Article by Jeff Hedback 

It is very likely that low frequency issues cause the greatest loss of profit to studio owners.  Mixes that travel too thick or too thin, mixes that are stamped final hoping the mastering house can fix and re-mixes due to poor low frequency balances are common occurrences in far too many commercial studios.  If you accept the hard earned money from your clients to produce a professional product, then you are accountable for the low frequency performance of your studio spaces.  It is possible to break down the aspects affecting the low frequency response of your space and gain control (at least a detailed awareness). So, roll up your sleeves, put aside thinking about plug-ins and sample rates for a moment and take a look at your room's bottom end.

There are three main areas to begin with: structure, treatment, and speaker/ear location. 

Structure: this means how are spaces constructed?  Also, what are the dimensions including Length, Width and Height.

• The construction of your space actually has a great bearing on the low frequency response.  Lightweight partition walls of 2x4 studs with single layers of gypsum board (drywalls) and no insulation will allow a great deal of the low frequency energy to pass through the structure.  Denser concrete block walls will reflect more of the energy below 200 Hz, but not in a linear manner that is relevant to detailed audio production.  It is no accident that the top commercial studios are not built like residential homes.  They are constructed with mass-layered structure, non-resonant framing with "dead" air gaps between partitions, and are built to exact standards.
• Dimensions relate to room modes.  The Length, Width and Height actually are your instrument, like the body of an acoustic guitar.  What we are really concerned with are resonant modes or how your room will resonate (ring-on) when excited by a source (your speakers for example).  Also, we are concerned about peaks and dips at various positions of the room?  Most everyone has heard such frequency response changes within a room.  Why does this happen?  Between two parallel surfaces a sound wave must be able to develop more than half its length to reduce resonance.  41.2 Hz is the low E of a bass guitar.  Its wavelength is 27.4'.  If you room does not have a dimension of at least 13.7' then this frequency is likely to have pronounced peak at the midpoint of the room and pronounces dips at the ¼ and ¾ marks (depends on actual dimensions).  Now…this gets complicated as rooms are actually in three planes (X,Y,Z) and so are the modes.  It gets further complicated when gear is added to the space.

There are three types of modes:

• Axial which reflect between two surfaces, front to back, side to side, floor to ceiling.
• Tangential which reflect off four surfaces: sidewalls, front/back and floor/ceiling, and sidewalls and floor/ceiling.
• Oblique modes reflect off six surfaces and not to of major concern.

Treatments: do you have bass traps?  If so, are they performing properly for your space?

• There are several types of bass traps including broadband, membrane, diaphragmatic and Helmholtz.
• Broadband traps offer even control across the frequency range.  These traps will also control mid and high frequencies well.
• Membrane, diaphragmatic and Helmholtz traps have center frequencies where they are most effective and a "Q" or bandwidth of effectiveness.  These are typically not effective in the mid and high frequency ranges.  Design depending, they can be very effective at the center frequency; or offer less control at the center frequency and greater overall control in the bandwidth.
• Location, location, location is more than a real estate mantra.  This is everything to bass trapping.  The modes mentioned above have pressure points.  Also, our ears are more affected by some modes than others.  So, a specific room needs the right traps placed in the right locations in order to offer its most accurate response.
• Referring back to the modes above, the goal is to control the primary axial modes if at all possible, and then address the tangential modes with corner trapping.

Speaker/Ear location:

• Where you place your speakers and ears will ultimately determine how the structure and treatments benefit your low frequency response.
• You can have a well-designed, constructed and treated space and place your ears in the wrong location and get fooled. 
• You can also "luck" into a perfect speaker and ear location in an otherwise bad space for audio and get accurate mixes.
• The two best analogies may be the tuning of a snare drum and the playing of harmonics on a guitar.  If the shell and the head represent the structure and acoustical treatment of your space, then the lug nuts and how they are tuned represent the location of speakers/ears.  In the case of a guitar, harmonics are related to the dimension of the string, like room modes.  You can play the same pitch at multiple locations on the guitar neck.  Likewise, you can locate your speakers and ears at multiple "correct" locations…as you can see, there are a lot of variables.

Let's look at an example.

