An Anechoic Chamber for SPCR

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SPCR's anechoic chamber could not hope to approach the above professional examples. There's the obvious budgetary constraint. Then, the practical issues: Firstly, the available room, already being used for testing, is far smaller, just 12' x 10' with an 8' ceiling. Secondly, it was unrealistic to consider any structural changes to the house. So what could be done? There are many web articles on soundproofing techniques for condo units and apartments, treating rooms for home theater use, etc. The information in many of these articles is relevant and highly educational, but there's nothing specific on the web about building an anechoic chamber. No one seems to have done it successfully and posted the procedure in detail on any publicly accessible web site.

Once I decided to embark on this project, I listed basic objectives:

  1. Better isolation from external noise so that low level audio measurements and recordings are easier and more convenient to achieve.
  2. Lower the noise floor of the room to better resolve differences between very quiet products.
  3. Reduce acoustic echoes to keep them from affecting SPL measurements.

The first objective was obvious, and we'll get to that soon enough. The second had other implications: Does SPCR's audio measurement and recording gear have the capability to deal with quieter sound levels? The short answer: Probably not. The long answer is in the form of a separate accompanying article, Test Equipment Upgrades: SPCR 2008.

A lot of research was done to study how these objectives could be achieved with minimal work and cost. Many people provided valuable advice and information that helped me understand better the issues involved.

There are four basic methods used in soundproofing:

1. Mass barrier
2. Air barrier
3. Insulation
4. Mechanical decoupling

Sorting through the mountains of information, the best practical options given the existing constraints appeared to be:

  • Build a mechanically decoupled room-in-a-room atop a floating floor with an airgap between inner and outer room.
  • Apply damping to the inside surface of the walls and ceiling to make it anechoic.
  • Use two doors and effective gaskets to minimize sound transmission through the doorway.

The room-in-a-room concept is not new, and it's been used in many applications. Ambitious home theater rooms employ this design, as do recording studios that need to block sound transmission, whether coming in or going out. One of the more detailed web articles on this topic was posted by Revolution Audio, a Canadian store for home recording studio products and services. Building a Home Recording Studio is in two parts, one on the Magical Floating Floor and another on the Sound Proof Walls. These articles describe an approach very similar to that adopted for SPCR.

The concept is simple enough:

  1. Build a floating floor over the existing concrete floor, with a 1~3" air gap from existing walls and ceiling. The floating aspect is provided by some type of resilient damping material.
  2. Build steel-stud / drywall walls with high STC (sound transmission class) value atop the floating floor, with no physical connection to the existing walls. Maintain the 3" gap mentioned above. Height should be several inches lower than the existing ceiling.
  3. Build a steel-stud / drywall ceiling with high STC atop the four walls, with no physical connection to the existing walls or ceiling.

Russ Kinder, now a project manager at an architectural firm, has helped SPCR since its inception. Russ provided a cutaway drawing of construction suggestions based on a group discussion with his mates.

Floating room-in-a-room construction suggestions.

Regardless of the fine details, after the inner room is built, the internal space would be up to a foot smaller in each of the three dimensions, which would result in a working area of about 9' x 11' with a 7.5' ceiling. AC outlets, a small closet in the room, lighting — all these would have to be dealt with. My brother in law Rob, a home builder with extensive experience building home theater rooms in high end homes, explained the procedures needed to build a room of this type. According to Rob, it is much easier to build from the inside out, but with a minimum crew of two, it is still doable. All the construction would have to take place step by carefully planned step inside the room.

Once this inner room is built, its walls and ceiling would be lined with sound absorbent materials to eliminate echoes. A 200 Hz cutoff frequency would require each sound absorbent wedge to be 1.6' tall, which would reduce the room width to under 6'. This is too small. A 300 Hz cutoff frequency would require wedges of about 14", leaving a working space of roughly 7' x 9' with a 6' ceiling. Tight, but workable. Foam wedges similar to the ones shown below would be ideal as they can be left uncovered, unlike fiberglass wedges.

Small anechoic chamber with foam wedges in a Vancouver corporate office. There are extra wedges strewn about.

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