All these variables make it impossible to know the impedance characteristic exactly. If a constant fraction of the signal being sent is subtracted from the signal on the two-wire loop, the hybrid balance will vary depending on the actual loop impedance, resulting in poor hybrid balance and echo for some loops. The degree to which echo is objectionable depends on echo loudness and total delay. Delay from transmission is in the range of 30 ms for transcontinental domestic calls, from 50 to 100 ms for international calls, and 600 ms for satellite calls. This delay is what affects the customer's perception of echo.
Larger delays lead to a subjective annoyance perceived as echo.
The larger the delay, the less masking there is by the direct speech and the more annoying the echo is. The average TELR of 30 dB would be objectionable to most subscribers at 35 ms of delay, and could be objectionable to some subscribers at 18 ms of delay. As previously noted, both echo annoyance and echo loudness increase with delay. One way of dealing with echo is to intentionally introduce equal loss in the TX and RX circuits so the desired signal sees this loss once and the echoed signals see it twice. The downside, of course, is that the desired signal is made weaker, so this can't be done too often.