever frequency canceled on the left side would add on the right side.   To do this, send the guitar through a delay line (usually 3 - 8 ms) and patch it into two channels.  Pan the two delayed channels fully left & right, phase reversing the channel that is panned right.  The mixing setup is shown in figure 6. 

Figure 6 - Mixing For Stereo Effect

You can actually adjust the delay time so that a scale will "walk across" the stereo spectrum - in any case you will hear some notes come from the right, some from the left and others in-between these extremes.   Go here to hear this effect.
Flange Effects
The flange is An effect caused by an approximately even mix of a modulated (varying) short delay with the direct signal.  As the delay time varies, back and forth from the modulation, the center-frequency of the cancellation notices also vary back and forth as shown in figure 6. 

Figure 6 - Flange Effect

The flange is a very dramatic type of swirling effect that can best be described by playing an example.  Go here to hear a flange effect on a guitar. 
Although you could manually vary the delay time back and forth - this is usually done by an oscillator that puts out a control voltage, called an LFO, and used to create delay time modulation.    The Low Frequency Oscillator gets its name from the fact that it puts out a frequency lower than the audible range - usually from .1 Hz to 10 Hz.  Delay time modulation is where a control signal will change the delay time of the delay unit. The delay time of the unit varies as the voltage of the control signal changes.
Figure 7 shows how the delay time of the unit could change with the control voltage put out by the Low Frequency Oscillator. The unit is set to a 10 ms. delay time. When the control voltage is zero (at 0 degrees, and 180 degrees) the delay time is at the front panel setting. When the amplitude of the control voltage is at maximum (at 90 degrees), the delay time shifts to 15 ms. When the amplitude of the control voltage is at its negative peak (at 270 degrees), the delay time shifts to 5 ms.

Figure 7 - Delay Time Modulation By LFO

The control that sets the amount of modulation has several names. The most common name is modulation "Depth." Depth of modulation is how much control the LFO will have. Alternate labels for the control are Width, VCO Level, LFO Level, Clock Level or simply Modulation Level. Whatever the label is, the control will change the amplitude of the control signal. The effect of changing this control is shown in figure 8 - raising the control will increase the amount of delay time variance.

Figure 8 - Low And High "Depth" (Level of LFO)

Another modulation control is called "Rate" or "Speed."  This control sets the frequency of the LFO.  The rate will vary usually between  0.1 Hz (one cycle of the LFO and therefore one sweep of the flange occurring over 10 seconds) and 10 Hz (10 cycles or sweeps per second).  Flange effects usually have a fairly low rate setting with a high amount of depth. 
Basic Delay Line Controls
Flange effects can be obtained by adjusting the controls on a delay device.  The common controls that you would find on a simple delay line are shown in figure 9. 

Figure 9 - Basic Delay Line Controls

1. Input Level: Sets the level of the audio signal coming in (normally set to 50% - 70% up). Too high of a level would usually trip an input overload indicator, making it flash.
2. Invert: This switch phase-reverses the delay signal. Inverting (switching) the phase of the delay changes which frequencies cancel.
3. Output mix: This control determines the mix of the delay and the direct sound. Turning the control completely one way causes only the direct signal to be at the output of the device. Turning the control completely the other way causes only the delayed signal to reach the output. For flange effects,  put the control in the middle to get a mix of 50% delay and 50% direct signal.
4. Delay Time: This control determines the delay time of the delay in the unit. There is often two controls: a "course" adjustment (sometimes called "Delay Range") and a "fine" adjustment. With this control or controls, the delay time can be set from 1/2 ms. to 1200 ms. or higher.
5. Depth:  Sets the amount of delay time modulation caused by the LFO.  Flange effects generally  use high amounts of depth. 
6. Speed:  Sets the frequency of the LFO and therefore the speed of the flange's sweep. 
Feedback and Short Delays
Feedback in a delay device consists of sending the delayed signal back into the input of the delay line through a level control (usually labeled "feedback").  At long delay times, this gives a repeating echo effect, often called a space echo. More feedback causes the echo to last longer; less feedback causes a quicker decay of the signal.  
The comb filter actually consists of both cancellation (at 1kHz, 3 kHz, 5 kHz in the example) and reinforcement (at 2 kHz, 4 kHz, 6 kHz in the example).  When you apply feedback to a delay line causing a comb filter the reinforcement and cancellation is made more extreme as shown in figure 10.  (The illustration only shows 2 reinforced frequencies - but all frequencies would be reinforced.) 

Figure 10 - Feedback Added To A Comb Filter

When large amounts of feedback are applied to the comb filter, the reinforced frequencies begin to "ring-out" and you actually have a tone generator that is generating tones with a harmonic relationship.  If this is applied to an instrument with indefinite tone, such as a snare drum, the generation of tones becomes obvious.  Click here to hear a guitar "dramatic flange" (one with feedback) and click here to hear a snare drum with a feedback flange.
Delay Time Settings For Different Instruments
The flange has the most sweeping effect when the combs fall at key frequencies in the instrument's range.  Since the fundamentals and harmonics are in different octaves for different instruments, the starting delay times should be set differently for different instruments.  A chart of recommended starting delay times is shown in figure 11.  
   Instrument    Initial Delay
   Cymbals, Shakers, Etc   1/2 to 2 ms. 
   Horns, Lead Instruments, Vocals    1 - 4 ms.
   Guitars, Rhythm Instruments     3 - 12 ms
   Bass Guitar & Low-Frequency Instruments     7 - 20 ms

Copyright 2001, Robert Dennis, ALL RIGHTS RESERVED