Building Great Bass Response In Your Home Theater

If you are searching for the utmost listening experience in your room with your system it is time you consider how your space is constructed. It is the purpose of this article to show you how the construction relates to your audio experience. There is an inverse relation between sound isolation (STC) and sound absorption (NRC). The greater the isolation of a surface the more sound energy is going to remain in that space. This applies directly below 200 Hz where the resonance of room modes is a primary factor of your systems character. It becomes compounded when you add multiple sound sources with low frequency information…you got it, your room.

Definition Time

STC stands for ''Sound Transmission Class.'' This is a single-number rating that can be used to compare the acoustical isolation of materials or partition constructions. The STC number is intended to have use for speech application (500 Hz to 2KHz), not music. You can have a high STC that does not address low frequencies.

TL- Transmission Loss is the resulting difference in sound level when the ''after'' test is subtracted from the sound level ''before'' plugging the opening (at the testing lab). These are taken at 1/3 octave readings.

NRC- Noise Reduction Coefficient is a comparative number showing how absorptive a material is. For a convenient, single-number rating, the Sabine alphas for 250, 500, 1000 and 2000 Hz are averaged and the result is the Noise Reduction Coefficient, or NRC…The higher the number, the more sound is absorbed.

A standard interior wall system of ½” drywall on both sides of a 2x4 non-insulated stud frame has an STC of approximately 33. The TL at 125 Hz is approximately 12 dB. This will result in a substantial amount of sound energy below 200 Hz passing through the structure, paper-thin is a common description. We know this can be a bad thing for your wife and neighbors. But is this good for the sonics of your room or not?

If your goals are a truly high-quality listening experience, the answer is a swift, NO. The energy transfer through the structure is not precise. It has no specific relevance to the modal distribution of your room and it results in a less defined experience from seat to seat. What about concrete block or poured foundation that will reflect more of the energy below 200 Hz back into the room? This is not a great deal better. The room modes are excited stronger, but below 125 Hz the TL becomes increasingly non-linear. This creates more dramatic peaks and dips than drywall partitions that are not easily addressed. Then what about acoustical treatments, well hold on to that thought…

The goal is to build structure for critical audio. The demanding THX reference standard requires partitions to achieve 50dB of attenuation at 63Hz. This is extremely difficult to obtain in a typical residential setting. But the possible benefits to your audio system are as tangible as your speakers themselves.

Think about room surfaces that have mass-layers, trapped air, and are built with exquisite detail…sealed like a bag of chips! Your walls are part of the instrument you call your room. You want the lowest frequencies with the longest wavelengths to be detoured at the boundary. The energy that does pass through the initial boundary to encounter high-density insulation, non-resonant framing, a “dead” air space, and another partition structure (be it concrete or stud/wallboard). This type of construction weakens the low frequency energy resulting in minimal external transmission and minimal return into your space. Back to the instrument analogy, it is the difference between building the soundboard of a violin out of construction grade yellow pine verses an aged piece of spruce.

Highlighting the sonic benefits of such room construction, you can expect:

• A room that expresses modal resonances in a clear and defined manner.
• Yes, you will need room correction via acoustical treatments, active DSP, and proper placement of speakers and listeners.
• However the results of such efforts are exponentially more dramatic and musical than in a typically built space.
• A room with problematic dimensions will improve. A room with good modal distribution based on dimensions may now be a superb listening environment.

What can be done in the face of practical realities of a typical residential space? Let’s consider a space near and dear to Audioholics, Gene DellaSala’s Reference System 1. The floor is the topic. Due to a construction issue, the framing warped. The solution by the builder ended up being in Gene’s best interest. They used HardiBoard rather than OSB subflooring. HardiBoard is a cement based construction material. The density of this material is in the range of 75#/cu ft while OSB has a density in the range of 45#/cu ft. This is a major difference maker to the sonic result of Reference System 1. The RBH LCR’s have massive weight and significant coupling to the floor. Were there to be OSB subflooring instead of the cement board, the room would start with an anemically unpredictable amount of low frequency energy.

Now you see the type of construction (in relative terms) needed for critical audio. The bass waves will bring the yacht to the wall. It will rise with the pressure against the surface and then gently lower when the wave cycle ends. This is how bass waves should behave.

There is no doubt that the detailed integration process, the acoustical treatments and the speakers themselves are a large part of the performance of the space. However the higher density subflooring is helping Gene successfully counter the asymmetrical shape of his space.

What are some available options for retro situations as well as new build:

• Any time you can double the mass of the system it is an effort worth taking. If you already have two layers of drywall, don’t simply place a third layer as it will not yield any benefit in relation to the cost and effort.
• Even a 1” airspace (minimum) and a new partition wall can yield substantial TL increase below 100 Hz. Especially with concrete, such airspace is absolutely needed. The greater the air gap the greater the TL increase. Note: you should not place wallboard materials in the airgap. This can reduce the amount of isolation and create strangely unpredictable resonance.
• Float your floor. Not the installing hardwood flooring type of float…but the isolation device, new joists, insulation, multiple subflooring layers type of float.
• Vary the type and thickness of multiple layers within a system. Don’t add ½” drywall to ½” drywall. Add 5/8” drywall or MDF or other.
• Limp mass barriers are very effective in this regard. As are viscoelastic damping compounds. There are isolation hangers that offer great enhancements. The key is getting a plan that matches your needs and properly installing the plan.

Editorial Note: in the most controlled and designed environments, designers may use the low frequency absorption properties of lightweight partition walls to tune a room…this is a very detailed process and part of a balanced and designed approach. Doesn’t happen by accident.

In summary, higher TL numbers of your surfaces (walls, floor and ceiling) below 200 Hz will result in greater room performance. Great bass leads to great audio and the converse is exceptionally true. A construction effort that is properly designed to address transmission below 100 Hz (especially 50 Hz) is an investment in system performance not simply sound isolation. If you are looking for the type of bass response that drops effortlessly from guitars to bass to kick drum…from orchestral score to rampaging sound effects you should consider construction as a this year’s system improvement.

Many thanks to Jeff Hedback of http://www.HedbackDesignedAcoustics.com for contributing this informative article to our site.