Acoustic Delay vs. VoIP Quality

What is Acoustic Delay Line?

acoustic delay line: A device that introduces a delay in the propagation of an electrical signal by (a) employing a transducer to convert the signal into an acoustic wave, (b) propagating the acoustic wave through a medium such as a column of mercury or a carbon or ferrite rod, and (c) by means of another transducer, converting the acoustic wave back to an electrical signal. Note: An acoustic delay line may be used for temporary storage of information, e.g., a digital data stream.

Source: http://www.atis.org/tg2k/_acoustic_delay_line.html

How does Acoustic Delay effect VoIP quality?

There are two forms of echo on voice networks, hybrid echo and acoustic echo. Hybrid echo is a linear electrical signal reflection that occurs at the 4-wire to 2-wire conversion point in a PSTN network (usually found in a Class 5 PSTN switch). Hybrid echo can enter the VoIP network wherever there is a connection between VoIP and PSTN networks. Acoustic echo is non-linear and is caused by poor acoustic isolation between the speaker and the microphone of user's device (e.g., handset, headset, softphone, speakerphone). It can enter the VoIP network from any source.

Both forms of echo become more noticeable and annoying to the caller with the added delay of the IP network. In fac, the added VoIP-induced delay can make what would formerly be considered minor echo annoying enough to cause users to abandon the call.

Source: http://www.ditechnetworks.com/solutions/appnotes/AN_Echo_VoIP.pdf