Part 3: The Sound Stage

Do you have a stereo playback system in your home?

Did you read ITU-R BS 775 and set up your speakers accordingly? Good.

soundstage image1

image borrowed from the ITU-R standard

So your speakers are positioned to the left and right of the listener at an angle of 30o from the centre axis, and this produces a wonderful sound picture from left to right. You are able to localize each instrument in the image and you get an impression of the overall ambiance. You can tell that the violin is in a large concert hall or church, and that the kick drum sounds dry with little reverberation.

In the car, the stereo image is often known as the sound stage, perhaps because you are recreating the stage in front of you on the dashboard.

Unless you are driving one of these…

soundstage image2soundstage image3

You are going to positioned to one side of the car, which means it is not possible to sit in the “sweet spot” between the left and right speakers. It is therefore impossible to create a perfectly symmetrical stereo image for both front passengers.

If you imagine drawing the sound image, it might look something like this:

car audio asymmetry(1)

The sound image is affected by so many things including:

  • Loudspeaker position & angle

  • Number of loudspeakers

  • How well the sound is dispersed

  • Shape of the dashboard and control panel

  • Angle of the Windscreen
  • Dimensions of the car cabin

  • How absorptive the surfaces are

Regardless of all of these variables, the sound will be asymmetrical, and you will always experience some near-side bias, i.e. you will perceive more energy coming from the side you are sitting closest to.

What can we do about it?

There are several techniques that can be used to improve the sound staging significantly.

  1. EQ. By removing large resonances in the bass and lower mid-range, the sound stage will appear higher and your attention less drawn to the car doors where the speakers are positioned.

  2. Delay. This is what I really wanted to talk about this whole time. I hope you enjoyed the build up.

Delays

We can use the rear loudspeakers to help us create good sound staging for the front seat passengers by adjusting the delay between front and rear channels. Say what?

Yes seriously. You will not believe the difference when you set your delays properly. The sound stage will improve, the bass performance will improve and life will just be better.

Digital time alignment allows us to delay certain channels so that the sound from all loudspeakers arrives at our ears at the same time.

Think about it. In most cars, your head will actually be closer to the rear loudspeakers, and according to the laws of psychoacoustics, if we hear the rear loudspeakers even a little bit before, our brain will localize the sound from behind us. Not cool!

For more facts, check out the section on the precedence/Haas effect in the chapter called “Localization B” in this psychoacoustics course. acousticslab.org

If we delay the arrival of sound from the rear loudspeakers, we will hear the sound coming from the front speakers and the sound stage will improve.

How do I set my delays?

For now, I will consider the example of a tuning that is optimized for both front passengers. I will write a blog post about sound tuning for selfish drivers in the coming months.

In this case, the delays we use will be symmetrical, i.e. the same for left and right side. There are a few techniques and I’ll go through each.

Method 1: Use a delay calculator

Get one of those free delay calculator apps, like JLAudio I mentioned before. Type in the distance between the loudspeaker and the reference point for each channel, and kapow, it will tell you what your delay should be.

What is the reference point, I hear you say. In this case, the reference point is in the front of the car in between the two passengers, at head-height (i.e. the average position between their two heads).

If you want to think of it as an equation:

(distance of Front speaker to ref point)

  • (distance of Rear speaker to ref point)

= Distance you must delay the rear loudspeakers.

soundstage image4
But this is a distance, and delay should be in time?

It depends on what your radio or tuning tool uses, but you can calculate the time delay in one of the delay calculator apps.

This is the method used by most people and included on the instruction manual with your radio. And it works, sort of. I was brought up to use my ears damn it….that brings me to Method 2.

Method 2: Use your ears for goodness sakes

Choose a music track that you know pretty well. The quality of the production should be great, with clear localizable instruments across your sound stage, and a lush natural-sounding vocal smack bang in the middle. There should also be some bass content, but nothing too extreme.

Take the delay value you got from your app as a rough starting point, and put that into your system. The two rear loudspeakers should be delayed by the same amount. If possible, you should link the rear loudspeakers together as this will speed up the process.

