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Standing waves influence the sound field

Maximum stationary waveis shown in Fig. The curve represents the area of ​​the room where specified frequency is audible. At minimum , the frequency is represented at a much lower level (sometimes -40 dB compared to the maximum ) .

If the room has the same dimensions as length – width – height is very difficult to obtain a uniform sound distribution.

How to cancel standing waves?

In rooms will avoid parallel walls.Thus the most powerful ways will be canceled. When monitors are placed it is important to be excited  fewer oscillation modes. For this reason the monitors should not be placed in a maximum of standing waves. At low frequencies can be considered that a monitor radiate sound waves in all directions. 


 

Placing the monitor close to a rigid wall, sound energy which had radiated to the wall will be radiated conversely to the  half of the front space. The sound pressure will be doubled in half of the space , respectively + 6dB.

 

Placing the monitor near two walls, for example, a limited corner of 2 walls, it will radiate in a quarter of space. Now the sound pressure is doubled twice resulting an increase of + 12dB. 


 

Placing the monitor near three walls – for example a corner formed by two walls and the floor it will radiate only 1/8 of spherical space. Comparing the acoustic pressure with it, will see an increase of + 18dB.

 

In practice, by placing the monitor close to the walls and floors, will influence the frequency range from 125-150Hz. 


 

Filtering function that arises when a signal is gathered himself, after a time delay, it’s called combining filter “comb filter”. The resulting frequency response has the shape of “comb = comb” and hence the name.

 

Two 500 Hz sinusoidal tones gathered. The second tone is delayed with 1 ms and their sum will be 0. Two 1kHz sine tones are gathered. The second tone will be delayed with 1 ms and in this case the amount will be doubled (ie + 6dB ).

Comb – filter function is almost never intentional, but it is always heard in sound productions, where it appears acoustic and electric. Acoustic it occurs when the sound, on its way from source to listener, partly directly and partly by indirect way because of a single reflective surface. The reflection must be attenuated at least 10 dB and preferable 15dB to have no effect on the sound field next to the listener. Electric, the phenomenon  arises when two microphones that have a certain distance between them capture the same signal and the signal from both mirofoane has the same order of magnitude.


Two typical situations in which comb -filter appears acoustic or electric.  In general, all digital processing takes time and delays the signal. This means that in practice the comb -filter effect occurs when a processed signal is mixed with the original.

 


dB LevelFrequency–Hz

An example of a comb -filter made ​​by combining two signals with the same simplicity but with a delay of 1 ms between them. It is obvious that there is a minimum of cancellation at 500Hz, 1,5KHz, 2,5KHz  etc. It is also seen that the two signals gather at twice their value  ( + 6dB ) at low frequencies and at a delay equal to the wavelength at  1kHz, 2kHz, 3 KHz etc.

 

DipFrequency

Cancellations occur because comb -filter effect, at any frequency at which the two signals have opposite phase.  This happens when the delay time includes durations ½, 1½, 2½ etc. At 1kHz the period is 1ms. Half time is 0.5 ms. If the delay time is exactly 0.5 ms will occur cancellations not only to 1kHz but at 2kHz, 3kHz etc.

 

 

 

Frequency cancellation

 


When the monitor is placed at a certain distance in front of a wall, wall reflections can appear to influence the perceived frequency response. It may contain a comb filter at all the frequencies produced by the monitor. But the monitors are typically omnidirectional at low frequencies. The result of the reflections is the appearance in front of the monitor of one or more null points. Frequency response may look like: a null to a frequency that has a wavelength of four times the distance to the back wall.

Cancellation frequency is dependent on the distance to the wall.   If the distance is 1m then first cancellation will be at the frequency that has a wavelength equal to 4m.

At a nearest position will result a cancellation at a higher frequency.  This limits the frequency at which the monitor becomes directional and do not radiate back.

 A farthest position it will translate into a lower frequency. This phenomenon is limited by the distance at which the reflected wave decreases due to the longer distance driven.