Length: 14'-7.25"

Width: 13'-.75"

Height: 8'-1.5"

The graph shows the modal response below 400 Hz.  400 Hz is about the transition zone where sound becomes less directional and more resonant.  The Bonello Distribution is an analysis method derived that shows how the modal density relates to low frequency response.  Without getting into all the details in this article, you want the blue lines to be higher than the red lines in each 1/3 octave band.  This would mean that there is an even distribution of the modes and not too many modes grouped at any one frequency band.  In our example you can see that at 100 Hz the red exceeds the blue.  This typically means that there will be problems at the next 1/3 octave band lower, in this case around 80 Hz.

Looking at the 80 Hz region shows: X= Length, Y= Width, and Z= Height.

The primary axial modes in this space are

So what does this mean and what could be done to improve this:

• These 1's 2's and 0's indicate how strong the mode is.  The lower the number the stronger the mode.  The X,Y and Z show how the mode is traveling in this space.
• The 80 Hz issue will make it tough to mix anything in the Key of E at the very least.  80 Hz is also a magical point for the union of kick drum and bass.  If you are hearing too much or too little due to these modes, your mixes will suffer.
• Sometimes you can locate your speakers and ears to minimize some axial mode issues.  But, this would not be the same for any clients.  You could be a in a position you trust and get clients asking for more of this or less of this due to these types of modes.  The floor to ceiling mode (0,0,1 at 68.9 Hz) can be a real problem to some people depending on their actual ear structure (yet another variable).

What can be done? 

• You could build structure to help.  This may mean an airspace and new partition wall with very specific layers.  This would eat up an already small control space…not a practical solution.
• You can add bass traps.  What type of trap?  To address the axial modes you will need either a super duty broadband trap or better yet one of the "tuned" type traps.  The design and placement must relate to these modes to help.  Properly applied, you can address the axial modes.  You will likely eat up significant space (anywhere from 6"-24" off surfaces).  Honestly, this is a tough goal to achieve.
• Most commercial bass traps that are designed to span across 90 degree corners will attack tangential modes around 80 Hz.  In this specific case you are best to locate traps in the wall ceiling junctures of all surfaces as well as the vertical corners.  This will minimize the two tangential modes shown above.  You still need the vertical corners trapped for good general application practices.

What about speakers and ears?

Here are two graphs.  The black lines show the modal density of our example space.  Note how the lines occur in dark bands and then widely spaced gaps below 100 Hz…this is not good.

The red line shows the frequency response (relative SPL)of two speakers and a listener.  Note: this does not factor acoustical treatments or surface types in the space, just location relative to the L, W and H.  Also, every 6dB reduction in SPL is perceived as a 50% reduction to the listener.

The example #1 places the left speaker near the front wall, about 4' from the left wall and the listener about 5' from the front wall (speakers are placed symmetrically).  The example #2 places the left speaker about 2.5' off front wall and about 3' from the left wall (and 3' from the right wall) while the listener is now 7' deep in the room.  In real world thinking, the first example will sound like what you would expect from Mackie 824's and the second from Yamaha NS-10M's…but we are not changing the speakers, just the location.  This shows how your selection of monitor and where you locate them is critical to producing accurate mixes.  I think it is also important to mention that the speaker responses curves shown in your manual are in free-field anechoic testing spaces.  They are helpful to compare one speaker to another, but you can't expect such flat response in a real room.

Position #1: Speakers against front wall, 4'4" in from walls. Listener approx 5' off front wall

Position #2: Speakers 2.5' off front wall, 3' in from sidewalls.  Listener approx 7' off front wall

BTW- the Pos #1 is going to offer more of a "flat" response after acoustical treatments than Pos #2 on the right.  But, this is a fine balance that needs to work with imaging and midrange detail.  Also, one engineer may prefer one response over another.

So let's pull all of this together.  The bass response of your room is directly related to your profitability.  You should look at the structure (including dimensions), acoustical treatments and the speaker/ear locations to see how your space performs.  Even if you do not understand all of the physics involved above, you are now aware that there are methods available to analyze and improve the low frequency response of most any room.  Start with the easiest part to change, the position of your monitors and ears.  Next look at trapping and then take a critical look at your room itself and judge if it truly meets your needs.  Taking your monitoring environment seriously will make you more money and create better music!  I think that says it all.