Method 2: Use your ears for goodness sakes

Choose a music track that you know pretty well. The quality of the production should be great, with clear localizable instruments across your sound stage, and a lush natural-sounding vocal smack bang in the middle. There should also be some bass content, but nothing too extreme.

Take the delay value you got from your app as a rough starting point, and put that into your system. The two rear loudspeakers should be delayed by the same amount. If possible, you should link the rear loudspeakers together as this will speed up the process.

Let’s say the delay value from your app was equal to 0.8m. Compare a delay of 0 with a delay of 0.8m. Listen to your track, and ask yourself the following questions.

  1. Does the bass sound full and warm?
  2. Where is the sound stage? Try drawing it to help you remember where you hear the individual instruments
  3. What differences can I hear between delayed and not delayed?
  4. Are all the instruments clear and focused?
  5. Is there a good surrounding effect from the rear loudspeakers?
  6. Is this sound more enjoyable for me?
  7. Is the bass punchy?

Now try a delay of 0.5m with 0.8m and repeat the process. Next try a delay that is much larger, e.g. 1.2m. Write some notes each time to help you remember what you liked and didn’t like about each setting. This can be really difficult at first, but have faith, with the right music and a bit of practice, you will start to hear the differences easily and begin to work out your own listening preferences.

Sound Stage Vs Bass Performance

Some delay values will work well for the bass, giving a full, punchy sound, whilst others will do more to enhance the sound stage and pull the sound up on to the dashboard in front of you.

In some cars I work in, the loudspeakers are so low in the doors and there are no tweeters, so I know it is almost impossible to create a brilliant high stage in front of me. In this case, I’ll focus on the bass performance, and choose a delay setting that works for the bass.

Some clients are not so bothered about the bass, so I can use the front-rear delay to create a beautiful sound stage, with clear focused sources at the front, and an immersive feeling provided by the rear speakers.

How can I be sure that I have selected the right delay for great bass?

I’m glad you asked.

Method 3: The Magic Combo

Of course my ears always have the last say, but I love to use an RTA to help me set delays. I use TrueRTA on my laptop. It has good resolution in the frequency domain, (24 bands/octave) and it lets me store several measurements at once for easy comparison. Let’s go through the process.

  1. Start with the delay value from your calculator for the rear channels
  2. Make a frequency response measurement using pink noise
  3. Store the measurement
  4. Apply a new delay value, perhaps 10 cm more or less
  5. Make a new frequency response measurement and store
  6. Repeat this process for 6-10 delay values

Here is an example of several frequency response measurements with different delay values. For this example I will just examine 4 as the graph becomes messy otherwise.

delay part 3

Take a closer look at the low frequency part of the spectrum. It is inevitable that at some frequencies, the front and rear loudspeakers will reinforce one another, and at other frequencies they will cancel out. Look at these simple diagrams and keep that somewhere in your mind when tuning. www.mediacollege.com

You will be able to see that there is a lot of cancelation with some delay settings compared to others. You should choose a delay setting where there is little or no cancelation occurring. In fact, choose 2 or 3.

Now for the important part

Get your ears out and start comparing the delay values with a good bass response. Listen to a number of different tracks and choose your favourite value for each one. Follow the listening steps in Method 2, but now with the knowledge you gained from making the measurements.

You should be getting closer to the perfect delay! Leave it at this value for now, and you can come back to it again for a round of tweaking at the end.

A final piece of advice.

Playing around with the delay between two loudspeakers affects the phase between them. Phase is a mysterious beast, and sometimes a delay value will just sound shitty, even though your frequency response measurement says otherwise. In this case, go with your ears and your heart. Choose the value that works for:

  • The bass

  • The sound stage

  • Clarity & detail of the image

That’s all for now

Is there something that wasn’t clear? Send me a mail.

shelley@echo-friendly.com

Wechat: DecibelShell

In the next installment you will learn about how to tune for listening LOUD!

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