Audible frequencies inside a car

Factual Questions
1

Inspired by this thread about overly loud bass heard from car stereos. To repeat my anecdote from that thread, and pose my question:

I once had a friend, an EE, tell me much of the bass from a woofer in a car stereo is inaudible inside the vehicle. He told me that it’s because the low-frequency waves were literally too long to fit inside the car.

In that other thread, I started some back-of-the-envelope calculations, and found that wavelengths below 138hz are larger than the passenger space inside a car, and by my friends theory, inaudible. 138hz seems like an absurdly high frequency rolloff – you’d barely hear any bass at all if that were the case. But perhaps I’ve botched the math.

So, my questions:

  • Is there any truth to the assertion that, given a capable subwoofer, some low frequencies will be louder outside a vehicle, and in- or less-audible inside the vehicle?

  • If so, is this really because of the wavelength v. the vehicle size? Or something else?

  • If wavelength v. listening room is the issue, at what point does a frequency start being less audible for a given room size? Less than one wave? 1/2 wave?

2

IANAEE, but taking this logic one step further, it seems you should not be able to hear frequencies where the wavelength is too big to fit into your ear. So my guess is that this is BS.

3

Well, you have to be able to drive the air inside the car to deliver those frequencies to your ear, no? I guess the idea is that if the given air pocket can’t be driven at, say, 40hz, then you wouldn’t hear that frequency. Dunno, that’s why I posed the question.

4

Well, if I can hear the bass inside my car (with the windows rolled up), 50 feet away from your car (also with the windows rolled up), then I would have to assume you can hear it, too. We’re both inside cars, right?

The whole premise seems silly to me. Does this mean that the closer I get to the speaker, the less bass I can hear? If not, could I just roll down the windows in my car to hear more bass? But wait–the sound waves are traveling through the window glass already, so that wouldn’t help.

5

Your EE friend was living in “calculation-ville” not “reality-land.” Of course you can hear tones from your subwoofer that measure less than 138Hz.

Shoeless gets a (+1, Insightful).

6

When come back, bring reading comprehension. I calculated that, not the EE, which made me doubt it was true.

The claim was that the bass below a certain frequency was in- or less-audible. My napkin calculation was that you take length of a passenger compartment as 2.5 meters, and the speed of sound as 340m/s, then 136hz was the longest was that fits in that space.

Anybody else want to jive in with unsupported opinions and assertions? Or are we still in GQ?

7

Sure, I’m willing to take a stab at it. :slight_smile:

I started to write a long, complex explanation, but erased it. I’m still going to ask about HP, though.

You’re not listing to RF energy, and your ear isn’t something that has to be at some multiple of the wavelength to pick up. Your ear works like a speaker in reverse. Signal vibrates speaker which vibrates fluid molecule which vibrates fluid molecule ad nauseum until it reaches your eardrum, which similarly vibrates. This induces a signal which then goes to your brain. It’s all mechanical.

To suggest that the bass from a speaker can only be heard outside the car would imply that the speaker is capable of putting molecules into motion outside of the car without putting into motion the intermediate molecules. Since contact with the speaker is essential to get things started, then no, it’s total and utter B.S. that the sound can’t be heard inside the vehicle.

As for HP, do they still make electronics? Things like signal generators? Or just cheap printers and computers?

8

Fair enough, but allow me to reframe the question:

You’re sitting inside a room of certain dimensions – say, the size of a broom closet. You have with you a subwoofer, emitting booms from say 100hz to subsonic. Does the size or shape of the room you’re in limit the ability of the speaker to “drive” the air in the room, so that some frequencies will be stronger than others, and conversely, some will be less audible to you?

It would seem to me that, based on the room length, some frequencies would be reinforced. Others, perhaps, canceled.

Whoosh. You’re the first in the thread to mention HP. :confused: To answer your question, HP spun off the signal generator division into a company named Agilent.

9

Yes, the size and shape of the space can cause reflections and standing waves which can reinforce sound in some areas, and dampen it in others, but it would be very unusual to find the lower frequencies completely damped out in a car.

