THE GENTLE ART OF SUBWOOFING
The likeliness of having a pair of floorstanders that can play down to the lowest frequencies with smooth in-room response is equal to the likeliness of spotting Santa Claus in the sky during a winter night.
If you own a few CDs of movie scores with recorded sounds of an airplane crashing to the ground or dinosaurs hunting their prey around the forest then you probably want to hear them in full; quite understandably you also speculate that because you do not hear the dinosaurs’ stomps in your living room you may miss something in ´normal´music material too - something that could be brought to the light by a subwoofer. You start considering buying one. Irrespective of how much such an assumption is correct for the dinosaurs, it is not correct at all as far as the music is concerned. Therefore, in this article we tried to address some of the myths and issues that are usually connected to subwoofers.
Conquering the low frequencies
Otherwise neat proportions of a room are the worst enemy of smooth bass response. The most difficult room shape is square. It is therefore very vital for the bass as well as for other frequencies to have a room as irregular as possible, however symmetrical, to avoid build up of soundwave resonances and not to ruin soundstaging qualities of a system.
The larger the room the smaller problems with the bass you can expect. Normally, it is thought that the floor surface of aproximately 4 x 8m or more is quite a good start for a listening room. Unfortunately we have to work with real spaces that are usually far from the ideal – still it is possible to get the bass right.
Lightweight walls (like plaster boards) allow some frequencies to escape from the room without being bounced back. The same effect can be achieved by wooden panelling, false walls or thick layers of rock wool (I really mean thick, like 30cm). You can consider adding studs and one layer of plaster boards to a wall (or walls) of the room to aid absorption of the bass.
Forget about a passive subwoofer. Any modern active subwoofer is usually based on Class D amplifier technology which means the power of hundreds Watts and high efficiency, a compact size and almost no heat produced.
Forget about how your room had measured in low frequencies before you added a subwoofer. One more independent source of ultra deep frequencies in the room redefines completely what you were familiar with. Therefore do not make any evaluations before you find the best placement for your subwoofer – only then evaluate the response and start fiddling with its knobs to make the response be as smooth as possible.
Placement of a subwoofer
A subwoofer‘s cabinet should not vibrate as it decreases its efficiency and possibly induces mechanical resonances. The same rules as for main loudspeakers apply: try either absorbing feet or decoupling spikes to keep the vibrations under control.
There is no general rule where to place a subwoofer so we encourage you to experiment. Very often it is recommended to place a subwoofer in the corner of a room behind main speakers to gain the strongest bass ouput. However, the bass volume often does not mean the best bass value so we recommend to try other placements too to prevent mighty but muddled sound.
As you move a subwoofer closer to a wall, the bass output wil become stronger and vice versa. Use a test CD with bass warbles or frequency sweeps to identify the frequencies that make your room play. Furniture resonances, rattling windows and glasses in cupboards all add their signature to the sound. Eliminate them before you proceed to fine tune a subwoofer.
A properly placed subwoofer(s) may help significantly enhance the soundstaging capabilities of your system. In the stereo mode the placement of the subwoofer(s) to the outside of the main speaker(s) and on the plane of the speakers or a bit behind is usually the safest point to start with.
In case just one subwoofer is to be added to a system we recommend to use the music you listen to as a guide for the placement. In classical music the most deep frequencies come from the right where the bass instruments are typically located on a stage. In rock music most bass energy comes from a kick drum so
why not to move the subwoofer to the inside of a main speaker. However, do not place the subwoofer close to the axis of a room and as a rule avoid placing it in the center of a room.
When evaluating the bass response be sure the room is in the configuration that it will be in when you typically listen – doors and windows closed, carpet on floor etc. Do not bother about your precise sweet spot – few inches back or forth do not make a difference for a low frequency wave.
An elegant way to find the correct placement quickly is the following:
Put the subwoofer in your listening chair and raise it to the apropriate height where your ears usually are. Play a frequency sweep from a test disc or a well-known and bass heavy track. Then crawl around the floor to identify the spot where the bass sound smoothest and tightest – you may find few such spots so mark them well. Move the subwoofer to the position(s) you have marked.
