You have a single 9V battery to power the amplifier. The amplifier is biased so that its output rests at +4.5V so that the output can swing the same amount up to +6V or more and can swing the same amount down to +3V or less.
If you connect the speaker directly to the amplifier output and ground then the speaker has DC current in it which moves its cone forward or backward which heats it and heats the amplifier (wasting battery power) and causes the speaker to produce distortion.
So an output coupling capacitor is added between the +4.5V output of the amplifier and the speaker to block the DC. Then the speaker can swing the same amount up (positive) and can swing the same amount down (negative). It is normal for a speaker to be driven positive and negative.
Measure the speaker's resistance which will be only a little less than its impedance. An LM358 cannot drive a speaker, its datasheet shows a load that is 2000 ohms minimum .
Why are you amplifying 15kHz? Many speakers cannot produce a frequency that high.
The LM386 little power amplifier can drive an 8 ohm speaker to 6V p-p when powered from a 9V battery. It is already biased so that its output is at half the supply voltage and it already has negative feedback resistors for a voltage gain of 20 or 200. All its details are in its datasheet.
If you connect the speaker directly to the amplifier output and ground then the speaker has DC current in it which moves its cone forward or backward which heats it and heats the amplifier (wasting battery power) and causes the speaker to produce distortion.
So an output coupling capacitor is added between the +4.5V output of the amplifier and the speaker to block the DC. Then the speaker can swing the same amount up (positive) and can swing the same amount down (negative). It is normal for a speaker to be driven positive and negative.
Measure the speaker's resistance which will be only a little less than its impedance. An LM358 cannot drive a speaker, its datasheet shows a load that is 2000 ohms minimum .
Why are you amplifying 15kHz? Many speakers cannot produce a frequency that high.
The LM386 little power amplifier can drive an 8 ohm speaker to 6V p-p when powered from a 9V battery. It is already biased so that its output is at half the supply voltage and it already has negative feedback resistors for a voltage gain of 20 or 200. All its details are in its datasheet.