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Thread: How to chose the right speaker

  1. #1
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    How to chose the right speaker

    Hi,

    I have a Teensy 3.2 and an audio shield and I would like to connect a minimum quality speaker to be clearly heared 5 meters far.
    I don't know how to chose the speaker and if I have to preamplify the line out or use the earphone out. I don't really know how to calcul the speakers spec' I need.

    I know the earphone out is 33ohm and the line out need to be preamplified if I connect it to a passive speaker but I don't know to do the calcul to wisely chose the speaker I need.

    Thanks a lot and I apologize for my english.

  2. #2
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    Speakers have a sensitivity spec, in dB for 1W at 1m distant. Level drops off outdoors as 1/distance^2, and
    somewhat less rapidly indoors due to room reflections.

    So if you have a 90dB@1W,1m speaker and a 3W amp, you will get 95dB at 1m (5dB is x3), and down
    to 81dB at 5m (5^2 = 25, 25 is a 14dB ratio).

    What can be heard clearly depends on the ambient environment and competing noises/sounds.

    Complex signals have much higher peak power than average power, so that needs to be factored in too, you
    may need 10x the amp power than a naive calculation suggests.

  3. #3
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    MarkT that was an excellent introduction for a beginner like me. Thank you!

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    Thank you MarkT, I will look at that.

    Do you know how much power can give the earphone out of the audio shield ?

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Ankermannnn View Post
    how much power can give the earphone out of the audio shield ?
    About 30 mW with ordinary headphones. It's only just enough for good listening on headphone, but nowhere near the power level needed for a loudspeaker! You absolutely will need an amplifier to drive the speaker.

    For clear listening at 5 meters distance in a reasonably calm indoor space with ordinary height ceiling (like a home without a TV playing or children running around), you probably need an amplifier rated at 3 to 5 watts.

    For a larger indoor space with high ceilings and many people talking (like a demo at a crowded convention center), you'll probably need at least 20 watts.

    For outdoor use, you might need even more power, especially if used in a city or near a highway or on a windy day.

    The quality of the speaker and the type of enclose it's mounted in makes a huge difference. Even a very good speaker will fail to impress when used without an enclosure.

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    Thanks for your precise answer Paul.
    My question now is : can the teensy 3.2 handle a 3-5W amplifier or I need an external source of power?

  7. #7
    Senior Member PaulStoffregen's Avatar
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    You'll definitely need external power.

    USB is limited to 500 mA at 5V, which is only 2.5 watts total power. Some class-D type amplifier are around 80-90% efficient, so you could maybe get about 2 watts if you use one of those amplifiers. The one on the prop shield is a good example.

    For 5 watts, you definitely need more power than normal USB can provide. Most of those amplifiers also need higher than 5 volt power.

    Class-AB amplifiers generally have higher quality than Class-D, and sometimes they're less expensive (though in these modern times it's not at clear-cut), but they are much less efficient. Usually they're around 50% to 60%. So if you use one of those, plan on needing a DC power source rated for about double the amp's rated power.

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    Senior Member+ Frank B's Avatar
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    I'm not that sure that I'm doing it right - Most of the time I use a pair of active speakers intended for PCs. I connect them with the 3.5mm plug to the earphone connector. It works for me...

    Super simple solution, and you can buy these speakers everywhere.. in all price categories

    @Paul: Is it OK to use the earphone connector?

  9. #9
    Senior Member oddson's Avatar
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    It works, sounds ok and doesn't seem to damage anything but you should never ever use the phone jack as a line out because... er, it's wrong?

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    The SGTL5000 on the audio shield's headphone output is definitely not designed to be a line-output, don't try to use it for this, use the lineout pads
    instead, they are actually referenced to ground, the headphone output is not.

  11. #11
    Senior Member+ Frank B's Avatar
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    Psssst......Good that my speakers and the shield do not know that they are not designed for that and work great.*
    Over the last years I killed one Audioshield - but that had an other reason.

    *note: possible that it does not work for you.

  12. #12
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    Well the common wire to the headphone jack is not at ground, and is actively generated by an internal amp as
    far as I can judge. They do this to avoid needing large electrolytic DC-blocking caps on the headphone outputs.

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    Senior Member+ Frank B's Avatar
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    I know that and I am aware of it.. seems not to be a problem in my case. For my active speakers, it's just a ground it seems. Then, there is no other ground connection to the teensy.

