Improving room acoustics

Improving room acoustics is as much of a challenge as providing soundproofing solutions. In a previous application note, Room Acoustics Testing and Analysis, we discussed the performance of a reverberation test, and how such a test can be used to determine the reverberation time within the room.   The reverberation time relates to the time that a sound wave will extinguish to an unintelligible level (usually 60 dB less than the incident sound level) within a room, and is often referred to as RT60.  If a room has too little reverberation, it will sound “dead” and sound will not carry or propagate.  If a room has too much reverberation, the reflected sound will continue to interfere with the incident sound and speech will be unintelligible and music will sound “muddy”.  The optimum reverberation time for speech is about one second, and for music, it is about two seconds.  Reverberation times over three seconds is undesirable for either speech or music, and should be addressed with sound attenuation materials.

Typically, most rooms have too much reverberation, and it is necessary to add sound deadening or absorbing materials to the room to improve room acoustics.  The questions then, are these:  “Which materials should be used?”, “What quantity of the materials are needed?” and “Where should these materials be placed?”  Although it is relatively easy to find acoustic blankets, baffles or panels to place on the walls, or to add carpeting to the floors, there is actually a method which can be followed, that we simplify to enable a layman to answer the above questions with a degree of confidence with the objective of improving room acoustics.

The acoustic reverberation time is defined by the well-known Sabine equation:

Picture 1

where:  RT60 is the reverberation time

V is the volume of the room

Si,j are the Sabine absorption coefficients for each surface in the room at each octave band frequency

Aj are the surface areas of the different materials in the room

There are numerous websites where a user can input the dimensions of the room and the types of materials and their surface areas to calculate a room’s reverberation time.  However, based on experience, it is generally difficult to match experimental measurements of reverberation time with the Sabine equation.

Improving room acoustics

In our first approximation, to accomplish the objective of improving room acoustics is to assume that we can address the “worst case” reverberation time in our RT60 measurement.  In our example, we used the mobile application, iAudioTools (JJ Bunn) shown in Figure 1.  Here, we see RT60 measurements (5-20 dB) at about 4 seconds and below, except for the 250 Hz and 500 Hz octave bands, which indictes an RT60 of 7.4 and 7.7 seconds.  It is desired to obtain a reverberation time of about 2 seconds in the restaurant to improve the intelligibility of speech and music.

In our second approximation, we assume that the room has an “overall” absorption coefficient, so that by knowing the dimensions of the room, it is possible to calculate this overall value to obtain the measured RT60 in the 500 Hz band.

In our third approximation, we assume that the absorption coefficient of the added material (i.e., acoustic blankets, baffles or panels) is much greater than the absorption coefficient onto which the material is placed.

Our example case for improving room acoustics is a restaurantthat measures 40’ x 30’, with a 16’ ceiling.  Calculating the “overall” absorption coefficient knowing that our reverberation time is about 7.7 seconds, we obtain a value of 0.027.  This is reasonable, since the test was conducted when the restaurant was empty, with a metal ceiling, hardwood floors, and glass, brick and wood paneled walls.

Knowing the “overall” room absorption coefficient, it is now possible to back-calculate the required surface area for sound absorption materials knowing that the desired reverberation time is 2.0 seconds.  Here, we postulate that acoustical energy is reflecting off of the metal ceiling, and that sound absorption materials suspended horizontally from the ceiling will be most effective in reducing noise in the restaurant.

Our choice for sound absorption materials are Sonic-Fiber baffles, which are 2” thick sheets of rigid mineral wool encapsulated within a canvas cover.  This material has a noise reduction coefficient (NRC) of 1.0 at 500 Hz.  The NRC is essentially the Sabine absorption coefficient of a material, and indicates its ability to absorb (or more accurately, to not reflect) acoustical energy.  A material with an NRC of 0.0 (like smooth concrete) will reflect all of the acoustical energy, and a material with an NRC of 1.0 (like Sonic-Fiber) will not reflect acoustical energy.  Using an NRC of 1.0, we find that the required surface area is about 288 square feet.

In many cases, consumers purchase acoustic baffles and panels and mount them directly to the wall or ceiling with the objective of improving room acoustics.  This is not the optimum location for sound absorbing materials because the acoustic wave velocity at a boundary wall or ceiling is zero.  The ideal location for sound absorbing materials will be where the acoustic wave velocity is at its maximum value, usually at a distance of a quarter wavelength from the nearest boundary surface.  The acoustic wavelength at 500 Hz, it is 2.25 feet (29 inches), so in order to effectively attenuate this wave, the offset of the acoustical baffle should be no less than 7 inches from the surface of the ceiling.  At 250 Hz, the acoustical wavelength is 4.5 feet, so an offset distance of 14 inches would be required.  Our preference would be to place half of the acoustical ceiling baffles at an offset of 7 inches and the other half at 14 inches to obtain a good balance of attenuating sound in both the 250 Hz and 500 Hz frequency bands to achieve the objective of improving room acoustics.

Sonic-Shield has an engineering staff with advanced technical capabilities who can assist individuals and building owners in the testing, analysis and solution of room acoustic problems.  We can perform the required room acoustic tests and analyses to arrive at a solution, as well as develop and supply the appropriate sound abatement materials and products necessary to improving room acoustics.