Though it might surprise you, human hearing is significantly more sensitive to some frequencies than others. You can think of this phenomenon as our built-in frequency response. However, unlike a speaker or amplifier, variations are not something we want to compensate for in an audio system—at least, not directly. Let’s discuss how human hearing works with respect to different frequencies and why we need to compensate for this when making sound or product specification measurements. All of this will tie together perfectly with an explanation of weighting curves.

Human Hearing and Frequency Response

Did you know that human hearing is most sensitive around 3.5 kHz? This is due to the dimensions of the ear canal, which typically resonate between 2 and 5 kHz. Even a faint sound at 3.5 kHz is easy to detect. A sound might need to be 10 dB louder at 350 hertz to be perceived as having the same loudness.

In 1933, Harvey Fletcher and Wilden Munsen performed a set of measurements to quantify human hearing concerning frequency and intensity perception. Their paper, “Loudness, Its Definition, Measurement and Calculation,” included what became known as the Fletcher-Munson curves.

The lines on the chart are separated into amplitude levels called Phons. The Phon is the unit of measure used to describe sounds perceived as equal in intensity. As such, they follow the equal loudness level contours proposed by Fletcher-Munson and subsequent iterations.

It shouldn’t be surprising that the test equipment used to generate test tones wasn’t as precise in 1933 as it is today. Similar testing in 1937 by Churcher and King and again in 1956 by Robinson and Dadson produced significantly different results.

Introducing the Equal Loudness Level Curves

The International Organization for Standardization (ISO) took over the creation of reference loudness curves in 2003. A study by Tohoku University, Japan, and the Research Institute of Electrical Communication showed errors as large as 15 dB from the original data. These new tests became the ISO 226:2003 Standard. Don’t think it’s over yet. These have since been revised again to the ISO 226:2023 Standard.

Of course, the above curves are averaged across a wide selection of people of different shapes and sizes. Everyone’s hearing will be slightly different in the areas where it is most sensitive. However, this data provides an excellent overview.

If anyone references the Fletcher-Munson curves, they are at least four generations behind in their data. When discussing the perception of sound levels versus frequency, the proper reference is the ISO 226:2023 Equal Loudness Level Curves.

Interpreting Equal Loudness Level Curves

If you look at the 1 kHz point on each trace, you’ll see that this point coincides with the reference SPL value. So, a sound at 70 dB at 1 kHz is perceived as being at 70 Phons. However, it takes about 74 dB of energy at 1.5 kHz to seem as loud. Further, it takes only 67 dB of energy at 3 kHz to seem as loud.

What matters here are the extreme ends of each trace. We can make some generalized assumptions about human hearing based on the reduced sensitivity in these regions. Staying with the 70 dB trace, we would need to hear a sound that’s 83 dB at 10 kHz to be perceived as being as loud as 70 dB at 1 kHz. Further, a sound at 93 dB at 63 hertz is also perceived to be as loud as 70 dB at 1 kHz.

Though we haven’t consulted with an audiologist (yet), the issue is less about attenuation at opposite ends of the audio spectrum and more about an increase in sensitivity in the middle. As mentioned, the ear canal resonates around 2 to 5 kHz. Furthermore, the outer ear, called the pinna, also amplifies sounds in this frequency range.

The middle ear bones, the ossicles, are more efficient at transmitting mid-frequency sounds. An effective impedance mismatch between the air and the fluid in the cochlea further accentuates this frequency range.

There are additional mid-frequency sensitivities in the cochlea due to where different frequencies peak.

Audio System Equalization

We’ve seen many amateurs try to equalize their audio systems, more often in a home environment than in a vehicle, to compensate for the shape of these curves. That’s not the purpose of the information. Our perception of hearing is static. In short, we hear what we hear. We accept that the sound of a trumpet or saxophone is what it is. We don’t want to change that presentation to compensate for being more sensitive in one range versus another.

There is an exception to this statement. Regarding headphones and earbuds, flat response doesn’t sound accurate. This is because we’ve eliminated some of the frequency filtering caused by the pinna. As such, a modified response curve sounds best. The team at Harman International has devoted significant time and expense to creating a target curve for headphones based on similar experimentation that created the Equal Loudness Level Curves.

Analyzing Headphone Target Frequency Response

There are two critical pieces of information to extract from the above response graph. First, we can see the boost around 3.5 kHz that coincides with the boost the pinna of our ears adds. Without this, headphones would sound dull and flat. Second, there is a boost in low-frequency information. Part of this will be due to the Equal Loudness Level Curves, and part will be listener preference. We all know that many people prefer bass information boosted in their listening systems. The Harman headphone curve combines the science and mechanics of human hearing with extensive listener preference. They even have details on the percentage of people who prefer more bass and less bass.

