# Simple intercom

#### Jonathan M

Joined Oct 23, 2021
2
I coach a boat of rowers and two are hard of hearing.
I need to put speakers near them and for me to have a mike.
It need not be wireless
Commercial products cost $600 -$1,000.
But we are a bunch of "getting older by the day" just trying to enjoy an activity in the fresh air.
A solution is needed. I could assemble bits - but haven't got much any idea of where to start.

And thank you

#### Wendy

Joined Mar 24, 2008
22,646
How much electronics experience do you have? It's actually pretty easy. Have you thought about cheap walkie talkies?

#### DickCappels

Joined Aug 21, 2008
7,954
Another thought: a car radio receiver -they run from batteries, no inverter required. They make some that can pump out a lot of watts and I think not terrible expensive. If there is no aux audio input you could still use a FM wireless mic, which might be nice since it is wireless and can be very cheap.

#### Ian0

Joined Aug 7, 2020
3,754
I fixed a number of these during my time at university - known as a "Cox box". Car-mounted PAs are quite readily available. You just need some waterproof speakers wired in series-parallel to make up the right impedance. Eight 8Ω speakers wired two-in-series four-in-parallel will make 4Ω, and most automotive products are designed for a 4Ω load.

#### Audioguru again

Joined Oct 21, 2019
3,828
I fixed a number of these during my time at university - known as a "Cox box". Car-mounted PAs are quite readily available. You just need some waterproof speakers wired in series-parallel to make up the right impedance. Eight 8Ω speakers wired two-in-series four-in-parallel will make 4Ω, and most automotive products are designed for a 4Ω load.
Car speakers are 4 ohms for maximum output power when the battery is only 12V. The resulting 14W in a 4 ohm speaker from a modern bridged amplifier is way more than enough power to blow your ears off if the speaker is as close as 1m from you.

The car amplifier works perfectly with 8 ohm speakers (half the power), or 16 ohm speakers (1/4 the power).

If the microphone can hear the speakers then the resulting feedback howling will blow everybody's ears off.

#### Wendy

Joined Mar 24, 2008
22,646
KISS ( keep it simple stupid ) applies. Off the shelf is best unless you just want to learn electronics. Cheaper too.

#### MisterBill2

Joined Jan 23, 2018
9,792
Several questions before any answer will be useful: How hard of hearing are the folks? And could they wear ear phones?
Does the system need to be on constantly or only when giving instructions? That affects battery life and the operation mode.
Will the louder sound bother the rest of the rowing team? That may be a big concern.
In a boat being rowed I am thinking that there is no power source on board normally, and so battery power will need to be part of the system, meaning that size and weight will matter.
One option could be a cassette tape recorder with a monitor speaker connection. There have been a few of them around, a used one might possibly work.
The other simple option could be to assemble just a small amplifier, perhaps five watts, with the small horn speakers, which produce a much greater volume level for the power driving them. Radio Shack used to sell small amplifier kits that would work for this application. Perhaps other places still sell such things.

#### Jonathan M

Joined Oct 23, 2021
2
I'm going to try an android phone with 2 bluetooth 5 speakers. Perhaps the latency will be annoying ....

#### MisterBill2

Joined Jan 23, 2018
9,792
Still I have that question or two. Does the application need constant communication? and can those rowers wear earphones? For earphone use, a very simple carbon microphone and battery setup can work, no active electronics needed.

#### Ian0

Joined Aug 7, 2020
3,754
I doubt earphones would last long before the leads got caught up in the seat-slides, and generally the cox has to yell “stroke” 20 to 30 times a minute.

#### MisterBill2

Joined Jan 23, 2018
9,792
I doubt earphones would last long before the leads got caught up in the seat-slides, and generally the cox has to yell “stroke” 20 to 30 times a minute.
Good points indeed! And if the TS is going to try a bluetooth phone, they need to be aware of pairing. I am not aware of bluetooth having a broadcast mode where it can communicate with multiple devices. That could be a serious problem. And the latency is an unknown quantity. And also, I find it difficult to imagine a leader on a boat holding a phone.
I use bluetooth capable hearing aids, so that might remove the need for the bluetooth speakers. But pairing will still be an issue.

#### MrSalts

Joined Apr 2, 2020
712
If they are hard of hearing, why not use a sound detector that outputs light flashes for them to keep the stroke?

#### Ian0

Joined Aug 7, 2020
3,754
If they are hard of hearing, why not use a sound detector that outputs light flashes for them to keep the stroke?
Even simpler - the cox could wear a hat with a lamp on it, and press a switch to illuminate the lamp for every stroke.

#### MisterBill2

Joined Jan 23, 2018
9,792
Even simpler - the cox could wear a hat with a lamp on it, and press a switch to illuminate the lamp for every stroke.
That implies no delay in the trigger, and that the only command is to stroke. Probably not the case. Besides, that would also mean watching the light. A small, cheap, PA amplifier and two of the 5 inch trumpet speakers will be quite enough. An over-the-ear mike for the coxswain, or even a cheap throat mike.

#### MrChips

Joined Oct 2, 2009
24,609

#### MrSalts

Joined Apr 2, 2020
712
That implies no delay in the trigger, and that the only command is to stroke. Probably not the case. Besides, that would also mean watching the light. A small, cheap, PA amplifier and two of the 5 inch trumpet speakers will be quite enough. An over-the-ear mike for the coxswain, or even a cheap throat mike.
With nice trumpet speakers and a good amplifier, the rest of the crew will soon be deaf.

#### MisterBill2

Joined Jan 23, 2018
9,792
With nice trumpet speakers and a good amplifier, the rest of the crew will soon be deaf.
Actually, a small trumpet speaker is fairly directional, and the volume does not need to be high in a large area. Small trumpets are quite directional, at least they can be.

#### MrSalts

Joined Apr 2, 2020
712
Mine get gross enough without exercising. The headphones would be gross in no time.

#### DUFFER

Joined May 3, 2013
12
Agreeing with Wendy's KISS advice, May I suggest the the coxswain use a mike (over the ear or throat) connected to a FM transmitter and the users employ over the ear BT earphones (not buds for the reason mentioned by Mr Salts), paired to individual FM receivers. That way, each user can have the volume he requires now and in the future and no cords.

#### AnalogKid

Joined Aug 1, 2013
9,479
ebay modules, cheap and reliable. One mic preamp module driving multiple speaker amplifier modules, one for each speaker location. If there are only two speakers, then a small stereo amplifier module can drive both with individual volume controls.

One small food storage container can house everything including the battery. Or, one for each module gives each listener their own volume control, but now you have three batteries. Ziploc containers probably are too flimsy, but any supermarket will have something big enough and rugged enough.

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