How to generate standing waves? Solution to low-frequency roar and stuffy voice in conference room

When debugging the audio and video of the conference room, many masters have encountered this kind of unsolvable situation:

The parameters of the whole set of audio equipment are normal, the microphone has no noise, and the processor has been debugged correctly, but there is always a low-frequency resonant sound in the conference room, and the voice is heavy, stuffy and vague, which makes people feel tired after talking for a long time.

What is even more strange is that standing in a certain position in the room is particularly noisy. If you move one or two meters, the sound becomes clear immediately.

Most people mistakenly think that the sound quality of the speaker is poor and the low frequency is too heavy. Repeatedly cutting the low frequency and adjusting the balance, the result is that the roar is slightly relieved, but the voice is dry and weak, and the overall sound quality is completely deteriorated.

In fact, this is not a device problem at all, but the most easily overlooked acoustic stealth killer in the conference room-standing wave.

Today, I will talk about the principle of standing wave, self-examination method, low-cost debugging scheme, radical transformation skills, and the whole process of engineering practice, which novices can understand.

1. What exactly is a standing wave? Why is the meeting room the easiest to appear?
Simply put, standing wave is a fixed resonance phenomenon formed by the reflection and superposition interference of sound waves in a closed space.

Conventional meeting rooms are basically regular rectangles and squares, and the walls, ceilings and floors are all hard materials: latex paint walls, glass, tiles and gypsum board ceilings.

These materials do not absorb sound, but only reflect.

When the low-frequency sound wave emitted by the sound box hits the parallel wall, floor and ceiling, it will produce reverse reflection wave.

Forward sound wave+backward reflected sound wave = superposition resonance

When two sound waves with the same frequency and opposite directions interfere with each other, a stable "standing wave" will be formed.

It has two core features, which perfectly correspond to the failure phenomenon of the conference room:

Antinode (resonance point): The sound is superimposed and amplified, with local low-frequency roar and strong vibration.

Nodes (silencing points): The sounds cancel each other, and the sounds in some areas are light and thin.

Small and medium-sized conference room standing wave concentrated frequency band: 60 Hz–200 Hz

This frequency band just covers the low-frequency base of human voice, which is the core reason why the standing wave exceeds the standard, and the human voice will be directly bored, turbid and the clarity will plummet.

The key truth: the standing wave is a room acoustic problem, not the equipment failure of speakers, amplifiers and processors! Simply changing equipment and adjusting parameters can only cure the symptoms, not the root cause.

Second, five characteristics, quickly judge whether there is standing wave interference in the conference room.
Without instruments, you can check yourself with the naked eye and ear on the spot:

1. The difference between points is obvious: the fixed position roars seriously, and the sound quality returns to normal instantly after moving 1-2 meters.

2. The voice is stuffy and turbid: the voice feels "pasted together", the speech is unclear, and the low frequency tailing is serious.

3. Vacant rooms are more noisy: the roar is most obvious when the conference room is empty, and it is slightly relieved when the tables and chairs are full and the people are seated.

4. There is a contradiction in cutting the low frequency: lowering the low frequency, reducing the roar but drying up the voice; Keep the low frequency, and the resonance roar is serious.

5. symmetrical space aggravation problem: the standing wave problem is the most serious in the conference room with square, flat wall, no soft clothes and large area of glass.

Third, practical engineering solutions: from zero-cost debugging to radical transformation.
According to the engineering sequence of "first simple, then complex, first free, then modified", the cost performance is the highest.

Option 1: Adjust the speaker position (zero cost, priority operation)
The core of standing wave is regular parallel reflection. Breaking symmetry can greatly weaken resonance.

1. It is forbidden to install the speaker close to the corner and wall. The corner is the area with the highest low-frequency pressure, which is the easiest to excite the standing wave.

2. The left and right speakers should not be completely symmetrically mounted on the wall, and the positions should be slightly shifted to break the regular reflection path.

3. Raise the height of the speaker appropriately to avoid direct reflection of the unit on the middle of the wall.

4. Long and narrow conference room, to avoid the speaker facing the remote wall, and reduce the long-distance low-frequency reflection superposition.

Scheme 2: Accurate balanced remedy of audio processor (general palliative scheme on site)
Many masters are mistaken: open up Qualcomm, cut the low frequency across the board.

The correct operation is: do not blindly cut the low frequency, only accurately press the formant.

1. Use the test microphone and acoustic software to scan the fixed low-frequency resonance frequency points in the room.

2. Equalize with narrow Q value, and slightly attenuate the standing wave frequency by 3-6dB.

3. The Qualcomm frequency is reasonably set at 80Hz-100Hz, and it is fine-tuned according to the space size, and the vocal base is reserved.

4. It is forbidden to greatly attenuate the whole low frequency, so as to avoid the loss of human voice thickness, hair floating and dry.

Note: Equalization can only suppress the peak value of the standing wave, but it cannot completely eliminate the standing wave, so it belongs to the field emergency remedy scheme.

Option 3: Optimize indoor layout (low-cost optimization)
Empty, hard and symmetrical space is a hotbed of standing waves;

1. Don't leave the meeting room vacant for a long time. Conference tables, fabric chairs and soft ornaments can effectively break up the sound wave reflection.

2. The single-sided wall is equipped with exhibition boards, cabinets and decorative shapes to break the regular structure parallel to all sides.

3. Curtains can be installed on large-area glass walls to weaken hard and strong reflective surfaces.

Option 4: acoustic transformation (the only radical solution, new construction/renovation must be done)
In order to completely solve the problems of standing wave and stuffy voice, we must start with the acoustic structure:

1. Install low-frequency traps

The four corners of the room are the places with the heaviest accumulation of low-frequency energy. Low-frequency traps are installed to specifically absorb the excess low frequency of 60-200Hz and accurately solve the low-frequency roar.

2. Install acoustic diffuser on the wall.

Break up the regular directional reflected sound waves, avoid the superposition of positive and negative sound waves to form a standing wave, and make the sound field more uniform.

3. Reasonable collocation of sound-absorbing materials

Don't spread sound-absorbing boards all over the house, which will easily lead to dry sound and no layering. Combine local sound absorption with diffusion to balance reverberation time.

Fourth, the three fatal misunderstandings that engineering masters are most likely to step on
1. Blindly raise Qualcomm

In order to avoid the standing wave, the low frequency was forcibly cut off, and the roar disappeared, but the voice of the host and spokesperson was weak and the sound quality was seriously degraded.

2. Using feedback suppressor to deal with standing wave.

The feedback suppressor is only aimed at high-frequency howling, but it is completely ineffective for low-frequency standing wave and resonant roar, which is a waste of effort.

3. Hoping for high-end equipment.

Standing wave is a spatial acoustic problem, which has nothing to do with the grade of speaker, microphone and processor. No matter how good the equipment is, the problem will still exist.

V. Summary of the full text
90% of the low-frequency roar, stuffy voice and abnormal local hearing in the conference room are caused by standing waves.

The superposition of acoustic reflection in regular hard space is the core source of standing wave, which does not belong to equipment failure.

Optimal priority of field debugging:

Adjust the speaker position → optimize the indoor layout → accurately and evenly remedy the processor → radically improve the acoustics.

Daily debugging can alleviate problems by optimizing parameters. In order to achieve long-term stable, clear and transparent vocal effects, it is necessary to optimize the acoustic structure of the room.

Doing audio and video engineering, knowing equipment and acoustics, can really solve the difficult problems on site.