As stated in an earlier article, room modes cause standing waves that can cause three acoustical problems: a level boost at some frequencies, an extent of the duration of sound at those same frequencies (resonance) and. some profound dips at other frequencies.

Why are room modes bad? Room modes accentuate specific frequencies. When the neighbor is listening to music, and you always hear some bass notes louder than the rest of the music, it might be caused by room modes in your or your neighbors apartment.

Room Mode Formula: If you want to know the axial mode for the room length, p=1, q=0, r=0. If you want to know the 2nd axial mode, p=2, q=0, r=0. To find a tangential mode, use 1 in 2 of the variables from the room mode calculator. So, if you want the first tangential of the length and width, p=1, q=1, r=0.

Axial Modes involve just two parallel surfaces – opposite walls, or the floor and ceiling. In other words, an Axial mode consist of waves resonating only along one dimension such as the length, width or height of the room. Normally the axial modes have the most strength while the oblique modes have the lowest strength.

The free spectral range of an optical resonator (cavity) is the frequency spacing of its axial (Gaussian-shaped) resonator modes. It is therefore also called axial mode spacing.

Here is a step-by-step guide:

1. Print out your room mode table and play a sine wave sweep from 20 to 350 Hz. Mark the frequencies in the table at which the sine tone becomes noticeably louder or quieter.
2. Now play each marked frequency statically as you move around in your room.
3. Find a suitable listening position.

Critical Frequency is the lowest frequency at which the coincidence effect occurs in a partition and the incident sound wave travels parallel to the surface of the partition.

A coincidence phenomenon occurs when the wavelengths of longitudinal sound waves in air and bending waves in a plate are equal. The frequency range is called the coincidence effect-controlled region, and the frequency is called critical frequency, which is determined by the following equation.

A Weighted Sound Reduction Index or Rw, is the rating used to measure the level of sound insulating abilities of walls, floors, windows and doors. It is expressed in decibels (dB), and is used for a partition or single component only.

The (DnTw) rating describes the acoustic performance of a completed part of a building. The method for calculating weighted values is defined in BS EN ISO 717-1:1997.

Noise Reduction Rating (NRR) is a unit of measurement used to determine the effectiveness of hearing protection devices to decrease sound exposure within a given working environment. The higher the NRR number associated with a hearing protector, the greater the potential for noise reduction.

weighted sound reduction index

There are two parameters that are used to describe the sound insulation of a partition – Dw and Rw. Dw is a term that relates to the sound insulation between rooms on-site. Put simply, it is the noise level in the source room minus the noise level in the receiving room, the level difference as it’s termed.

Rw represents the lab tested sound insulation of an element making up a partition wall/floor type. Due to flanking and other factors, lab rated sound reduction levels will not be achieved on-site. Conventionally, there is a 5 – 10 dB reduction between a Rw lab tested figure and an on-site Dw figure.

Please note that in the case of airborne sound insulation, DnTw is the measurement used, however as low-frequency sound is the most difficult frequency range to stop a Ctr (correction factor) is used to take this into account. LnTw is measurement used to measure the impact sound insulation of floors (on site).

STC & Rw are laboratory test results and both mean the same as they relate to measuring sound, and are the terms used for acoustic ratings. The STC abbreviation means Sound Transmission Class, it is the measurement of internal sound transmission, eg high frequencies.

Some professionals prefer Rw because it corresponds to the decibel scale. So an Rw 50 rating means you could expect the noise you want to ‘block’ to be reduced by 50 decibels.

An STC rating roughly equals the decibel (dB) reduction in noise volume a wall or partition can provide. For example, if an 80dB sound on one side of a wall/floor/ceiling is reduced to 50dB on the other side, that partition is said to have an STC of 30.

30 to 34

The STC rating goes all the way up to 65. Most products don’t promise anything above 55 and even at a rating of 60, it still doesn’t guarantee complete soundproofing. Nothing is completely soundproof. You cannot stop outside noise completely from entering your space.