Like most owners of small dedicated listening rooms (mine measuring 5m x 3m x 2.8m L,W,H) the clean reproduction of deep bass presents a real challenge. My recent focus has been on the selection of acoustic treatment for the rear wall. The final decision to adopt a mixture of absorption (read - broadband bass trapping) and 2D diffusion was primarily motivated by a desire for bass ‘support’ whilst diffusion increased the soundstage, aided ambience and improved tonal definition of instruments. The combination of the two acoustic tools created the impression of being in a larger more natural environment.
Be that as it may, room measurements, notably XTZ spectrogram measurements showing room reverberation time in the low frequency band (16 – 250Hz); still vividly pointed out that further improvement in the low bass should be investigated and this is an update to that end.
BASS LIKE GOD IS EVERYWHERE
Looking around the room one could be forgiven for thinking that sufficient bass trapping was in place. After all, three of the four room corners have GIK Tri-traps stacked two high (specified as effective from 50Hz to 5KHz); each of the sidewalls have GIK Monster Elite traps also stack two high and finally the rear centre wall has a stand mounted GIK Monster trap deliberated spaced from the rear wall to aid its low bass performance.
While sometimes difficult to interpret room measurements don’t generally lie and as evidenced by the spectrogram below issues remain. The idea of the spectrogram is to show the decay time as a function of time and frequency. An uneven colour spectrum suggests an uneven frequency response in the room. High amplitudes (warm colours) reaching far into the right of the chart show that there are problems with long decay times. The problem areas are centred around 35Hz and 60Hz, not surprisingly being room length, width and height modes.
Graph 1: Spectrogram (2D Waterfall) shows long decay times at high amplitudes The burning question was now – what acoustic tool could be used to address bass issues at 35Hz and 60Hz? With so much broadband bass-trapping in place and this being largely ineffective at these low frequencies lent itself to the prospect that some form of tuned bass trap was needed.
THE DEEP BASS CHALLENGE
Thick boomy bass is a common affliction that can be difficult to control. It is often the result of room resonance modes – but other possible causes include poor loudspeaker placement, poor loudspeaker performance or not enough low-frequency absorption in the listening room. The site of the listening chair can also mitigate or exacerbate bass bloat.
Not wishing to leave any stone unturned I once again embarked on a campaign of tedious ‘furniture movement’ moving the speakers incrementally forwards and backwards and the listening chair forwards and backwards carefully measuring and recording acoustic results of each movement. Those readers who have tried this exercise will understand the frustration of such a task. One usually sees improvement in frequency response in one area of the spectrum only to find another area of the spectrum now measures worse. Further, room nodes (dips) can be stubborn pesky beasts, refusing to meaningfully change despite your best and most patient efforts in placements. At the end of a day’s effort the placement of the listening chair and loudspeakers remained largely unchanged from where I started – this location producing the smoothest bass – but still an unsatisfactory room decay measure.
Satisfied that my loudspeaker and listening chair placement was in order I now surmised that I needed to acquire some truly low frequency absorbers. It is important to note that the purpose of such absorbers is not to reduce the amount of low end you hear in a room, but to reduce destructive reflections and thereby even out the level fluctuations that occur at different frequencies within the room. The objective becomes to attenuate those reflections that might otherwise cancel out the direct sound from the loudspeakers, causing those unwelcome dips and peaks (being measured) in the room response. Done right, evenness in the bass should improve with no obvious bass ringing or resonances.
A CALL TO GIK ACOUSTICS
While the landscape is dotted with various acoustic panel manufacturers; GIK Acoustics – in my opinion – is difficult to beat on a value for money basis and the fact that their team are very accessible and liberal with their acoustic advice / consultancy services and importantly prepared to modify products for specific needs rather underpins why I have a room full of their gear.
GIK’s latest Scopus range peaked my interest because it seemed to offer a good solution for my needs. As their marketing blurb reads “The GIK Acoustics Scopus Trap takes low end control to new level with maximum targeted absorption using a membrane design with an air tight chamber. Think of them like a drum in reverse. When you strike a drum, it makes a sound based on the mass of the head and the size and depth of the drum body. In the Scopus Trap, the sound strikes the ‘head’ and causes the membrane to move, absorbing the energy based on the mass of the membrane and the depth of the sealed cavity.
Figure 1: GIK Scopus tuned trap design. Source: GIK website accessed Nov 2012The Scopus Traps come in 3 different options that cover a frequency range from 35Hz to 125Hz. Centre frequencies are 40Hz, 70Hz, and 100Hz. It is a perfect addition to rooms that already have broad band control, but are still require targeted control without absorbing above 125Hz. All of this in a package that is as thin as 10cm!”
The T40, centred on 40Hz is advertised with an absorption range of 30Hz to 60Hz. Since my particular challenge was a 35Hz room mode I enquired whether the T40 could be modified to have a lower centre frequency? Yes, came back the prompt reply we already have a T38 model with the essential difference being it is 1 inch thicker than the T40. I ordered two units to try out thinking that if I could measure improvements I could subsequently order more.
THE SCOPUS T38 IN ACTION
Installed at the base of the rear wall the initial measurements were a disappointment. Using the high resolution mode of the XTZ Room Analyser II Pro, which focuses on the frequency range of 16-250Hz with 12 points per octave measure smoothing, the frequency response showed NO change at 35Hz, the peak being unchanged from the earlier arrangement with just the Monster trap in place. What had changed however was a reduction in the peaks at the F2 & F3 resonant modes.
Photo 1: Initial rear wall configuration – GIK T38 to the left & right of listening chair with stand mounted GIK Monster trap in-between. Other panels visible in photo include GIK Tri-traps stacked in right corner, Vicoustic (Portugal) Multifusor DC2 2D diffusors on rear wall & Vicoustic Wavewood absorbers on left & right sidewalls.I fed the results back to GIK and they replied that first, 35Hz is a huge wave and therefore coverage is an important factor (read – more panels would be beneficial); second, the effect of the T38 would be felt more in decay times rather than frequency. GIK further noted that, “We have also found that keeping the units together rather than spread apart does a better job in both decay times and in frequency response - assuming that they are positioned at the location of the problem for frequency.”
Taking the advice on-board I reconfigured the rear wall – joining together the T38’s at the base of the wall and sitting the Monster trap (centred) on top. The measurements of this configuration can be found below and show significantly improved decay times at and either side of 35Hz thereby demonstrating the efficacy of Scopus in the context of this small listening room.
Graph 2: Spectrogram (2D Waterfall) shows much improved decay times
Graph 3: 3D Waterfall (300ms/slice, 200ms duration, sliding CSD)
SUMMARY
Although it seems counter-intuitive, a device that traps low frequencies will in fact increase the amount of bass a room can produce. When the cancellations caused by standing waves are eliminated, or at least reduced, the most noticeable effect should be improved bass evenness, bass quality, and clarity from your loudspeakers.
One of the most rewarding aspects of acoustical room analysis is that significant measured changes always translate to identifiable and meaningful audible changes. In my next update I will detail my observations on what was heard as a result of the reconfigurations as described in the preceding text.
More details on my room can be found on my blog
http://www.audioenz.co.nz/forums/showthread.php/11492-Music-from-a-farther-room
