Out-of-phase dipole cancellation strengthens as source spacing drops below a wavelength.

Evidence

Figure 7.26 demonstrates the importance of the distance between the two out-of-phase point sources relative to the wavelength. If they are within a wavelength of each other, they partially cancel one another, more nearly so the closer they are. The sequence in this figure corresponds to moving the sources farther apart for a fixed wavelength. However, exactly the same result is reached by leaving the sources a fixed distance apart and decreasing the wavelength, which follows from the law of similarity. — Eric J. Heller, Why You Hear What You Hear- An…, 613