This app note is part of a series on car audio sound. The top level app note and list of all parts is:
Contents
- Sound "philosophy" 1: Staging
- Sound "philosophy" 2: House curves in automotive
- Frequency ranges and their challenges
- Noise of the car and interior acoustic effects
Sound "Philosophy" 1: Staging [Top]
Overview
The staging of the played back content in the car is not created "naturally" through the physical positioning of the speakers or design of the crossover, but rather by tuning design decisions in the DSP. There are two predominant philosophies when it comes to staging.
EU style
The EU style of imaging could also be called "right behind the steering wheel". It can often be found with European car manufacturers. The phantom center of the stage is perceived to be directly in front of the driver and passenger. This sound is created in its core by delaying all sources to the driver position in the car. The imaging is automatically mirrored on the passenger side.
US style
The US style of imaging could also be called "middle of the dashboard". This can often be found in US and some Asian manufacturers. The phantom center of the stage is perceived to be in the middle of the dashboard. This sound is created in its core by delaying all sources to the middle of the car.
Width of imaging
A decision completely based on your personal taste and independent from the front imaging style is the width of the stereo image in the car cabin. The palette ranges from "A-pillar to A-pillar" up to "B-pillar to B-Pillar." In general, it can be said that very wide stages require support through rear fills, surround speakers and center speakers. If these speakers are not present, a narrower staging is the best choice. If you have plenty of speakers, you can widen your image more easily.
Although this app note is not supposed to be about setups, I must give one piece of advice here: integrate a center speaker! A simple 100mm or 80mm speaker in the middle of the dashboard adds wonderfully to the overall height of the image and stabilizes the perceived center image.
Using polarity flips, mid/side processing, phase shifts and allpass filters, the perceived width of the stereo image can be changed.
Sound "philosophy" 2: House curves in automotive [Top]
Flat and linear are relative terms in automotive. During the tuning process you will encounter weird and wonderful frequency responses. That is normal and you will acquire a feeling for working with graphs over time.
To give you an overview of the problematic areas, I have averaged the benchmark measurements I have made over the last few years from several cars with premium sound systems. The linearity analysis shows the average (blue) and the ±3 dBr line (green). Perhaps to your surprise, this curve is far from being flat and linear:
Note that the overall frequency response is tilted and loses about 1 dB per octave from 150 Hz on. This is good practice in a lot of cars and helps with ear fatigue over long listening periods and gives a modern sound. The shape of this curve goes back to the work of Binelli and Farina (http://pcfarina.eng.unipr.it/Public/Papers/245-AES125.pdf).
The bass area below 150 Hz is boosted above the ±3 dBr window to up to +6 dBr max. The general shape of this curve resembles the noise curve of the car itself and is therefore ideal for masking the car's noise.
Note that you can define your own target house curve in REW under Preferences – House Curve.
An even more in-depth article by Floyd Toole on house curves in general and automotive sound in special can be found here (free article): https://www.aes.org/e-lib/browse.cfm?elib=17839.
Frequency ranges and their challenges [Top]
This graph shows the frequency response presented earlier, annotated with the different frequency ranges to consider for car audio tuning:
0: Subbass
Probably the reason why you are here. The lowest reproducible frequency in the car cabin is determined by the diameter and excursion capabilities of your subwoofer/woofer speaker(s). Premium factory built-in systems go down to 25 Hz (f3), standard sound to about 38 Hz. The lower, the better.
Bass perception is – for laymen as well as for sound nerds – an important indicator for perceived quality. So, you want to aim for a good tuning response here. Besides the midbass, this is probably the second most EQed frequency range in the whole audio spectrum.
It has been proven that several low-frequency sources placed in different locations in the car give the best overall performance. That is, it is better for the overall quality impression to have two 8" woofers and one 10" subwoofer with proper mono bass tuning, rather than having all frequencies below 150 Hz handled by one 12" subwoofer in the trunk.
1: Bass
Car cabins and bass... this is inherently not a match made in heaven. The small size of the cabin makes it hard to get an even bass response: room modes conspire to make the unoptimized frequency response look like a stylized picture of mountains. The pressure chamber effect boosts the complete bass region like a shelving filter. A lot of EQ is needed to lower the peaks and even out the dips.
Within this region, you will probably have your crossovers from sub to midbass.
2: Midbass and low mids
This is without a doubt the most difficult range to tune. Most EQ bands are needed here. Our ear draws a lot of information from the frequency range of 50 Hz to 150 Hz since many instruments have their fundamental frequencies in this area.
Clean timing and phase relations between the speakers are necessary against acoustic cancellations.
Be aware of possible null points in this frequency area!
For practical advice, I recommend PSSound's Youtube channel: https://youtube.com/channel/UCKJUj746WOIUNjv5imYs6yA.
