|Max dB with Doug Blackburn
Back Issue Article
This month Im going to drop on you most of the "facts of bass" Ive managed to collect over the years from manufacturers, books and personal experience. Some of the things you read here may contradict what youve read in other magazines. I wish it were just as simple as telling you the information youll see here is right and the other stuff youve read is wrong. Unfortunately, I cant prove to you what Ill tell you is true unless you read the same books, experience the same products, and speak in earnest with the same manufacturers as I have. One thing I can promise you, I am not going to make up anything you read here. Its all as true as I can make it based on my study and experience. Some of the information will be brief factoids, other subjects will take some detailed explanation. The article isnt going to read as an organic whole due to the nature of the subject. So be ready to jump around a bit as I discuss some of the realities and misconceptions about bass and how to get it in your system.
The nature of bass
As far as equipment goes, good bass is expensive. There is no quick and inexpensive way to get good bass. Musically, the bass octaves are undoubtedly the most expensive octaves in terms of equipment costs. The bass octaves are: deep bass -- 20Hz to 40Hz; bass -- 40Hz to 80Hz; and upper bass -- 80Hz to 160Hz. These may not match up with the octaves on an organ or piano keyboard, but you can arbitrarily make an octave be whatever you want it to be for purposes of discussion. Pick any number and double it and you have an octave for purposes of discussion. The octaves on musical instruments are based on musical notes which may have frequencies like 42.3Hz, unnecessary complication for purposes of this months discussion of bass.
Sound travels though air as waves. The length of the wave gets longer as the frequency decreases. How long are the wavelengths? Hard question to answer. The length of the wave of any particular audio frequency varies with air density/pressure -- just as the speed of sound varies with atmospheric conditions and altitude. In fact, it is because the speed of sound varies significantly over time that the wavelengths of audio frequencies in air varies. Lets say the speed of sound at your altitude and current atmospheric pressure happens to be 1150 feet per second. To calculate the length of a 100Hz sound wave, you want the answer to come out in feet per cycle, so make 1150 the numerator and the divisor will be 100(Hz). You get 11.5 feet for the length of a 100Hz sound wave. Tomorrow the speed of sound for you might be 1100 feet per second, which will make 100Hz sound waves 11 feet long. Does this make a difference to the sound of your system? You bet it does! Ever wonder why you like the sound of your system more on some days than on other days? You think it is related to temperature? Sure, it could be. Think it is related to humidity? Yes again, that could do it. And electricity? Yes, another source of time-to-time differences. But Ill bet not all that many audiophiles have considered how day-to-day changes in the speed of sound affects the sound of their systems. Wavelengths change and the acoustics of the room will change a bit with the wavelengths. Ill even go so far as to speculate that setting up a system to sound good at 5000 feet elevation is different than setting up a system to sound good at 500 feet because of the significant change in the speed of sound between those two elevations -- assuming the same room and speakers in both locations, of course.
So we know 100Hz sound waves are going to be in the range of 11 to 12 feet long or so. How about 50Hz? Double the length -- 22 to 24 feet. This means that if one dimension of your room is 23 feet, on some days youll be able to contain a fully developed 50Hz wave within the room and on other days you wont! Pretty interesting, eh? Im not saying there will be a huge qualitative or quantitative difference in the sound of 50Hz in that room on different days, but it is inevitable that some sonic difference will arise from the change in wavelengths from day to day.
Twenty-five Hertz waves in air are twice as long as 50Hz -- 44 to 48 feet. And 20Hz, the lowest frequency were supposed to be able to hear, would clock in around 55 to 59 feet long. Do you need a room with at least one 60-foot dimension in it to hear a real 20Hz in the room? No. Your ears actually pick up sound in a different way, reacting to the compressions and rarefactions that happen in the air as the sound propagates though the room. Twenty Hertz creates 20 compressions and rarefactions per second and your ear will pick that up even if you are listening to headphones that respond to frequencies that low. Otherwise your ear canal would have to be 60 feet long -- we would look rather odd if our heads were 60 feet wide.
If you can still hear 20Hz even though your room is too small to contain a 20Hz sound wave, why does it matter that the 20Hz sound wave is almost 60 feet long? For two reasons. The wavelength defines the size of objects which are capable of acting on a 20Hz sound wave in a meaningful way (like 5dB or 10dB or more). Those objects really need to be at least 1/2 the length of the frequency they are trying to control in order to have more than a very small effect on the bass frequency in question. This has to do with the physics of waves. What this means is that you cant make a passive device the size of a softball or shoebox that would profoundly and directly affect a bass frequency. The softball-size or shoebox-size device may have some collateral room-tuning effect that could be interpreted as having some control over the bass frequency. But the degree of the control will be quite a bit below the 5dB, 10dB or more that a purpose-designed and sufficiently large device will have. This is an important concept. Smaller items like Tube Traps, Pressure Zone Controllers, on-wall tuning panels or other products that touch a wall or floor or ceiling in the right way at the right position can alter the resonant property of the wall or other surface in a way that may have some smaller (yet still perhaps useful) effect on low frequencies. But to act on a low frequency (for purposes of this discussion, lets assume 100Hz and lower) in a profound way (5dB, 10dB or more), that strongly amplifies or damps the wave, you need devices that are 1/2 the wavelength or larger. Around six feet for 100Hz and around 28 feet for 20Hz. This physical law is the reason horn-loaded loudspeakers have to be physically huge to reproduce bass and deep bass octaves; a horn would have to be at least 1/2 the length of the lowest frequency in order to reproduce that frequency. A horn good down to 20Hz would have to be nearly 30 feet long. Likewise, a bass trap that would have a significant impact on a 20Hz sound wave would have to be almost 30 feet long. But there would still be some compromise. Making the horn or bass trap the full length of the sound wave would further increase control over the bass frequency.
