Confusion About DB Values Below 750 Hz

@MikeF said:

I’m a little confused about what should be the DB values below 750 Hz. From my understanding, Typically an audiologist looks for 0 DB at 250 and 500 Hz and normally does not check hearing below 250 Hz.

However the folks at digital-recordings.com seem to think that the ideal hearing at 750 Hz should be -1.5 DB, at 500 Hz should be -3.5 DB, at 250 Hz should be -11.7 DB, and at 125 Hz should be -25 DB. See chart below of their ideal curve.

What confuses things even further is what Behringer (maker of audio compressors) appears to think. I was checking my MDX1600 audio compressor yesterday to see when compression kicks in and with the threshold set at -10 DB, the compressor started compressing at 52 DB for all frequencies 750 Hz and above. For 500 Hz, it started compressing at 55 DB, at 250 Hz it started compressing at 58 DB, and at 125 Hz it started compressing at 67 DB. Therefore they start compressing audio 15 DBs earlier at 750 Hz and above than at 125 Hz. Also with the peak limiter set to + 3 DB, the audio at 750 Hz and above, hit the peak limiter 15 DBs earlier than did the audio at 125 Hz. The test was not based on hearing but was visual where the input to the compressor was fed different frequency tones increasing the output to the compressor by 3 DB until the compression and peak limiter lights would light and then recording the results.

Therefore it seems that Behringer may think they are using the correct hearing curve and therefore starting compression at different times depending on the frequency. Also it seems they think it is ok to exceed the maximum DB at 125 Hz by 15 DB more than at 750 Hz and above without doing hearing damage.

My current hearing curve at 125 Hz is 7 DB above the digital-recording.com ideal hearing curve and 3 DB below the Behringer compressor response curve. If I adjust my equalizer so that curve below 500 Hz remains the same as it was originally and add up to 12 DB gain to frequencies above 750 Hz to try to create as flat of a curve as possible above 500 Hz, the nasal (muddy) sound through my Bose headphones disappears and clarity improves significantly but the pitch of voices are slightly too high.

If I reduce the gain of the 125 Hz frequency to become closer to the ideal curve from digital-recordings.com, the pitch of voices skyrockets and becomes extremely annoying. If I increase the gain of the 125 Hz frequency to move closely resemble the curve from Behringer, voices tend to sound lower pitched and more natural without any apparent loss of clarity. If I increase the gain at 125 Hz even further, voices tend to become even warmer and more natural.

Am I interpreting the Behringer curve correctly by using when compression starts? Is it also true that the ears can take more DB SPL at lower frequencies than at higher frequencies? Is there an accepted standard for hearing below 250 Hz?

I’m wondering if the issue here is a confusion of the dB scale.

There are a wide range of dB scales (dB SPL, dB A, dB C, dB HL etc), each with a specific purpose in mind. So if you start trying to compare HL vs SPL you could certainly run into confusion.

When hearing is tested, the scale is such that every single frequency should be along the 0dB HL line at every frequency. However minus 10dBHL to plus 20dBHL is still considered in the normal range.

@ZCT said:

I’m wondering if the issue here is a confusion of the dB scale.

There are a wide range of dB scales (dB SPL, dB A, dB C, dB HL etc), each with a specific purpose in mind. So if you start trying to compare HL vs SPL you could certainly run into confusion.

When hearing is tested, the scale is such that every single frequency should be along the 0dB HL line at every frequency. However minus 10dBHL to plus 20dBHL is still considered in the normal range.

This was my thought as well.
Their values of -12dB have no meaning when you don’t know what’s in relation to. What’s the dB scale they are using?

Also, a side note. When an audiologist tests hearing we aren’t looking for 0dB. 0dB is not an absence of sound since the dBHL scale is a logarithmic calculation to be able to translate dBSPL into a graph where normal hearing would be displayed in a fairly flat line. So there is no such thing as perfect hearing. Only normal or not normal.

To give you a little better idea of what I am up against, the following two jpeg files are plots of my left and right ears. In the plots is a line which is the ideal hearing curve according to digital-recordings.com1 and I am trying to get as close as possible to that line by using equalizers.

Over the next couple of days, I will be setting up my 15 band equalizer and my 31 band equalizer to try to create the ideal curve. The curve is in relation to -66 DB (the best hearing of my left ear at 500 Hz).

