reading signal analizer

[MrDEB]

Am running all sorts of filter configurations for a DIY hearing aid.
My audiologist says 250hz to 4.3khz with 60-70 db loss.
well in running simulations in TINA I can kinda figure out the bandwidth and center frequency but why is the db measurment say -40 etc. ALL minus signs?
Here is a screen shot of a speech filter combined with a sallen filter. Looking at the screen I think it tells me the green trace (both combined with summing amp) that I have a 140 - 4khz bandwidth or ??

 

[MrDEB]

did I interpid this right??

230 bandwidth but what is the db gain being mius??
why??

 

[audioguru]

Do you want no bass and no treble?
Don't you want "normal" frequency response instead of this muffled and thin telephone quality?
The gain is low because adding a narrow bandwidth highpass filter with a lowpass filter reduces the gain at all frequencies.

 

[JimB]

A quick ramble about decibels.

A decibel is one tenth of a Bel, the bel being a large unit, tenths of a Bel are more convenient for practical use.

The decibel is the ratio if two powers, expressed logarithmically.
So we can use decibels to express the gain or loss of an amplifier, attenuator, transmission line etc.

0dB represents a gain of 1, ie the same power output as the input, a short length of cable will have a gain of 0dB, the power coming out is the same as the power going in.

+10dB represents a gain of 10, 1 watt in and 10watts out = a gain of 10dB
-10dB represents a loss of 10, 1 watt in and 100mW out = a loss of 10dB or a gain of -10dB

When stating the bandwidth of an amplifier or some other system, it is usual to state the bandwidth between the "half power points", ie where the power has dropped to half its mid band value.
-3dB is a loss of 2, half the power, hence the expression "3dB bandwidth".


Useful ratios to remember:

0dB 1:1
3dB 2:1
6dB 4:1
10dB 10:1
20dB 100:1
30dB 1000:1

To find the overall gain or loss of a system, it is easier to add and subtract the dBs rather than multiplying and dividing the ratios.

dBs can also be used as a measure of absolute value rather than being used as a ratio.

dBm = dBs relative to 1 milliwatt
dBW = dBs relative to 1 watt
There are others but these are the ones which are most familiar to me.

An example:

An audio signal has a level of +3dBm (power = 2mW)
it passes through an amplifier with a gain of +16dB
along a transmission line with a loss of 18dB (it is a very long line)
at the far end ther is another amplifier, this time with a gain of +10dB
What is the signal level at the far end?

 +3dBm      (the original signal level)
plus +16dB (the near end amplifier
plus -18dB  (negative dB, the line has loss)
plus +10dB (the far end amplifier)
 
+3 + 16 - 18 + 10 = +11dBm.

So, getting back to your original question (one of them anyway)

but why is the db measurment say -40 etc. ALL minus signs?

Because there is a lot less coming out of the filter(?) than going in, it has lots of loss.
This is probably good on the skirts of the filter, what the loss is mid band I cant easily see as I am typing this.

I hope this has been helpfull.

There are some people on this forum who think that decibels are a device of the devil invented by some twisted mathematical pervert for the torture of students.
This may be true!
But, give them a chance and they are very useful.

JimB

 

[MrDEB]

I will see if I can add a High Pass filter

What is considered a LOW PASS? anything below say 200khz?
If so Maybe I can add to filter.
I assume using a summing amp to combine all 3 filters together?
Found another filter that looks very promising.
see attached. It goes from 300hz to 2.8hz.
Need to compare the OPA 347 vers the OPA 364 op amps

 

[MrDEB]

revised wave form

circuit and wave form in TINA
wish I could get Filter Pro working. Keep getting an error??
Is this what we want? A low pass filter (red) and a mid-range frequency filter?green
Using the trial and error method of selecting a filter for hearing aid I am planning on building.

