Pulsing noise in the phone speakers?

Thread Starter

xchcui

Joined May 12, 2014
191
Hi.
I have a new wired phone which is connected only to the phone line(no power line).
The phone has a speakers phones.When i use the speakers phones there is a pulsing noise.
The pulsing noise is continuous but it stops in the moments that someone is talking,
but the pulsing still appears between the sentences.
Those pulsing noise doesn't heard in the handset speaker,only in the phone speakers.
What can it be?can i fix that?

Thanks.
 

Audioguru again

Joined Oct 21, 2019
2,358
A speakerphone has a circuit that when it detects sounds in the mic then it turns off the speaker to avoid acoustical howling feedback. Maybe your speakerphone is placed under a shelf or is in a room full of background noises then when the distant person stops talking, the noise heard by the mic causes the circuit to turn off the speaker with a thump that is heard by the mic which causes the speaker to be turned off again and it repeats.

Turn down the volume control and/or move the speakerphone away from background sounds.
 

Thread Starter

xchcui

Joined May 12, 2014
191
Thanks for your advices and for the explanations.
Dodgydave,when i block the microphone out,the pulsing noises
stop.So,i guess that,as you said,this is a feedback from the internal microphone indeed.
Audioguru again,i turned down the volume of the speakers to minimum,
i canceled any background sounds(quite room)and i picked up the phone and moved it to all possible directions,but it still makes this pulsing noises.
In the quite room,since it's quite,the pulsing noises are more noticeable.
Is there anything else that i can do to fix that?
If i leave it as it is,can these pulsing noises get worse with time?
or can this situation cause something else to fail?
 

Thread Starter

xchcui

Joined May 12, 2014
191
Then the only suggestion is to move the microphone away from the speaker or turn the mic gain down.
When you says"...to move the microphone away from the speakers...",
you meant as a general solution when it can be done,right?
Since i can't see how i can do that in my case,the outer shell of the phone are made to fit the inside components.But i assume that turn the mic gain down
is possible.In that case,how should i do that?do i need to turn left/right some kind of component with screwdriver?
 

Audioguru again

Joined Oct 21, 2019
2,358
Many years ago I made a speakerphone with a Motorola speakerphone IC. It worked perfectly.
It measured the level of background noise then subtracted it from the switching threshold.
 

Thread Starter

xchcui

Joined May 12, 2014
191
Many years ago I made a speakerphone with a Motorola speakerphone IC. It worked perfectly.
It measured the level of background noise then subtracted it from the switching threshold.
The issue here is that there isn't any background noise
and even when the speakers don't emit any sound at all,those
pulsing noises are still there.
If i leave it as it is,will it get worse with time?could it cause to
something else to fail?
 

Dodgydave

Joined Jun 22, 2012
9,837
It won't get worse as I said earlier it's a feedback problem, the gain is too much, can you alter the gain or reposition the microphone, best to take pictures of the inside..
 

Thread Starter

xchcui

Joined May 12, 2014
191
It won't get worse as I said earlier it's a feedback problem, the gain is too much, can you alter the gain or reposition the microphone, best to take pictures of the inside..
I attached a photo of the corded phone and i marked the speaker+mic. locations.Doesn't the microphone far enough from the speaker?
It is a brand new telephone and i am not that keen on disassembling
a new phone unless changing the gain can be done by a simple adjusting like turns something as a potentiometer.Do i need to make any soldering and change component's value in order to achieve the optimal gain?
If it was old unit,i would have already disassembled the phone.
He says it has "speakers". More than one speaker?
I wrote"speakers",but i don't really know how many speakers it has.
I didn't attach importance to it,but as i see it,it looks like it has only one speaker(besides the one that in the handle).
 

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Audioguru again

Joined Oct 21, 2019
2,358
The speakerphone is defective or it was poorly designed.
How many pulses in 1 second? Only a few pulses per second or many pulses per second making a buzz? The buzz might be caused by not enough power from the telephone line feeding the speaker amplifier. All speakerphones I have seen have a separate power supply.
 

Thread Starter

xchcui

Joined May 12, 2014
191
The speakerphone is defective or it was poorly designed.
How many pulses in 1 second? Only a few pulses per second or many pulses per second making a buzz? The buzz might be caused by not enough power from the telephone line feeding the speaker amplifier. All speakerphones I have seen have a separate power supply.
There is not a buzz.It is about 1-3 pulses per second.
 
Last edited:

Dodgydave

Joined Jun 22, 2012
9,837
The speakerphone is defective or it was poorly designed.
How many pulses in 1 second? Only a few pulses per second or many pulses per second making a buzz? The buzz might be caused by not enough power from the telephone line feeding the speaker amplifier. All speakerphones I have seen have a separate power supply.
He's already proved what's causing it, it's the microphone pickup , it stops when covered up.post#5
 

Audioguru again

Joined Oct 21, 2019
2,358
I think the pulses start at the speaker's amplifier and its power supply, creating a pulse sound that is picked up by the mic. Then the half-duplex cuts off the speaker making another pulse.
Maybe the speaker's amplifier makes a pulse which drops the power supply voltage that powers the electret mic causing the mic to make a pulse.
Pulse sounds are low frequency "motorboating" caused by poor filtering of a weak battery. The battery for a phone line could be far away.
 

Thread Starter

xchcui

Joined May 12, 2014
191
He's already proved what's causing it, it's the microphone pickup , it stops when covered up.post#5
Yes,it stops when i covered the hole at the bottom of the phone where the microphone located and i would like to add that in order to stop the pulses
i needed to cover the hole completely.Even small space in my cover made the pulses continue.
I think the pulses start at the speaker's amplifier and its power supply, creating a pulse sound that is picked up by the mic. Then the half-duplex cuts off the speaker making another pulse.
Maybe the speaker's amplifier makes a pulse which drops the power supply voltage that powers the electret mic causing the mic to make a pulse.
Pulse sounds are low frequency "motorboating" caused by poor filtering of a weak battery. The battery for a phone line could be far away.
When you say power-supply do you refer to the 50v from the phone line?
This corded phone is not connected to the main power(110v/220v),it just connected to the telephone line(50v)and it doesn't have a battery.
 

Audioguru again

Joined Oct 21, 2019
2,358
If your phone is a long distance from the telephone company's 50V battery then the resistance of the wires reduce the 50V and the current to the amplifier to almost nothing.
A phone line is designed to power a low current handset, not a speakerphone. Maybe the speakerphone manufacturer hopes that you are near the telephone company's battery like most people will be.
 

Thread Starter

xchcui

Joined May 12, 2014
191
Okay,thanks alot for your help.
If these pulses noises,as you said,shouldn't get worse or affect something
else to fail,i guess i leave this new phone as it is.When the time will come
i will dismantle it.
Thanks again.
I wish you all
Merry Christmas and a Happy New Year!
 

Thread Starter

xchcui

Joined May 12, 2014
191
Hi again.
I would like to add something new that i noticed about the pulses noises.
As we said before,when i covered the hole,where the microphone is located,the noises stop,but i have also noticed that,when i am rubbing the plastic around that hole(without covering it),which makes a rubbing noises near that microphone hole,the noises also stop.I think that the rubbing noises that i made are a kind of background noises simulation,which in contrast to what you said,stopped the noise rather than caused it.May that new information add something to the source of the problem?Does this new information reinforce the claim that decreasing the microphone gain will stop the pulsing noises?
 
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