One of Two Speakers Has a Noticeable Vibration/Sound

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Clearly5000
 
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One of Two Speakers Has a Noticeable Vibration/Sound

Post by Clearly5000 »

Hello (hopefully this is the right forum),

I recently purchased and set up the "Stereo 20W Class D Audio Amplifier - MAX9744" this weekend and used the recommended 20w speakers to test everything out.

Music plays back fine, the amp itself works great. However, one of the two speakers has a very noticeable and unexpected vibration/sound when playing back music with any bass to it. To be clear, it's not a constant electrical buzz, it's only when low bass tones play, there is an audible vibration that increases the more the volume is applied. It kicks in subtly around volume level 40-45 (volume levels from the Adafruit tutorial for the amp, using digital control). I'm using my iPhone for source music with the volume about half way, but also tried using an alternate device (old MP3 player) with the same result.

I figured maybe the speakers didn't handle bass very well, however the second of the two speakers plays flawlessly at the same volume levels. It even plays fine when the volume is increased to 50-60. At these levels the vibration/sound is clearly coming from the other speaker.

I have tried mounting the speakers in a speaker box, letting them sit unmounted on my desk and holding the problem-speaker in my hand. I have tried switching the left/right outputs. I have tried powering the amp with 9v and 12v power sources. The Arduino is powered separately by USB. The vibration/sound can still be heard in all instances in the same speaker.

Should I request a replacement? Thanks for any info you can provide. Let me know if you need any other information.

Class D Amp
https://www.adafruit.com/product/1752

Speakers
https://www.adafruit.com/products/1732

Tutorial (where I'm using the code from)
https://learn.adafruit.com/adafruit-20w ... al-control

Code: Select all

#include <Wire.h>

// 0x4B is the default i2c address
#define MAX9744_I2CADDR 0x4B

// We'll track the volume level in this variable.
int8_t thevol = 31;

void setup() {
  Serial.begin(9600);
  Serial.println("MAX9744 demo");
  Wire.begin();
  
  if (! setvolume(thevol)) {
    Serial.println("Failed to set volume, MAX9744 not found!");
    while (1);
  }
}


// Setting the volume is very simple! Just write the 6-bit
// volume to the i2c bus. That's it!
boolean setvolume(int8_t v) {
  // cant be higher than 63 or lower than 0
  if (v > 63) v = 63;
  if (v < 0) v = 0;
  
  Serial.print("Setting volume to ");
  Serial.println(v);
  Wire.beginTransmission(MAX9744_I2CADDR);
  Wire.write(v);
  if (Wire.endTransmission() == 0) 
    return true;
  else
    return false;
}

// Read in + and - characters to set the volume.
void loop() {
  if (! Serial.available()) return;
  
  // read a character from serial console
  char c = Serial.read();
  
  // increase
  if (c == '+') {
    thevol++;
  }
  // decrease
  else if (c == '-') {
    thevol--;
  }
  // ignore anything else
  else 
    return;
    
  if (thevol > 63) thevol = 63;
  if (thevol < 0) thevol = 0;

  setvolume(thevol);
}

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adafruit_support_mike
 
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Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by adafruit_support_mike »

Those sound like symptoms of minor damage to the speaker's cone or membrane.. either a dent that creates an unexpected harmonic or a loose connection at an edge.

Could you post a photo of the speaker that's misbehaving (800x600 images usually work best), showing the "sounds comes out here" face please?

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Clearly5000
 
Posts: 6
Joined: Fri Aug 15, 2014 11:57 pm

Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by Clearly5000 »

Sure thing, thanks for the quick reply!
speaker_photo001.JPG
speaker_photo001.JPG (214.7 KiB) Viewed 401 times
This is the problem-speaker.

It looks in pretty good shape, compared to the working one, I can't see any major differences.

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adafruit_support_mike
 
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Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by adafruit_support_mike »

Thanks..

Just to hit the obvious things, double-check all your screws. A connection that feels solid can vibrate a fraction of a millimeter, which is more than enough to make noise. The best option would be to put plastic washers on both sides of the mounting tabs.. those will squish down a little and provide a solid mechanical connection while still absorbing vibration.

If that doesn't help, get something like a pencil or chopstick and do a 'tap test'.. basically low-impact drumming. Hold the stick about 6" back from the tip and lightly rap it against the cone around the perimeter. It doesn't take much force, you just want to hear how the cone sounds when it receives an impulse at various points. It's basically the same as the old 'knocking on the wall to find the secret door' trope from movies. If there's a gap in the connection between the cone and the frame, you'll probably be able to hear it.

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Clearly5000
 
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Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by Clearly5000 »

Yup, made sure the screws were tight, but at this point I've gone back to un-mounting the speaker so other objects causing the vibration isn't a factor. I've gone so far as to rest the speaker on a sheet of soft foam, but the vibration seems to be coming from within the speaker itself rather than any object it might be resting on.

Tried out the tap test. The area around the mid-right to top-right does sound slightly more "hollow" than the rest of the perimeter, the rest seems "tighter". Is that what I'm listening for?

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adafruit_support_mike
 
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Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by adafruit_support_mike »

That's the bunny.

The hollow sound means the cone has come loose from the mounting near that point. That's one of those problems which is impossible to see and might not be noticable in a standard test.

Send a note to [email protected] with a link to this thread and the folks there will get you a replacement speaker.

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Clearly5000
 
Posts: 6
Joined: Fri Aug 15, 2014 11:57 pm

Re: One of Two Speakers Has a Noticeable Vibration/Sound

Post by Clearly5000 »

Thanks, much appreciated for all your help.

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