Small Rooms - High Frequency Control

 

TAXI is the leading independent A&R company helping unsigned bands, artists and songwriters get record deals, publishing deals and placement in films and TV shows. WSDG is creating a series of articles on small listening / production room design and acoustics.

 

 
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Article 2: "Small Rooms - High Frequency Control"

 

In the last article (#1), we discussed low frequency control in small listening rooms -- the types of rooms that are typical for many TAXI passengers. Quick review: The best low frequency response will usually take place in rooms that have good proportions, remember?

 
Now we're going to cover the control of high frequencies (300-400Hz and up) in such rooms. The result we strive for is in an even distribution of sound in the primary listening position. The good news is that this is normally easier to understand, and deal with, than low frequency control!
 
Again, the best solution starts with good geometry, which in turn will give us good reflection control.
 

Comb Filtering

When you listen to recording playback, you hear two different types of sounds: direct and reflected. "Direct" is what comes to you from the speakers, and "reflected" is what arrives at your ears after bouncing off one or more surfaces.
 
Many small studios (actually I would venture to say MOST small studios) use "near field" monitoring. The monitor speakers often end up on top of the console or very close to the console bridge.
 
"Near field" monitoring (where the listener sits quite close to the monitoring speakers) is often used to accomplish an important monitoring goal: to have the direct sound from the monitoring system arrive accurately at the listening position, and not be "clouded" with high level reflections that arrive too soon.
 
When this reflection confusion (or "clouding") happens, "comb filtering" takes place. In fact, poor reflection control will almost always cause some sort of "comb filtering."
 
The name "comb filtering" describes a pattern of frequency response that looks something like a comb: very high and low discrepancies in the frequency response. (see figure 1) This is the opposite of the smooth (and accurate) frequency response that we want to hear!
 







figure 1
 

The comb filtering effect comes in many shapes and sizes and is certainly a subject a bit larger than we can tackle in a short article such as this. But following is one of the simplest examples (and one that is a common "mistake" in small production studio control room environments).

 

Test Your Studio

Want to find out if "comb filtering" is happening in your studio? Try this simple experiment:
 
In your primary mixing position, listen carefully to a recording, particularly in the vocal range of frequencies. Then move the speakers back about 12", and do the same critical listening. You will most likely notice three things:
  1. a clarity in the frequency range;
  2. better stereo separation; and
  3. more accurate fidelity.
 

This is happening because there is no longer a conflict between the direct (accurate) response from the speaker, and a harsh first reflection: one that is arriving a few milliseconds behind the direct sound. The frequency response associated with each of these two positions is shown in the figures below. These are the frequency response graphs of what you hear! (see figures 1 and 2)



 
 
 
 
figure 2
 

Possible Causes

This same reflection control (or lack of reflection control) can happen with side room walls that are too close to the critical listening position. It can also happen with hard ceiling surfaces that are angled incorrectly (i.e. sloped downward from the front of a room towards the listener).
 
The technical term for this phenomenon is Time Delay Gap (TDG): the time between the arrival of the direct sound from a monitor (speaker) and the moment of the arrival of the first important reflection. (see figure 3)
 


figure 3

 
In small control room environments like the ones we are discussing, the TDG should be no less than 10 - 15 milliseconds. If sound is moving at roughly about one foot per millisecond (actually 1.1 feet per millisecond), then (again roughly) we can calculate 10 - 15 milliseconds to be about 10 - 15 feet.
 
Please note that a correction has been made from the original article at taxi.com. Sound does not travel at one foot per second. It in fact travels at about one foot per MILLISECOND (actually 1.1 feet per MILLISECOND) as stated above.
 

Possible Solutions

You can see how solving this reflection control problem with good geometry is possible (i.e. angled walls, ceilings, etc.). For examples, see figure 4. When this is not feasible, then surface applied acoustic treatments provide the only remaining answers. We have two choices:
  1. Absorption -- which does not change the reflection pattern but reduces the level of harsh reflection
  2. Diffusion -- which does not change the level of the reflections, but changes the pattern.
 
 
 
 
 
 
 
 
 
figure 4
 

There are many other examples of projects in our website. Take a look around.

 
In the next article we will discuss the pros and cons of these surface applied treatments. In most "existing" rooms -- the kind of spaces that many TAXI drivers will be using for their production spaces -- the only solution is applied surface treatments.
 
Have fun!
 
For more information about TAXI go to www.taxi.com.
 
 
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