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Subwoofers - do you really understand what you are doing?

A post by Mike Leader
Tuesday May 25, 2010
Mike Leader of Leader Cinema systems, manufacturers of world class large-scale systems for both music industry and motion picture industry professionals, tells you more about subwoofers than you thought you could possibly want to know...
Subwoofers - do you really understand what you are doing?

by Emmy Award Winner Michael K. Leader, President, Leader Cinema Systems, Inc

Ex VP Studio Systems Group Electro-Voice, Inc

© Leader D-Cinema Systems, Inc, 2010. All rights reserved.

It is most unfortunate that many of today’s mixers and engineers, music producers and performers have never experienced a truly superlative monitoring system in an exceptionally well designed control room. This is part of the problem with much of today’s music recording. I speak from a few decades of experience in the engineering of very large scale monitoring and presentation systems for both multi-channel music to picture, Digital Cinema and Alternate Content, Tracking and music playback. I am none too sure either that today’s monitoring systems are anywhere near the level they need to be either. And the truth of the matter is, with the proliferation of compact monitors Yamaha et al, in the budget price range, attempting to purchase a so-called accurate monitor has the same number of subjective headaches as purchasing an audiophile speaker system at any price range for their homes. I personally find it laughable across both markets the nonsense talked about in terms of accuracy and uncolored sound. If this really is the case, why do all the speakers in a similar size and price range sound sooooo different? Think……

The ridiculous concept of running a single “sub-woofer” to extend the limited acoustical power performance from baby sized monitors is preposterous. Sure it’s done. But do you really understand what you are doing?

The greatest fallacy foisted upon us from the movie industry, is that low frequencies are directionless, as the wavelengths are so long that we do not lock on to a direction. This is only somewhat true for sustained energy….traffic rumble, low frequency filtered pink noise, single frequency sine wave..pick a frequency say 30 Hz…even I doubt I would quite know from where the source was located. Should you be fortunate enough to be in a real studio, have someone wind up and do some hard kicks on a drum set. Deliberately place the kick on the left or right channel and low pass the kick say at 70 Hz. Then route the kick to a single “sub-woofer”…you will immediately realize that the impact of the beater on the skin creates a pulse which has direction. Same for electric bass, string bass and symphony orchestras and your deep synth runs.

What does one require in a full range monitor system:

SOFFIT MOUNTED Systems are a must. If the system is not soffit mounted, the random energy rolling around behind the box, off to the sides, and top to bottom becomes delayed energy arriving behind the first wave front has passed your ears. Boomy bass lacking definition and source location are all major issues with non mounted monitor systems. In addition, SOFFIT mounting smoothes out the response, provides you with free 5 to 6 db of low frequency gain, and a degree of bass smoothness that can not be matched through equalization..no matter who made it or how many digital bits and sampling it might have. If you have monitor speakers with the vent in the rear of the enclosure…you are out of luck.

At a minimum, the low frequency section needs to reach down to 25 Hz..flat in power bandwidth. This is not quite the same as frequency response. Well it is frequency response…but with attitude…it means that this box as specified “should” get down to 25 Hz as listed on the data sheet. The attitude is, can you do this at its long term average or continuous power handling….I’ll bet you can’t, and this is why you cant hear the real depth and power of the bass. Be prepared to spend real money. A 10 inch 200 watt 2 cu ft box is a toy, as you will see later.

Power Compression ideally should be zero. All woofers, even the 18 inch, well designed woofers by EV and JBL in the 1000 watt 4 inch voice coil range will suffer from power compression if you pound them with more than its long term average power rating. Power Compression is due to heating of the voice coil. As the heat builds up, so does the resistance of the voice coil. The higher the resistance, the less power the woofer draws. You turn it up…and it begins to sound lousy..and after a few minutes you are aware of a very strange expensive smell, as a unique shade of blue smoke emerges from the box. So, if your have a baby “sub-woofer” its overall power handling with be quite limited. A cone woofer’s reference efficiency will not be much greater than the 3% range. So, with lets say 100 watts power handling for a 10 or 12 inch woofer, @ 3% efficiency (on the high side) the output in Acoustical Watts will be 3 acoustic watts. A popular 8 inch 2 way, American made good monitor typically found in broadcast studios has a ½ space reference efficiency of 00.6% with long term power handling of 30 Watts the system will play at 107 dB SPL above 80 Hz. Its output in Acoustic Watts is 00.18 watts. More on this follows, but power compression is a significant issue.

