Best Practice - Getting Great Bass
Probably the most difficult thing to achieve in private cinema design is a consistent experience of fast, deep, detailed bass across all seating positions. Whilst much of the conversation on The Internet around getting good bass centres around which brand of subwoofer is the best, the end result is dominated by the room, its acoustics, how and where the sub(s) are mounted, room EQ and calibration, and where you sit your listeners.
Some people spend a lifetime studying bass in small rooms. It’s a HUGE PHD level subject that I can’t hope to even touch upon in a short article. Here, however, are six things to think about for delivering great bass in a small room.
Sound Pressure Level Matters.
When a subwoofer manufacturer specifies the useful bandwidth of a subwoofer (usually with -3db or -6db) points, they rarely specify at what sound pressure level this is at. You can’t get away from the physics of reproducing low frequencies which says that to produce the same sound pressure level (SPL) at half the frequency (i.e. 80hz and 40Hz) you need to move four times the amount of air. Air movement is a combination of driver surface area, and driver piston excursion. This can also be augmented with different bass loading systems such as reflex or transmission line, but the principle remains.
So, when that 10” subwoofer specs sheet say that its -3db point is 18Hz, question at what SPL that is because I guarantee you that it will not be able to reproduce the same SPL at 18Hz as it does at 50Hz.
There is no substitute for size when it comes to subs. Some manufacturers are now making ‘Infra Subs’ specifically designed to take over the handling of ultra low frequencies below around 40Hz. These can have drivers of 32” and above. Due to this massive surface area, and high piston excursion, these subs are designed to deliver infra-bass at usefully high SPLs. If you are using an infra-sub to handle ultra low frequencies, ensure that your processor has the required bass management functionality to do this.
The Ascendo SMSG32 is a 32” subwoofer designed for high SPLs at ultra low bass frequencies.
Two is better than one. Four is better than two.
Every high performance room using subwoofer(s) should have at least two. The reasons are -
Multiple subwoofers combine to deliver more SPL and thus, as per the above conversation, will allow the system to reproduce ultra low frequencies at the required SPLs.
There are specific recommended locations for positioning of subwoofers that will help smooth a room’s modal response. Picture courtesy of Harman and taken from the below referenced paper.
Multiple subwoofers correctly positioned will deliver more even bass across the seating area. This is critical as although bass can and should be EQ’d, EQ works best when the physics of the room already delivers relatively even bass across all seats; EQ for the prime viewing position will then also work to optimise all seating positions. The placement, bass management and EQ of multiple subs is a complex subject. This is a GREAT introduction to the subject by Todd Welti of Harman who’s research on the subject is some of the best - https://www.harman.com/documents/multsubs_0.pdf
Ideally, subwoofers should be baffle wall mounted
SBIR (Speaker Boundary Interference Response) causes comb filtering due to sound emanating from the speaker as an omnidirectional monopole, hitting a (back) wall then bouncing back and combining with the direct sound from the speaker. At the frequency = to a wavelength 4 X the distance from speaker to wall, you will see a big dip in the frequency response due to cancellation effects. This can be mostly eliminated by mounting the sub in a baffle wall. This type of mounting also has the advantage of increasing the efficiency of the speaker by 3db. And don’t think that comb filtering produced by SBIR can be ‘EQ’d out’. It can’t.
The performance benefits of baffle wall mounting speakers an subwoofers has been common practice in the highest performing commercial cinemas for decades.
Don’t put subwoofers in resonant chambers
The space around a subwoofer mounted into a cabinet or soffit must be highly damped by filling it with acoustically absorbent material. Failure to do this will create a resonant chamber around the sub or speaker, the result of which will be impossible to correct for using EQ. If you mount a subwoofer into furniture, try and engineer it so the sub is sitting on the floor and not touching the cabinet. Fill any space between the sub(s) and cabinet with acoustically absorbing material.
To get better bass, absorb it
It may sound counter-intuitive that if you absorb bass, you can potentially get more of it. This phenomenon is caused by room modes. These are caused by sound reflecting off opposite (Axial modes) or adjacent (tangential modes) surfaces and exciting resonant frequencies. So, if you absorb much of this bass energy before it has a chance to reflect back and cause the room mode, you will greatly lessen the peaks (positive interference causing certain frequencies at certain seating positions to sound bloated and boomy) and nulls (destructive interference causing certain frequencies at certain seating positions to become almost inaudible) of the room’s bass frequency response.
There are many strategies to absorb bass. Many manufacturers have dedicated ‘Bass Traps’ for this. At HTE we build our walls in a way that effectively, all four walls are working as massive broadband bass traps. Some clever cinema designers turn seating risers, and the space above false ceilings into bass traps. The ‘room treatments of life’ such as lossy walls (any wall that has give in it will absorb bass. The issue is that this is extremely difficult to predict and when the wall ‘springs back’, this can feed more unpredictable bass energy back int the room), chairs and furniture will all absorb bass energy to some degree though this is difficult to predict. If the bass energy is left unabsorbed in a room, it just bounces around augmenting room modes, and making the bass sound uncontrolled and ‘slow’. No amount of any electronic eq or optimisation can solve this, though it can absolutely help. More on that below.
HTE’s Acoustic Interior Design™ system turns every wall into a broadband bass trap. Whatever method you use, absorbing broadband bass is key to smoothing a room’s modal response and achieving a similar listening experience at all seats.
Every room needs some EQ at bass frequencies
When using subwoofers, correct setup and EQ is absolutely essential. Setup includes configuring bass management and crossover frequency/slope, and setting delays between groups of subwoofers.
Calibration on the other hand is a deep subject. Many AV receivers and processors have one button automatic EQ functionality. The more advanced ones giving the user far more control over what the EQ is doing. A professional calibrator will do far more than just set up a microphone and let the AV processor handle everything themselves. They will take multiple different measurements of the room to first understand what is going on before then coming up with and executing a calibration strategy. To properly calibrate a 4-subwoofer setup using an advanced room EQ system should take at least a full day, with measurements taken and recorded at every step to then compare the before and after.
A great place to begin your learning is to take a course from The Home Acoustics Alliance - HAA
The best processors, like the Trinnov Altitude shown here, have amazing active crossover and bass management functionality that enables you to setup and calibrate the system with a very fine degree of precision.
Of course great bass needs great speakers and great subwoofers, but it also needs correct subwoofer(s) specification, correct subwoofer placement and mounting, correct listener positioning, great acoustical design and professional calibration. If you think great bass is easy, you’ve probably never heard great bass (-: