100 Things #17: Simulated Free Field Measurements Without an Anechoic Chamber

With SoundCheck you can make simulated free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. By performing near field and far field measurements, we are able to utilize the measurement strengths of each technique; near field measurements for low frequencies and room reflection immunity, and time-windowed far field measurements for high frequencies. Splice these measurements together, and the result is the free field response of the loudspeaker. All without an anechoic chamber, and all within a single SoundCheck sequence.

Simulated Free Field Measurements Without an Anechoic Chamber

Learn more about free field measurements with SoundCheck

In this online seminar, Steve Temme explains how to use simulated free field measurements to provide accurate free-field measurements across the entire audible frequency range without an anechoic chamber. This seminar talks about setup and equipment considerations, testing in different environments, and analysis of results.

Try simulated free field measurements for yourself with our Splice sequence, available in our sequence library.

Video Script:

Free field loudspeaker measurements are often included in loudspeaker specifications. These measurements are usually made in an anechoic chamber. However, anechoic chambers are not cheap, particularly those large enough to give accurate measurements at low frequencies. This means that many engineers don’t have easy access to a suitable chamber, even more so if they are working from home.

With SoundCheck you can make free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. Here’s how it’s done.

First, measure the near field response of the loudspeaker using a stepped sine sweep or Stweep, and placing the microphone very close to the low frequency driver. If the loudspeaker is ported, you also need to make a measurement at the port or ports. This near field measurement is accurate at low frequencies as it’s unaffected by room reflections, but it does not accurately represent the free field response at high frequencies. 

Next, place the microphone in the far field and measure the time-windowed frequency response using a continuous log sweep with the Time Selective analysis algorithm. The far field, time-windowed measurement is unaffected by room reflections, but it is not accurate at low frequencies as the room size limits the width of the time window and therefore the frequency resolution.

Now, if you examine the two responses you can see an overlap range in the middle where the shapes of the curves align. Using this, you can select a precise frequency to splice the two halves of the measurement together and display a response over the entire frequency range. This is implemented in several post-processing steps which include calculating the impulse response, and automatically correcting for differences in amplitude and phase.

Here you can see the simulated free-field response measured using this method compared to the same speaker measured in an anechoic chamber. In fact, the simulated free field response is actually more accurate than the anechoic chamber measurements because of its small size and 120Hz cut-off frequency

We can also compare our measurements to the manufacturer’s published data. Here we see a tight correlation across the whole frequency range, even at low frequencies – likely because a much larger chamber was used.

If you want to know more, there’s a detailed seminar covering both the theory and practical aspects on our YouTube channel, and a pre-written test sequence for use with SoundCheck can be downloaded from our website for free!