KnowHOW: Choosing and placing a subwoofer
KnowHOW: Choosing and placing a subwoofer
Gordon Moore looks at nuances of human hearing and the impact our hearing has on the placement of subwoofers
The human ear is a funny thing. It has incredible sensitivity to rather high-pitched sounds that would threaten our existence (such as a snapping twig, rustling leaves, things sneaking up on us) from which we can run away or sounds that enhance or insure our lives (such as a baby crying, the sizzle of meat cooking). Yet our ears do not detect as well the low-frequency sounds that could also threaten us but may be more difficult to escape (earthquakes or thunder). In fact, at low levels, the human ear has a considerably lower detection of low frequencies than the mids and highs.
In Figure 1, the threshold of hearing (perceived loudness) across the range of frequencies shows you that we are most sensitive at 3.5kHz to 4kHz but, to be able to say that we hear the 62.5Hz signal at the same perceived loudness, we need an additional 35dB of level (over 2,000 times louder).
Additionally, the lower frequencies, due to their long wavelengths, are also very difficult to localise – our ears are too close together for accurate phase and spatial detection. Said simply, we can’t tell where a low-frequency signal is coming from very easily.
So, humans cannot easily hear lower frequencies and we cannot tell from which direction the sound is coming.
Yet, we enjoy hearing a deep bass note. Our entire cultural history dates back thousands of years where we used large drums for communication (low frequencies propagate very well over long distances), for ceremonial music (the drum possibly being the first instrument) and for entertainment. We love our bass.
Our love of bass, and our seeming inability to properly hear it, seem diametrically opposed. And that brings us to this column’s topic: the selection and placement of subwoofers.
Most PA speakers are not designed to optimise your bass response. They are designed to deliver the mids and highs efficiently and in a well-balanced manner. Low frequencies require more specialised drivers and amplifiers to deliver optimal performance. In the circuit designs, impedances get generally lower as frequencies drop, so making the amplifier specifically for subwoofer use is a preferred route.
Hence the rise of the subwoofer. In this article, we will be using the term subwoofer to describe a complete speaker system (driver, cabinet, venting, etc.) – the finished product. The discussion about building a subwoofer assembly from scratch entails a greater number of additional factors that would take far more space than this column can possibly offer.
The subwoofer is a speaker designed specifically for reproducing the low frequencies – the bass part of the audible spectrum. In the past, subwoofers were generally very large diaphragm speakers with large magnets. Today, modern materials and magnet metallurgy have allowed the subwoofer to be a smaller diaphragm unit with long excursion (range of motion) capabilities that allow the longer, slower stroke required to reproduce the sound. High volume levels, however, still require large surface areas. In some subwoofer designs, the surface area is made up of many smaller transducers where the more traditional speaker will have a large single diaphragm driver.
Regardless of physical design, the choice will be subject to a series of parameters.
Where do you want to place your speaker?
Flown (hung in the air via cables) with the line array – adding a subwoofer to a flown line array is generally simple. Nearly all flown line array systems offer a compatible subwoofer cabinet designed specifically for the array. Both from a visual point of view and a compatibility point of view, it is advisable to stay within the same family of products. If you try hanging a sub made by Brand X and a linear array by Brand Y, the probability is a mismatch of hardware, fly points and appearance. Keep the combination within a single brand.
Properly hanging a subwoofer/line array allows careful alignment of coverage patterns and lends to better safety practices. Remember, hanging speakers is not for the untrained installers – it is not a do-it-yourself project. This type of installation suspends literally tens to hundreds of kilos of deadweight at height which, if it fails, can quite literally kill anyone under it. Use licensed and insured installers to hang speaker clusters. Properly installed, they will need little maintenance and will avoid the buzzing and rattling of nearby objects. After installation, run a long sweep tone, from 30Hz to 20kHz, and listen for any buzzing or rattling. If you hear any ‘buzziness’, first make certain you are not overdriving the system at any point along the gain staging. I was involved in an installation where there was a severe buzz from the hanging cluster which was 8m in the air. After renting a lift and several hours aloft, we found no problem with the speakers. It turned out that the input trim for the test tones was too ‘hot’ and the input was distorting just a little bit – it sounded like a mechanical rattle. Had we listened to the mixer via headphones, we would have discovered the error early on and avoided the lift rental and time in the air. If your signal is clean and you still hear a buzz during the sweep tone, then carefully examine all the hanging hardware to make certain it is not rattling. I found a chain suspended speaker once that had a loose link. Taping the link with gaffer tape stopped the aggravating noise.
