The response can be modified electronically and physically to whatever is desired, and setups can be recalled in the future if needed.
Digital signal processing provides a low-cost, powerful way to modify the response.
Dummy heads can cost many thousands of dollars, but the cost is easily justified for researchers that need the benefits.
Human Mics. One way to make a “poor man’s” dummy head is to utilize your own (no offense intended). Everything is already in place except the microphones. I’ve seen numerous mic placement mechanisms over the years, including eyeglass mounts, wires, and even earrings.
Possibly the most clever and realistic approach to date is the In-The-Ear (ITE) recording technique pioneered by Don and Carolyn Davis in the late 1980s.
Figure 3: The Countryman B6 lapel mic makes an excellent “At-The-Ear” microphone. The foam insert is from a Shure E1 ear bud.
This involved placing probe mics at the surface of the ear drum. This technique captured the outer ear response, including the ear canal resonance. The resonance was removed with an inverse filter during playback.
A variation on this technique that sacrifices some accuracy for practicality is to place small mics at the entrance to the ear canal. I will call this “At-The-Ear” to distinguish it from the previous technique.
The mics are held in place by some foam inserts (Figure 3).
The two mics have XL male connectors that can connect directly to my data recorder.
I normally survey an auditorium without wearing the mics to determine the measurement positions, and then return to the seats with mics in place to gather data.
Figure 4. The impulse response and frequency magnitude of the B6 mic placed in a free-field and At-The-Ear. The impulse response of the At-The-Ear placement has been offset for clarity. Note the stark contrast between accuracy and realism in gathering data. (click to enlarge)
Figure 4 shows a comparison between a free-field measurement and the “At-The- Ear” placement in both the time and frequency domains.
The responses have been overlaid for comparison.
The methods used to gather data are determined by the intended use of the data.
This often requires more than one technique, each preserving or enhancing the information in a way that yields more insight into the particular problem being solved.
When making measurements, arrive equipped to acquire both accurate data and realistic data, and then let the question being pondered determine the preferred perspective.
Pat Brown teaches the Syn-Aud-Con seminars and workshops. Synergetic Audio Concepts (Syn-Aud-Con) has been a leader in audio education since 1973. With nearly 15,000 “graduates” worldwide, Syn-Aud-Con is dedicated to teaching the basics of audio and acoustics. For more information visit their website.