Stereo 2 channel low frequency energy. This is forgotten, that there is significant “movement” in music below 50 Hz region. In live music, in this case lets take classical, the row of double basses sawing away creates energy that both reaches the ears of the listeners directly along with highly desirable reflected and delayed sound emerging from the side and rear walls of the stage area. This energy moves across the stage, and given the recording technique, this very low frequency content is there. It simulates sustained low frequency energy. But, if you have any doubt, mono the two channels during a session and experience the room collapse. Have you heard the term about control rooms “breathing”? There you have it. It IS a part of the performance. It is a part of the art form of music. And it is a critical element missing in over hyped consumer speakers at all price range…and even more so in studio systems as we will see.

It is fascinating to listen to sound effects libraries, recorded either in two channel stereo or Ambisonic. Out door atmosphere is dramatic in two channel…if! Your systems can get down..way down to below 30 Hz. Even a gentle wind will create movement. Listen to the deep growl prior to a thunder clap using two high performance systems that can get down. The switch to mono…either sum both channels…or mute one. The difference is like listening to a 1970’s car radio.

Matching your 5-inch, 8-inch 10-inch whatever number of inches you have (!) compact monitor to a sub woofer is a hugely challenging experience, as there is a massive difference in the output capabilities…er.. that's right measured in Acoustic Watts. So, your 5 inch..ok 8 inch box, for the most part has no energy in the region where you need to augment its output. So, even with our simple example above with the 3% efficient cone woofer, the disproportionate acoustical power is the reason that there is such controversy in matching a woofer to a compact monitor. No Amount of time delay or parametric EQ will fix the miss-match. In addition is is highly unlikely your acoustical space is good, and the monitor system is not Soffit Mounted. The best you can do, is to make sure the system is at least in phase. If you like and have the experience, you can sweep very slowly with a sine wave generator, or use pink noise with a good RTA. Watch for holes in the transition as you are crossing over between the two. If augmenting and the compact monitor is running full range, you run the risk of bottoming the woofer in the compact monitor. Then find a good solo recording of a cello.. hopefully you will know what a live cello sounds like. Cellos are difficult to maintain the power balance in studio systems…the great systems can do it seamlessly….try it…. You might be surprised. You can also use a keyboard….run some scales…and also run scales in different keys…you will be most surprised with the shift in density.

You will experience a benefit with the small compact systems even with a baby sized woofer extension in each channel. If you do apply an active crossover between the compact full range monitor, set the crossover about ½ octave above the specified 3 dB down point. This will insure that the full range system will not be stressed with low frequencies below the tuning frequency of the full range system. Once you have everything balanced, the improvement in clarity should be evident in the mid band. This is due to removing low frequency energy and at high levels, the passive crossover network in many passive systems can have the coils saturated if the coils are not air core type. In any event, the removal of low frequency content from the woofer in the full range monitor, will improve the sound quality.

I am amazed how, that so many project studio owners have the need to run out and purchase Neumann, AKG, Sanken, Schoeps and other exotic microphones costing thousands of dollars. You have a big or small session. Now lets see.. a few expensive microphones….what $ 10,000, maybe a few instruments.. a Strad or two, maybe and Amati and Guarneri…so maybe $ 30 million of instruments in the trio. Or your session has a signed original Les Paul or other million dollar treasure…along with the group…say a modest $ 2 million of instruments. The mics are in place….the record button is pressed… ok you hard drive has been recording everything as your terabyte of space only costs pennies in comparison to 10 inch reels of tape. The down beat…..the music…the passion..the artistry of music..the professionalism…..and you are monitoring it on speakers that cost less than your array of microphones.