Harman uses very specific test equipment to measure headphones. Specifically, a head and torso simulator accurately and repeatably simulates how humans perceive sound.

It’s worth noting that Harman has revised its target curve from the one shown above. Unlike this first iteration, they are not releasing this new curve to the public for endless debate and whining (insert sarcastic wink here!). They will use it to fine-tune the performance of their AKG, Mark Levinson, Harman Kardon, and JBL consumer and professional products.

Speaker Evaluations in Free Field Conditions

If we measure the frequency response of a conventional loudspeaker using a sine sweep or pink noise, we should end up with a fairly flat line, assuming the speaker can play from 20 Hz to 20 kHz with good accuracy. Most floor-standing home speakers roll off below 30 Hz. It would be best to have a dedicated subwoofer to fill in that bottom octave.

The chart below represents a nearly perfect speaker’s ideal frequency response. Do you think if we suck up to KEF enough that they will loan us a set of Blade 2 Meta to use as our reference review speakers? Here’s hoping!

As noted in Stereophile Magazine’s review of the Blade 2 Meta, the boost in the bass is primarily due to the close-micing technique used for measurements.

A-Weighting Curves in Measurements

Let’s get to the nitty-gritty of this article. Look at the Equal Loudness Level Curve chart above and analyze the 40-phon trace. Information similar to this was used to create what’s known as the A-weighting curve. This curve is intended to be applied to a sound pressure measurement so that the energy in the measurement correlates to how we hear. It was actually the Fletcher-Munson 40-phon curve that was used to create the A-weighting curve. Thankfully, the ISO 233-2032 40-phon curve is quite similar.

To show you the same data in a format you might be more familiar with, we dug out the Sony XM-4ES amplifier we reviewed a few years back. We performed a frequency response test with all the settings flat and again with the A-weighting filter activated in the QuantAsylum software.

 

Where We Use Weighted Measurements

The ANSI/CTA-2006-D standard for measuring car audio amplifiers calls for applying the A-weighting curve to the measurement after the reference level is set. Up to this point, we’ve shown the measurements as unweighted. The result is slightly lower values, indicating the presence of more noise. As we move to further comply with ANSI/CTA-2006-D, we’ll start using the A-weighting curve to evaluate the signal-to-noise ratio of the source units’ amplifiers and signal processors we test.

We can see that the Signal-to-Noise Ratio measurement of this Sony XM-4ES amplifier is specified as being 71.23 dB.

Now, if we turn on the A-weighting filter and apply it to the measurement, the low- and high-frequency information is attenuated, which reduces its effect on the measurement.

The QuantAsylum software has revised the SNR measurement to -76.11 dBA. The addition of the letter A after dB indicates the use of A-weighting. Given the published measurements on the CTA TECH website, which show -76.5 dBA for the XM-8ES and -80.8 dBA for the XM-6ES, we are comfortable saying our data aligns with those numbers.

So, the next time you see a signal-to-noise ratio measurement with the amplitude specified in dBA, you will understand how and why that rating system is used. Finally, a higher SNR number means that the noise is further below the test signal. A level of -75 dBA is about the minimum you’ll want to consider for an amplifier in a car audio system that will drive midrange and high-frequency speakers.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

A reader requested that we compare the sound quality performance of wired and wireless Apple CarPlay. This is a great idea, as very little information is provided about the communication standards used to send audio signals wirelessly to a source unit. So, let’s look at distortion characteristics and frequency response to see if there is a performance difference.

The Test Configuration

While our lab has an impressive selection of car radios, we didn’t have one that supports both wireless and wired CarPlay. We contacted our friend Lee Mattason at Burlington Radioactive for help. He graciously lent us a Kenwood eXcelon DMX908S to make some measurements. If you live in the Golden Horseshoe region of Southern Ontario and are looking for a car, boat, or motorcycle audio upgrade, Lee and his team are a great choice.

We are going to perform two different tests using four connectivity criteria. We will analyze sound quality by playing a 0 dB 1 kHz test tone and then analyzing it for harmonic content. The second test will evaluate frequency response by analyzing a white noise track. In both cases, the test tracks have 44.1 kHz sampling rates and are stored with 16 bits of depth. To ensure Apple didn’t alter the file while uploading it to the phone, we used the Onkyo HF Player and copied the tracks as files to a specific folder on the iPhone. The device is an Apple iPhone 14 Pro running iOS 18 Beta 2. Please don’t ask why the owner tries beta software on this phone—it’s a terrible idea.

