July 22, 2011
by Grant Clauser
We hate math, but there are times when you really need to just break out the calculator in order to get your A/V gear optimized for your room, or in order to ensure you’re getting the right product in the first place. The Bassaholics over at the Audioholics web site have come up with a system to calculate the room size a subwoofer is best suited for.
Let’s face it, most people make their speaker decisions based on aesthetics, recommendations from friends, company reputation, dealer suggestions, price and in-person demos—but sound is a science, and you can apply a scientific method to determine how a device is going to operate in a specific environment. The procedure takes several elements into account including room size, room gain, listening position, and it includes some assumptions about what levels people typically listen to their subwoofers.
In this system, a small room is determined to be less than 1,500 cubic feet, while a large room is 3,000 to 5,000 cubic feet.
Here’s a little of the Audioholics methodology:
“The standard calibration “Reference Level” (RL) is 75dB at the listening position. The goal is to ensure the system calibrated at 75dB can hit clean 105 dB peaks for each of the speaker channels and 115 dB peaks for the LFE channel. Note that the LFE channel is boosted 10 dB over the speaker channels. Technically speaking, most people redirect the bass from other channels to the subwoofer, which in conjunction with LFE could in theory ask the sub for a 123dB peak signal. However this is NOT a common scenario and most people don’t listen at reference levels (especially if they value their long term hearing). It is much more common for A/V enthusiasts to listen between -15 to -10db from reference. Thus a 115 dB peak at the listening seats in-room is a more realistic benchmark goal for large rooms. Anything higher is icing on the cake and will earn a manufacturer our “Bassaholic” rating for their sub.
Next we need to determine the test frequency that the sub must hit the 115dB peak at in order to meet our “large room” goal. CEA burst signals are a reasonably good approximation of how a sub will be stressed with real program material. Since we are using CEA peak SPL data which is measured in 1/3 octave increments, we can either choose anywhere from 20Hz to 63Hz in 1/3rd octave step sizes. The tactile bass we feel and hear happens mostly in the 30-60Hz range. Room gain typically reinforces ultra low end bass frequencies (below 40Hz) - so at first glance, it seems reasonable to average our CEA test data from 25Hz to 63Hz to determine a subwoofer’s room size capability.
Annex A of the CEA 2010 standard recommends adding SPL data in dB to average over the critical bands they refer to as “Ultra Low” (20Hz to 31.5Hz) and “Low” (40 to 63Hz). It is mathematically incorrect to average logarithmic based numbers (such as decibels) as it will bias the outcome to the lowest number in the data set. The correct way to average dB’s is to first convert them to Pascals (a linear, not logarithmic, measure of pressure) to properly average the data before converting back to dB’s. There is a problem averaging like this, however, since our ears don’t hear loudness equally for different frequencies. Averaging in Pascals will bias the average to the highest measurement in the data set. Properly averaging in Pascals (Pa) will make a sub with just one good SPL # still have a good average score. Averaging in dB’s (instead of Pascals) will make a sub with just one bad SPL # look bad.”
To read the entire article, along with the complete testing procedures, go here.
Grant Clauser has been covering home electronics for more than 10 years with editorial roles in several consumer and trade magazines. He's done ISF-level damage to hundreds of reviewed products and has had audio training from Home Acoustics Alliance and Sencore. He's also the author of the book The Trouble with Rivers
. Follow him on Twitter @geclauser.