Bass Traps

We've been pioneering, building and improving bass traps since 1984. Our flagship then remains the same today....the TubeTrap

We've been pioneering, building and improving bass traps since 1984. Our flagship then remains the same today....theTubeTrap

TubeTrap audiophile Listening Room Bass Trap Avalon speakers

Bass Traps absorb low frequency sound. They are mostly used in small rooms in which a high power audio system is being played, such as recording studio control rooms, mastering rooms and audiophile listening rooms. There are many types of bass traps.

Traditionally acoustic designers or acousticians designed bass traps to be built into the construction of the room. In the mid '80s the first portable bass trap became available, invented by Art Noxon, an acoustic engineer. The TubeTrap is a cylinder shaped bass trap that is frequently found in the corners of high power audio playback rooms.

It's the corner of a rectangular room that funnels bass pressure into a high pressure zone. Bass traps are usually placed in the corners of the room. The TubeTrap is a unique type of bass trap because it includes a suspended sheet of material that has been perforated in such a way that it backscatters the treble range of sound, diffusing it into the room.

The curved treble diffusers act to retain room ambience, while enhancing the sense of spaciousness in the listening room. The corner loaded bass traps act to minimize the reverberant buildup of bass energy in the room. Used together, the feeling of being in a small room disappears, replaced by the feeling of being in a large ambient space.

Basic Concepts

Why are Bass Traps prerequisite to quality audio playback in small rooms?

Bass traps, especially the TubeTrap type of bass trap, are used to control the “room gain” effect in the bass range of high end high power audio listening rooms.  Properly sized and positioned bass traps can reduce the temporal distortion and tonal masking of music caused by room gain in the musical bass range.  

Room gain is rapidly created when loudspeakers are positioned between parallel surfaces in a room.  A  substantial portion of sound emitted from the speaker is repeatedly reflected between the parallel surfaces, getting louder with each subsequent pass. This results in a rapid buildup of stored sonic energy which is ordinarily very slow to dissipate compared to the rapid dynamic or sound level changes in music.  The result is blurred musical dynamics and the masking of musical fidelity. 

Bass range room gain comes from the horizontal and vertical components of the spherical bass wave front emanating from the speaker and their multiple reflections off the floor, ceiling and side walls in the front of the listening room.  Each musical event produces some sort of sustain that lasts easily 1/8th second. During that time the sonic wave front travels 140 feet, which is across the room 9 or 10 times and vertically over 15 times.  This causes a rapid increase in stored energy and sound level in the front of the room which slowly expands out into the rest of the room, engulfing the listener in pulse of nonsensical sound that sounds just like what the loudspeaker had just been playing.   It acts to blur the otherwise rapidly changing sound level and musicality contained in the direct signal. 

A loudspeaker is designed to be a linear sound reproducer. The manufacturer tests the speaker in an anechoic chamber for a flat frequency response, something like what we get if we play the speaker outdoors.  Additionally, loudspeakers tend to be directional with the mids and highs predominately being emitted in the forward "cardoid" direction and the bass emitted "omni" equal power in all directions.  This is why the relatively instantaneous room gain effect is a bass buildup problem in the front of the room.  

Piano Keyboard Frequency Hz bass treble tones

Musically speaking the bass range of sound is defined by the notes to the left of middle C on the piano, the treble range are the notes to the right of middle C. Bass ranges between 45 Hz and 260 Hz and treble ranges between 260 Hz and 4,000 Hz.

Below the bass range is the sub-bass range, sounds below 50 Hz. Human hearing extends down to about 20 Hz. The Sub-bass range of sound is 1 ½ octave range between 20 Hz and 50 Hz. Below 20 Hz is the infrasonic range of sound where people feel the effect of sound pressure but cannot hear it which tends to create the feeling of anxiety. Subwoofers generate sub-bass and some infrasonic sound.

When a loudspeaker is played outside the waves created expand away from the speaker box and never return. However, when the same sounds are played in a room, the surfaces of the room reflect the sound back into the room. These reflections increase the loudness of the sound in the room which is room gain.