In fact, at least in part because the space is much smaller than the wavelength of the lowest frequencies, you don’t get reflections that are in phase or directly out of phase with the main wave. When the phase of the reflections are essentially random with respect to the phase of the primary, you get very little reinforcement or cancellation.

I’ve heard this same thing many times over the years, it seems to be a kind of audio urban legend. As near as I can tell, the reasoning seems to be “the only reason those idiots play the music so loud is that it’s actually quieter in the car than outside”. I can tell you from direct experiment and measurement that as loud as it sounds from twenty feet away, it’s even louder inside the car, low frequencies included.

For low frequencies in small spaces, consider headphones. Mine ( Senheiser HD-600) are rated down to about 20 Hz, and the space is much smaller than a car. Also consider that many medium and high end car sound systems will easily reproduce frequencies down below 50 Hz. See this website:

http://www.roadgearmag.com/article.asp?section_id=11&article_id=636

where it says:

Based on these things, I think the “car is too small to hear the soundwave” theory is shot.

10

Exercise for the reader: Using this concept, calculate the ‘cutoff’ frequency inside a pair of sealed headphones. That’ll tell you right there that there’s more going on than being able to produce a full wavelength.

11

RJKUgly, thanks for the feedback. You’re probably right, it was a crackpot theory. I’ll tell my EE friend to lay off the cheap vodka at the dorm.

Speaking of reading comprehension, Sam, my “napkin” calculation was given to show that I was skeptical of my friend’s claim. And I see RJKUgly beat you to the headphone comparison by 11 minutes, sans sarcasm.

12

No sweat, it is kind of an interesting question. And I’ve heard it so many times, I always wonder again if maybe I’m just missing something. I don’t think so, but I’m just an experienced hobbyist, not an engineer of any nature.

There is another reason for sound reinforcement that I originally had in my last post but took back out during my final edit. Resonance.

Every closed in space has a resonant frequency, which (engineers go easy on me, I’m going from old memories) depends on its size, compliance, and mass. If you pump sound into a space, and vary the frequency up and down but don’t change the amplitude of your input, the volume will go up as the frequency you use approaches the resonant frequency, peak at the resonant frequency, then drop off again as you go by.

So at the resonant frequency for an automobile’s interior, it seems to me the car body and frame are vibrating at their maximum amount, and this will put more sound outside the car, with the body of the car acting as a large passive radiator.

But those same vibrations are inside the car as well, and you’re closer to the source. In speaker talk, you’re in the “near field”, and your ears are going to be very closely “coupled” acoustically to the speakers. Believe me, you not only hear but feel those bass notes.

So even though the volume can be vary by frequency because of the size and shape of the car, it’s still always going to be louder inside the car than outside from a few yards away.

And I’ve actually measured this with test CDs and sound pressure level meters. Regardless of the frequency, I can’t ever remember getting a louder reading outside of the car than in.

13

Is there any realistic possibility of a sonic “dead spot?” College physics taught me how to forecast the location of dead spots around speakers, but that was a long time ago.

14

Actually, I wasn’t aiming that at you, and it wasn’t intended to be sarcastic. Just light in tone. My apologies if you took it the wrong way.

15

Humans can detect both sound and vibration. Heck, bouncing your suspension at 1Hz is inaudible to your basilar membrane (which only detects >20Hz) but you can still feel it. Similarly, “bass” vibrations (of less than 60Hz) affect both your basilar membrane and physically shakes about your stomach cavity and eardrum. As RJK points out, the “length” of the propagating longitudinal wave (and note that this is rather an abstraction when it comes to vibration, merely corresponding to periodicity in contractions and expansions) is only important in understanding interference effects.

16

Totally private, inside joke that means nothing to anyone but me. My “long” explanation that I started out with included reference to signal meters and other interesting HP equipment.

17

Sonic “dead spots” are more likely to appear in home applications where you’d have two speakers in a symetrical room void of furniture where the waves are somewhat controlled and predictable. Have the waves disperse in a controlled symetrical pattern from two points and you’ll find some dead spots.
However, in a car there are just way too many curved surfaces and obstacles for the waves to make any normal looking pattern. One dispersed they start reflecting and absorbing all over the place. Pretty hard to locate a dead spot in that mess.