The four communication methods we’ll use are playing the files from a USB memory stick, over a Bluetooth connection, through Apple CarPlay with the phone connected to the radio with a Lightning cable, and finally, using Wireless Apple CarPlay.

USB Media File Playback

The first test is to play the 1 kHz test tone track directly from the USB stick. We’ll note that with all the audio settings turned off and the source-specific levels flat, we saw an output of 4.539 volts on the front RCA output. The signal had a THD+N measurement of 0.01086%, which is excellent for a consumer-grade source unit. The signal-to-noise ratio (SNR) came in at -85.39 dB, unweighted.

Next, we have a 10-minute white noise track. We cranked up the averaging on the QA403 audio analyzer to produce as flat of a measurement as possible. The goal in interpreting the data below is to average the frequency response in your mind. We’d call this flat from below 20 Hz to 22 kHz on the top end. This is precisely what you’d expect from this measurement.

These measurements will serve as a solid baseline to compare with other playback media.

Bluetooth File Playback

We all know that the iPhone isn’t the go-to for Bluetooth sound quality. The use of Bluetooth 5.3, specifically with the SBC codec, really limits performance compared to an Android-based device that offers something spicier, like LDAC or aptX HD. For now, this is the test, and we’ll hunt down an Android phone and give that a go in a future article.

Let’s start with the distortion measurement. In a word, we can sum this up as being subpar. We can see all sorts of artifacts and harmonics starting at -46 dBV. This gives us a THD+N specification of only 0.14655% and a signal-to-noise ratio of 56.83 dB.

In terms of frequency response, the curve remains nice and flat, so overall, music would sound generally the same. The increase in distortion shown above hampers clarity and could add some emphasis. However, the raw amplitude measurements are similar. Looking closely, you can see that the high-frequency information rolls off at about 17.5 or 18 kHz. None of us can hear anything above 18 kHz, so that’s not a massive concern.

Wireless Apple CarPlay

We deleted the Bluetooth pairing from the radio and phone and reestablished it to turn on Apple CarPlay. We were cautious in all the tests to ensure the communication used our desired method.

Starting with the 1 kHz tone, we can see that the Wireless Apple CarPlay connection uses Bluetooth to transmit audio to the source unit. The distortion numbers are slightly worse at 0.15072%, and the SNR is -56.62 dB unweighted. If you are fanatical about sound quality, this isn’t the best way to get music from your phone to your radio.

Using Wireless CarPlay, the high-frequency information is attenuated a little more than when using a Bluetooth-only connection. We’d say the audio information stopped around 17-17.5 kHz. Once again, this won’t be audible to most people. However, it’s repeatably measurable and very real.

Wired Apple CarPlay

Last but certainly not least, it’s time to see how the combination works with a wired connection between the iPhone and the Kenwood radio. We deleted the phone pairing from both devices and plugged the phone into the micro-USB port on the back of the chassis.

For the 1 kHz tone, we saw a THD+N measurement of 0.01042%, which is the best of this test session. The signal-to-noise ratio came in at -85.21 dB. Both numbers are inconsequentially different from the original USB test. Nobody would be able to hear the difference in clarity or background level between Wired Apple CarPlay and playing an audio file from a USB memory stick.

We also have the spectral analysis of the white noise when played over a wired CarPlay connection. It looks nearly identical to the wireless connection. Again, even a variation from 17.5 to 20+ kHz isn’t audible to humans. Your pet cat, Fluffy or Wayne, or the neighborhood bat might disagree.

Conclusions on Apple CarPlay Audio Playback Quality

Based on this specific test, we’d say there is an audible difference in clarity between wired and wireless connections. Whether you are using Bluetooth or Wireless Apple CarPlay, you leave a significant amount of audio clarity performance on the table compared to Wired CarPlay or a USB memory stick. However, this is a single test with one specific radio and phone combination. It’s possible that an Android phone would fare much better wirelessly. We’ll make that test happen soon!

It’s also worth noting that the clarity difference might be easy to hear when sitting in the car with the engine off but much harder when you’re driving down the Interstate. Background noise masks a lot of distortion. However, we’d use a wired connection every chance we could.

