Damping

Spectrally, damping means broadening the peak around the sine wave's frequency. As damped sine waves do not have an exactly sinusoidal form, damped sine waves have more complex spectra than undamped sine waves.

The walls of the vocal tract are soft and they absorb sound energy produced by the glottis. The vocal tract resonances (or formants) therefore have certain bandwidths because the frequencies are damped. The formant bandwidths of nasal sounds are wider than those of nonnasal sounds, because the vocal tract then has a greater surface area and volume, and thus more energy is absorbed.

Link
Utrecht Lexicon of Linguistics