Sound panels reduce the strength of the treble range reflections while bass traps deal with the bass range reflections. Both are used in audiophile listening rooms, recording studio control rooms and mastering studios to reduce the room gain effect and help keep the music being heard more faithful to that on the original recording. They are also used in live rooms, where microphones are set up and recording takes place, to minimize sonic affects, the coloration of sound due to room gain, from being recorded along with the desired sounds of the musical talent.

ADSR sound envelope attack transient

There are many types of bass traps just as there are many types of sounds. In general a single sonic event begins with an Attack transient, a rapid growth in sound level. This is the single most important sound we hear when identifying the source of the sound. Following the Attack is the Decay, a rapid drop in sound level which leads to a lower level Sustained tone. Finally the sound is Released and the instrument quickly drops to quiet hence the acronym ADSR symbolizes the lifeline of a sonic or musical event.

ADSR attack transient time sustain sound envelope

Some bass traps are fast acting. Fast traps are good at minimizing the room gain effect during the fast changing parts of a sonic event, the attack Transient and Decay and of course, they work well as the slower parts of the sonic event, the Sustain and Release.  TubeTraps are fast bass traps

Typically membrane, spring plate and Helmholtz bass traps are slow bass traps.  All "mass loaded absorbers" take time to come up to speed and cannot keep up with the fact changing parts of a sonic event.  They are generally effective during the slower moving segments of the sonic event, the Sustain and Release. Slow traps are  best suited to handling the RT60 aspects of sound and not suited to handle the attack transients.  

Fast traps can work within the early, transient stages of the sound envelope.  They can control overshoot of the Attack transient and variations in the Decay transient due to room gain mechanisms. Both Slow and Fast traps operate on the Sustain and Release, the difference being that Slow traps tend to be tuned bass traps, frequency or bandwidth selective while fast traps are broadband.  .

TubeTrap bass trap audiophile listening room acoustics

Only during the Sustain is a tone played long enough in the room to develop a room mode. Slow traps are tuned to the mode frequency and are located at pressure zones of the mode so as to add damping to a particular mode. Fast Traps are broadband; they operate at any frequency within their response curve.  They require fluctuating air pressures to operate and they are typically placed in the corners of the room because the bass sound  pressures are stronger in the corners than elsewhere.  

The primary location for fast bass traps, TubeTraps, are the front two corners of the room, the corners behind the speakers. The low frequency wave front expands in hemispherical shape away from each speaker, 1/3rd of the energy in the vertical direction, 1/3rd in the lateral direction and 1/3rd in the axial or front to back directioon.  2/3rds of the wave front gets trapped by repeated vertical and side to side reflections, creating a nearly instantaneous buildup of reverberation in the front 1/3rd of the room. High speed bass traps placed floor to ceiling in the front corners dampen the speed and strength of the build-up.  Additional traps are placed in the plane of the speakers and elsewhere along the side walls in the front of the room.  Each floor to ceiling tram absorbs side to side and floor to ceiling reverberation.  

Avalon speakers with TubeTraps for bass traps

This intense room tuning with bass traps up front is not done to improve the frequency response curve of the room. It is done to insure that the high speed dynamics in the music can be heard in the room. Attack transients can easily change sound level by 80 dB in a short period of time such as 20 ms, a dynamic speed of 4000 dB/second. Fast bass traps are needed in the front of high power audio rooms to keep dynamic transients from being muddled and blurred, sound masking by the otherwise uncontrolled rapid growth of bass range reverb in the front of the room and subsequent expansion down the length of the room, and past the listening position.

Once the built-up head end ringing expands to fill the whole room, the energy becomes  whole room reverberation. What fast traps in the front of the room accomplish is not only to reveal more of each dynamic transient but as they pull energy out of the unwanted buildup in the front of the room they are simultaneously reducing the loudness of the whole room reverberation, before it became room reverb.