Low frequency distortion. You would not like to see the distortion specs for any ones low frequency system. 15 inch, 18 inch 24 inch, the really really good stuff, no you still do not want to see the distortion specs. But do not worry. The human hearing system being much less sensitive at low frequencies (Fletcher Munson) masks the annoyance of distortion curves that do not look pretty. Keeping the low frequency section large, will provide you with greater sensitivity. If you are fortunate to have a dual 15 inch or dual 18 inch system tuned to 26 Hz or so, for each channel and soffit mounted, you will truly experience another dimension, and one that all of the exotic makes of snake-oil price range speakers have never experienced. Two woofers in an enclosure couple and sound superior to a single driver. The next challenge is in finding top flight amplifiers to provide the control with high damping factor. I use American made Crown amplifiers in our systems, with DF > 3000:1 I also provide amplifiers with more than 6dB of headroom over the woofers Long term power handling. So, a 800 watt LTA woofer will have 3200 watts peak capability per cone, as I apply a single amplifier channel per cone.

X Max and X Max Limit. This is a very long story. X Max is the specification that produces 3% distortion. It is safe to operate a woofer to X max. depending on the voice coil and heat sinking, I would not recommend operating continuously at X max. We design our systems for the max level we require at 10 dB below X max. Remember, that for every increase of 3 dB you are doubling the amplifier power. This becomes a very costly and demanding design feature if you want to have your system to operate at military levels of mission critical performance. Ok..so you want to lean on it and play BlackHawk Down at realistic in chopper levels…no problemo. The music will sound effortless and musical too. X Max Lim..is the danger zone. Lim is the limit at which expensive mechanical damage will occur…followed by that annoying expensive smell.

How will you know how great your low frequency performance is? A few ways. Find a classical recording made in a church or auditorium. A budget recording will possibly do. If you can feel or hear the air conditioning system, at a very low level, your system certainly is reaching down there. Possibly the same recording, if done in a church, will exhibit low level rumble from traffic outside and or a combination of traffic and HVAC…this is hi-fidelity! Obviously play music…the extension should reach deep easily to 35 Hz..flat…without any silly humps between 60Hz to 120 Hz, if anything slightly notch 70Hz to 110Hz due to room combining.

Last test, and I am serious. The motion picture Master and Commander. The first battle sequence between the British and French ships. If you are unable to hear a difference in pitch and impact between the French and British cannons, you do not have an accurate woofer system at all. The Walt Disney Post Production sound department first experienced this phenomenon on one of our Leader Hollywood Format™ 4 way very large format systems in the Aidikoff Screening Room in Beverly Hills California. There is a difference…what a test…and try playing it at 120 dB SPL…well we can…bring two check books. But it is possible.

Finally, you might be wondering who can hear down this low and deep…. Prince’s studio Paisley Park Minneapolis, the control room is equipped with an astonishing system (not ours) by Westlake Audio SM-1 5 way..each enclosure is equipped with dual 18-inch woofers….Output in acoustic watts we calculate at 58 Acoustic Watts per enclosure at low frequencies.. Kinoshita Monitors were specified by Tom Hidley for Bop Studios South Africa. System reaches down to 9 Hz…for this you will require three check books www.reyaudio.com Our clients have it as well….we have a passion, so visit us at www.leadercinema.com We manufacture world class large scale systems for both music industry and motion picture industry professionals.

Good luck in your search for “sub-woofers” for your compact monitors….but first visit the drug store for a bottle of aspirins.

Musically Yours,

Michael Leader,

© Leader Cinema Systems, Inc 2010. All rights reserved.

A post by Mike Leader
Tuesday May 25, 2010 ARCHIVE
David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)