3: Low-mids and mids
In the low-mids and mids, the side effects of automotive acoustic measures can be found. Acoustic dampening material that is used to keep road noise out of the car also affects the interior entertainment sound. This can be seen in the figure above as a dip between 350 and 600 Hz. The deepness of the dip also goes back to the insufficient number of midrange speakers in the tested cars. 3-way-systems are a must in automobiles for a good sound.
Besides that, null points can also occur here.
4: Upper mids
In the upper mids we are well above the Schroeder frequency (room modes are random here). Most cars are fine to tune in this area. But, due to the high position-dependency of the measurement, multipoint measurements and array measurements are a must to make good tuning decisions here.
Within this region, you will probably have your crossovers from midrange to tweeters.
5: Presence and high ear sensitivity area
The human ear is the most sensitive in this area. Due to the individual size of pinna and ear canal, every human being has a slightly different frequency response, in-ear resonance and perception in this frequency area. If you do the car tuning for yourself, it makes sense to adjust it to your personal liking.
Take care of your hearing, especially when tuning and listening in this area. Our body can protect itself with the stapedius reflex. A muscle twitches in the hearing and causes a click-sound and the hearing immediately dampened by ca. 20 dBr. If you hear that clicking noise, be careful and turn down the levels. The stapedius reflex only protects our perception in the nerve system, not the hearing mechanisms!
Often a certain "trademark" frequency response can be found in this area. Since every audio brand tries to distinguish itself from one another, here is one of the places where they do so. So do not be surprised if you measure irregularities in this area.
6: Highs
The high frequencies above 5 kHz hold challenges with tweeter resonances and off-center use of speakers. If you are using hard-dome tweeters or broadband speakers, it is likely that you will encounter resonances that make the sound harsh and edgy. This can be countered with tuning.
Often tweeters cannot be used on-center in cars and are in the vicinity of reflecting surfaces. The loss of high frequencies must be encountered with high frequency boosts, while broad combing effects can be countered with slight boosts.
Noise of the car and interior acoustic effects [Top]
Driving noise
To give you an impression of how much noise and in which region your car produces noise, I have recorded an upper-midrange car of German origin with a six-cylinder Diesel engine with the miniDSP EARS at 30km/h, 80 km/h and 130 km/h. During the measurements, the entertainment system and air conditioning were switched off, so only the engine, powertrain, tire and wind noise is recorded.
The energy is mostly contained in the low frequency domain and drops sharply at higher frequencies. The broadband dip centered at around 500 Hz can be attributed to the acoustic dampening foam used to improve ride comfort. The bass dip and the slope of this curve are a relevant factor for the form of the previously shown house curves.
As you can see, quite significant amounts of noise are produced by wind, tires, powertrain and chassis.
Since there is nothing we can easily do about this boundary condition we must accept it and respect it in the DSP parameter creation. Since the noise is static, only slightly modulated and often not of a tonal nature, it is typically not perceived as disturbing.
Buzz, squeak, rattle, hum, oil-canning
Noise sources can also be found in the interior. Since these acoustic events are often of a temporary nature, our ear can easily detect them. Often trim pieces that have become loose vibrate in the audible range. This can be especially a problem when you are pumping a lot of acoustic energy into your cabin.
To get the chassis and interiors quiet from acoustic effects such as buzz, squeak rattle, hum and oil-canning, additional acoustic measures such as alubutyl, butyl, foams and mass loaded vinyl can be used to stop resonances from interior and chassis parts.
This article gives a good overview of products and areas of application: https://noico.info/articles/sound-deadening-main-areas/.
Pressure chamber effect and room modes
The small volume of car cabins has two effects on the playback of low frequencies. The low frequencies are boosted due to the sealed environment of the car.
Additionally, a lot of room modes are added, causing the frequency response to look like mountains instead of a gentle flat curve. Due to the geometric complications of a car interior, it is very time consuming to calculate the effect of the cabin with finite-element methods.
To give you an impression how severe this effect is, here is a comparison of a midbass driver measured in an anechoic chamber against the same driver in a car:
The enclosed environment of the cabin traps the bass and causes a shelving filter-like boost in the low end. In the example above you can see up to +20 dBr boost of the frequencies below 150 Hz.
Reflections
Without glass in the car, it would be very windy, or it would be hard to see anything at all. So, unfortunately, we cannot get rid of it. But we can work our way around it.
Glass and hard materials create strong reflections in the sound field. These often appear in measurements as dips and peaks in the mid and high frequencies. Fine EQ can be used to tame problems here. Be aware not to use filters with overly high Q.
Closing [Top]
That concludes this part! Jump back to the top:
Related products - Car Audio DSP platforms
.jpg "Harmony DSP 8x12")