You will notice that my worst ear (right ear) most closely follows the ideal hearing curve below 750 Hz. Getting my ears balanced below 750 Hz as well as trying to obtain the ideal curve below 750 Hz at the same time is quite complex and time consuming.

You can see that my left ear curve is fairly flat below 750 Hz giving me loud muddy sounds for the speaking voice (85 Hz-250 Hz) where as my right ear follows the curve more closely giving me quiet but fairly normal sounds for the speaking voice.

Just finished setting up my equalizer and the results for the left ear are in the attachments at the bottom. The first plot is for the initial left ear, the second is the plot of the left ear after the 15 band equalization, the third is a plot of the left ear with the 15 band equalizer feeding the 31 band equalizer, and the forth is a plot of the 15 and 31 band equalizers enabled and a 9 band equalizer is enabled to add bass (62.5 Hz, 125 Hz, and 250 Hz bands are pulled up with all other bands at 0 DB).

Since only a maximum of 5 attachments is allowed, the right ear plots will be in the following post.

The 9 band equalizer was implemented to improve the sounds of music (added depth), mellow the sounds of tv programs, and to increase the loudness of the speaking voice (fundamental frequency between 85 Hz and 250 Hz). All the equalizers have a button to enable or disable that equalizer so therefore I have a choice of which equalizer(s) I want to use but should have at least the 15 band equalizer enabled since that does the majority of the work to balance my ears as well as develop the correct curve.

With a reduction of 21 DB for my left and a 24 DB gain at the higher frequencies, there is a maximum equalization between the speaking voice and it’s harmonics of about 45 DB with the 15 and 31 band equalizers enabled.

With limited testing with only the 15 and 31 band equalizers enabled, the clarity of movies has dramatically increased. Prior to the equalizers, my comprehension of dialog in movies was extremely low when using headphones/earphones. My comprehension level was so low that normally I would have to turn on “closed caption” to follow the plot of a movie. Having my ITE hearing aids in my ears as well as the headphone on helps but my comprehension level is still a struggle and I can normally follow the plot but I miss a lot of the dialog. With the equalizers enabled, my comprehension level increases to about 95%-99%. I watched segments of several movies that I previously had extreme difficulty hearing the dialog even with my hearing aids and headphones combined. One was the “The Social Network” where I watched the most difficult scene where they were at a nightclub with loud background music playing and they were speaking softly. Before I didn’t understand a word that was said but now I heard the vast majority of the words. Another small segment that I watched was one of the Hannibal Lecter movies which always gave me extreme difficulty in understanding the dialog but with the equalizers, I have very few problems.

The biggest problem with using equalizers is maintaining the perception of loudness. Because of that issue, I have a sound level meter to check the loudness level before I use the system. Although a sound level meter may not give an accurate level when testing headphones, I have never lost the perception of loudness and therefore have the ability to determine what is a comfortable safe sound when using the headphones without the equalizer being enabled. Once I determine that level, I use the sound level meter to determine what is that level. For example, I can play a bunch of songs from Spotify and set the volume to be comfortable. If the compressor is enabled, the songs will likely be compressing (a red light on the compressor will be lite), I can then check the sound level meter to see what it indicates. As long as I don’t increase the volume level beyond that level when the equalizers are enabled and compressing, I’m safe.

Perception of Loudness Issues

Over Driving the Equalizers

Equalizers have to work within a certain range of amplitude and when 12 DB is added to any frequency, the equalizer will tend to intermittently be over driven producing a sound artifact that sounds like “lip smacking” when someone is speaking. Therefore the input level of the equalizer must be reduced by about 5 DB (a level control is on all equalizers). With two equalizers, this causes a 10 DB loss in gain. Just turning up the gain on the headset or a headphone amplifier by 10 DB does not solve the problem since the compressor is set to the threshold level that was previously set. In my case, the threshold level was set to -10 DB which basically restricts the maximum stream level to about 75 dBu, By turning up the volume on the headset by 10 DB SPL with the compressor still set to -10 DB, a loud sound could drive the headset to 85 DB SPL.

To solve the problem, the compressor must be turned down by another 10 DB to -20 DB restricting the stream to about 65 dBu. Then by increasing the volume on the headset or headphone amplifier by 10 DB will not cause any problems since the compressor will kick in early restricting the headset to about 75 DB maximum.

