IP SubStations, Acoustical design

IP SubStations - Acoustical design

The IP-SubStation audio is based on the following:

1. The output is 1W with an 8 ohm speaker, from a Clas D amplifier that runs on 5V.
2. The speaker and mic. must be 11 cm apart.
3. The mic must be pressed against the frontplate or mounting arrangement (no air gaps), with a gasket around it.

One Watt can be much sound pressure. So if one really wants all of it, the mic must be more than 11 cm away to lower the feedback.

If the feedback is too high, the DSP software in the station automatically starts reducing the output level. The DSP is not very good at that, the volume goes up and down and is generally rather low.

Bad mounting of the mic. ( there is air around it ) makes high volume and voice-switching impossible. Often the mic must be re-entered and pressed good into the mounting gasket or front-plate to fix this. Only the centre of the mic should be open.

Zenitel has three “types of stations” that can illustrate this

- Separate 120 mm 8 ohm speaker box, station speaker removed. Placed 1m away from the station (and mic) this gives almost home theatre effect. Very loud.

- The 8041 Euro substation, with mic 10cm from speaker. Small 50mm 8 ohm speaker, quite good output, no “swaying” of the output level.

- The 8051 3Gang substation, with mic 7cm from speaker. Medium 66mm 8 ohm speaker, mic is too close to the speaker and speaker is too good. Must insert 15 ohm to reduce power into speaker. ( using a 25 ohm speaker probably would give the same effect.)

Lowering AlphaPro volume level doesn't help

That is because feedback margins are the same for small and large signals. Example; if a small signal returns 3 dB above feedback margin, the station DSP must reduce the output level by 3db, even if it is a whisper. One could think this is the same as increasing speaker resistance, but as said above, the DSP seems to overcompensate and also drive volume up and down. This should not be the normal operating mode of the DSP. ( Note; Positive gain/feedback can not be alloved, even a small singal will increase in the audio loop and eventually end in howling.)

Audio design tips

Can use a tunnel to the mic to increase speaker distance.

To check feedback limit, flush mount the station in a wide baffle and place a book below the speaker. If the speaker volume increases due to longer sound travel, the DSP was in output reduction mode.

Summing up

A station must be designed below feedback limits. At present there is no “standard speaker” for the IP-stations. The speaker must match the feedback from the cabinet. If the cabinet is too small or the mic is too close, it’s not possible to use the available 1W. One can either increase speaker impedance or place a resistor in series.