    I think a bigger problem are those 3.5mm headsets with 4 connections. Normal headphones are good - I have some headsets - some with buttons - all with mics.. i can hear the left channel on the right and vice versa. Not sure if they are healthy for the SGTL. Not sure which connections the 4-rings make...

    It's not that easy to find "old" headphones with the 3-rings jacks.

  14. #14
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by Frank B View Post
    I know that and I am aware of it.. seems not to be a problem in my case. For my active speakers, it's just a ground it seems. Then, there is no other ground connection to the teensy.

    I think a bigger problem are those 3.5mm headsets with 4 connections. Normal headphones are good - I have some headsets - some with buttons - all with mics.. i can hear the left channel on the right and vice versa. Not sure if they are healthy for the SGTL. Not sure which connections the 4-rings make...

    It's not that easy to find "old" headphones with the 3-rings jacks.
    Though you can get splitter cables that convert a 4-pin TRRS cable to a pair of 3 pin TRS cables:


    Or the reverse cable that takes 2 3-pin TRS cables and makes a 4-pin TRRS cable:

  15. #15
    Senior Member+ Frank B's Avatar
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    If there ever will be a rev "E", it should perhaps use a 4 pin connection..

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    Senior Member PaulS's Avatar
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    To make life more complicated: apparently there are 2 versions of the TRRS [4-pin] jack?

    Click image for larger version. 

Name:	TRRS OMTP CTIA.jpg 
Views:	20 
Size:	187.4 KB 
ID:	23671

    Yuck...
    Paul

  17. #17
    Senior Member PaulS's Avatar
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    Just checked my Acer laptop and it supports the CTIA "standard". GND on the second ring, counted from the tip.
    More info here.

    Paul

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    Quote Originally Posted by PaulStoffregen View Post
    You'll definitely need external power.

    USB is limited to 500 mA at 5V, which is only 2.5 watts total power. Some class-D type amplifier are around 80-90% efficient, so you could maybe get about 2 watts if you use one of those amplifiers. The one on the prop shield is a good example.

    For 5 watts, you definitely need more power than normal USB can provide. Most of those amplifiers also need higher than 5 volt power.

    Class-AB amplifiers generally have higher quality than Class-D, and sometimes they're less expensive (though in these modern times it's not at clear-cut), but they are much less efficient. Usually they're around 50% to 60%. So if you use one of those, plan on needing a DC power source rated for about double the amp's rated power.
    OK this is great info
    I'm hoping my project can do with less power:

    Building a portable audio player. Sound quality doesn't need to be great. I'll just be playing mono voice files indoors. But I'm hoping it will sound better and louder than for instance an iphone on speaker mode.

    I have:

    - teensy 4.1
    - teensy audio shield
    - class D amp https://www.adafruit.com/product/987
    - 1 or 2 small 2.5" 8 ohm speakers: https://www.visaton.de/en/products/f...k-64-wpt-8-ohm

    Would it be possible to power this setup from a single lipo battery or battery pack of some sort ?

    And second question: can this be powered from the USB output on my laptop?

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    Quote Originally Posted by Jeroen View Post
    OK this is great info

    I have:

    - teensy 4.1
    - teensy audio shield
    - class D amp https://www.adafruit.com/product/987
    - 1 or 2 small 2.5" 8 ohm speakers: https://www.visaton.de/en/products/f...k-64-wpt-8-ohm

    Would it be possible to power this setup from a single lipo battery or battery pack of some sort ?

    And second question: can this be powered from the USB output on my laptop?
    That amp can only do 3.7W into 3 ohm speakers, it'll be more like 1.5W into 8 ohm - however you could parallel two
    or three 8 ohm speakers to get more from it. It needs the full 5V supply to get that level, and you should avoid
    using a switch-mode supply for a class D amp lest the switching frequencies beat into the audio range (this will make
    a loud unwanted tone). So maybe 2S LiPo into 7805 regulator for supply?

  20. #20
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    Thanks Mark. I got into this microcontroller stuff as a programmer, I don't have much hardware electronics knowledge. So I had to google most of your message
    But yeah great feedback!

    I do think my setup is maybe a bit overkill for what I'm trying to achieve. I also have https://www.adafruit.com/product/1381 this and an Arduino Uno.

    But probably the power consumption of the board/shields is negligible compared to the amp/speakers.

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