If you are shopping for a new radio with Apple CarPlay and Android Auto, drop by a local specialty mobile enhancement retailer and ask what they have that will fit your vehicle. Be sure to bring your phone to connect it and ensure the radio works exactly how you want.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Unlike home audio systems, car stereo upgrades have to withstand harsh conditions. Your car, truck or SUV subjects your system to vibration, bumps and massive temperature changes. At home, as long as there isn’t a flood, everything should work fine for decades with regular dusting. Your car stereo system might not be quite as foolproof. Let’s look at five quick and simple tips to ensure that your system works great

1. Radio and Touchscreen Maintenance

The device you will interact with the most is, of course, your car radio. If you have a multimedia receiver, then the touchscreen and volume knob will be the primary contact points. A slightly damp cloth is likely the best way to keep these items looking good. If there’s something stubborn on a touchscreen, a product like Whoosh! is a perfect choice. Spritz a little of this cleaner on a soft microfiber cloth, and you’ll be able to get it looking like new. You can wipe any dust off the instrument cluster while you’re at it.

Never spray any liquid directly on the screen. If dust is caught in the corners of the screen, pick up a makeup brush from a dollar store. These are soft enough to prevent any damage to the screen. This cleaning process also works great for laptops and smartphones.

While checking the screen, make sure the radio is still solid and secure in the dash. Push on the corners of the chassis (not the screen) with a finger. If it moves, something might need tightening behind it. Please drop by the shop that did the installation and book an appointment to have them check it out.

2. Amplifier Maintenance

In most cases, a well-designed amplifier will happily play without trouble for years or even decades. You will want to start by ensuring that the amplifier or amplifiers remain secure in the vehicle. Once again, please give them a gentle push or tug. If they move, get with the shop that installed them.

Next, inspect the amplifier and the area around it for signs of water damage. If an amp is mounted in the corner of the trunk, water from a leaky seal can cause problems. Water and salt damage from slushy boots can cause trouble if the amp is under a seat. If you see signs of water on the amp, find out where it came from. Make sure everything is dry, especially a wooden amp rack.

Check all the wiring to and from the amp. Do the connections look solid? Do you see any signs of excessive heat? If anything looks like it might be loose or if plastic has started melting, consult with the installer immediately. Some types of wiring, especially copper-clad aluminum, can loosen over time and cause poor connections. Terminal blocks can get very hot when these connections get hot, and plastics will melt.

3. Power and Ground Connections

Though this is an extension of an amplifier inspection, pop the hood and look at the connections to the battery. Pull on the power wire (gently). If any connectors move, then something needs to be tightened.

Check the fuse holder. Is it still secure? Was it ever secure? The electrical connections will be stressed if it’s flopping under the hood. Make sure a reputable shop mounts the fuse holder securely.

Check the ground connection in the trunk or under a seat. These are notorious for corroding, resulting in amplifier failure from power starvation.

If you see any signs of green, white or blue corrosion on the wiring or battery terminals, there might be an issue with the battery or water getting into the wiring.

4. Subwoofer Enclosures

While the other items are likely safe to check once or twice a year, this one should be done every month. If you have a subwoofer in the trunk or cargo area of your car, truck or SUV, make sure it’s secured solidly to the vehicle. In the event of a severe accident, a subwoofer enclosure flying through the vehicle could be enough to seriously injure you or a passenger.

Using a simple hook-and-loop fastener isn’t enough. In an accident, forces from the deceleration can easily exceed 10 or even 20 Gs. This would make a 30-pound subwoofer enclosure act like it weighs 300 to 600 pounds. Little plastic hooks (on the hook-and-loop fastener) won’t keep it in place.

While you’re inspecting the subwoofer enclosure, push gently and evenly on the subwoofer cones. They should move smoothly. If their motion is rough or scratchy, you have overheated the voice coil and damaged the speaker. Don’t replace the subwoofer with an identical model. Clearly, you need something that’s more capable in terms of power handling and output capability.

5. Speakers

Without taking your car or truck apart, checking the condition of your car’s audio speakers can be difficult. The best test is to give them a listen. Put in some good earplugs and play music with a lot of bass at a moderate volume level. Long, drawn-out bass notes from an organ or bass guitar are better than drums. Get up close to the speaker and listen for buzzes and rattles. These could be signs that the cone is interfering with the grille or that the surround has failed. The speaker could also have come loose from its mounting. If you hear anything abnormal, have the installer check it out.

Please never do this without proper hearing protection. You only get one set of ears, and if you damage them, you’ll regret it for the rest of your life.

Car Audio System Maintenance Improves Longevity

Thoroughly inspecting your car’s audio system should take about 10 to 15 minutes. If anything seems even the slightest bit abnormal, return to the shop that installed it and have it inspected. Fixing a loose connection now could prevent you from having to replace an amplifier later.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.