If the room´s response is not good in bass even after careful placement the only things you can do is to fix the room´s acoustic properties or use a subwoofer with DSP processor that can equalize its output (Velodyne subs are great in that). If you use a subwoofer with the DSP than first try to find the location with minimum number and severity of ´valleys´in the frequency response curve as peaks unlike valleys can be addressed by the DSP later.
Hooking up a subwoofer
If possible, avoid high level input for connecting a subwoofer – that is do not feed a subwoofer from main loudspeakers terminals. There are few exceptions among subwoofers but they are rare. If you do not have spare line level outputs on your amp then use speaker-level outputs from the amplifier.
If your subwoofer has only mono low level input (just one RCA or XLR input) it is not a problem – you can use some high quality signal splitters (Cardas manufactures good ones) to connect the subwoofer to both channels of your amplifier.
Do not underestimate the importance of power filtration for a subwoofer - each active sub is also an amplifier. If you ever experienced how the bass of a system tightened/loosened with the use of a good/bad power conditioner then you know what we mean.
Be prepare for the investment in a quality pair of interconnects (or speaker cables if you use speaker-level connection). Do not believe the people who say that it does not matter which type of wire has been used for the connection. If you use a quality wire you should be able to hear it.
Two or more subwoofers can be daisy-chained. Good subwoofers (Dynaudio for example) usually feature a master/slave switch that enables all the chained subwoofers to be controlled from one common place.
In complex installations consider using external crossover – subwoofer manufacturers, pro audio companies or specialized high-end manufacturers (like Bryston, for instance) can help you to maximize the performance of your system.
Setting up a subwoofer
It is great if you can use measurements for initial set up of a subwoofer. Do not underestimate psychoacoustics. If you know that your system measures perfect it gives you peace of mind and encourages you to experiment further. It is likely that you will end up with slightly different settings but still your system will remain to measure near-to-perfect.
The human ear is the best judge of what is important in the bass region. If you like the sound stay happy with it. Do not hunt for flat frequency response if your ears say ´no´. Do not listen to friends or audio critics – each of them has a different pair of ears. If you can’t hear the difference and they can, who cares?
If you do not have Clio, ATB PC or other measurement instruments it is perfectly OK to set up a subwoofer by ears only. The detailed guide written by Bob Katz can be found here.
Every subwoofer should have at least 3 controls: volume (gain), phase switch (0 to 180°) and low pass crossover setting. If a subwoofer does not have any of these controls, do not buy it.
Some subwoofers offer many additional control options – no problem, you do not have to use them in real life and they are usually bypassable.
Do not confuse the phase switch for a polarity switch. Unlike in the case of a power socket the phase in subwoofing means a time delay, i.e. to what extent the sound of a subwoofer is delayed comparing to main speakers. The phase is measured in degrees °.
Not every subwoofer enables smooth change of the phase in the range 0-180°. While simple ´reversal´the phase may be sufficient in certain cases it will be insufficient in other. We recommend to look for a subwoofer that can adjust the phase continuously or in increments (0/90/180/270°) at least.
Almost every audio set up disc comes with low frequencies sweeps or warbles – they are extremely useful to judge the smooth bass response of a subwoofered system as a music signal varies too fast in time – our recommendations for some can be found as a footnote of this article. Do not try to set up a subwoofer for ultra-low bass. The frequencies like 20Hz are very rare in recorded music and it is likely that you will end up with overdone settings. Rather go for mid bass (60 to 100Hz for example) to judge the proper gain (volume) of the subwoofer.
The manuals that accompany subwoofers give usually a comprehensive information on how to set individual knobs and toggle switches on a subwoofer. Again, the article of Bob Katz is a perfect guide to the art of subwoofing when you are in doubt.
