Treating a Room for Bass Anomalies

By Shaun Snaith, Advanced Acoustics Every person's home is their castle, and while some castles are larger than others, for those who have the desire and the space for it, a dedicated listening room or home cinema room is a must. It is an opportunity to listen to music or watch films in the comfort of your own castle without someone sitting in front of you talking through all of the best bits or crunching noisily on the overpriced cinema popcorn. While many fulfil their desire to have a supreme home cinema room or dedicated listening room, the results can often fall short of that perfect sound normally experienced at the cinema or when listening to live music. It is because one vital aspect of the build has not been taken into consideration - the room. A project that hasn't taken the room into account can suffer from boom, peaks and nulls, lack of clarity or poor soundstage. Thankfully, these problems are easy to solve with the right acoustic treatment. Acoustic treatment is not a minefield - it just needs a little planning and forethought, and can add considerably to the room both acoustically and visually. The most disruptive and difficult-to-treat problem is that of bass anomalies. These anomalies could be bass build up, bass boom, or bass suckout - collectively known as room modes or standing waves - and they are always directly related to the room and its dimensions. Room modes are room resonances which occur if the room has parallel walls and when the room is excited by a source such as a loudspeaker or subwoofer. [caption id="attachment_970" align="aligncenter" width="400"] Diagram showing room modes (standing waves) as a result of bass frequencies.[/caption] Estimating problem frequencies Room modes mainly occur between 16Hz and 350Hz, depending on the size of the room, when sound waves travelling in one direction interact with those travelling in the opposite direction, resulting in them either cancelling each other out or reinforcing each other depending whether they are in phase or not. When they cancel each other out, you get bass suckout, and when they reinforce each other, you get bass build up or bass boom. When you consider that in a listening room or cinema room you have six surfaces all reflecting sound waves, there are going to be certain frequencies which will collide. It is possible to calculate what these frequencies will be. You can either estimate them using online calculators such as the one found at Dr. Jörg Hunecke's site or you can test the room itself, using dedicated equipment such as the XTZ Room Analyzer. Treatments Knowing what the problem frequencies are is only part of the battle. You still have to treat them. When it comes to treating these bass anomalies, there a few different methods you can use. You can treat them passively with acoustic treatment products such as bass-absorbing acoustic panels, or you can cure it actively with electronics such as DSP (digital signal processing) room correction or parametric room correction (analogue correction). Acoustic treatment Firstly, we will discuss acoustic treatment and its pros and cons. Bass-absorbing panels such as the Advanced Acoustics Corner Panel or Real Trap can reduce the build up of standing waves in a listening room or cinema room. They act as broadband absorbers and are not specifically tuned for a certain frequency. When bass-absorbing panels are installed in-room, the frequency response of that room is much flatter and tighter, and peaks and troughs even out. You also get a more consistent sound throughout the room. So they can be a cost-effective and simple solution, if done correctly. For bass-absorbing panels to be effective however, they have to be big. Low-frequency sound waves are long and strong, and it takes a lot of mass and energy to reduce and slow them down. So the drawback is that acoustic panels are not always discreet, but if it is a dedicated room and the priority is to get the best sound reproduction possible, then such room treatment could be acceptable. If done correctly, room treatment can become a feature of the room as opposed to an afterthought. [caption id="attachment_971" align="aligncenter" width="600"] A demo room with bass-absorbing panels as a feature.[/caption] DSP treatment Next, there is DSP room correction, for example the XTZ DSP Subwoofer Amplifier. The biggest pro with DSP is that you can do away with large bass trapping panels and treat the problem electronically at the source by having a black box doing all of the work electronically. It means that you can get a perfectly flat response by using the DSP to fill in the troughs and take out the peaks. The problem however, is that more often than not, a DSP unit will add its own signature to the sound. For those who want a neutral sound and have carefully selected speakers and electronics to get that sound, all the work can be undone. That is why a DSP subwoofer is more favourable to a DSP AV receiver. Parametric room correction The final option is parametric room correction using a product such as the Rives Audio PARC. Again, the pro is that you can do away with a lot of the bass trapping in a room and use the electronics to do all of the work. The advantage of parametric correction as opposed to digital, is that there is much less likelihood of the unit adding its own signature, as it is an analogue correction, and as long as very high-quality components are used, it will be a neutral unit. But again, the drawback is that you are putting another box in the system which can affect the sound quality. [caption id="attachment_972" align="aligncenter" width="425"] The Rives PARC parametric room correction system.[/caption] Conclusion Acoustic panels are only so effective, for example, they would have to be extremely large to be able to reduce a 35Hz resonance. Yes, they will have some impact, but the amount of absorption they can offer is reduced the lower the frequency. With electronics such at the Rives PARC however, you can effectively control right down to 18Hz. The benefits of treating bass anomalies are without doubt one of the most important factors in getting the best set-up possible for your clients. The difficulty comes in selecting what is the best product. When we specify for projects such as cinema rooms, our advice is simple, and follows the same rule as many acoustic treatment manufacturers and room correction manufacturers will tell you, namely do as much as possible with effective room treatment first, and treat the final bit with room correction. That way, you have the room working in balance, and if you use the room correction, which can add its own signature, the less electronic equalization you use, the better the results. It must be remembered however, that overdamping a room can be detrimental to the room and bass trapping might not always be practical. No two rooms are ever going to be the same, and every room will require a different approach. Don't let that you put off solving the problem; it won't go away if you simply ignore it. Instead, make the most of the many different solutions available, remembering that implementation of two different methods can give you the result you require. Shaun Snaith is responsible for Product Development at Advanced Acoustics, manufacturer of acoustic treatment solutions for the domestic, professional and commercial sectors, and UK importers of room analysis equipment and room correction modules.

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