By Fred Gilpin
Owning your own studio is great. Having neighbours complain about hearing your next big hit all night isn’t so great. This is where I always start my studio designs: how much sound isolation exists and how much do we need? When we talk about sound isolation, what we measure is the “transmission loss” (T.L.) of a partition (i.e. walls, ceiling, doors, windows, etc.).The basics of measuring T.L. are: 1. Measuring and storing the response in the studio at your normal listening level (90dB SPL is pretty standard) 2. Measuring the response outside your studio 3. Subtracting the outside response from the inside response, giving you the T.L. of your existing partitions.
There are a number of free software audio analyzer packages you can use to make these measurements. To use these with a degree of accuracy, you will need a measurement microphone. There are a number of 1/4-in. omni-directional measurement microphones available for around $50 that will work just fine for this application. Most use the same miccapsule so the responses are very similar.
To calibrate the measurement system, use the software to measure the gain of your mic preamp. Enter the preamp gain and the sensitivity value that came with the microphone into the software and you can measure “absolute” SPL. (If you are going to use these inexpensive mics to look at the response of your monitors and room, don’t trust the top octave. It’s not uncommon to see a 6dB error in the top octave, even between mics from the same manufacturer.)
If you don’t want to get into the whole analyzer learning curve, the following is a simple measurement technique that only requires your monitor loudspeakers, subwoofer, sound level
meter, and a pink noise source to give you a reasonable idea of where you are at. You’re going to want to do this when it is quiet so you know you are measuring the sound coming through the walls.
Inject pink noise into a channel of your system. Put a 12 or 18 (preferred) dB/oct high-pass filter at around 150Hz on this pink noise channel. Play this through your loudspeakers (subwoofer off) and set the volume to 90dB SPL at the listening position (this is a typical studio monitoring level). With your sound level meter set to “A” weighting, go outside the room and measure the level at several locations about 2-3 m from each wall and above the ceiling. Subtracting the outside reading from the 90dB SPL will give you the mid/high frequency transmission loss of the existing partitions.
Now, repeat the same procedure using only the subwoofer without the high-pass filter and set the sound level meter to “flat” or “C” weighted. This will give you the low frequency transmission loss of the existing partitions.
If you’re using a software analyzer, you can see the entire spectrum at the same time, so you don’t need to do the sub separately.
Next, you need to measure the “ambient” or “background” noise in the areas surrounding your studio. Turn your speakers off and take your sound level meter, set to “C” or “Flat” weighting (or your measurement mic), into the surrounding space while there is normal activity going on and measure the noise level.
If you take the original 90dB SPL, subtract the existing transmission loss you calculated earlier, and then subtract the ambient noise level just measured, you are left with how much additional transmission loss you need in your partitions.
Your ears are the most important tool you have in audio. Listen as you do these measurements. Hearing a lot of high frequencies indicates small air leaks, mid frequencies indicate larger air leaks, and low frequencies are likely due to direct coupling or insufficient mass of the walls between the two spaces. You can also decide if the amount of noise escaping your studio is going to bother the people around you.
In part two in the June issue of Professional Sound, we will talk about construction techniques to meet the required T.L. you just calculated. If you can’t wait for the next issue and would like additional tips, read the expanded version of part one at www.professional-sound.com/soundadvice/.
Fred Gilpin is an acoustic consultant, designer, and the owner of FGA Electroacoustics. He has 35 years of experience and expertise designing studios and providing acoustical solutions at facilities around the world. Fred is based out of Abbotsford, BC and can be reached at firstname.lastname@example.org.