The second reason that the length of bass sound waves is important is that below a certain frequency (room dependent), where you place the subwoofer in the room really makes little difference in bass balance within the room. This is very different from what youve read in other publications for years. Loudspeaker placement is still important because the loudspeaker produces frequencies that are much higher with shorter wavelengths than subwoofers. So loudspeaker placement is not really governed by this "rule." But with subwoofers -- lets say your room is 13 feet wide and 17 feet long with an eight-foot-high ceiling. If the speed of sound is 1150 feet per second, the width of your room can support wavelengths down to 88.4Hz, the length can support wavelengths down to 67.6Hz and the height can support wavelengths down to 143.75Hz. If your subwoofer crossover point is 60Hz, almost everything coming out of the subwoofer will be larger in wavelength than any dimension in your room. You can put the subs in the center of the room, or just about anywhere in the room with little or no qualitative difference in the sound of the bass. One thing though, as the subwoofer approaches a two-sided or three-sided corner, the wall/floor/ceiling surfaces will "horn load" the subwoofer and the bass may sound a bit louder near a two-surface or three-surface corner. But the quality wont change much and the balance wont change much. The higher the crossover frequency for the subwoofer, the more concern there may be for where the subwoofer is located. So you can use the walls and corners of the room to "lift" the subwoofers bass energy in the room a little if you need it.
One subwoofer or two?
Generally, the smaller the main loudspeakers, the higher the crossover point needs to be. The higher the crossover point, the more you need two subwoofers. Any time a significant portion of the subwoofers operating range goes over 100Hz, you really want to seriously consider two subwoofers. Use this rule of thumb to decide if you really need two subwoofers for imaging reasons: Double the subwoofers crossover point, and if that number is higher than 100Hz, an argument can be made that two subwoofers will be better for stereo imaging than a single subwoofer. If the doubled crossover point number is higher than 100Hz and you can only deal with having one subwoofer for space or budget reasons, that single subwoofer should unquestionably be placed in the center of the two main loudspeakers for imaging purposes. Placing a single subwoofer off to one side when it will be reproducing any sounds above 100Hz will pull the image to the side. Not many subwoofers crossover as low as 50Hz or lower. This implies that not many subwoofers are going to perform optimally when you only use one. You can use one subwoofer, just know that there will be a compromise as compared to using two subs.
Home theater is an exception. In a dedicated home theater, reasonably full-range main speakers and a single subwoofer can be quite adequate if the subwoofer is connected to the LFE (low-frequency effects) channel of a surround processor. This setup will do nothing for music reproduction though. If your goal is integrating a music system and a home-theater system, read on.
Here are the "rules." Feel free to break them for any number of legitimate reasons, but understand that by doing so, a compromise of some sort and some magnitude will be introduced. You may find the consequence inaudible or inconsequential. That doesnt mean it is unimportant or incorrect; it just means that in your system, it wasnt a significant factor.
dBs rules for great bass in a combined music and home-theater system
Special home-theater considerations
LFE subwoofers should not have a crossover per se. They should have a natural and/or cabinet-assisted roll-off on the top end, not an electronic filter to roll off the top-end response. The LFE output of a surround processor limits the frequencies delivered to the subwoofer. So not having a crossover means fewer parts in the signal path and better sound.
Some surround processors dont deal with bass correctly. If you arent using an LFE subwoofer, the LFE bass disappears from some surround decoders. If you are using full-range main speakers or main speakers with subwoofers, you may not be getting all the bass on the soundtrack if the LFE bass is not redistributed at the proper level to the main left and right channels. Theres no easy way to identify this in advance. Perhaps the Video Essentials DVD includes an LFE-only track that would easily reveal whether a decoder was properly redirecting LFE bass to the main speakers when there was no LFE sub connected. I have not yet seen the Video Essentials DVD, so I cant confirm that it has this capability.
Five-point-one-channel processors add additional crossover "poles" when you select "small" loudspeakers for any of the speakers in the system, and this may actually make the speakers sound worse than calling them "large" speakers so that the processor does not add another crossover "pole." This is true for main, center and surround speakers. Unless they are VERY small speakers, you may get better sound by calling the speakers "large" even if the speakers are only modest in size.
Subwoofers designed primarily for home theater will typically suck for music. Dont spend a lot of time and money trying to force a home-theater subwoofer to sound good playing music. You just wont be able to make it happen. Youll get bass and you may even find it reasonably entertaining. But an LFE sub will rarely sound as good as a "music sub" when playing music.
Thats about it for bass this month -- hope you found the information useful. See you next month.
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