 

[audioguru]

I am not deaf so I do not understand why you want to remove important high speech frequencies from 2.8kHz to 20kHz.
What about music? And bird sounds? A squirrel hissing at you from a tree? FM radio and TV audio is flat to 15kHz. A cheap electret mic is also flat to 15kHz.
Doesn't a hearing aid try to correct a hearing deficiency so sounds are normal and natural?

Words like sunny, money, funny and bunny all sound the same without the natural and important high frequency consonants sounds.
There are thousands of other words that have high frequency consonants that are not recognised without the high frequencies. There was a discussion about "articulation" on the internet where missing high speech frequencies caused hillarious confusion.

Maybe deaf people can "read lips" to see the differences in those words.

 

[Vizier87]

audioguru proves that he's still at the top at his game!

 

[BrownOut]

1) Hearing loss is not the same as deafness.

2) Hearing aids are intended to restore the natural sound by emphasizing the frequencies that are suppressed in a patient's hearing, thus filtering and amplification are required. Simple amplifiers that reproduce all frequencies in the audio spectrum are of little use, as they only make the sound louder, and don't correct for the unevenness in hearing loss. Believe me; I've tried all different methods of hearing augmentation.

 

[audioguru]

Most deaf people who abused their hearing with guns, motorcycles and acid rock music have a hearing loss at high audio frequencies.
But here the OP wants to remove the frequencies that need to be boosted.
Why?

Maybe I understand his hearing loss wrongly. Maybe he can hear deep bass and sizzling highs better than my normal hearing but can't hear the narrow range of frequencies produced by a telephone.

An audiologist doctor does not measure all normal hearing frequencies. They don't care about speech articulation, music and other natural sounds, they only care about basic grunts and groans like heard on an AM radio. They do not measure above only 8kHz because the test headphones and hearing aid transducers cannot accurately go higher.

People with hearing aids complain about background noise maybe because their hearing aid does not produce the important high frequency consonants sounds in speech. Background noise is muffled, it is not crisp and clear like normal sounds. Without the important high frequencies then every sound is the same as background noise.

 

[MrDEB]

My hearing loss is from 200hz to about 4khz with 60-70 db loss.
Most hearing aids stop at 4-5 khz. the ultra high priced ones maybe go to 13khz but nothing higher.
as for background noise, my plan is to have mics facing forwards and back (a collar arrangement. a button to turn on front, back or both.
looking at using an OPA 4364 quad op amp. very low voltage requirements, single supply and a 7mhz bandwidth. 90db gain min.
very low noise. I just need to design a filter for optima hearing. The filters I am working with are in a TINA example file. Just changing component values to get a filter for lows, highs and in betweens. Lets just say experimenting as I am lousy at math. The speech filter is where I am starting and trying to get a wider bandwidth. Plan to use a summing amp to join all the filters together. Would like an auto cut off when the ambient noise (a gun shot or ?) occurs The OPA4364 has an enable pin so going to look into that as well??

 

[MrDEB]

Enable feature

I knew Audioguru is at the top of his game. That's why I listen to ANY and ALL of his suggestions.
Now I saw this feature on the OPA4364 and wonder if I can use to turn off the hearing aid upon a loud noise?
How to determine at what noise level to turn off?? and how. Could use a PIC perhaps??
ADC method??

 

[audioguru]

Hi MrDEB,
If you want to amplify frequencies from 200Hz to 4kHz then do so.

You have a 2nd-order 300Hz highpass filter feeding a 2nd-order 2.8kHz lowpass filter. Its bandpass output is from 300Hz to 2.8kHz like a telephone or AM radio.
Then why do you have a separate 3rd-order 12kHz lowpass filter?

You cannot sum filters together. When you do, you get a notch filter (due to phase-shifts in the filters) like you see at 90kHz.

The signal can be fed through a resistor normally without attenuation. Then when a very loud sound is detected by a comparator it turns on a transistor or Jfet that shorts the signal to ground, attenuating it. The comparator can have an adjustable reference or a dynamic reference.