Freestanding – these units allow you more flexibility in the selection of your subwoofer. Remember that very low frequencies are difficult to localise by the human ear. This human drawback can be used to your advantage in your system design. You can select a pattern, a cabinet size and place the speaker in any position you wish. If the subwoofer is most conveniently set off to one side of the sanctuary, remember that most of the listeners will have no perception of the direction of those low-frequency sources. Indeed, all localisation of the signal will be based on the mid- and upper frequencies. If you are installing an L-R or L-C-R system, placement of your mid- and upper-range drivers are more important than the sub. You have much greater leeway in the placement of the subwoofer.
On the same plane as the main speakers – whether on the floor or hanging, keeping the speaker on the same plane as the main speakers will help keep the signal in phase.
Wall-mounted – these subwoofers gain a boundary effect, so energy that would be directed to the back of the speaker will be reflected forwards and effect a small increase in level.
2-way corner – placing the subwoofer in a 2-way corner (against the wall and floor or the wall and ceiling) offers additional boost to the perceived levels of the speaker.
3-way corner – this can either be a wall, ceiling and other wall, or wall, floor and other wall, and can offer an additional boost to the level of the subwoofer.
What is the coverage pattern of the subwoofer? Once you have decided where you will place your speaker, then you can look at the pattern. Remember that subwoofers tend to have a very broad area of coverage due to the long wavelengths. Generally speaking, selecting a very broad pattern suits most installations. If you need more selective coverage, there are line array choices that can give you better pattern control.
If you are not flying a subwoofer cabinet as a part of a line array or speaker cluster, try experimenting with the cabinet in different locations throughout the room. Room modes are frequencies that will develop standing waves – spots in the room where the frequency will be either very audible (much louder) or inaudible (a dead spot). They are determined by the room dimensions as a function of the length, width and height of the room. The shape will also affect it. By placing the speaker in different locations, you will notice a shift in the placement of those hot/dead spots. Select the position that gives you the best possible pattern (dead spots in the main aisle, for example, instead of in the seating area). Different frequencies will have different room modes. Experiment before committing to a particular location. You may find the best location might also require some delay dialed in on the signal processor to keep everything in phase. Keep the speaker, whenever possible, in the same vertical plane as the main speakers.
Remember that the human ear requires a much greater level of volume for bass frequencies than the higher registers. That is why the typical home 5.1 surround sound system can have excellent loudness with only 25W to each of the surround speakers but need 500W for the subwoofer. For a house of worship, you will find the subwoofer requires much greater power than the remainder of your house system.
Sensitivity is the rating that indicates the level of volume your subwoofer will deliver at 1W (a higher number is more sensitive and can create more sound). Every 3dB difference means changing the amplifier power by double or half. For every +3dB (better sensitivity), the amplifier can be half as powerful. For every –3dB (less sensitivity), the amplifier must be twice as powerful.
If you are buying a powered speaker, then the specifications will only include maximum levels since the amplifier is integrated and already matched up.
Powered versus passive?
A self-powered subwoofer will have the amplifier built into the cabinet. The design engineers will have taken the time and calculations into their hands to deliver to you a matched system. You don’t have to answer the hard questions – did I buy a large enough amplifier? Will it deliver enough level?
A passive enclosure will have only the speaker itself – you must specify and provide the amplifier. Your speaker manufacturer can often provide you with guidance in selecting a good amplifier based on your installation and program needs.
Remember, it is easier to burn out a speaker with an inadequate amplifier than it is with an amplifier that is rated for more than the speaker can handle. It is better to err on the side of ‘too much’ rather than ‘too little’. ‘Just right’, of course, is always the best answer.
Enjoy your newfound deep base response. A subwoofer is often one of the more economical improvements one can bring to a house of worship audio experience.