Equalization and Loudness

In order to get my hearing near the ideal curve, I reduced the gain of my left dominant ear by 21 DB at the 125 Hz frequency range (fundamental range of the speaking voice). This reduces loudness but dramatically increases the harmonic ratio between the harmonics and fundamental voice sound. To get the perception of loudness back, I can increase the curve below 500 Hz but for every 1 DB that it is increased, I would lose 1 DB of the harmonic ratio in relation to the fundamental voice sound and would therefore lose clarity.

In my case since my left dominant ear was 21 DB above the ideal curve at 125 Hz and probably about 10-15 DB above the normal hearing curve, I alway kept the loudness of the tv at a relatively low level (in the 40 DB to low 50 DB range). Someone with the ideal hearing curve would likely have the volume set to produce 60-70 DB. Therefore I am just going to have to initially use the sound level meter until I have adjusted my perception of loudness.

Having significantly better hearing than the ideal curve in both ears below 75 Hz has been a curse. Those are normally the sounds of hum in stereo systems, vibrations and floor rumble caused by speakers, hum caused by high tension lines, road noise, and certain noises caused by motors. For the last five years I have lived in the top floor of a condo and all the electrical equipment is placed on the roof (air compressors, heat pumps, etc.) and every once in a while, one of those with bad bearings or loose mountings starts up just as I am falling asleep and I am jarred awake by the hum. Rolling over on my other ear doesn’t help since both ears have superb hearing ability in that frequency range.

Frequencies below 75 Hz is such a problem for sound stage or studio recordings that much of the sound stage equipment has a 75 Hz cutoff switch (equalizers, crossovers, etc.) to reduce loudness below 75 Hz.

Compressors/Equalizers and the Perception of Loudness

A compressor is needed to protect your ears primarily from sounds that you don’t hear (harmonics) or you hear a sound at a low level (musical instrument at a high frequency) while not realizing that a large DB SPL may currently be applied to your ears. By increasing the gain of equalizers at the higher frequencies, you risk the fact that a musical instrument could be playing at that frequency at 70 DB and an equalizer gain of 24 DB would produce a 94 DB SPL sound to your ears without the protection of a compressor. The same can occur with harmonics.

This is a major problem since the compressor compresses all frequencies and not just the sound that is exceeding the threshold. Therefore if a sound of 94 DB is produced (including the gain of the equalizer) and the compressor has a threshold of 75 DB and a compression ratio of 8:1, all frequencies (including the fundamental voice sounds) will have their gain reduced by about 17 DB. As you add more equalization to increase the gain of the high frequencies, the problem becomes worse.

To try to solve this problem, hearing aids have multiple crossovers and compressors. The newest most expensive hearing aids have as many as 16 crossovers and 16 compressors. Crossovers just divides the full frequency range into bands and feeds those bands to different compressors. Now when compression occurs, the compressor only compresses frequencies in it’s band. After compression is completed, all the compressors feed a mixer to put the bands back together into one big band that covers the full frequency range. I suspect that the crossovers in hearing aids produce a fixed size band. So therefore it is likely that if a hearing aid was supporting frequencies between 0 Hz and 8 KHz and had 16 crossovers and compressors, each band would be 500 Hz (8,000/16) and each of those bands could move up and down in gain independently of the other bands.

Therefore to solve my problem, I need multiple crossovers and compressors and a mixer. I currently have 4 compressors (2 per channel) and am currently using two (one for each channel). I also have a 2 way crossover (4 bands total or two bands per channel), and a mixer but am waiting on the cables to test out the crossover to make sure that sound artifacts aren’t produced at the crossover point. If i get a three way crossover and another 4 channel compressor, this will allow me to create 4 bands per channel. That may be enough compressors and crossovers since they are primarily needed on the declining hearing slope and therefore I would likely create a band between 0 Hz and 1.5 KHz, another between 1.5 KHz and 2.0 KHz, another between 2.0 KHz and 2.4 KHz, and the final band to include 2.4 KHz and above. If that is not enough, another 3 way crossover, another 4 channel compressor, and another mixer would be needed to support 6 bands.

Part of what I am doing is to help me hear better but the engineering part of me is doing it to see how far I can push equalization without any major problems. Although one two way crossover would likely solve my current problem, having the system work with more compressors and crossovers may help solve my hearing problem as my hearing gets worse in the future.

The following are the plots for my right ear that I couldn’t put in the previous post due to the limitation on the number of attachments.