Room Boundary Simulator

Version: 1.3 (2023-05-02)
Software platform: Windows

Room Boundary Simulator is a simplified speaker placement simulator. Reflections from the floor, ceiling and from the wall behind the speaker can be analyzed.

Room Boundary Simulator

Software features:

Important notice: This simulation does not substitute in-room measurements.

Version: 1.3
File Format: exe (32-bit, portable, no installation required)
Operating System: Windows 2000 or later
License: freeware

New in v1.3:

Revision history:

Comb filtering pattern and steady state response (early and late reflections)

When the first reflected wave (usually from the floor) hits the microphone or the ear drum, it creates a strong comb filtering pattern. Then, the second reflection creates a similar pattern, but due to the longer path way the pattern moves down in frequency. Depending on the path difference the second reflected wave partially fills the notch created by the first reflection. This process is repeated over and over with longer path ways and decreasing amplitude. As a result, large notches created by early reflections are partially filled in the steady state response.

In the simulation we can see this process by selecting "Gated frequency response" and increasing the gate value from 1 msec to 50 msec.

Baffle step in rooms

In sound-reflecting spaces baffle step (6 dB rise in the frequency response in free-field) is modified by boundary interference. The only case where a cabinet frequency response similar to the free-field response is when the cabinet is set up "far" from the walls and the microphone position is close to the cabinet (closer than the half of the room height, but farther than the width of the cabinet).

Baffle step and rear radiation are closely related phenomena. Rise in the front response corresponds attenuation on the rear. Also, baffle step and cut-off frequency of the rear radiation are controlled by the dimensions of the baffle. Since cabinets have very little sound radiation above 500 Hz to the rear, placing diffusers and porous sound absorbers on the wall behind the speakers is useless.

Electronic room correction - does it work?

In a typical "untreated" living room the in-room frequency response of a speaker changes from place to place. We can't fix this chaotic behaviour with an equalizer or a room correction software.

The other problem is the directional behaviour of our hearing system. Measurement microphones have omnidirectional characteristics at least up to 10 kHz (depends on the diameter of the capsule), which means that measurement microphones capture sounds evenly from all directions. However, the directivity of the human ear gradually increases from 500 Hz. In other words, the ear emphasizes some directions and attenuates others above 500 Hz. The deviation from the free-field response caused by the room will be different in the ear and in the microphone. A dummy head measurement could solve this problem, but we are still faced with the chaotic behaviour of the reflections mentioned above.

For every rule there is an exception. In our case the exception is the reflection from the wall behind the loudspeaker. The notch in the frequency response created by the reflection from the wall behind the speaker doesn't change with the listening position, besides the reflection comes from the same direction as the loudspeaker. It's possible to correct this notch with an EQ. (Though it's hard to isolate this reflection from the rest even in the gated response.)

Of course, small correction is acceptable below 500 Hz. As a rule of thumb, boosting of frequencies must not exceed 5 decibels.

Problems at high frequencies

As previously mentioned the human ear is not equally sensitive to sounds coming from different directions and the direction dependence depends also on frequency. A diffuse sound field causes a slightly different response in the ear than pure frontal waves [1]. The difference is small and mainly affects the frequency range above 5 kHz. Around 10 kHz the diffuse-averaged ear response (diffuse HRTF) is 5 decibels more than the frontal response. This might explain why loudspeakers with dome tweeters sound a bit brighter in a small untreated room than in a large or damped room. (Diffuse field equalized headphones have a slightly brighter sound than free-field equalized headphones.)

Room absorption affects the magnitude of the 'stereo dip' at 2 kHz as well. What makes the stereo dip really interesting is that the dip is not present in measurements made with one microphone, only in 'artificial head' measurements. Without room reflections the magnitude of the stereo dip in a dummy head measurement is about 10 decibels, in a "live" room the stereo dip is lowered to 5 decibels due to reflections [2].

Conclusion: two identical frequency response captured by one microphone can cause different response in the ear. Two identical frequency response can sound slightly different.

[1] Fig. 8.2. on page 205 in ‘‘Psychoacoustics - Facts And Models‘‘, Zwicker, Fastl, 2007
[2] Chapter: 9.1.3 An Important One-Toothed Comb - A Fundamental Flaw in Stereo in ‘‘Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms‘‘, Floyd E. Toole (Amazon link)

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