What to expect from a subwoofer
The deep bass can be described as the range from 20 to 60Hz. The bass below 20Hz is normally not present in a recorded music – not only that professional tape recorders historically hardly worked below 50Hz but also because, with the exception of the cathedral organ, there is no instrument with dominant energy in this range. Such a statement would be very simplistic as there are unknown territories yet: the ear can hear frequencies up to ca 18kHz, however, with the introduction of new wide-range formats, like SACD, it was proven that people can feel much higher oscillations. Similarly, the deep frequencies do not have to be heard to be felt: they exist and are important for the proper tonal balance of instruments like a double bass or piano or even human voice.
Unlike popular misconception, classical music contains very little of the ultra deep bass. The impact of orchestral crescendos is achieved by immense dynamic range rather than by earthshaking lows and the heart of orchestral bass is somewhere between 80 and 200Hz. The problem with the symphonic music is that, apart from the aforementioned tonal balance, there is a lot of sounds and rumblings in a concert hall that are not caused by the instruments – therefore a subwoofer helps to recreate the atmosphere of the live performance and as such it is indispensable for real high-end systems.
Do not expect just bass from a subwoofer. A lot of deep bass. If you do, then buy the cheapest boom box in supermarket and do not bother about high end. There are more important qualities to the bass like its tightness, speed, resolution and definition. This is what we should be looking for when adding a subwoofer to an existing audio system.
Silence is important. The response of the cone of a subwoofer to a transient signal should be fast and clean. If the cone is not fast enough or the body of the subwoofer resonates then it will fill the silent gaps in music by sonic garbage. You will never hear the silence and you will throw low frequency veil on the played music too.
A 50cm woofer is better than a 30cm woofer – the former generates lower bass through its pistonic movement. However, there are some limits to the size of a woofer as extremely large cones are not stiff and lightweight enough, which means slower acceleration and decceleration. Do not attempt to buy the largest subwoofer as the quantity is often at the expense of quality.
In the article published in the Journal of the AES (Audio Engineering Society, June 1988 – reprinted by Stereophile in 1989) Louis Fiedler and Eric Benjamin examined 13 carefully selected CDs (mainly classical music) to find out the minimum audible frequencies present on the CDs based on a predefined threshold of hearing. To summarize their findings, the frequencies below circa 22Hz are very rare in well recorded music. If they are there at all, they are generated by cathedral organs, synthesizers, special effects or environmental noises. Again, they add to the atmosphere rather than to relevant music content. On top of that, if we want to hear such frequencies than the music has to reproduced at minimum 120dB peak SPL in a room, which is comparable to a live performance´s peak loudness.
The response of an audio system in a room does not have to be absolutely flat on the screen. According to revised Fletcher model the human ear can detect the changes in energy output below 300Hz only in the bandwiths about 100Hz wide. This means that a peak or valley below 300Hz should not be broader than 100Hz in total. But how deep or high? In idealized conditions (headphones or anechoic chambers) a listener can hardly detect the peak or valley smaller than +/-2dB. The lower the frequency the lower the sensitivity of the ear. Therefore peaks and valleys not exceeding 100Hz width and 2dB height or depth will make no real difference comparing to the ruler flat response. Music is not static unlike pink noise signals so the irregularities will remain unnoticed.
Conclusion
Do not bother anymore about the low frequency response of a speaker. When buying a speaker rather go for the best midrange and treble you can afford with your budget. Then use a good subwoofer to add anything below 200Hz what is missing. Satisfaction guaranteed.
Recommended test CDs:
Stereophile Test CD 3 (Stereophile) – available at www.stereophile.com
My Disc (Sheffield Labs) – available at internet mailorders
Nordost System Set-Up Disc (Nordost) – available at Nordost authorized dealers
Rebecca Pidgeon – The Raven (Chesky) – available at internet mailorders
Related articles:
Bob Katz: How to Accurately Set Up a Subwoofer
Velodyne SPL-800 Ultra - Review
Bowers & Wilkins DB-1 - Review
Audiodrom © 2010 MJ