And then finally, it acts on the whole room reverb to reduce the decay time rate of the room reverb.  Typically from the natural room's 1.5 second RT60 down to a more desirable RT-60 of 0.75 seconds.  But it is not the reverb time that matters, nor does the flatness of the response curve.  What the golden ear most prefers and appreciates is musical clarity.  Blur-free dynamics in the transient response of the room is much more important to the audiophile and recording engineer than a flat room or fast decay rate, which is why the ASC MATT test was developed.

How do bass traps and TubeTraps work?

There are three classes of bass traps; Passive, Reactive and Active. Passive traps are broadband bass traps, relatively easy to build, easy to setup in a room and easy to get good results. Reactive traps are tuned, resonance based bass traps, relatively difficult to build and set up but the results can be very good but only at a particular frequency. Active Traps are powered bass traps. They can be based on sound canceling, very difficult to build but broadband in the bass range and easy to use, or based on an electronic circuit version of the reactive or resonance trap except with adjustable frequency and bandwidth, also very difficult to build and setup but done right gets good results. ASC manufactures a wide variety of passive and reactive bass traps and one model of an active bass trap.

Here we will be discussing non-electronic bass traps. The more common bass traps are the Functional Bass Trap, the Helmholtz Bass Trap, the Membrane Bass Trap, the Diaphragm Bass Trap and the Quarter Wavelength Bass Trap. Although they all absorb bass energy they work in different ways. In order to DIY make, commercially manufacture or even use any of these bass traps to control sound in small audio rooms, one has to know two things; how each bass trap works and how sound works in small rooms.

It is always important to begin at the beginning. For starters, technically speaking, there is no such thing as a “sound wave” despite the fact that everyone calls it a “sound wave” Let me explain: A “sound wave” is just the Pressure part of an Acoustic Wave. An acoustic wave has two parts, an air pressure part and an air movement or velocity part. We hear the air pressure part of the acoustic wave and call it a sound wave. We can’t hear the velocity part of the acoustic wave and most people don’t even know that it exists. 

All waves are the same in that they all have some sort of pressure effect coordinating with some kind of movement effect. Imagine for a moment being in a boat when an ocean wave passes by. An ocean wave must be a combination of water pressure and water movement. When the wave is high, the boat is pushed forward, in the direction the wave is traveling and when the wave is low, the boat is pulled back against the direction the wave is traveling. 

The energy of any wave is comprised of some combination of potential energy (pressure) and kinetic energy (velocity). A bass trap is a device that can absorb energy out of the pressure part of the acoustic wave or the velocity part, or both.

Functional Bass Trap

The functional type bass trap is the fastest acting, most powerful type of bass trap credited to Harry Olson an acoustic engineer at RCA Labs. He published a book in 1950 that remains even today the "bible" of audio acoustic engineering. It's full of acoustic devices and acoustic circuits and one of these is called the "Functional Sound Absorber" page 508 Fig 11.4 of his book Acoustical Enginering. The TubeTrap is an improved and readily available version of the original Functional Bass Trap.

harry olson functional bass trap and tubetrap

This type of bass trap is based on acoustic circuit design. It is not a 1/4 wavelength bass trap, it is 1/4 the size of a quarter wavelength trap. It is comprised of an acoustic resister in series with an acoustic capacitor and the resistor is impedance matched to the radiation impedance of a sound wave. The acoustic resistor is the wall of the bass trap and the acoustic capacitor is the air cavity inside. For comparison, the outline of a modern bass trap, the TubeTrap is shown along side the functional bass trap.

When an acoustic wave passes by the bass trap excess pressure fluctuations alternatingly push air through the wall into the cavity and pull it out of the cavity through the walls of the cavity. The walls of the cavity are composed of air flow resistive material, typically quite similar to what fiberglas sound panels are made of.

When there is a pressure difference across the wall of this type of bass trap there is a corresponding flow of air through the wall. The ratio of this pressure to current is called acoustical impedance. This impedance is pure resistance. When an acoustic wave travels through the air, the air experiences pressure changes, which is sound, accompanied by back and forth air movement, much the same as what happens to the air that lies in front of a loudspeaker.

