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Building Great Bass Response In Your Home Theater

by Jeff Hedback May 20, 2007
Imagine this being a typical interior wall system and the water represents the bass waves from your speakers.  You can easily see the yacht will topple the stud wall upon contact.

Imagine this being a typical interior wall system and the water represents the bass waves from your speakers. You can easily see the yacht will topple the stud wall upon contact.

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.

Bass 2What 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.


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Recent Forum Posts:

Sasquatch posts on September 15, 2015 11:53
So I could improve the acoustics of our TV room by adding a layer of ordinary foam insulation to the back wall and covering that with a layer of Hardiboard? That would sort of mimic the the effect of the inner wall often built in studios? The back wall of our room is normal timber/drywall construction and adjoins the garage. Does the garage space then factor into the room design? Could I add anything to the garage side of the wall to improve the TV room acoustics?

Our TV room is very small. So I can't sacrifice a ton of space building extra walls and adding massive traps everywhere. And the couch has to be on the back wall. So there isn't room behind the seating to add much of any treatment to the back wall.
tsmessi posts on August 02, 2009 22:30

This is an interesting discussion. thank you for sharing :
bigeric posts on December 10, 2007 10:20
Into the details …

This is a great thread, exactly what I need to make the next step forward in my construction planning.

I'm building a theater in my basement on a shoestring budget but, being an engineer, I want to get the bones of the room right so that I have a solid foundation for the theater of my dreams when I can afford it.

I'm looking for a bit more detail on how to make my room have good acoustical performance. Here are some of the things I'm working with based on a lot of reading on the forums:

Front wall, poured concrete. I've been planning to stud in a wall and have a false, acoustically transparent screen wall. Total flexibility in materials and distance of the stud wall from the concrete.

Left side wall, poured concrete. Total flexibility in materials and distance of the stud wall from the concrete.

Right wall, stud wall. Total flexibility in materials and some flexibility on location to balance room width and audio performance. Backs to a storage room and hallway, no room directly adjacent.

Back wall, stud wall. Total flexibility in materials and location. Backs to a utility room / workshop.

Estimated room dimensions:
front third 12-12 1/2 feet wide
rear two thirds 14-14 1/2 feet wide, door on right side aisle on right side

Estimated screen size:
8-9 feet wide

Estimated viewing locations:
row at 11 feet from screen
row at 16.5 feet from screen
bar behind second row

Thanks in advance,
Jeff Hedback posts on May 26, 2007 21:57
Hi Jeff.

Thanks for the read and the comments!

I appreciate you bringing forth the STC range. I was attempting to highlight how the STC rating by itself without futher details of the TL numbers at different frequencies is most helpful for typical speech or common applications, not the complex needs of a dedicated music space.

I really appreciate you expanding on the use of drywall to “tune” a room. I included the editorial because it needed to be mentioned, but would be beyond the needs of that article to go into detail. You brought it forth in terms of balance, meaning: if there is an ultimate goal of performance and aesthetics…which choices are the best for each person??? Nice emphasis.

I like this forum a great deal. It allows points to be brought into the light and better focused. What a tremendous way to communicate.

I guess I'm giving kudos to Gene and Audioholics for some great structure and planning.

Hope you're well!
Savant posts on May 24, 2007 17:53
Great piece, Jeff. Just a couple of thoughts:

- An STC rating is got by curve-fitting the lab-measured, 1/3-octave-band TL data from 125 to 4000 Hz; a wider range than the 500 to 2000 Hz indicated in the article.

- I was happy to see editorial mention of utilizing the absorbent properties of drywall at low frequency to advantage in small room design - and how difficult a process it is! While difficult, if a designer is already involved, the right combination of a high-STC wall, like poured concrete, with an interior “finish” of studs and drywall (leaving the appropriate airgap, of course) could yield outstanding results. It's not as easy as it sounds though. The variables are the mass of the drywall, the depth of the cavities, the amount of absorption in the cavities, the spacing between the studs, etc. In all likelihood, all these will have to be varied considerably to make the results worth the trouble. Of course, while this could help reduce the need for “tweaks” like DRC or overly-large “traps,” the design and construction costs will also be substantially higher, thus introducing another (sometimes the most important) variable…

These two comments aside, nice work!
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