 

[crutschow]

If I understand correctly, the OP would want a circuit with a flat frequency response for the high and low frequencies and a gain for the frequencies between 250Hz and 4.3kHz, where there is a hearing loss. This could be achieved by connecting a single 250Hz to 4.3kHz bandpass filter output into a two-input summing op amp that has the unaltered signal as its other input. By adjusting the relative gains of the two inputs, you should be able to achieve the proper frequency response to compensate for the hearing loss.

An easy way to do the filter design is to use the FilterPro free software from TI. Use the "Wide Band-Pass' option.

Below is a filter from FilterPro with a summing amp. The filter consists of a Salen-Key high-pass and low-pass filter in series to achieve the bandpass. This is summed with the input signal to give the flat response outside the bandpass. You likely will have to adjust the filter and gain parameters to get the exact response you want.

Note: Depending upon the relative phase of the filter output, you may have add an additional inverting op amp to invert the phase of the signal to avoid a dip at the edges of the bandpass where the summed signal transitions to a gain of one. I had to do that on the attached circuit.

 

[BrownOut]

Once again, hearing loss is not the same as deafness. Neither is hearing loss necessarily from people abusing their hearing with loud music or motorcycles. My own hearing loss appears to have a hereditary origin, as most of my family suffers to some degree from hearing loss. Further, my audiologist treats patients in Africa who suffer loss, and none have any exposure to damaging sound levels. Some hearing loss comes with age in many people. I heard of a proposal to keep teenagers from loitering by blasting high frequencies that only those under about 20 years old can hear. As with most things, the natural loss in hearing that comes with age varies widely between individuals. Other causes are medical in nature. Hearing can be damaged by disease or can be defect by birth.

My audiologist tested me over the entire audio spectrum, and I was fitted with hearing aids that are designed to augment any part of the spectrum. They are programmable, so that they may be customized to the patient. They contain a speech processor, which is supposed to be the best technology available. In spite of that, it remains difficult for me to filter out background noise and hear what I want. They even have a switchable mode that cancels sound originating from behind me, which helps, but does not eliminate the issue. They also feature the ability to automatically attenuate loud noises, which can be both a blessing and a curse. Slamming doors and such are nice to have filtered out, but sometimes, very low frequencies, which I would not normally hear that well anyway ( rather I would "feel" them ) cause the amplification to temporally shut off, and I miss things I would have otherwise hear. It would be nice if I could switch the feature on and off.

 

[audioguru]

I am an old geezer. I used to be able to hear "ultrasonic" burglar alarms but now I an lucky to hear the 15.75KHz from a CRT TV.
Speech and music still sound perfect. My tweeters still "sizzle".

I am like most guys. We can completely filter out nagging from the wife. But I already took the garbage outside and vacuumed the house (but not this week). I bought the beer yesterday, not today (but I drank it all today, burp).

 

[BrownOut]

HAHA! Consider yourself lucky to have had good hearing. Mine has been bad for as long as I can remember.

PS, as I type this, I can only hear the key clicks if I am wearing my hearing aid. I kind of like the sound of it.

 

[audioguru]

Lucky? No.
I took care of my hearing.
Other guys laughed at me when I covered my ears before a gun blast. Now they are deaf and I hear almost perfectly.
I used ear-plugs when shooting a rifle in the Air-Cadets and near a noisy engine.
I covered my ears when a stupid noisy motorcycle was within about 20 miles.
When I measured 120dB in a disco then I did not go there anymore.
I never listened to acid rock noise.

 

[BrownOut]

Yeah, I did those things too, but I've never experienced normal hearing.

 

[audioguru]

When I was young I could hear "ultrasonic" burglar alarms. They were very loud.
Then of course I could also hear the harmonics of everything. Crisp and clear. Sizzling. A live orchesrta sounded wonderful. (My grandfather conducted an orchestra).
I could hear the difference between manufactureres of violins. A Stradivarius sounded great.
I loved classical music until I heard it all then I became accustomed to the beat and horrible sounds of pop music.