There were actually four microphones - captain, first officer, cockpit and observer (see page 46, Figure 9). Four receivers is sufficient to determine the sound origin at a point in space, with an associated error volume (three receivers only allows you to determine a curve in space).<p>See <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Multilateration" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Multilateration</a>