Functional sound absorber tubetrap bass trap pressure gradient

The ratio of pressure change to air movement is also acoustical impedance, but this time the impedance is made up from the weight of the air and it's elasticity. The important point in making DIY or commercial bass traps is that the impedance of the bass trap matches the impedance of the acoustic wave that it is interacting with. This type of bass trap is called a pressure or pressure zone bass trap because it requires acoustic pressure to push and pull the air within the frictional walls of these bass traps.

The pressure zone bass trap does not directly absorb energy out of the pressure changes. There are very few pure or direct pressure zone bass traps. A viscoelastic closed cell foam does absorb energy out of pure pressure changes when the shape of the foam cells change dimension under pressure. A good reference for learning about many types of bass traps and how they relate to room acoustics, written in 1991 for dB magazine, can downloaded here.

Notice the flat walls of the early functional bass trap compared to the cylinder shaped walls of the TubeTrap. The walls of the TubeTrap are curved which, like a pressure tank, do not change shape under pressure. The flat panels of the functional bass trap easily cave in and out under acoustic pressures. For general purpose low level noise such as in a gymnasium or atrium the sound levels are relatively low, in the range of 50 or 60 dB,A and with noise, no particular tone plays for very long period of time. Under these conditions the flat panel trap is not strongly vibrated. However in high power audio playback rooms and control rooms, where the sound level can easily reach 100 dB,C and long sustained tones are frequently played, along with extreme dynamic percussive pressure impulses, the flat panel is easily driven into vibration, while the cylinder trap resists being deformed under pressure.

Whenever there is a pressure difference on either side of a wall the wall tends to move in the direction of the pressure gradient. Sound absorbing fiberglass panels are no exception. These panels are lightweight and do have some strength and when you thump a freely suspended sound panel it freely twangs for some time. Despite the fact that is absorbs sound it can also vibrate. A sound panel that moves in the air provides a resistance to air moving through it. A vibrating sound panel acts like a loudspeaker, a dipole loudspeaker, which is a panel that moves back and forth, creating coordinated air pressure and air motion changes which become a sound wave. Yes, a vibrating sound panel makes noise. If the walls of the bass trap are unsupported, some low frequency tones or impulses will push and pull the panel into sympathetic vibration, which becomes an unwelcome audible sound om a room.

This type of bass trap works best when their walls stand still, for yet another reason; which is so they can absorb sound. If sound pressure pushes against the unsupported sound panel, the panel moves away from the pressure which means it is not letting as much air pass into the panel And this means less efficient sound absorption. It is in details such as these that DIY and commercial bass trap construction can lead to frustrating results instead of a high efficiency bass trap.

bass trap DIY cutaway tubetrap diffuser diffusor

The TubeTrap is more than a functional bass trap, it comes with a built in treble range diffuser. In addition it has bulkheads which compartmentalize the trap into a number of smaller traps and to further stabilize the walls of the TubeTrap. As with all acoustic products made from fiberglass, the entire unit has to be sealed fiber tight. These traps are used in small rooms with minimal air conditioning or drafts. They experience years of daily buffetting when used in high power audio applications, which can shake fibers out of an unsealed bass trap or sound panel. Not sealing DIY bass traps and sound panels fiber tight is one of the most common mistakes home builders of bass traps make.

The fabric covering has to be acoustically rated 100% transparent. Most fabrics are woven too tight and air cannot pass through them. Thread counts of about 12 threads per inch or less are acoustically transparent. Note, fabrics are not used for fiber seals, they are only voiceless, reflection free cosmetic covering.

Its not enough to make a bass trap, they have to be positioned, often stacked one atop another and hung down from the ceiling. Mounting bass traps require very sturdy fasteners, ones which do not pull out under the most rough conditions of use. TubeTraps are sturdy enough to support a grown man, over 200 pounds standing on the top of each one. It is not unusual to find applications where 6 TubeTraps are stacked one atop the other, creating columns some 18 to 24 feet tall.