Bass Trap FAQs

Bass Trap FAQs are intended to explain one of the areas we are about most frequently- Bass Traps- hence our FAQs below.

Bass traps are used in listening rooms, theaters, and auditoriums to suppress or eliminate low frequency acoustic standing waves within the room.  A variety of materials and techniques are used to create bass traps, each with different claims of effectiveness.  This application note provides some background on bass traps, describes from a technical standpoint how they work, examines some of the claims, and recommends best practices for using bass traps in listening rooms.

Why would you need a bass trap?-Bass Trap FAQs

The acoustic volume within a room has a mass and compliance which will have several natural modes of vibration.  When the frequency of a sound source, such as a loudspeaker, coincides with one of the room natural acoustic modes, standing waves can occur which amplifies the sound at that frequency.  Although a room will have numerous acoustic modes, the lower frequency modes appear to be dominant, since the modal density is lower at lower frequencies, making these frequencies stand out since they do not interact with other room acoustic modes.  The net effect is that certain bass frequencies will appear to “boom”, which can drown out music at other frequencies.  To address the booming bass, it is possible to attenuate (or equalize) the output of the loudspeakers at troublesome bass frequencies, however, over-equalized music tends to sound dull and lifeless.  A better approach is to treat the room by eliminating the potential for standing waves through the use of one or more bass traps.

How do bass traps work?-Bass Trap FAQs

Transmission Curve
Bass Trap FAQs Figure 1

It is possible to better understand how bass traps work by idealizing the room as behaving like a single degree-of-freedom oscillator as shown in Figure 1.  Here we assume that a room has a single acoustic mode of high amplitude (shown in red).  A bass trap essentially acts like a tuned mass damper, that is capable of reducing the magnitude of the single peak into two peaks (shown in magenta), where the lower peak is the vibration mode of the coupled system, and the higher peak is the room acoustic mode.  This curve would be representative of a bass trap that has no damping.  With greater damping,  a greater amount of attenuation can occur, as shown by the green peaks. At an optimal amount of damping, the coupled vibration response would be represented by the blue curve, which has a magnitude of about 14, and is more than four times lower than the original red peak.  This is primary mechanism by which bass traps work.

What are the different types of bass traps, and are they effective?-Bass Trap FAQs

There are several different types and styles of bass traps on the market, some of which are effective and some are not.  Excluding Helmholtz resonators, which are not practical due to size and may be difficult to tune, bass traps are typically boxes or panels filled with sound absorption materials or diaphragmatic absorbers.  We described sound absorption materials in a previous application note (see Sound Absorbtion FAQs), and diaphragmatic absorbers are simply panels that are tuned to certain room acoustic frequencies.

To illustrate the effectiveness of a bass trap, assume that a media room in a home theater has a length of 20 feet.  This corresponds to an axial acoustic mode of about 28 Hz.  A loudspeaker could therefore excite acoustical standing waves at the 28 Hz fundamental or at its harmonics.  If we assume that we are exciting a standing wave at the fourth harmonic of the room (113 Hz), the corresponding wavelenth would be about 10 feet.  According to the “quarter wavelength rule”, a sound absorber will need to have a depth of at least a quarter wavelenth to properly attenuate this sound, corresponding to a depth of the sound absorber material of 2.5 feet, or 30 inches.  As you can see, bass traps that include only sound absorber materials may not be effective in eliminating low frequency room resonances.

Bass traps that include only sound absorbing materials that are only a few inches deep may therefore not be effective in attenuating low frequency standing waves within a room.  It is believed that such traps might be effective in reducing the amount of reverberation in the room, which may appear to improve the room acoustics, but may not necessarily eliminate room standing waves.

An alternative approach involves the use of one or more panels within the bass trap that are tuned to the room standing wave frequencies.  Such bass traps, called diaphragmatic absorbers, behave in a similar manner to Helmholtz absorbers, and can be more effective than bass traps that rely only on sound absorbing fibers or foam.  Going back to our single degree-of-freedom oscillator analogy, a properly tuned panel will vibrate at a complementary frequency and effectively damp out a room acoustic mode by creating a resonant peaks that are each lower than the original resonant peak of the room.  Since panels have several vibration modes over a wide frequency range, it is possible to tune and damp a panel in a manner that will attenuate several room acoustic modes over a wide frequncy band.  Because they do not rely on the oscillation of a volume of air (like a Helmholtz absorber) or on the depth of sound absorbing insulation, diaphragmatic absorbers can be compact and packaged in a relatively small panel.

Bass traps that incorporate diaphragmatic panels must be tuned sufficiently low to enable cancellation of low frequency room acoustic modes.  Many diaphragmatic bass traps on the market incorporate a sheet of plywood, which may not have sufficient compliance to attenuate the lowest room acoustic modes.

Where should bass traps be installed?-Bass Trap FAQs

It is a well known fact among audiophiles that bass frequencies tend to pool in corners of a room.  Higher frequency noise will tend to interact with several surfaces within a room and decay at a much faster rate than low frequency noise.  Acoustical energy does not necessarily propagate in directions that are orthogonal to a room (i.e., parallel or perpendicular to walls, ceilings and floors), and will take the paths of least resistance, with some waves propagating at tangential and oblique angles to the walls.  In fact, an examination of room acoustics will indicate that a significant amount of acoustical energy, particularly at low frequency, will concentrate in the corners, since corners contain boundaries of multiple surfaces.

The type and style of bass trap used will dictate the placement of the traps within a room to obtain the best sound attenuation performance.  Bass traps that are not highly damped are best used closer to the corner, as the corners are locations of highest acoustic pressure (with low wave velocity).  Bass traps with a significant amount of damping are best used at a distance of a quarter wavelength from the corner, as this location has the highest wave velocity.  Bass traps with a combination of diaphragmatic and absorber elements should be placed somewhere in between the surface of the wall and the quarter wavelength distance.

Sonic-Shield has an engineering staff with advanced technical capabilities who can assist our residential, commercial and industrial customers in the selection and installation of soundproofing materials and products to solve their noise and vibration problems.